Technical Papers

Thermoelectric Performance Model Development and Validation for a Selection and Design Tool, Thomas Nunnally, Devin Pellicone, Nathan Van Velson, James Schmidt, Tapan Desai, 2014 IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), Orlando, FL, May 27-30, 2014.

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Thermoelectric Performance Model Development and Validation for a Selection and Design Tool

A thermal model has been developed to simulate the performance of thermoelectric cooling for two avionics scenarios, where utilizing commercial off the shelf (COTS) components is highly desirable. Modeling predictions were validated through a series of experiments which studied the two scenarios at varying heat loads and heat sink thermal resistances. In these experiments, component temperatures were shown to be reduced by up to 15% with the addition of a thermoelectric cooler. Furthermore, in both scenarios, the model predicted the temperature of the cooled components within 3-10% accuracy. Further development of the model could result in a tool, which is not currently available, for optimizing system performance and determining the applicability of thermoelectric cooling in a given scenario.

High Heat Flux Heat Pipes Embedded in Metal Core Printed Circuit Boards for LED Thermal Management, Dan Pounds, Richard W. Bonner III, 2014 IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), Orlando, FL, May 27-30, 2014

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High Heat Flux Heat Pipes Embedded in Metal Core Printed Circuit Boards for LED Thermal Management

As LED applications continue to expand beyond lighting and sensors, the power levels and heat dissipation requirements will also continue to increase. Thermal management is becoming a major design issue for highpower LED systems. The size and weight of conventional bulk metal heat sinks cannot satisfy shrinking packaging constraints. Active cooling methods, such as forced air cooling or even pumped liquid, can provide acceptable performance but at the expense of increased energy consumption, reliability and most notably noise. Passive phase change (liquid to vapor) cooling devices, such as heat pipes, are well established in the electronics industry as a very effective and reliable way of removing excess waste heat at low thermal  resistance. Successful application of heat pipes in general solid-state lighting (SSL) and other higher intensity lighting products will require adapting these heat pipe technologies to the form-factor, material and cost requirements unique to SSL products. This paper describes a recent development effort that integrates heat pipes with novel wick structures into metal core printed circuit boards (MCPCB) for high power LED devices. The novelty of the advanced wick structure lies in a low evaporative thermal resistance, which was engineered to address the high heat fluxes associated with LED devices. The embedded heat pipes use water as the working fluid, allowing the MCPCB to significantly improve heat spreading capability over conventional PCBs and MCPCBs. Experimental results show an average of 35 – 45% reduction in thermal resistance from typical MCPCB sizes and materials, which agrees with numerical modeling. The advanced wick structure was engineered to maximize the evaporative heat transfer coefficient near the heat input area (>8 W/cm2-K) while maintaining high heat transport limits (>30 Watts per heat pipe). In this paper, the continuing study on heat transfer enhancement in a single-diode LED assembly is reported. Future development efforts will integrate the design in practical applications including arrays, address manufacturing issues and improving cost efficiency.

Enhancing Thermal Performance in Embedded Computing for Ruggedized Military and Avionics Applications, Darren Campo, Jens Weyant, Bryan Muzyka, 2014 IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), Orlando, FL, May 27-30, 2014.

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Enhancing Thermal Performance in Embedded Computing for Ruggedized Military and Avionics Applications

Embedded computing systems used in many military and avionics applications are trending toward higher heat fluxes, and as a result performance is being hindered by thermal limitations.  This is intensified by the high ambient conditions experience by today’s modern warfighter. In many applications liquid cooling is replacing air flow through chassis for both thermal and  environmental benefits. Although liquid cooled solutions prevent contaminates from being introduced into the electronics cage, it does introduce leak risks, particularly with line replaceable  units that are consistently swapped in and out of the card cage. For this reason it’s preferred to attach the liquid cooling loop externally to the card cage and often at the base. As a result, there are several components in the thermal path that contribute to the overall thermal resistance including the thermal interface material, conduction card, wedge lock interface, and card cage wall. This paper outlines a series of passive thermal improvements which are easily integrated into legacy, or existing, systems and can provide a 3-4x increase in dissipated power. The first area for improvement is the conduction card. Heat pipes may be incorporated into conventional conduction cards to significantly reduce thermal gradients as heat is transferred from the electronics to the wedge lock connection at the card cage wall. Likewise, heat pipes may also be used to enhance the thermal performance of the card cage wall by embedding them directly into the chassis itself, or by bolting on heat pipe embedded heat spreaders. Both solutions are particularly amenable to retrofitting and new designs. Embedding heat pipes within the conduction cards and cage provides significantly higher effective thermal conductivity with minimal size, weight, and cost consequences, especially when compared to copper and annealed pyrolytic graphite alternatives.

A Corrosion and Erosion Protection Coating for Complex Microchannel Coolers used in High Power Laser Diodes, Tapan G. Desai, Matthew Flannery, Angie Fan, Jens Weyant, Henry Eppich, Keith Lang, Richard Chin, and Aland Chin, 2014 IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), Orlando, FL, May 27-30, 2014

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A Corrosion and Erosion Protection Coating for Complex Microchannel Coolers used in High Power Laser Diodes

With efficiencies in the range of 50-75%, significant amount of waste heat is generated during the operation of high power laser diodes. In order to maintain consistent optical performance, the temperature of the laser diode needs to be managed effectively. Single phase copper microchannel coolers (MCC) with high purity and low electrical conductivity de-ionized water (DIW) at high velocities are used to dissipate the heat. Though thermally beneficial, the high velocity, high purity DIW coolant exposes the copper MCC to erosion and corrosion damage [1]. This substantially decreases the thermal performance of MCC and requires costly replacement within a short duration. This paper presents the experimental validation of a new vapor deposition coating, developed by Advanced Cooling Technologies, Inc., (ACT) for enhanced protection of MCCs in high power laser diode systems. MCC samples with coating were exposed to accelerated erosion and corrosion testing in high velocity, high purity water that simulates laser diode operating conditions. Evaluation of thermal resistance, pressure drop, and optical properties of MCC stacks demonstrated significant improvements in erosion/corrosion protection provided by the coating without impeding the cooling performance of the MCCs. Hence, the coating will increase the lifetime of MCCs and reduce costly maintenance intervals.

Effect of Crosslink Formation on Heat Conduction in Amorphous Polymers, Gota Kikugawa, Tapan G. Desai, et al., Jounal of Applied Physics 114, published online July 16, 2013

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We performed molecular dynamics (MD) simulations on amorphous polyethylene (PE) and

polystyrene (PS) in order to elucidate the effect of crosslinks between polymer chains on heat

conduction. In each polymer system, thermal conductivities were measured for a range of crosslink

concentration by using nonequilibrium MD techniques. PE comprised of 50 carbon atom long

chains exhibited slightly higher conductivity than that of 250 carbon atom long chains at the

standard state. In both cases for PE, crosslinking significantly increased conductivity and the

increase was more or less proportional to the crosslink density. On the other hand, in the PS case,

although the thermal conductivity increased with the crosslinking, the magnitude of change in

thermal conductivity was relatively small. We attribute this difference to highly heterogeneous PS

based network including phenyl side groups. In order to elucidate the mechanism for the increase

of thermal conductivity with the crosslink concentration, we decomposed energy transfer into

modes associated with various bonded and non-bonded interactions.

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Variable Conductance Heat Pipe Cooling of Stirling Convertor and General Purpose Heat Source, Calin Tarau, et al.,11th International Energy Conversion Engineering Conference (IECEC), San Jose, CA, July 15-17, 2013.

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In a Stirling Radioisotope Power System (RPS), heat must be continuously removed from the General Purpose Heat Source (GPHS) modules to maintain the modules and surrounding insulation at acceptable temperatures. The Stirling convertor normally provides this cooling. If the Stirling convertor stops in the current system, the insulation is designed to spoil, preventing damage to the GPHS at the cost of an early termination of the mission. An alkali-metal Variable Conductance Heat Pipe (VCHP) can be used to passively allow multiple stops and restarts of the Stirling convertor. In a previous NASA SBIR Program, Advanced Cooling Technologies, Inc. (ACT) developed a series of sodium VCHPs as backup cooling systems for Stirling RPS. The operation of these VCHPs was demonstrated using Stirling heater head simulators and GPHS simulators. In the most recent effort, a sodium VCHP with a stainless steel envelope was designed, fabricated and tested at NASA Glenn Research Center (GRC) with a Stirling convertor for two concepts; one for the Advanced Stirling Radioisotope Generator (ASRG) back up cooling system and one for the Long-lived Venus Lander thermal management system. The VCHP is designed to activate and remove heat from the stopped convertor at a 19 °C temperature increase from the nominal vapor temperature. The 19 °C temperature increase from nominal is low enough to avoid risking standard ASRG operation and spoiling of the Multi-Layer Insulation (MLI). In addition, the same backup cooling system can be applied to the Stirling convertor used for the refrigeration system of the Long-lived Venus Lander. The VCHP will allow the refrigeration system to: 1) rest during transit at a lower temperature than nominal; 2) pre-cool the modules to an even lower temperature before the entry in Venus atmosphere; 3) work at nominal temperature on Venus surface; 4) briefly stop multiple times on the Venus surface to allow scientific measurements. This paper presents the experimental results from integrating the VCHP with an operating Stirling convertor and describes the methodology used to achieve their successful combined operation.

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High Temperature Heat Pipes for Space Fission Power, Kara L. Walker, et al.,11th International Energy Conversion Engineering Conference (IECEC), San Jose, CA, July 15-17, 2013.

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Future space transportation and surface power applications will require small fission reactors for power generation. The fission reactors would generate up to 13kWt of power for the energy conversion system. The heat generated by the reactor would be collected and transferred by high temperature alkali metal heat pipes to a series of Stirling convertors or thermoelectric convertors for power generation. These heat pipes would be relatively long, 2 meters for a Stirling convertor design, and 4 m for a thermoelectric convertor design. Previous alkali metal heat pipe designs for spacecraft have used arterial, annular, or crescent wicks. All of these wicks can be difficult to fabricate for long heat pipes. Two other wick designs were examined in this study, grooved wicks, and self-venting arterial wicks. Artery de-priming by trapped vapor or non-condensable gas in an artery has been a single point failure for traditional arterial heat pipes. A self-venting arterial heat pipe has a screen artery that contains small venting pores in the evaporator section that allow for any trapped vapor or non-condensable gas (NCG) to escape. A trade study was conducted to compare the maximum transport and specific power for the self-vented artery, arterial, and grooved heat pipe designs. In all cases for a given diameter, the self-vented artery design carried the highest power, and had the highest specific power. Two 1-m long heat pipes were fabricated and tested, a self-venting wick and a grooved wick. The 1m long, 0.75in. (1.91cm) outer diameter sodium self-venting arterial heat pipe is capable of carrying 3.4kW of power at adverse elevations of up to 3.0in (7.62cm) without drying out. At an elevation of 5.0in (12.7cm), the self-venting arterial heat pipe is capable of carrying a maximum transport power of 1.4kW. Grooved heat pipes are typically used for spacecraft thermal control, since any NCG in the grooves can easily escape. The 1m long, 0.75in (1.91cm) outer diameter sodium grooved heat pipe is capable of carrying 846W, 546W and 346W at adverse elevations of 0.1in (0.25cm), 0.6in (1.52cm) and 1.0in (2.54cm), respectively.

 

Variable Conductance Heat Pipe Radiator for Lunar Fission Power Systems, William G. Anderson, et al., 11th International Energy Conversion Engineering Conference (IECEC), San Jose, CA, July 15-17, 2013.

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 Nuclear power systems for long-term Lunar and Martian missions present many challenges to thermal management systems, such as variable thermal loads, large temperature swings between day and night, and freezing of the working fluid. The radiator to reject the waste heat must be sized for the maximum power at the highest sink temperature, but is then oversized for other conditions, such as the Lunar/Martian night, or periods when the power to be rejected is low. A Variable Conductance Heat Pipe (VCHP) radiator can passively accommodate changing thermal loads and environments. A variable conductance thermosyphon radiator was developed and tested, with heat supplied by a single-phase pumped loop. The radiator is designed to operate in the 370 to 400 K temperature range, which is above the operating range of standard aluminum/ammonia radiator panels. To operate at these higher temperatures, the radiator has a titanium heat exchanger, titanium/water thermosyphons, and graphite fiber reinforced composite radiator panels. The radiator is capable of: 1) Accommodating changes in power and sink temperature, 2) Successfully starting up from an initially frozen state with excess water frozen in an arbitrary location, and 3) Shutting down, freezing, and then successfully restarting. A low mass design was developed that accommodates the coefficient of thermal expansion (CTE) mismatch between the titanium heat exchanger and the graphite radiator panel. A full-scale VCHP radiator was fabricated and tested in order to demonstrate functionally at a larger, more representative size and determine maximum heat rejection.

 

Ammonia and Propylene Loop Heat Pipes with Thermal Control Valves – Thermal/Vacuum and Freeze/Thaw Testing, Kara Walker, et al., 43rd International Conference on Environmental Systems (ICES 2013), Vail, CO, July 14-18, 2013.

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It is often desirable to partially or completely shut down a Loop Heat Pipe (LHP), for example, to maintain the temperature of electronics connected to the LHP on a satellite during an eclipse. The standard way to control the LHP is to apply electric power to heat the compensation chamber, reducing the pressure differential across the system and decreasing LHP flow. The amount of electrical power to shut down an LHP during an eclipse on orbit is generally reasonable due to the short duration in the cold environment. On the other hand, for LHPs on lunar and Martian landers and rovers, the electrical power requirements can be excessive, since the Lunar night lasts for 14 days. For example, the Anchor Node Mission for the International Lunar Network (ILN) has a Warm Electronics Box (WEB) and a battery, both of which must be maintained in a fairly narrow temperature range using a variable thermal conductance link. During the lunar day, heat must be transferred from the WEB to a radiator as efficiently as possible. During the night, heat transfer from the WEB must be minimized to keep the electronics and batteries warm with minimal power, especially with a very low (100 K) heat sink. A mini-LHP has the highest Technology Readiness Level, but requires electrical power to shut-down during the 14-day lunar night, with a significant penalty in battery mass: 1 watt of electrical power translates into 5kg of battery mass. Ammonia and propylene LHPs with Thermal Control Valves (TCVs) were developed to provide passive variable thermal conductance without electrical power. The TCV routes vapor to the condenser, or bypasses the condenser and routes the vapor back to the compensation chamber, depending upon the environmental temperature conditions. Thermal vacuum testing of both LHPs with thermal control valves demonstrated the ability of the TCV to passively maintain a warm evaporator during roughly 24 hours of operation at a 0W power input and a -60°C sink. For lunar applications, the sink temperature during the lunar night could reach as low as -223°C. It is possible for ammonia to freeze, potentially causing structural damage as the ammonia melts and expands. Freeze/thaw testing of a vertical condenser on an ammonia LHP with TCV was performed that showed negliglbe change in condenser dimensions after 9 freeze/thaw cycles.

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Intermediate Temperature Heat Pipe Life Tests and Analyses, W. G. Anderson, et al., 43rd International Conference on Environmental Systems (ICES 2013), Vail, CO, July 14-18, 2013

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There are a number of applications that could use heat pipes or loop heat pipes (LHPs) in the intermediate temperature range of 450 to 750 K, including space nuclear power system radiators, fuel cells, geothermal power, waste heat recovery systems, and high temperature electronics cooling. Since 2004, we have been conducting life tests at temperatures up to 550 K with water and Commercially Pure Titanium Grade 2 (CP-Ti), titanium alloys, Monel 400, and Monel K500 heat pipes. Since 2006, life tests have been conducted at temperatures up to 673 K with titanium and Hastelloy B-3, C-22, and C-2000 envelopes paired with AlBr3, GaCl3, SnCl4, TiCl4, and TiBr4 halide working fluids. Recently, roughly half of these heat pipes were selected for destructive evaluation. The working fluids were analyzed, and sections of the heat pipes were examined to determine the type and amount of corrosion in the wicks and heat pipes. The results showed that Titanium/water and Monel/water heat pipes are suitable for temperatures up to 550 K. Analysis of titanium/water heat pipe cross-sections using optical and electron microscopy revealed little if any corrosion even when observed at high magnifications. Copper depleted zones, as well as copper surface nodules formed on the Monel 400 screen wick, but not on the Monel K500 envelopes. An analysis of the water working fluids showed minimal pickup of metals. The long terms tests also established that Titanium/TiBr4 at 653 K, and Hastelloy B-3, C-22 and C-2000/AlBr3 at 673 K were compatible. Hastelloy C-2000 underwent little corrosion when used with TiCl4 working fluid. Hastelloy C-22 exhibited a 5-10 micrometer thick dual corrosion layer when tested with AlBr3 working fluid. The results indicate that the tested envelope materials and working fluids can form viable material/working fluid combinations.

 

A Non-Catalytic Fuel-Flexible Reformer, Chien-Hua Chen, et al., 8th U. S. National Combustion Meeting, hosted by the University of Utah, May 19-22, 2013

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A compact, non-catalytic, thermal partial oxidation fuel reformer was developed based on the Swiss-roll heat-recirculating combustor concept with the reforming reactions take place at elevated temperature in the centrally-located combustion chamber of a spiral heat exchanger. In this reformer, the reactants (hydrocarbon fuel and air) are preheated by the partial oxidation products of combustion resulting in a reaction temperature that is higher than the adiabatic flame temperature which accelerates chemical reaction rates and leads to partial oxidation product compositions approaching the chemical equilibrium state, yielding high H2 and CO (syngas) concentrations without requiring an external heat source. No catalyst is needed, thus eliminating undesirable issues associated with catalytic based reformers including sulfur poisoning and carbon deposition. In addition, the Swiss-roll combustor is compact and thermally efficient due to the high effectiveness of the spiral heat exchanger. Using a 6-turn Swiss-roll reformer (5 cm tall, 8 cm overall diameter) and a rich propane-air premixed feedstock with equivalence ratio = 3, experiments showed about 73% of input chemical enthalpy remains in the reformate (18% H2 and 18% CO). Consequently, of the total input chemical enthalpy of 2400 W, only about 650 W is released as thermal enthalpy during partial oxidation to self-sustain the reaction. The effect of fuel-to-air ratio was studied to evaluate its effect on the H2 and CO yields. Fuel flexibility was tested using different fuels (propane, n-heptane and JP-8); at the same equivalence ratio and flow rate, similar results were obtained for all three fuels. No visible soot formation was observed in the flame/flare at the exit from the reformer when reforming propane or n-heptane. Current work aims to optimize the reformate product for portable solid oxide fuel cell generators.

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Variable Conductance Thermal Management System for Balloon Payloads, Calin Tarau and William G. Anderson, 20th AIAA Lighter-Than-Air Systems Technology Conference, Daytona Beach, FL, March, 25-28, 2013

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Variable Conductance Thermal Management System for Balloon Payload

Calin Tarau1 and William G. Anderson2
Advanced Cooling Technologies, Inc., 1046 New Holland Ave. Lancaster, PA 17601 U.S.A. 717-295-6066, calin.tarau@1-ACT.com

 While continuously increasing in complexity, the payloads of terrestrial high altitude balloons need a thermal management system to reject their waste heat and to maintain a stable temperature as the air (heat sink) temperature swings from as cold as -90°C to as hot as +40°C. The current solution consists of copper-methanol Constant Conductance Heat Pipes (CCHPs). The problem with these devices is that the conductance cannot effectively be reduced under cold operating or cold survival environment conditions, so an active heater requiring significant energy is required to maintain the instruments in their normal operating range. 

This paper presents the development of a low cost Variable Conductance Heat Pipe (VCHP) that allows the thermal resistance to increase passively under cold operating or cold survival environment conditions, keeping the instrument section warm with minimal electric heating. This VCHP is based on smooth-bore, thin-wall stainless steel tubing, with methanol, toluene or pentane as working fluids, and is capable of passively maintaining a relatively constant evaporator (payload) temperature while the sink

temperature varies between -90°C and +40°C. Two configurations were developed, a cold reservoir one (reservoir is attached to the condenser) and a hot reservoir one (reservoir is attached to the evaporator). Both onfigurations were tested with the above mentioned working fluids and the experimental results were consistent with the modeling results. In all experimental cases, the evaporator temperature was maintained within the required interval of -10ºC…+50ºC while the sink temperature varied between -90°C and +40°C. 

The hot reservoir configuration showed the tightest temperature control. For example, the pentane based hot reservoir VCHP allowed the evaporator temperature to change only 3.7ºC from the coldest to hottest heat sinks. The largest temperature variation observed was 32.6ºC for the pentane based cold reservoir VCHP, still meeting the design requirements. Survival tests were also carried out but only for the toluene based cold reservoir VCHP. A duration of 13,000 seconds was needed by the evaporator to cool from 49ºC down to 20ºC while the power was shut down and the sink (condenser) was continuously as cold as -90ºC.

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Variable Conductance Heat Pipe Radiator Trade Study for Lunar Fission Power Systems, William G. Anderson, Bryan J. Muzyka, and John R. Hartenstine, Nuclear and Emerging Technologies for Space (NETS-2013), Albuquerque, NM, February 25-28, 2013.

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Variable Conductance Heat Pipe Radiator Trade Study for Lunar Fission Power Systems

Nuclear power systems for long-term Lunar and Martian missions present many challenges to thermal management systems, such as variable thermal loads, large temperature swings between day and night, and freezing of the working fluid. The radiator to reject the waste heat must be sized for the maximum power at the highest sink temperature. This radiator is then oversized for other conditions, such as the Lunar/Martian night, or periods when the power to be rejected is low. A Variable Conductance Heat Pipe (VCHP) radiator can passively accommodate changing thermal loads and environments. A trade study was conducted to select the best design for the heat exchanger/heat pipe interface. The interfaces that were examined were an annular evaporator, bent tubes inserted into the coolant channel and a POCO Foam saddle that engulfed the coolant channel and evaporator portion of the VCHP. The trade study showed the highest specific power was for the annular evaporator, roughly 10% higher specific power than the competing interface designs.

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Alkali Metal Backup Cooling for Stirling Systems – Experimental Results, Carl Schwendeman, Calin Tarau, William G. Anderson, and Peggy A. Cornell, Nuclear and Emerging Technologies for Space (NETS-2013), Albuquerque, NM, February 25-28, 2013.

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Alkali Metal Backup Cooling for Stirling Systems

In a Stirling Radioisotope Power System (RPS), heat must be continuously removed from the General Purpose Heat Source (GPHS) modules to maintain the modules and surrounding insulation at acceptable temperatures. The Stirling convertor normally provides this cooling. If the Stirling convertor stops in the current system, the insulation is designed to spoil, preventing damage to the GPHS at the cost of an early termination of the mission. An alkali-metal Variable Conductance Heat Pipe (VCHP) can be used to passively allow multiple stops and restarts of the Stirling convertor. In a previous NASA SBIR Program, Advanced Cooling Technologies, Inc. (ACT) developed a series of sodium VCHPs as backup cooling systems for Stirling RPS. The operation of these VCHPs was demonstrated using Stirling heater head simulators and GPHS simulators. In the most recent effort, a sodium VCHP with a stainless steel envelope was designed, fabricated and tested at NASA Glenn Research Center (GRC) with a Stirling convertor for two concepts; one for the Advanced Stirling Radioisotope Generator (ASRG) back up cooling system and one for the Long-lived Venus Lander thermal management system. The VCHP is designed to activate and remove heat from the stopped convertor at a 19 °C temperature increase from the nominal vapor temperature. The 19 °C temperature increase from nominal is low enough to avoid risking standard ASRG operation and spoiling of the Multi-Layer Insulation (MLI). In addition, the same backup cooling system can be applied to the Stirling convertor used for the refrigeration system of the Long-lived Venus Lander. The VCHP will allow the refrigeration system to: 1) rest during transit at a lower temperature than nominal; 2) pre-cool the modules to an even lower temperature before the entry in Venus atmosphere; 3) work at nominal temperature on Venus surface; 4) briefly stop multiple times on the Venus surface to allow scientific measurements. This paper presents the experimental results from integrating the VCHP with an operating Stirling convertor and describes the methodology used to achieve their successful combined operation.

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Alkali Metal Heat Pipes for Space Fission Power, Kara L. Walker, Calin Tarau, and William G. Anderson, Nuclear and Emerging Technologies for Space (NETS-2013), Albuquerque, NM, February 25-28, 2013.

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Alkali Metal Heat Pipes for Space Fission Power

Future space transportation and surface power applications will require small fission reactors for power generation. The fission reactors would generate up to 13kWt of power for the energy conversion system. The heat generated by the reactor would be collected and transferred to a series of Stirling convertors or thermoelectric converters for power generation. This heat collection and transfer can be done by high-temperature alkali-metal heat pipes. Two 1m long sodium heat pipes were designed, fabricated, and tested; a self-venting arterial heat pipe and a grooved heat pipe. Artery de-priming by trapped vapor or non-condensable gas in an artery has been a single point failure for traditional arterial heat pipes. A self-venting arterial heat pipe has a screen artery that contains small venting pores in the evaporator section that allow for any trapped vapor or non-condensable gas (NCG) to escape. ACT has demonstrated that a 1m long, 0.75in. (1.91cm) outer diameter sodium self-venting arterial heat pipe is capable of carrying 3.4kW of power at adverse elevations of 0.1in (0.25cm), 0.3in (0.76cm), 0.6in (1.52cm), 1.0in (2.54cm) and 3.0in (7.62cm) without drying out. The sodium self-venting arterial heat pipe is capable of carrying a maximum transport power of 1.4kW at an adverse elevation of 5.0in (12.7cm). Grooved heat pipes are typically used for spacecraft thermal control, since any NCG in the grooves can easily escape. ACT has also demonstrated that a 1m long, 0.75in (1.91cm) outer diameter sodium grooved heat pipe is capable of carrying 846W, 546W and 346W at adverse elevations of 0.1in (0.25cm), 0.6in (1.52cm) and 1.0in (2.54cm), respectively.

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Syngas Production by Thermochemical Conversion of CO2 and H2O Using a High-Temperature Heat Pipe Based Reactor, H. Pearlman and Chien-Hua Chen, SPIE Solar Hydrogen and Nanotechnology VII, Proceedings of SPIE Vol. 8469 San Diego, CA, August 12-14, 2012.

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Syngas Production by Thermochemical Conversion of CO2 and H20 Using a High-Temperature Heat Pipe Based Reactor

The design of a new high-temperature, solar-based reactor for thermochemical production of syngas using water and carbon dioxide will be discussed. The reactor incorporates the use of high-temperature heat pipe(s) that efficiently transfer the heat from a solar collector to a porous metal oxide material. Special attention is given to the thermal characteristics of the reactor, which are key factors affecting the overall system efficiency and amount of fuel produced. The thermochemical cycle that is considered is that for ceria based material. Preliminary data acquired from an early stage reactor, operated at temperatures up to 1100°C, is presented and efforts are now underway to increase the operating temperature of the reactor to 1300°C to further increase the efficiency of the thermochemical fuel production process.

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Diode Heat Pipes for Venus Landers, Calin Tarau et al., 9th Intersociety Energy and Conversion Engineering Conference (IECEC), San Diego, CA, July 31 - August 3, 2012.

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Diode Heat Pipes for Venus Landers

Cooling during normal operation of the Long-lived Venus Lander can be provided with a radioisotope Stirling power converter that energizes Stirling coolers. High temperature heat from roughly 10 General Purpose Heat Source (GPHS) modules must be delivered to the Stirling convertor with minimal ΔT. In addition, the cooling system must be shut off during transit to Venus without overheating the GPHS modules. This heat is managed by a High Temperature Thermal Management System (HTTMS). During normal operation, waste heat is produced at both the cold end of the main Stirling converter and the hot end of the highest rank Stirling cooler. It is critical for this waste heat to be rejected into the environment also with a minimal ?T to maintain a high efficiency for the cooling system. A passive Intermediate Temperature (~520°C) Thermal Management System (ITTMS) that will reject this waste heat is under development. During transit, the cooling system rests and no waste heat is generated. In turn, the HTTMS will reject high temperature heat bypassing the Stirling converter’s heater head and heating the ITTMS. Diode heat pipes are required so that heat will not be transmitted in the reverse direction, from the radiator heat pipes to the cold end of the Stirling converter and hot end of the highest rank cooler. A gas charged alkali metal Diode Heat Pipe (DHP) is under development for this purpose. Two proof of concept potassium DHPs that differ in the size of their reservoir connecting tube were tested at 525°C transporting a power of 500W. The pipes worked in both Heat Pipe Mode and Diode Mode intermittently as the power was applied at the evaporator and condenser, demonstrating the concept. The DHP with a larger diameter reservoir connecting tube showed faster transients during the returning from the Diode Mode to the Heat Pipe Mode.

Long-Lived Venus Lander Thermal Management System Design, Rebecca Hay et al., 9th Intersociety Energy and Conversion Engineering Conference (IECEC), Atlanta, GA, July 30 July-August 1, 2012.

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The thermal management system for a Long Lived Venus Lander is critical for mission success. The difficult operating environment on Venus, 460°C and 92 atm pressure, presents significant thermal design and implementation challenges. No previous mission has operated for more than two hours on the surface of Venus. Missions with longer operating duration on the Venus’ surface are needed for a more consistent science return. Dyson et al. have proposed a Stirling system that generates power as well as provides cooling to allow the lander’s instruments to survive for more than one Venusian year in the harsh environment of Venus’ surface. It consists of a multi-stage cooling system using several Stirling engines in reversed cycle, and an energizing radioisotope Stirling power converter that is coupled (electrically or pneumatically) to the Stirling coolers. During operation, heat is supplied to the main (energizing) Stirling converter at approximately 1000°C. This heat is provided by the High Temperature Thermal Management System (HTTMS). The converter generates electric power, and powers the different stages of the Stirling coolers. The waste heat from the Stirling converter is at roughly 500°C, and must be rejected to the Venusian environment via the Intermediate Temperature Thermal Management System (ITTMS). The Venus lander has to operate successfully in four different modes: (1) space transit with the Stirling deactivated, (2) precooling in space prior to entry, (3) nominal operation on Venus, and (4) stoppage of the Stirling on Venus. This is accomplished using a variable conductance heat pipe (VCHP) as the primary component of the HTTMS and a diode heat pipe (DHP) for the ITTMS. Systematic system level thermal analysis has been carried out on the HTTMS and the ITTMS simultaneously due to their strong thermal interaction. To reach thermal solutions for each mode, this analysis was carried through several iterations. The goal was to observe critical temperatures and thermal resistances within the system when applying boundary conditions that correspond to each mode. These temperatures were then used in subsequent iterations to more accurately design HTTMS and ITTMS components (ex. VCHP reservoir, DHP reservoir, VCHP radiator size, etc.).

Variable Conductance Heat Pipes for Variable Thermal Links, W. G. Anderson et al., 42nd International Conference on Environmental Systems (ICES 2012), San Diego, CA, July 15-19, 2012.

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Variable Conductance Heat Pipes for Variable Thermal Links

Variable Conductance Heat Pipes (VCHPs) for spacecraft thermal control typically have a cold-biased reservoir for the Non-Condensable Gas (NCG) at the end of the condenser. During operation, electrical heat is supplied to the reservoir to provide ±1-2°C temperature control over widely varying powers and sink temperatures. A second application for VCHPs is as a variable thermal link. Applications that can benefit from using VCHPs as variable thermal links include Lunar and Martian Landers and Rovers, Research Balloons, and Lunar and Space Fission Reactors. The applications that can benefit from variable thermal links normally have: 1. Variable system loads resulting from intermittent use, 2. Large variations in the sink temperature, and 3. Limited electrical power. Since the lowest sink temperature can be below the freezing point of the working fluid, many applications with variable thermal links also need to consider freeze/thaw and start-up from a frozen state. Fortunately, the NCG in the heat pipe also helps when the pipe is frozen, and during start-up. An aluminum/ammonia VCHP was developed to act as a variable thermal link for lunar landers and rovers, passively minimizing heat losses during the lunar night, without requiring electric power to shut off. The reservoir was located near the evaporator, rather than near the condenser, to prevent the reservoir temperature from dropping during the lunar night, without requiring electrical heaters. Variable thermal links were also developed for high altitude, research balloons. Two VCHP configurations (hot and cold reservoir) were designed, fabricated and successfully tested, with methanol, toluene, and pentane as the working fluids. Both configurations provide a variable thermal link without electrical power. The warm reservoir VCHP has a 4.8°C temperature control band, while the cold reservoir control band is larger, at 21°C.

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Pressure Controlled Heat Pipe Applications, W. G. Anderson et al., 16th International Heat Pipe Conference, Lyon, France, May 20-24, 2012.

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Pressure Controlled Heat Pipe Applications

In a Variable Conductance Heat Pipe (VCHP), a Non-Condensable Gas (NCG) is added to the heat pipe to vary the condenser length, and hence the conductance. A Pressure Controlled Heat Pipes (PCHP) is a modified VCHP, where the heat pipe operation is controlled by varying either the gas quantity or the volume of the gas reservoir. This paper will discuss two applications for PCHPs: 1. Precise temperature control, and (2) Switching thermal power between multiple sinks. A prototype aluminum/ammonia PCHP was built and tested to demonstrate the capability of controlling the temperature of the evaporator section of an aluminum/ammonia pressure controlled heat pipe to milli-Kelvin levels over an extended period of time, while the heat sink temperature and evaporator power were varied. In a second program, a heat pipe solar receiver was designed to accept, isothermalize and transfer the solar thermal energy to reactors for oxygen production from lunar regolith. The receiver has two PCHPs and two Constant Conductance Heat Pipes (CCHPs) to supply heat to two reactors. During operation, one reactor is producing hydrogen at low solar power, while the other reactor is warming up a fresh batch of regolith. The PCHPs are used to switch power between the two reactors as required.

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Variable Conductance Heat Pipes for Variable Thermal Links, W. G. Anderson et al., 16th International Heat Pipe Conference, Lyon, France, May 20-24, 2012.

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Variable Conductance Heat Pipes for Variable Thermal Links

Variable Conductance Heat Pipes (VCHPs) for spacecraft thermal control typically have a cold-biased reservoir at the end of the condenser. During operation, electrical heat is supplied to the reservoir to provide ± 1-2°C temperature control over widely varying powers and sink temperatures. A second application for VCHPs is as a variable thermal link for lunar landers and rovers, while minimizing the required electrical power. During the long lunar day, the VCHP must remove waste heat from the electronics and batteries to prevent overheating. During the long lunar night, the variable thermal link must passively limit the amount of heat removed from the electronics and radiated to space since little to no power is available for temperature regulation. A VCHP was developed to act as a variable thermal link for lunar landers and rovers, passively minimizing heat losses during the lunar night, without requiring electric power to shut off. In addition to acting as a thermal link, the VHCP was able to withstand multiple freeze/thaw cycles without performance degradation. Short-duration, full-power bursts were demonstrated during -60 °C and -177 °C cold shutdown. Startup of the VCHP with a frozen condenser was also demonstrated.

The Affect of Device Level Modeling on System-Level Thermal Predictions, Jens Weyant, et, al., ITerm, San Diego, CA, May 30, 2012,

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The Affect of Device Level Modeling on System-Level Thermal Predictions

Thermal management is important for the performance and reliability of today’s high power and high density electronics systems. The thermal architecture between the device and heat sink can quickly become very complex when designing for ideal operating temperatures. In order to predict the temperature rise, it is desirable to have a simple modeling technique which reduces the amount of time and effort required to obtain accurate results. Often, the heat flux of the device is based on either the die area or the case area. Complication occurs when simplifying the contact area of a given component.

Detailed analyses have been performed for two different cases that show the importance of die-level modeling. In the first case, models of an insulated gate bipolar transistor (IGBT) attached to a cold plate are compared to determine the cold plate temperatures when assuming uniform heat flux, and when modeling from the device level. The different analyses results in a heat sink ΔT that differs by 33%. In the second case, a heat spreader is used to cool several high power components. The heat generation areas of the components are significantly smaller than the case footprint. A detailed look at the device level spreading reveals a difference in maximum temperature of 14.5°C between the results of the different modeling techniques used.

Integration of a Phase Change Material for Junction-Level Cooling in GaN Devices, Daniel Piedra, et, al., Semitherm, San Jose, CA, March 2012

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Integration of a Phase Change Material for Junction-Level Cooling in GaN Devices

Daniel Piedra, Tapan G. Desai, Richard Bonner, Min Sun, Tomás Palacios Dept. of Electrical Engineering and Computer Science, Massachusetts Institute of Technology 77 Massachusetts Avenue, Rm. 39-623 Cambridge, MA 02139 Advanced Cooling Technologies, Inc.,1046 New Holland Ave.Lancaster, PA 17601

Next generation gallium nitride (GaN) RF power transistors offer higher power, higher efficiency and wider bandwidth than competing Si technologies. However, the high power densities available in GaN power transistors create new challenges for heat dissipation. This paper presents a novel micro-scale thermal storage design that involves phase change material (PCM) filled grooves etched in the substrate to remove the heat generated in the active regions of a pulsed mode GaN transistor. High electron mobility transistors(HEMTs) were fabricated on a GaN-on-Si wafer. Backside patterning and etching were done to thin the Si substrate under the active channel region of the selected transistors. A phase change material (PCM) with a melting temperature of 118°C was deposited in the etched grooves. Electrical measurements were carried out to compare the performance of transistors with and without PCM filled grooves. It was found that the groove etching did not degrade the transistor performance under low power conditions where the junction level heating is not enough to start the PCM melting process. From the current-voltage characteristics at different temperatures ranging from 25°C to 120°C, it was found that at higher temperatures, the current density in the PCM-enabled device was larger than in the reference device, due to the enhanced thermal management. The role of PCM was confirmed when measurements at temperatures well above the melting temperature of the PCM did not show signs of increase in the current density. The maximum current density in the device with PCM material was found to be much more stable under pulsed conditions than in current state-of-the-art devices.

An Innovative Passive Cooling Method for High Performance Light-emitting Diodes, Angie Fan, et, al., Semitherm, San Jose, CA, March 2012

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An Innovative Passive Cooling Method for High Performance Light-emitting Diodes

Angie Fan, Richard Bonner, Stephen Sharratt and Y. Sungtaek Ju, Advanced Cooling Technologies, Inc. 1046 New Holland Ave, Lancaster, PA, University of California, Los Angeles, CA. Presented at Semitherm 2012, San Jose Ca, March 2012

Thermal management challenges are becoming a major roadblock to the wide use of high-power LED lighting systems. Incremental improvements in conventional bulk metal heat sinks and thermal interface materials are projected to be insufficient to meet these challenges. Active cooling methods, such as forced air and pumped liquid cooling, may provide better performance but at the expense of higher cost and energy consumption. Passive phase change (liquid tovapor) cooling devices, such as heat pipes and thermosyphons, are well established in the electronicsindustry as a very effective and reliable way of removing excess waste heat at low thermal resistance. Successful application of heat pipes and thermosyphons in solid-state lighting (SSL) products will require adapting the technologies to the form-factor, material and cost requirements unique to SSL products. This paper describes a recent development effort that integrates a planar thermosyphon into a printed circuit board (PCB) for LED devices. The planar thermosyphon/PCB uses a dielectric fluid as the heat pipe working fluid, achieving significantly improved heat spreading performances over conventional PCBs. Analytical modeling showed a more than 50% thermal resistance reduction from typical metal core PCBs. A low temperature electroplating technique was also investigated to fabricate wick structures onto PCB surfaces to enhance the boiling heat transfer performance of the dielectric fluids. Test results showed that a boiling heat transfer coefficient of 20,000W/m2-K can be achieved with the 3M Novec fluid. In this paper, the preliminary study on heat transfer enhancement by using the PCB planar thermosyphon in single LED assembly was reported. Future development efforts will verifythe design in practical applications, address manufacturing issues and improve the cost efficiency.

Ultra High Temperature Isothermal Furnace Liners (IFLs) For Copper Freeze Point Cells, Peter Dussinger and John Tavener, 9th International Temperature Symposium, Anaheim, CA, March 2012

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Ultra High Temperature Isothermal Furnace Liners (IFLs) For Copper Freeze Point Cells,

P.M. Dussinger, Advanced Cooling Technologies, Inc., J.P. Tavener, Isothermal Technology Ltd, Pine Grove, Southport, England, presented at 9th International Temperature symposium, Anaheim, CA.

Primary Laboratories use large fixed-point cells in deep calibration furnaces utilizing heat pipes to eliminate temperature gradients. This combination of furnace, heat pipe, and cell gives the smallest of uncertainties. The heat pipe, also known as an isothermal furnace liner (IFL), has typically been manufactured with Alloy 600/601 as the envelope material since the introduction of high temperature IFLs over 40 years ago. Alloy 600/601 is a widely available high temperature material, which is compatible with Cesium, Potassium, and Sodium and has adequate oxidation resistance and reasonable high temperature strength. Advanced Cooling Technologies, Inc. (ACT) Alloy 600/Sodium IFLs are rated to 1,100°C for approximately 1,000 hours of operation (based on creep strength). Laboratories interested in performing calibrations and studies around the copper freeze-point (1084.62°C) were frustrated by the 1,000 hours at 1,100°C limitation and the fact that expensive freeze-point cells were getting stuck and/or crushed inside the IFL. Because of this growing frustration/need, ACT developed an Ultra High Temperature IFL to take advantage of the exceptional high temperature strength properties of Haynes 230.

High Heat Flux, High Power, Low Resistance, Low CTE Two-Phase Thermal Ground Planes for Direct Die Attach Applications, Peter Dussinger, et, al., GOMACTech 2012, Las Vegas, NV, March 2012

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High Heat Flux, High Power, Low Resistance, Low CTE Two-Phase Thermal Ground Planes for Direct Die Attach Applications,

Peter Dussinger ACT, Dr. Y. Sungtaek Ju & Dr. Ivan Catton University of California Los Angeles, Dr. Massoud Kaviany University of Michigan presented at (GOMACTech 2012) in Las Vegas, NV, March 2012

A low coefficient of thermal expansion (CTE) vapor chamber for heat transport and spreading was developed for thermal management of high-power, high heat flux silicon, gallium arsenide, or gallium nitride microelectronics chips. The development effort focused on innovative wick structures and low-CTE envelope materials, specifically aluminum nitride ceramic with direct bond copper. The low CTE construction allows for direct die attach, eliminating the thermal resistance of the die substrate and associated interface.

Passive Control of a Loop Heat Pipe with Thermal Control Valve for Lunar Lander Application, K. L. Walker et al., 42nd International Conference on Environmental Systems (ICES 2012), San Diego, CA, July 15-19, 2012.

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Passive Control of a Loop Heat Pipe with Thermal Control Valve for Lunar Lander Application

It is often desirable to partially or completely shut down a Loop Heat Pipe (LHP), for example, to maintain the temperature of electronics connected to the LHP on a satellite during an eclipse. The standard way to control the LHP is to apply electric power to heat the compensation chamber. The amount of electrical power to shut down an LHP during an eclipse on orbit is generally reasonable. On the other hand, for LHPs on Lunar and Martian Landers and Rovers, the electrical power requirements can be excessive. For example, the Anchor Node Mission for the International Lunar Network (ILN) has a Warm Electronics Box (WEB) and a battery, both of which must be maintained in a fairly narrow temperature range using a variable thermal conductance link. During the Lunar day, heat must be transferred from the WEB to a radiator as efficiently as possible. During the night, heat transfer from the WEB must be minimized to keep the electronics and batteries warm with minimal power, even with a very low (100 K) heat sink. A mini-LHP has the highest Technology Readiness Level, but requires electrical power to shut-down during the 14-day lunar night, with a significant penalty in battery mass: 1 watt of electrical power translates into 5kg of battery mass. A mini-LHP with a Thermal Control Valve (TCV) was developed to shut down without electrical power. An aluminum/ammonia LHP which included a TCV in the vapor exit line from the evaporator was designed, fabricated and tested. The TCV could route vapor to the condenser, or bypass the condenser and route back to the compensation chamber, depending upon the temperature conditions. During test, the LHP condenser was cycled from approximately 30°C to -60°C and the power was kept constant; the evaporator remained above 19°C.

A Computational Model of a Phase Change Material Heat Exchanger in a Vapor Compression System with a Large Pulsed Heat Load, G. Troszak and X. Tang, Proceedings of the ASME 2012 Summer Heat Transfer Conference, Puerto Rico, July 8-12, 2012.

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A Computational Model of a Phase Change Material Heat Exchanger in a Vapor Compression System with a Large Pulsed Heat Load

Phase change materials (PCMs) use latent heat to store a large amount of thermal energy over a narrow temperature range. While PCMs are commonly used for thermal storage applications, they may be also used to dampen large pulsed heat loads, which are commonly generated by high-power electronics and direct-energy weapons. During a pulse the PCM absorbs some of the large heat load, and between pulses the heat is dissipated to a cooling system, which minimizes the instantaneous head load applied to the cooling system, reducing its physical size and power consumption.

To minimize the size of a PCM heat exchanger, a simple computational model that can capture the transient thermal response of the flat plate PCM heat exchanger in a vapor compression cooling system with a pulsed heat load was developed. Using this model, the effect of PCM thermal conductivity and melt temperature had the greatest impact on the PCM heat exchanger size. The ideal PCM heat exchanger would contain a relatively high thermal conductivity PCM with a melt temperature close to  the desired heat source temperature.

2-D Simulation of Hot Electron-Phonon Interactions in a Submicron Gallium Nitride Device Using Hydrodynamic Transport Approach, Angie Fan et, al., ASME 2012 Summer Heat Transfer Conference, Puerto Rico, USA , July 8-12, 2012,

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2-D Simulation of Hot Electron-Phonon Interactions in a Submicron Gallium Nitride Device Using Hydrodynamic Transport Approach

In this study, a thermal and electrical coupled device solver is developed to simulate the energy transfer mechanism within a GaN FET with a gate length of 0.2 mm. The simulation simultaneously solves a set of hydrodynamic equations (derived from the Boltzmann Transport Equation) and the Poisson equation for electron, optical phonon and acoustic phonon energies, electron number density, electric field and electric potential. This approach has been previously established for gallium arsenide (GaAs) devices [36,37], but has not been extended to GaN due to the lack of readily available property values for GaN devices that are required. Via extensive literature study, high-fidelity properties for GaN were collected in analytical forms with respect to many dependencies, e.g. lattice temperature, electrical field, electron number density, doping rate, defects rate. These properties are then implemented into the developed code to provide a high accuracy sub-micron GaN device simulation.

Simulations show that non-equilibrium heat generation is exhibited in a typical device while the drain current is reduced due to the decrease in electron mobility. Future analysis is needed to quantify the hot-electron effect on reducing the drain current and to discover more effective ways of heat removal.

 

Novel Junction Level Cooling in Pulsed GaN Devices, Tapan G. Desai, et, al., ITerm, San Diago, CA, May 30, 2012,

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Novel Junction Level Cooling in Pulsed GaN Devices

Gallium nitride (GaN) based RF power transistor technology offers the unique combination of higher power, higher efficiency and wider bandwidth. However, the extremely high power densities create new challenges for heat dissipation. In pulsed GaN devices, each duty cycle consists of an active period of heat generation followed by an inactive period. The device temperature oscillates causing thermal stresses leading to device fatigue and life reduction. In this paper, we present a novel junction level cooling technique based on a compact thermal storage design that involves phase change material (PCM) filled micrometer-sized grooves etched in the semiconductor substrate. PCM located close to the junction absorbs waste heat during the active period and dissipates the heat to a heat sink during the inactive period. Computational simulations proved the feasibility of the concept and showed reduction in junction level temperatures. High electron mobility transistors (HEMTs) were fabricated on a GaN-on-silicon wafer with micrometer-sized grooves. The selection of appropriate PCM (critical for concept’s success) to completely fill the grooves was done by performing wetting tests. DC and pulsed characterization of the new PCM-enabled devices showed up to 10% improvement in the electrical device performance (due to enhanced thermal management) compared to baseline GaN transistors.

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Intermediate Temperature Heat Pipe Life Tests, W. G. Anderson et, al., 16th International Heat Pipe Conference, Lyon, France, May 20-24, 2012.,

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Intermediate Temperature Heat Pipe Life Tests

There are a number of different applications that could use heat pipes or loop heat pipes (LHPs) in the intermediate temperature range of 450 to 750 K, including space nuclear power system radiators, fuel cells, geothermal power, waste heat recovery systems, and high temperature electronics cooling. Titanium/water and Monel/water heat pipes are suitable for temperatures up to 550 K, based on life tests that have been running for over 54,000 hours (6.1 years). At higher temperatures, organic or halide working fluids can be used. Long term life tests (currently 50,000 hours or 5.7 years) show that Titanium/TiBr4 at 653 K, and Superalloys/AlBr3 at 673 K are compatible. These results are confirmed by optical and electron microscopy, and working fluids analysis on heat pipes chosen for destructive examination.

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Passivation Coatings for Micro-channel Coolers, Richard W. Bonner III, Jens Weyant, Evan Fleming, Kevin Lu, Daniel Reist, APEC 2012, Orlando FL, February 1, 2012

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Passivation Coatings for Micro-channel Coolers

Many high heat flux electronics applications have surpassed the limits of air cooling and are moving towards liquid cooling as a method to remove waste heat directly from electronics packages. In applications such as power electronics, high liquid velocities along with highly corrosive coolants (DI water in particular) limit the reliability and performance capability of many liquid cooling solutions. Engineering solutions include the use of stainless steel in place of better performing copper or aluminum materials and the use of costly nickel and gold plating where the performance of copper is required. The problem is particularly difficult with copper micro-channel coolers (MCCs), as conventional plating techniques are not capable of creating conformal coatings in the micro-channels. In this paper, preliminary results on the passivation capability of nano-scale alumina coatings deposited by atomic layer deposition (ALD) are presented. ALD coatings are deposited in the vapor phase, one atomic layer at a time, resulting in unmatched conformality and coating thickness uniformity on almost any geometry. Experimental results for thermal cycling, erosion and corrosion passivation performance with salt water are presented for a baseline (copper), gold plated copper, and ALD coated copper micro-channel cooler. In all cases the ALD coated samples demonstrated superior passivation properties.

Pressure Controlled Heat Pipe Solar Receiver for Regolith Oxygen Production with Multiple Reactors, John Hartenstine, et, al., 9th Intersociety Energy and Conversion Engineering Conference (IECEC), San Diego, CA, July 31 - August 3, 2011

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Pressure Controlled Heat Pipe Solar Receiver for Regolith Oxygen Production with Multiple Reactors

John R. Hartenstine1, Kara L. Walker2, Calin Tarau3, and William G. Anderson4 Advanced Cooling Technologies, Inc., Lancaster, Pennsylvania, 17601, U.S.A.

Oxygen from lunar regolith can be extracted to provide breathable oxygen for consumption by astronauts during long term stays on the Moon. The regolith is heated using concentrated solar energy to 1050°C, and then hydrogen is introduced that reacts with the regolith, extracting oxygen in the form of water vapor. After several hours, the regolith is dumped, and fresh regolith is added. To minimize mass, it is desirable to supply thermal energy to multiple reactors with a single concentrator. Pressure Controlled Heat Pipes (PCHPs) can be used to transfer heat to the multiple reactors from a single heat source. During operation, one reactor is producing hydrogen at low solar power, while the other reactor is warming up a fresh batch of regolith. The PCHPs switch power between the two reactors as required. A high-temperature, demonstration system was designed, fabricated and tested using two PCHPs and two Constant Conductance Heat Pipes (CCHPs) to supply heat to two reactors. The system was fabricated with Haynes 230 as the envelope, sodium as the working fluid, and argon as the non-condensable gas. Tests demonstrated the use of PCHPs to switch power between two reactors as required, as well as provide a means to reject excess power.

Thermal Management System for Long-Lived Venus Landers, Calin Tarau, et, al., 9th Intersociety Energy and Conversion Engineering Conference (IECEC), San Diego, CA, July 31 - August 3, 2011

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Thermal Management System for Long-Lived Venus Landers

Calin Tarau1, William G. Anderson2 and Christopher J. Peters3, Advanced Cooling Technologies, Inc., Lancaster, Pennsylvania, 17601, U.S.A.

Long-lived Venus Landers require cooling, which can be provided with a radioisotope power converter and cooling system. Heat from a stack of General Purpose Heat Source (GPHS) modules must be delivered to the Stirling convertor with minimal Δ T. In addition, the cooling system must be shut OFF during transit to Venus without overheating the GPHS modules. The bypass heat can be removed by alkali metal Variable Conductance Heat Pipes (VCHPs) integrated with a two-phase heat collection/transport package (HTP) from the GPHS stack to the Stirling convertor. A five-feature flat-front-theory-based VCHP model was developed, and a four-feature proof of concept VCHP was designed, built and successfully tested. The five-feature VCHP model predicts that the Stirling convertor can: 1) rest during transit at ~100°C lower temperature than the nominal one (~1000°C); 2) pre-cool the modules before the entry into the Venus atmosphere, lowering the temperature by another ~85°C; 3) work at nominal temperature of ~1000°C on Venus surface; 4) stop working (for short periods of time on Venus surface with a relatively small vapor temperature increase of ~ 6-9°C and 5) reject excess heat during the entire mission if shortlived isotopes are used. The four-feature proof of concept test setup was a sodium-stainless steel VCHP. Proof of concept theoretical and experimental results for the Backup Cooling System are presented in this paper. The experimental data fully validated the model.

Pressure Controlled Heat Pipes, William Anderson, et, al., 41st International Conference on Environmental Systems, Portland, OR, July 17-21, 2011

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Pressure Controlled Heat Pipes

William G. Anderson1, John R. Hartenstine2, Calin Tarau3, David B. Sarraf4, and Kara L. Walker5
Advanced Cooling Technologies, Inc., Lancaster, PA, 17601, U.S.A.

In a Variable Conductance Heat Pipe (VCHP), a Non-Condensable Gas (NCG) is added to the heat pipe to allow the conductance to vary. A Pressure Controlled Heat Pipe (PCHP) is a VCHP variant, where the heat pipe operation is controlled bvarying either the gas quantity or the volume of the gas reservoir. This paper will discuss two applications for PCHPs: 1. Precise Temperature Control, and (2) Switching thermal power between multiple sinks. A prototype aluminum/ammonia PCHP was built and tested to demonstrate the capability of controlling the evaporator section of an aluminum/ammonia pressure controlled heat pipe to milli-Kelvin levels over an extended period of time. The external (simulated radiator or heat sink) temperature was varied and the heat input into the evaporator section was varied during those tests. Temperature set point changes were also demonstrated. PCHPs can also be used to switch power between multiple high temperature reactors. In a second program, a heat pipe solar receiver was designed to accept, isothermalize and transfer the solar thermal energy to reactors for oxygen production from lunar regolith. The receiver has two PCHPs and two CCHPs to supply heat to two reactors. During operation, one reactor is producing hydrogen at low solar power, while the other reactor is warming up a fresh batch of regolith. The PCHPs switch power between the two reactors as required.

Variable Conductance Heat Pipe for a Lunar Variable Thermal Link, Chris Peters, et, al., 41st International Conference on Environmental Systems, Portland, OR, July 17-21, 2011

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Variable Conductance Heat Pipe for a Lunar Variable Thermal Link

Christopher J. Peters1, John R. Hartenstine2, Calin Tarau3, and William G. Anderson4 Advanced Cooling Technologies, Inc., Lancaster, PA, 17601, U.S.A.

The Anchor Node Mission for the International Lunar Network (ILN) has a Warm Electronics Box (WEB) and a battery, both of which must be maintained in a fairly narrow temperature range using a variable thermal conductance link. During the lunar day, heat must be transferred from the WEB to a radiator as efficiently as possible. During the night, heat transfer from the WEB must be minimized to keep the electronics and batteries warm with minimal power, even with a very low (100 K) heat sink. Three different variable thermal links were identified that could perform this function: 1. A mini-loop heat pipe (LHP), 2. A mini-LHP with a thermal control valve, or 3. A Variable Conductance Heat Pipe (VCHP) with a hybrid wick. The mini-LHP has the highest Technology Readiness Level (TRL), but requires electrical power to shut-down during the 14-day lunar night, with a significant penalty in battery mass. The VCHP incorporates three novel features in order to achieve the design targets of the ILN program. The first is a hybrid wick, which allows the VCHP to operate with an adverse tilt in the evaporator. The second is locating the reservoir near the evaporator, rather than near the condenser, to prevent the reservoir temperature from dropping during the lunar night. Third, a bimetallic adiabatic section is used to minimize heat losses due to conduction when the VCHP is shut down. Testing included 1. Freeze/thaw, 2. Simulated lunar performance, with an adverse evaporator elevation, 3. Performance with a 2.54 mm (0.1 inch) adverse elevation, both for normal operation, and to demonstrate diode behavior when the condenser was heated. All of the tests were successful; however, the power with the heat pipe level was slightly lower than expected, probably due to problems with the hybrid wick interface.

Two-Phase Heat Sinks with Microporous Coating, T. Semenic and S. M. You, 9th International Conference on Nanochannels, Microchannels, and Minichannels, Edmonton, CA, June 19-22, 2011

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TWO-PHASE HEAT SINKS WITH MICROPOROUS COATING

To minimize flow boiling instabilities in two-phase heat sinks, two different types of microporous coatings were developed and applied on mini- and small-channel heat sinks and tested using degassed R245fa refrigerant. The first coating was epoxy-based and was sprayed on heat sink channels while the second coating was formed by sintering copper particles on heat sink channels. Mini-channel heat sinks had overall dimensions 25.4 mm x 25.4 mm x 6.4 mm and twelve rectangular channels with a hydraulic diameter 1.7 mm and a channel aspect ratio of 2.7. Small-channel heat sinks had the same overall dimensions, but only three rectangular channels with hydraulic diameter 4.1 mm and channel aspect ratio 0.6. The microporous coatings were found to minimize parallel channel instabilities for mini-channel heat sinks and to reduce the amplitude of heat sink base temperature oscillations from 6 °C to slightly more than 1 °C. No increase in pressure drop or pumping power due to the microporous coating was measured. The mini-channel heat sinks with porous coating had in average 1.5-times higher heat transfer coefficient than uncoated heat sinks. Also, the small-channel heat sinks with the “best” porous coating had in average 2.5-times higher heat transfer coefficient and the critical heat flux was 1.5 to 2-times higher compared with the uncoated heat sinks.

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Die Level Thermal Storage for Improved Cooling of Pulsed Devices, Richard Bonner III, et, al., Semitherm, San Jose, CA., March 2011

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Die Level Thermal Storage for Improved Cooling of Pulsed Devices

In many communications applications semiconductor devices operate in a pulsed mode, where rapid temperature transients are continuously experienced within the die. We proposed a novel junction-level cooling technology where a metallic phase change material (PCM) was embedded in close proximity to the active transistor channels without interfering with the device’s electrical response. Here we present multiscale simulations that were performed to determine the thermal performance improvement and electrical performance impact under pulsed operating conditions. The modeling effort was focused on Gallium Nitride (GaN) on Silicon (Si) chips with Indium (In) as the PCM. To accurately capture the microscale transient melting process, a hierarchical multiscale model was developed that includes linking of atomistic-level molecular dynamics simulations and macroscale finite element analysis simulations. Macroscale physics, including the melting process, were captured with a transient twodimensional finite element analysis (FEA) model. The FEA model also includes interfacial and contact resistances between the semiconductor materials and PCM. Nonequilibrium Molecular Dynamic (MD) simulations were performed to estimate the value of the interfacial resistances between the Si substrate and the In PCM, which included a new interatomic potential between In and Si that was developed from experimental scattering results available in the literature. The thermal modeling results indicate 26% more heat can be dissipated through the PCM enhanced transistor while maintain a safe operating temperature.

A separate electrical modeling effort showed that the metallic PCM layer did not create appreciable parasitic capacitances as long as the PCM was farther than 1μm from the active channel. The lower, more constant temperatures achieved by this technology can help improve the reliability and performance of future communication devices.

A 2-D Numerical Study of Microscale Phase Change Material Thermal Storage for GaN Transistor Thermal Management, Xudong Tang, et, al., Semitherm, San Jose, CA, March 2011

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A 2-D Numerical Study of Microscale Phase Change Material Thermal Storage for GaN Transistor Thermal Management

A novel thermal management technology was explored to lower the peak temperature associated with high power GaN transistors in pulse application. The technology involves the use of an embedded microscale PCM heat storage device within the chip (near the active channels of the GaN device), which effectively increases the heat capacity of the material by taking advantage of the latent heat of the PCM. In this study, 2-D transient thermal models were developed to characterize the thermal behavior of GaN transistors with micro-scale PCM heat storage device. The model is capable of computing the spatial-temporal temperature distribution of the GaN transistor as it is rapidly pulsed and captures the formation and evolution of hot spots that form within the device. The model also captures the PCM melting behavior and latent heat absorption during the transient.

The use of a PCM can effectively control the hot spot temperature by absorbing a significant portion of the transient heat input. As shown in this modeling study, the use of PCM heat storage in GaN transistors reduces the GaN hot spot temperature for a given heat input. Alternatively, the maximum allowable GaN heat input can be increased with the use of PCM. At a given heat input flux of 5×105 W/cm2, for example, the use of PCM heat storage can lower the peak temperature by 21~22°C, relative to transistors without PCM (baseline), regardless of the duty cycle ratio. In addition, a transistor with PCM heat storage can accommodate much higher joule heat generation without exceeding the maximum allowable temperature limit, 180°C. In this study, the modeling results show that by integrating a PCM that has a 140°C melting point in a 5m×6m groove configuration, the critical heat flux can be increased from 13.34×105 W/cm2(baseline) to 16.8×105 W/cm2(with PCM), a 26% improvement.

Key PCM design parameters were identified in this modeling study: (1) PCM amount; (2) PCM melting point; and (3) PCM groove structure. Their coupling and the impact on design optimization require further investigation.

Dynamic Response of Phenolic Resin and Its Carbon-nanotube Composites to Shock Wave Loading, Arman, et. al., Journal of Applied Physics, 109, 013503 (2011)

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Dynamic response of phenolic resin and its carbon-nanotube composites to shock wave loading

We investigate with nonreactive molecular dynamics simulations the dynamic response of phenolic resin and its carbon-nanotube (CNT) composites to shock wave compression. For phenolic resin, our simulations yield shock states in agreement with experiments on similar polymers except the “phase change” observed in experiments, indicating that such phase change is chemical in nature. The elastic – plastic transition is characterized by shear stress relaxation and atomic-level slip, and phenolic resin shows strong strain hardening. Shock loading of the CNT-resin composites is applied parallel or perpendicular to the CNT axis, and the composites demonstrate anisotropy in wave propagation, yield and CNT deformation. The CNTs induce stress concentrations in the composites and may increase the yield strength. Our simulations suggest that the bulk shock response of the composites depends on the volume fraction, length ratio, impact cross-section, and geometry of the CNT components; the short CNTs in current simulations have insignificant effect on the bulk response of resin polymer.

Loop Heat Pipe with Thermal Control Valve for Variable Thermal Conductance Link of Lunar Landers and Rovers, Loop Heat Pipe with Thermal Control Valve for Variable Thermal Conductance Link of Lunar Landers and Rovers, J. R. Hartenstine et al., 49th AIAA Aerospace Sciences Meeting, Orlando, FL, January 4-7, 2011.

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A number of proposed Lunar landers and rovers have a Warm Electronics Box (WEB) and a battery, both of which must be maintained in a fairly narrow temperature range using a variable thermal  conductance link. During the Lunar day, heat must be transferred from the WEB to a radiator as efficiently as possible. During the night, heat transfer from the WEB must be minimized to keep the electronics and batteries warm with minimal power, even with a very low (100 K) heat sink. A mini-LHP has the highest Technology Readiness Level, but requires electrical power to shut-down during the 14-day Lunar night, with a significant penalty in battery mass: 1 watt of electrical power translates into 5kg of battery and solar cell mass. A mini-LHP with a Thermal Control Valve (TCV) was developed to shut down without electrical power. An aluminum/ammonia LHP which included a TCV in the vapor exit line from the evaporator was designed, fabricated and tested. The TCV could route vapor to the condenser, or bypass the condenser and route vapor directly back to the compensation chamber, depending upon the temperature conditions. During testing, the LHP condenser temperature was decreased to -60ºC and the power input was decreased to near zero power: the evaporator remained above 0ºC. This paper will describe the LHP and TCV design, fabrication and testing details.

 

Electronics Cooling Using High Temperature Loop Heat Pipes with Multiple Condensers, William G. Anderson, et, al., SAE Power Systems Conference, Ft. Worth, TX, November 2010

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Electronics Cooling Using High Temperature Loop Heat Pipes With Multiple Condensers

In military aircraft, electronics are often subjected to operating environments well beyond their survival temperatures and with limited heat sinks. The current approach is to use a Liquid Cooling System (LCS) with either vehicle fuel or Polyalphaolephin (PAO) to cool electronics. However, advanced military platforms have found this approach limits their operational effectiveness. A thermal management system for electronics cooling in high temperature avionics environments is under development using Loop Heat Pipe (LHP) and heat pipe based technology. The system reduces thermal energy transport inefficiencies within electronics enclosures, identifies potential sinks to provide continuous heat rejection over the operating envelope of the platform, and provides passive thermal energy transport from the electronics enclosure to selected sinks.

The system developed to accomplish these tasks is divided into two subsystems. The first subsystem is responsible for improving thermal transport within the electronics enclosure and consists primarily of heat pipe assemblies. Model results of the first subsystem show considerable improvements over the current implementation. The overall temperature gradient within a generic electronics box decreased from 42.7°C (76.9 °F) to 17.8 °C (32.0 °F), increasing the allowable sink temperature from 66.7 °C (152.1 °F) to 91.7 °C (197.1 °F).This increase allows for more freedom in sink selection,which is typically limited aboard military platforms. The second subsystem transports thermal energy from the external surface of the enclosure to appropriate sinks and consists primarily of a LHP. At this stage, several sinks have been identified and evaluated. Final sink selection is underway. Depending on sink temperature and capacity throughout the operating envelope of the platform, multiple sinks may be used. During operation, the LHP will passively select the appropriate sink.

Development of Heat Pipe Loop Technology for Military Vehicle Electronics Cooling, Xudong Tang et, al., NDIA Ground Vehicle Systems Engineering and Technology Symposium, Dearborn, Michigan, August 2010

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Development of Heat Pipe Loop Technology for Military Vehicle Electronics Cooling

Current standard military vehicle thermal management systems are based on single phase air/liquid cooling. To meet increasingly stringent demands for high power electronics thermal control, two-phase cooling solutions show great potential and can satisfy the need for compact and high heat flux heat acquisition, transport and dissipation under vibration and shock conditions.

One novel two-phase cooling technology that has been developed in this work is a new Heat Pipe Loop (HPL), which exploits the advantages of both heat pipes and loop heat pipes while eliminating their shortcomings. Similar to heat pipes and loop heat pipes, the HPL operates on evaporation and condensation of a working fluid and uses capillary forces in the wick for the fluid circulation. Unlike in a heat pipe, the liquid and vapor in the HPL flow in separate passages made from smooth wall tubing. This results in a low pressure drop and consequently great heat transfer capacity and distance over which the heat can be transferred. The evaporator wick in a HPL is also made in-situ through a low cost manufacturing process and has a high thermal conductance, much like the low cost traditional heat pipe wick.

To demonstrate the HPL technology, a compact 3kW HPL thermal management system was successfully designed, built and tested in an environment representative of military combat vehicles. This system consisted of six compact plug-and-play HPL modules. Each HPL module was designed to transport 500W of waste heat from two discrete high power devices on an electronics board to a chassis level thermal bus that was a pumped liquid loop. The HPL evaporator (or heat source) temperature was maintained below 80°C with a heat sink temperature of 30-50°C. The advantages of the HPL technology include: (1) Passive operation and high reliability; (2) Low cost in-situ wick fabrication; (3) High conductivity evaporator wicks; (4) Long distance heat transfer capability; and (5) Insensitivity to vibration/shock and gravitational orientation.

Dropwise Condensation Life Testing of Self Assembled Monolayers, Richard Bonner III, IHTC14, Washington, DC, August 2010

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Dropwise Condensation Life Testing of Self Assembled Monolayers

The increasing thermal demand of electronics devices has pushed the limits of current two-phase thermal technologies such as heat pipes and vapor chambers. The most obvious area for thermal improvement is centered around the high heat flux generating chips including improved evaporators, thermal interfaces, etc. However, heat fluxes in the sink/condensing regions have also risen as the size of electronics packages has decreased. One way to reduce the thermal resistance associated with condensation is to promote dropwise condensation. In previous work, the condensation performance improvement using self-assembled monolayer coated surfaces (to promote hydrophobicity) has been shown. However, the question of the life of the self-assembled monolayer coatings needs to be addressed before the technology is adopted, as this has plagued other dropwise condensation coatings in the past.

Presented here is a general use of self-assembled monolayer coatings to promote dropwise condensation in electronics device applications, including a summary of recent work regarding dropwise condensation on gradient surfaces. Also presented is experimental data from a life test of self-assembled monolayers on copper and gold plated surfaces. In the life test, the surfaces have been continuously exposed to saturated steam at 60°C. Both surfaces have continued to promote dropwise condensation for over 9 months under conditions representative of heat pipe electronics cooling applications.

Heat and Mass Transfer in a Permeable Fabric system Under Hot Air Jet Impingement,, Sangsoo Lee et. al., International Heat Transfer Conference (IHTC14), Washington, DC, August, 2010

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Heat and Mass Transfer in a Permeable Fabric system Under Hot Air Jet Impingement

Personal Protective Equipment (PPE) was investigated using hot air jet impingement conditions to mimic the jet exhaust of Short Take-Off and Vertical Landing (STOVL) aircraft. The STOVL aircraft uses a thrust-vectoring nozzle of the jet engine and a lift fan in order to vertically land and take off a short runaway. The jet engine exhaust is a new kind of thermal hazard for military personnel operating within an affected zone of the jet exhaust.

An experimental approach was used to measure the thermal response of a fabric system consisting of permeable fabric samples and air pocket using a high-speed jet impingement. The jet impingement conditions consisted of two different temperatures: one of 100°C and another of 200°C at a jet impingement velocity of 32 m/s. Air was used as the working fluid. In this study, two permeable fabrics, (NOMEX IIIA and Cotton) commonly used for the Personal Protective Equipment (PPE) were investigated. The physical properties (porosity, permeability, Ergun coefficient, and density) and the thermo-physical properties (thermal diffusivity, thermal conductivity, and specific heat) of the fabrics were measured.

A one-dimensional, two-medium formulation assuming thermal non-equilibrium between solid (fabric) and gas (air) phases in the fabric layer was used for the numerical analysis. The measurement results from the fabric experiment were used to define boundary conditions and adjust various heat transfer correlations and input data used in the numerical model.

The experimental and numerical results of the temperatures of the fabric system were compared. The effects of the air temperature of the jet impingement on the thermal response of the fabric system were discussed.

Variable Thermal Conductance Link for Lunar Landers and Rovers, William G Anderson et. al., IECEC, Nashville, Tennessee, July, 2010

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Variable Thermal Conductance Link for Lunar Landers and Rovers

The Anchor Node Mission for the International Lunar Network (ILN) has a Warm Electronics Box (WEB) and a battery, both of which must be maintained in a fairly narrow temperature range using a variable thermal conductance link. During the Lunar day, heat must be transferred from the WEB to a radiator as efficiently as possible. During the night, heat transfer from the WEB must be minimized to keep the electronics and batteries warm with minimal power, even with a very low (100 K) heat sink. Three different variable thermal links were identified that could perform this function: 1. A mini-loop heat pipe (LHP), 2. A mini-LHP with a bypass valve, or 3. A Variable Conductance Heat Pipe (VCHP) with a hybrid wick. The paper discusses the advantages and disadvantages of each link. The mini-LHP has the highest Technology Readiness Level, but requires electrical power to shut-down during the 14-day Lunar night, with a significant penalty in battery mass. The mini-LHP with bypass valve and the hybrid loop VCHP require more development, but will require no electrical power for shut-down.

Sodium Variable Conductance Heat Pipe for Radioisotope Stirling Systems – Design and Experimental Results, Calin Tarau and William G Anderson, IECEC, Nashville, Tennessee, July, 2010

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Sodium Variable Conductance Heat Pipe for Radioisotope Stirling Systems – Design and Experimental Results

In a Stirling radioisotope system, heat must continually be removed from the General Purpose Heat Source (GPHS) modules to maintain the modules and surrounding insulation at acceptable temperatures. The Stirling converter normally provides this heat removal. An alkali-metal Variable Conductance Heat Pipe (VCHP) has been developed to provide backup cooling, allowing multiple stops and restarts of the Stirling convertor. Unlike standard VCHPs which maintain a relatively constant temperature, this VCHP has two different heat rejection surfaces. During normal operation, heat is transferred to the Stirling convertor heater head. When the Stirling convertor is stopped, the VCHP temperature increases by 30°C, and the gas front is pushed back, allowing the heat from the GPHS to be rejected to the Cold Side Adapter Flange (CSAF) using a low-mass, carbon-carbon radiator. The 880°C temperature when the Stirling convertor is stopped is high enough to avoid risking standard ASRG operation, but low enough to save most of the heater head life. The Haynes 230/sodium VCHP was successfully tested with a turn-on ΔT of 30°C in three orientations: horizontal, gravity-aided, and against gravity.

Sodium Variable Conductance Heat Pipe with Carbon-Carbon Radiator for Radioisotope Stirling Systems, Calin Tarau and William G. Anderson, 15th International Heat Pipe Conference, Clemson, SC, April 2010

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Sodium Variable Conductance Heat Pipe with Carbon-Carbon Radiator for Radioisotope Stirling Systems

In a Stirling radioisotope system, heat must continually be removed from the General Purpose Heat Source (GPHS) modules to maintain the modules and surrounding insulation at acceptable temperatures. The Stirling converter normally provides this cooling. An alkali-metal Variable Conductance Heat Pipe (VCHP) has been developed to provide back-up cooling, allowing multiple stops and restarts of the Stirling convertor. Unlike standard VCHPs which maintain a relatively constant temperature, this VCHP has two different heat rejection surfaces. During normal operation, heat is transferred to the Stirling convertor heater head. When the Stirling convertor is stopped, the VCHP temperature increases by 30°C, and the gas front is pushed back, allowing the heat from the GPHS to be rejected to the Cold Side Adapter Flange (CSAF) using a low-mass, carbon-carbon radiator. The 880°C temperature when the Stirling convertor is stopped is high enough to avoid risking standard ASRG operation, but low enough to save most of the heater head life. The Haynes 230/sodium VCHP was successfully tested with a turn-on ΔT of 30°C in three orientations: horizontal, gravity-aided, and against gravity.

Low-Temperature, Dual Pressure Controlled Heat Pipes for Oxygen Production from Lunar Regolith, Kara Walker et al., 15th International Heat Pipe Conference, Clemson, South Carolina, April, 2010

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Low-Temperature, Dual Pressure Controlled Heat Pipes for Oxygen Production from Lunar Regolith

Oxygen can be extracted from lunar soil by hydrogen reduction: the lunar regolith is heated using a solar concentrator to approximately 1050°C and exposed to hydrogen gas. Water is formed from the reaction and the oxygen is recovered by electrolysis of the water. To minimize mass, it is desirable for the solar concentrator to supply heat to more than one reactor. In a dual reactor system, one reactor is at 1050°C extracting oxygen. The second reactor, filled with fresh, cold regolith, uses the majority of the solar power to heat the regolith up to the extraction temperature. After one hour, the roles of the reactors switch, and the fraction of power supplied to each reactor must be switched. A method has been developed using Pressure Controlled Heat Pipes (PCHPs) to shuttle the solar power from one reactor to the other. The final system will use sodium as the working fluid, with a Haynes 230 envelope material. This paper reports on the design and testing of two lower temperature Monel/water systems that have been used to verify the performance and control scheme for the overall design.

Intermediate Temperature Fluids for Heat Pipes and Loop Heat Pipes, William G. Anderson, John R. Hartenstine, David B. Sarraf, and Calin Tarau, Advanced Cooling Technologies, Inc., Pennsylvania, 15th International Heat Pipe Conference (15th IHPC) Clemson, USA, April 25-30, 2010.

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Intermediate Temperature Fluids for Heat Pipes and Loop Heat Pipes

Potential working fluids for heat pipes and loop heat pipes include water, organic fluids, elements, and halides. The paper surveys life tests conducted with 30 different intermediate temperature working fluids, and over 60 different working fluid/envelope combinations. Life tests have been run with three elemental working fluids: sulfur, sulfur-iodine mixtures, and mercury. Other fluids offer benefits over these three liquids in this temperature range. Life tests have been conducted with 19 different organic working fluids. Three sets of organic fluids stand out as good intermediate temperature fluids: (1) Diphenyl, Diphenyl Oxide, and Eutectic Diphenyl/Diphenyl Oxide, (2) Naphthalene, and (3) Toluene. While fluorinating organic compounds are believed to make them more stable, this has not yet been demonstrated during heat pipe life tests. Finally several halides are suitable for temperatures up to 673 K (400°C). Life tests at temperatures up to 400°C were conducted with titanium and three corrosion resistant superalloys, and six different working fluids: AlBr3, GaCl3, SnCl4, TiCl4, TiBr4, and eutectic diphenyl/diphenyl oxide. (Therminol VP-1/Dowtherm A). Ongoing life tests with superalloys/TiCl4 and AlBr3 have been running for 28,000 hours. Ongoing life tests with up to 45,000 hours demonstrate that titanium/water and Monel/water heat pipes can be used at temperatures up to 550 K (277°C).

Intermediate Temperature Fluids for Heat Pipes and Loop Heat Pipes, William Anderson, et al., 15th International Heat Pipe Conference (15th IHPC) Clemson, USA, April 25-30, 2010.

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Intermediate Temperature Fluids for Heat Pipes and Loop Heat Pipes

Potential working fluids for heat pipes and loop heat pipes include water, organic fluids, elements, and halides. The paper surveys life tests conducted with 30 different intermediate temperature working fluids, and over 60 different working fluid/envelope combinations. Life tests have been run with three elemental working fluids: sulfur, sulfur-iodine mixtures, and mercury. Other fluids offer benefits over these three liquids in this temperature range. Life tests have been conducted with 19 different organic working fluids. Three sets of organic fluids stand out as good intermediate temperature fluids: (1) Diphenyl, Diphenyl Oxide, and Eutectic Diphenyl/Diphenyl Oxide, (2) Naphthalene, and (3) Toluene. While fluorinating organic compounds are believed to make them more stable, this has not yet been demonstrated during heat pipe life tests. Finally several halides are suitable for temperatures up to 673 K (400°C). Life tests at temperatures up to 400°C were conducted with titanium and three corrosion resistant superalloys, and six different working fluids: AlBr3, GaCl3, SnCl4, TiCl4, TiBr4, and eutectic diphenyl/diphenyl oxide. (Therminol VP-1/Dowtherm A). Ongoing life tests with superalloys/TiCl4 and AlBr3 have been running for 28,000 hours. Ongoing life tests with up to 45,000 hours demonstrate that titanium/water and Monel/water heat pipes can be used at temperatures up to 550 K (277°C).

Dropwise Condensation in Vapor Chambers, Richard Bonner, 26th IEEE Semi-Therm Symposium, Santa Clara, California, February 2010

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Dropwise Condensation in Vapor Chambers

Electronic devices continue to shrink in size while dissipating more heat. The size of the air cooled heat sinks required to remove this heat has increased while the size of the heat source has decreased. These trends have resulted in large conduction gradients across the base of the heat sinks, resulting in decreased thermal performance. A passive and reliable method of minimizing the spreading resistance in air cooled heat sinks is to embed a vapor chamber in the base of the heat sink. A vapor chamber is a two-phase heat transfer device that uses capillary forces to isothermally circulate a working fluid at saturated conditions. Provided that the vapor chamber is circulating fluid properly (within its capillary limit) the thermal resistance of the vapor chamber is limited by the evaporating and condensing processes in the vapor chamber. Much attention has been paid to the evaporating process since the heat flux of the evaporating process is generally many times higher than that of the condensing process. However, heat fluxes in the condensing regions of vapor chambers have risen to the point where they can’t be neglected. Described here is a novel method of improving the condensation performance in vapor chamber devices by using self-assembled monolayers to promote dropwise condensation. In other applications, dropwise condensation has been shown to improve the condensation heat transfer coefficient by an order of magnitude over the typical filmwise condensation surfaces found in vapor chambers. Presented here are condensation test data comparing the performance of filmwise and dropwise condensation surfaces in vapor chambers.

Sodium VCHP with Carbon-Carbon Radiator for Radioisotope Stirling Systems,, Calin Tarau, et al., Space, Propulsion and Energy Sciences International Forum (SPESIF), Laurel, Maryland, February 2010

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Sodium VCHP with Carbon-Carbon Radiator for Radioisotope Stirling Systems

In a Stirling radioisotope system, heat must continually be removed from the General Purpose Heat Source (GPHS) modules to maintain the modules and surrounding insulation at acceptable temperatures. The Stirling converter normally provides this cooling. If the Stirling convertor stops in the current system the insulation is designed to spoil, preventing damage to the GPHS at the cost of an earlier termination of the mission. An alkali-metal Variable Conductance Heat Pipe (VCHP) can be used to allow multiple stops and restarts of the Stirling convertor. A sodium VCHP with a Haynes 230 envelope was designed and fabricated for the Advanced Stirling Radioisotope Generator (ASRG), with a baseline 850°C heater head temperature. When the Stirling convertor is stopped, the heat from the GPHS is rejected to the Cold Side Adapter Flange using a low-mass, carbon-carbon radiator. The VCHP is designed to activate with a ΔT of 30°C. The 880°C temperature when the Stirling convertor is stopped is high enough to avoid risking standard ASRG operation, but low enough to save most of the heater head life. The VCHP has low mass and low thermal losses for normal operation. The design has been modified from an earlier, stainless steel prototype with a nickel radiator. In addition to replacing the nickel radiator with a low mass carbon-carbon radiator, the radiator location has been moved from the ASRG case to the cold side adapter flange. This flange already removes two-thirds of the heat during normal operation, so it is optimized to transfer heat to the case. The VCHP was successfully tested with a turn-on ΔT of 30°C in three orientations: horizontal, gravity-aided, and against gravity.

Advanced VCS Evaporators for Lunar Lander and Lunar Habitat Thermal Control Applications, Tadej Semenic, Space, Propulsion and Energy Sciences International Forum (SPESIF), Laurel, Maryland, February 2010

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Advanced Evaporators for Lunar Lander and Lunar Habitat Thermal Control Applications

Six 20.3cm x 20.3cm (8in x 8in) evaporators for Lunar Lander and Lunar Habitat thermal control applications were designed, fabricated and tested in a vapor compression loop at heat loads in the range of 3-6 kW. The evaporator heated area was 826cm2 and their total mass ranged from 1.0kg to 3.2kg depending on the design. The primary objective of the study was to investigate different evaporator designs and identify and characterize the evaporator design with the lowest temperature, most uniform temperature, smallest mass, and lowest pressure drop. The results obtained using serpentine evaporators showed excellent temperature uniformity (±3°C) across the evaporator surface at these relatively high heat loads. The temperature lift from the evaporator surface to the average condenser coolant temperature was also measured and ranged from 30 to 50°C depending on the heat load. The Coefficient of Performance (COP), defined as the ratio of the heat load to the compressor work at 6kW, was 1.9. The best evaporator out of six evaporators tested transferred heat at one half of the thermal resistance of the baseline evaporator, while maintaining the same system COP.

Modeling Initial Stage of Phenolic Pyrolysis: Graphitic Precursor Formation and Interfacial Effects, Tapan Desai, et, al., Polymer, 52, 2010

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Modeling initial stage of phenolic pyrolysis: Graphitic precursor formation and interfacial effects

Reactive molecular dynamics simulations are used to study the initial stage of pyrolysis of phenolic polymers with carbon nanotube and carbon fiber. The products formed are characterized and water is found to be the primary product in all cases. The water formation mechanisms are analyzed and the value of the activation energy for water formation is estimated. A detailed study of graphitic precursor formation reveals the presence of two temperature zones. In the lower temperature zone (<2000 K) polymerization occurs resulting in the formation of large, stable graphitic precursors, while in the high temperature zone (>2000 K) polymer scission results in formation of short polymer chains/molecules. Simulations performed in the high temperature zone of the phenolic resin (with carbon nanotubes and carbon fibers) show that the presence of interfaces does not have a substantial effect on the chain scission rate or the activation energy value for water formation.

Slip Behavior at Ionic Solid-fluid Interfaces, Tapan Desai, NDIA Chemical Physics Letters, 501, 2010, 93-97

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Slip behavior at ionic solid–fluid interfaces

Molecular dynamics simulation results showing that the slip behavior at the interface between an ionic solid and fluid depends on the crystal face exposed to the flowing fluid are presented. The boundary condition of fluid NaCl confined between charged octopolar (1 1 1) surfaces is ‘stick’, but, between neutral (1 0 0) NaCl surfaces is ‘slip’. Also direction dependent change from stick to slip boundary condition is observed for atomically flat, neutral dipolar (and atomically rough, neutral octopolar) (1 1 0) NaCl surfaces. Thus, by changing the surface orientation and/or its roughness, the boundary conditions at ionic solid–fluid interface can be changed from slip to stick.

Roles of Atomic Restructuring in Interfacial Phonon Transport, Seungha Shin et. al., Physical Review B, 82, 081302 (2010)

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Roles of Atomic Restructuring in Interfacial Phonon Transport

Phonon resistance can dominate the interfacial thermal resistance between hard and soft solids. Using ab initio calculation, molecular-dynamics simulation and the diffuse mismatch model for Si/In as example, we decompose phonon interfacial resistance into boundary and interfacial-region resistances. These show that the interfacial atomic restructuring as well as the cross-boundary interactions reduce the phonon boundary resistance by providing additional transport channels altering their phonon density of states and cause extra interfacial-region resistances due to additional phonon scattering.

Anisotropic Shock Response of Columnar Nanocrystalline Cu, Sheng-Nian Luo et. al., Journal of Applied Physics , 107, 123507 (2010)

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Anisotropic Shock Response of Columnar Nanocrystalline Cu

We perform molecular dynamics simulations to investigate the shock response of idealized hexagonal columnar nanocrystalline Cu, including plasticity, local shear, and spall damage during dynamic compression, release, and tension. Shock loading one-dimensional strain is applied along three principal directions of the columnar Cu sample, one longitudinal along the column axis and two transverse directions, exhibiting a strong anisotropy in the response to shock loading and release. Grain boundaries GBs serve as the nucleation sites for crystal plasticity and voids, due to the GB weakening effect as well as stress and shear concentrations. Stress gradients induce GB sliding which is pronounced for the transverse loading. The flow stress and GB sliding are the lowest but the spall strength is the highest, for longitudinal loading. For the grain size and loading conditions explored, void nucleation occurs at the peak shear deformation sites GBs, and particularly triple junctions; spall damage is entirely intergranular for the transverse loading, while it may extend into grain interiors for the longitudinal loading. Crystal plasticity assists the void growth at the early stage but the growth is mainly achieved via GB separation at later stages for the transverse loading. Our simulations reveal such deformation mechanisms as GB sliding, stress, and shear concentration, GB-initiated crystal plasticity, and GB separation in nanocrystalline solids under shock wave loading.

 

Heat Pipe Embedded Alsic Plates for High Conductivity-Low CTE Heat Spreaders, J. Weyant, ITHERM 2010, Las Vegas NV,

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Heat Pipe Embedded Alsic Plates for High Conductivity-Low CTE Heat Spreaders

Heat pipe embedded aluminum silicon carbide (AlSiC) plates are innovative heat spreaders that provide high thermal conductivity and low coefficient of thermal expansion (CTE). Since heat pipes are two phase devices, they demonstrate effective thermal conductivities ranging between 10,000 and 200,000 W/m-K, depending on the heat pipe length. Installing heat pipes into an AlSiC plate dramatically increases the plate’s effective thermal conductivity. AlSiC plates alone have a thermal conductivity of roughly 200 W/m-K and a CTE ranging from 7-12 ppm/°C. Silicon alone has a thermal expansion coefficient of 3 ppm/°C, which makes AlSiC a much closer CTE match than tradition copper (17ppm/°C) and aluminum (25 ppm/°C) heat spreaders. An equivalent sized heat pipe embedded AlSiC plate has effective thermal conductivity ranging from 400 to 500 W/m-K..

Pressure Controlled Heat Pipe Solar Receiver for Oxygen Production from Lunar Regolith, John R. Hartenstine, et al., AIAA Aerospace Sciences Meeting, Orlando, Florida, January 2010

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Pressure Controlled Heat Pipe Solar Receiver for Oxygen Production from Lunar Regolith

The lunar soil contains approximately 43% oxygen as oxides, which could be extracted to provide oxygen for future Lunar bases. One method of extracting the oxygen is hydrogen reduction: the lunar regolith is heated using a solar concentrator to approximately 1050°C and exposed to hydrogen gas. Water is formed from the reaction and the oxygen is recovered by electrolysis of the water. To minimize mass, it is desirable for the solar concentrator to supply heat to more than one reactor. ACT is developing a thermal management system using Constant Conductance Heat Pipes (CCHPs) and Pressure Controlled Heat Pipes (PCHPs) solar receiver for this process. A PCHP is similar to a Variable Conductance Heat Pipe and permits control over heat pipe operation by varying the gas quantity or the volume of the gas reservoir. The PCHP solar receiver is designed to accept, isothermalize and transfer the solar thermal energy through CCHP’s to multiple reactors for oxygen production. The final system will use sodium as the working fluid, with a Haynes 230 envelope material. This paper will report on the transient modeling and design and fabrication of a lower temperature system that was used to verify performance of the overall design.

Sodium Variable Conductance Heat Pipe for Radioisotope Stirling Systems, Calin Tarau, et al., 7th International Energy Conversion Engineering Conference, Denver Colorado, August 2009

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Sodium Variable Conductance Heat Pipe for Radioisotope Stirling Systems

In a Stirling radioisotope system, heat must continually be removed from the General Purpose Heat Source (GPHS) modules to maintain the modules and surrounding insulation at acceptable temperatures. Normally, the Stirling convertor provides this cooling. If the convertor stops in the current system, the insulation is designed to spoil, preventing damage to the GPHS, and also ending the mission. An alkali-metal Variable Conductance Heat Pipe (VCHP) has been designed to allow multiple stops and restarts of the Stirling convertor in an Advanced Stirling Radioisotope Generator (ASRG). When the Stirling convertor is turned off, the VCHP will activate when the temperatures rises 30°C above the set point temperature. A prototype VCHP with sodium as the working fluid was fabricated and tested in both “gravity aided” and “against gravity” conditions for a nominal heater head temperature of 790ºC. The results show very good agreement with the predictions and validate the model. The gas front was located at the exit of the evaporator when heater head temperature was 790ºC while cooling was ON, simulating an operating Advanced Stirling Convertor (ASC). When cooling stopped, the temperature increased by 30°C, allowing the gas front to move past the radiator, which transferred the heat to the case. After resuming the cooling flow, the front returned at the initial location turning OFF the VCHP. The “against gravity” working conditions showed a colder reservoir and faster transients.

Loop Heat Pipe Design, Manufacturing and Testing – an Industrial Perspective, William Anderson, et al., ASME 2009 Heat Transfer Summer Conference, San Francisco, California, July 2009

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Loop Heat Pipe Design, Manufacturing, And Testing – An Industrial Perspective

Loop Heat Pipes (LHPs) are two-phase devices that can passively transport heat over long distances relative to other passive two phase systems such as heat pipes. Most of the art of LHP fabrication is in the primary and secondary wick. The manufacturing steps for an LHP are described, including the tests to validate the LHP during manufacture. The tests include wick property testing (pore size, permeability, and thermal conductivity), secondary wick testing, and parallel flow balance design and testing. The required tests after the LHP is fabricated include low power starts, shutdown through compensation chamber heating, unbalanced condenser temperature tests, transient testing – both power cycling and condenser temperature changes, and maximum power tests.

Dropwise Condensation on Surfaces with Graded Hydrophobicity, Richard Bonner, ASME 2009 Heat Transfer Summer Conference, San Francisco, California, July 2009

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Dropwise Condensation On Surfaces With Graded Hydrophobicity

Dropwise condensation has shown the ability to increase condensation heat transfer coefficients by an order of magnitude over filmwise condensation. In standard dropwise condensation, liquid droplets forming on a sub-cooled nonwetting surface are removed from the surface by gravitational forces when the droplets reach a critical mass. The dependence on gravity for liquid removal limits the utilization of dropwise condensation in low gravity aerospace applications and horizontal surfaces. Presented in this study is a novel passive mechanism to remove droplets from a condensing surface using a surface energy gradient (wettability gradient) on the condensing surface. The wettability gradient creates a difference in contact angle across droplets condensing on the surface. The difference in contact angle across the droplets causes motion of the droplets to regions of increased wettability, without relying on additional forces. The movement of droplets away from the surface prevents flooding and allows for the condensation of new droplets on the surface. This paper presents an overall description of the wettability gradient mechanism and experimental condensation data acquired on surfaces with wettability gradients. A mechanism for creating the wettability gradients is also described, which involves varying the surface concentration of hydrophobic molecules through a self-assembled monolayer process.

Evaporators for High Temperature Lift Vapor Compression Loop for Space Applications, Tadej Semenic and Xudong Tang, ASME 2009 Heat Transfer Summer Conference, San Francisco, California, July 2009

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Evaporators For High Temperature Lift Vapor Compression Loop For Space Applications

An Advanced Vapor Compression Loop (AVCL) for high temperature lift for heat rejection to hot lunar surface during lunar daytime was developed. The loop consists of an evaporator, a compressor, a condenser, and an electronic expansion valve. Different types of evaporators were evaluated in this study: a circular tube evaporator, a circular tube evaporator with a twisted tape, a circular tube evaporator with a wick, and a circular tube evaporator with a wick and a twisted tape. The evaporators were tested with two different compressors. The first was a 0.5hp oil-less compressor and the second was a 5.3hp compressor that used oil as lubricant. A heat exchanger (recuperator) was used to subcool the high pressure liquid and to superheat the low pressure vapor. Tests were performed with and without the recuperator. Vapor superheat during the tests was controlled with an electronic expansion valve controller. The working fluid was R134a. The results show that the heat source-to-working fluid thermal resistance of the circular tube evaporator with the wick and the twisted tape was one-third of that of the circular tube evaporator. The recuperator was able to decrease the vapor quality at the evaporator inlet and increase the vapor superheat at the compressor inlet. The evaporators without wicks were able to operate at a heat flux of 5.7W/cm2 with the recuperator and vapor superheat set at 5°C. Evaporators with wicks reached dryout at lower heat fluxes when maintaining superheat at 5°C. However, the wicked evaporators reached a heat flux of 7.6W/cm2 when decreasing superheat below 5°C. A temperature lift of 70°C was achieved with the 5.3hp compressor.

Variable Conductance Heat Pipe Radiators for Lunar and Martian Environments, William Anderson, et al., Space, Propulsion and Energy Sciences International Forum (SPESIF), Huntsville, Alabama, February 2009

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Variable Conductance Heat Pipe Radiators for Lunar and Martian Environments

Long-term Lunar and Martian surface systems present challenges to thermal system design, including changes in thermal load, and large changes in the thermal environment between Lunar (or Martian) day and night. For example, the heat sink temperature at the Lunar equator can vary from 210 to 315 K. The radiator must be sized to reject the design power at the maximum temperature, but must also be able to accommodate both the changing heat sink temperature, as well as changes in power. Variable Conductance Heat Pipe (VCHP) radiators were examined for the main reactor of a fission surface power system, as well as the cavity cooling radiator. A VCHP radiator was designed for Lunar Equator that is capable of maintaining a 16K temperature drop with a 4% addition to overall mass. Without the VCHP the radiator would experience a 43K drop in temperature. This design is also capable of handling turndown on the power without an effect to the outlet temperature. At Shackleton Crater, the temperature drop for a conventional heat pipe radiator is small enough that a VCHP is not beneficial at constant power. However, a VCHP will allow turndown ratios of 5:1 or more. A conventional radiator can not be turned down more than 2:1, without valves to bypass part of the radiator. VCHPs are also easier to start than conventional radiators, since the gas-loading prevents sublimation from the evaporator when the condenser is frozen.

High Temperature Variable Conductance Heat Pipes for Radioisotope Stirling Systems, Calin Tarau, et al., Space, Propulsion and Energy Sciences International Forum (SPESIF), Huntsville, Alabama, February 2009

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High Temperature Variable Conductance Heat Pipes for Radioisotope Stirling Systems

In a Stirling radioisotope system, heat must continually be removed from the GPHS modules, to maintain the GPHS modules and surrounding insulation at acceptable temperatures. Normally, the Stirling convertor provides this cooling. If the Stirling convertor stops in the current system, the insulation is designed to spoil, preventing damage to the GPHS, but also ending the mission. An alkali-metal Variable Conductance Heat Pipe (VCHP) is under development to allow multiple stops and restarts of the Stirling convertor. The status of the ongoing effort in developing this technology is presented in this paper. An earlier, preliminary design had a radiator outside the Advanced Stirling Radioisotope Generator (ASRG) casing, used NaK as the working fluid, and had the reservoir located on the cold side adapter flange. The revised design has an internal radiator inside the casing, with the reservoir embedded inside the insulation. A large set of advantages are offered by this new design. In addition to reducing the overall size and mass of the VCHP, simplicity, compactness and easiness in assembling the VCHP with the ASRG are significantly enhanced. Also, the permanently elevated temperatures of the entire VCHP allows the change of the working fluid from a binary compound (NaK) to single compound (Na). The latter, by its properties, allows higher performance and further mass reduction of the system. Preliminary design and analysis shows an acceptable peak temperature of the ASRG case of 140°C while the heat losses caused by the addition of the VCHP are 1.8 W.

Heat Pipe Solar Receiver for Oxygen Production of Lunar Regolith, John Hartenstine, et al., Space, Propulsion and Energy Sciences International Forum (SPESIF), Huntsville, Alabama, February 2009

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Heat Pipe Solar Receiver for Oxygen Production of Lunar Regolith

A heat pipe solar receiver operating in the 1050°C range is proposed for use in the hydrogen reduction process for the extraction of oxygen from the lunar soil. The heat pipe solar receiver is designed to accept, isothermalize and transfer solar thermal energy to reactors for oxygen production. This increases the available area for heat transfer, and increases throughput and efficiency. The heat pipe uses sodium as the working fluid, and Haynes 230 as the heat pipe envelope material. Initial design requirements have been established for the heat pipe solar receiver design based on information from the NASA In-Situ Resource Utilization (ISRU) program. Multiple heat pipe solar receiver designs were evaluated based on thermal performance, temperature uniformity, and integration with the solar concentrator and the regolith reactor(s). Two designs were selected based on these criteria: an annular heat pipe contained within the regolith reactor and an annular heat pipe with a remote location for the reactor. Additional design concepts have been developed that would use a single concentrator with a single solar receiver to supply and regulate power to multiple reactors. These designs use variable conductance or pressure controlled heat pipes for passive power distribution management between reactors. Following the design study, a demonstration heat pipe solar receiver was fabricated and tested. Test results demonstrated near uniform temperature on the outer surface of the pipe, which will ultimately be in contact with the regolith reactor.

Variable Conductance Heat Pipe Performance after Extended Periods of Freezing, Michael Ellis and William Anderson, Space, Propulsion and Energy Sciences International Forum (SPESIF), Huntsville, Alabama, February 2009

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Variable Conductance Heat Pipe Performance after Extended Periods of Freezing

Radiators operating in lunar or Martian environments must be designed to reject the maximum heat load at the maximum sink temperature, while maintaining acceptable temperatures at lower powers or sink temperatures. Variable Conductance Heat Pipe (VCHP) radiators can passively adjust to these changing conditions. Due to the presence of non-condensable gas (NCG) within each VCHP, the active condensing section adjusts with changes in either thermal load or sink temperature. In a Constant Conductance Heat Pipe (CCHP) without NCG, it is possible for all of the water to freeze in the condenser, by either sublimation or vaporization. With a dry evaporator, startup is difficult or impossible. Several previous studies have shown that adding NCG suppresses evaporator dryout when the condenser is frozen. These tests have been for relatively short durations, with relatively short condensers. This paper describes freeze/thaw experiments involving a VCHP with similar dimensions to the current reactor and cavity cooling radiator heat pipe designs.

Loop Heat Pipe for TacSat-4, Peter Dussinger, et al., Space, Propulsion and Energy Sciences International Forum (SPESIF), Huntsville, Alabama, February 2009

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Loop Heat Pipe for TacSat-4

The TacSat-4 micro-satellite uses an aluminum/ammonia loop heat pipe (LHP) to transport 700 W of heat from the electronics to two radiator sections. In addition to the thermal requirements, there were additional specifications for the primary and secondary wicks, and the flow balancer between the two LHP condensers. This paper discusses the experimental test rigs designed to verify the LHP performance against these requirements. The measured LHP performance at various operating conditions including start-up, un-balanced condenser heat removal, transient power, high power, and shut-down is discussed.

Advanced Thermal Management Technologies for High Power Automotive Equipment, Jon Zuo, et al., National Defense Industrial Association Ground Vehicle Power and Energy Workshop, Troy, Michigan, November 2008

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Advanced Thermal Management Technologies for High Power Density Automotive Equipment

Waste heat in military and civilian vehicles continues to rise as more electronics are integrated into these vehicles. High density packaging results in high heat fluxes. Many military vehicles operate in extreme temperature, shock and vibration conditions, imposing additional constraints on the design of thermal management solutions.

Thermal management of a vehicle system requires different technologies at different locations. For example, the high heat fluxes within a semiconductor device package require an effective heat spreader to lower the heat fluxes to a level suitable for further transport and dissipation. Heat transport and dissipation tend to focus more on power consumption, transport line size and system weight. In some cases where the environmental condition fluctuates, thermal control is required to maintain a relative constant temperature for protection of the electronics.

This paper presents several advanced thermal management technologies that address three key categories of electronics thermal management: Heat Spreading, Heat Transport and Temperature Control.

Vibration and Shock Tolerant Capillary Two-Phase Loop Technology for Vehicle Thermal Control, Xudong Tang and Chanwoo Park, 2008 ASME Summer Heat Transfer Conference, Jacksonville, Florida, August 2008

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Vibration/Shock-Tolerant Capillary Two-Phase Loop Technology for Vehicle Thermal Control

Two-phase thermal management technologies are promising cooling solutions for the high performance electronics in the next generation military and commercial vehicles. However, vibrations (~ 10Grms in commercial automobile engines and transmissions) and shocks (30G to 1,200G in military combat vehicles, caused by gun firing, ballistic launch and abrupt maneuvering) present a severe challenge to any capillary-driven (i.e., passive) two-phase devices. A low-cost, vibration/shock-tolerant Capillary Two-Phase Loop (CTPL) technology was developed as a cooling alternative for the future military vehicles. Unlike the traditional two-phase cooling loops such as Loop Heat Pipes (LHP) and Capillary Pumped Loops (CPL), the CTPL offers the following advantages: (1) lower manufacturing cost by sintering the evaporator wick in-situ; (2) improved tolerance to vibrations and shocks due to the improved mechanical strengths of the in-situ sintered wick; (3) improved heat flux performance because of the non-inverted meniscus wick. Small-scale proof-to-concept CTPL prototypes were successfully tested up to 120W of heat input and under multiple, consecutive shocks of up to 6.6G.

NaK Variable Conductance Heat Pipe for Radioisotope Stirling Systems, Calin Tarau, et al., 6th International Energy Conversion Engineering Conference (IECEC), Cleveland, Ohio, July 2008

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NaK Variable Conductance Heat Pipe for Radioisotope Stirling Systems

In a Stirling radioisotope power system, heat must continually be removed from the General Purpose Heat Source (GPHS) modules to maintain the modules and surrounding insulation at acceptable temperatures. The Stirling convertor normally provides most of this cooling. If the Stirling convertor stops in the current system, the insulation is designed to spoil, preventing damage to the GPHS, but also ending use of that convertor for the mission. An alkali-metal Variable Conductance Heat Pipe (VCHP) was designed to allow multiple stops and restarts of the Stirling convertor. In the design of the VCHP for the Advanced Stirling Radioisotope Generator, the VCHP reservoir temperature can vary between 40 and 120°C. While sodium, potassium, or cesium could be used as the working fluid, their melting temperatures are above the minimum reservoir temperature, allowing working fluid to freeze in the reservoir. In contrast, the melting point of NaK is -12°C, so NaK can’t freeze in the reservoir. One potential problem with NaK as a working fluid is that previous tests with NaK heat pipes have shown that NaK heat pipes can develop temperature non-uniformities in the evaporator due to NaK’s binary composition. A NaK heat pipe was fabricated to measure the temperature non-uniformities in a scale model of the VCHP for the Stirling Radioisotope system. The temperature profiles in the evaporator and condenser were measured as a function of operating temperature and power. The largest ΔT across the condenser was 28°C. However, the condenser ΔT decreased to 16°C for the 775°C vapor temperature at the highest heat flux applied, 7.21 W/cm2. This decrease with increasing heat flux was caused by the increased mixing of the sodium and potassium in the vapor. This temperature differential is similar to the temperature variation in this ASRG heat transfer interface without a heat pipe, so NaK can be used as the VCHP working fluid.

Heat Pipe Cooling of Concentrating Photovoltaic (CPV) Systems, William Anderson, et al., 6th International Energy Conversion Engineering Conference (IECEC), Cleveland, Ohio, July 2008

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Heat Pipe Cooling of Concentrating Photovoltaic (CPV) Systems

Concentrating photovoltaic systems (CPV) utilize low cost optical elements such as Fresnel lens or mini-reflecting mirrors to concentrate the solar intensity to 200 to 1000 suns. The concentrated solar energy is delivered to the solar cell at up to 20 to 100 W/cm2. A portion of the energy is converted to electricity, while the remainder must be removed as waste heat. Solar cell cooling must be an integral part of the CPV design, since lower cell temperatures result in higher conversion efficiencies. A heat pipe cooling system was developed to passively remove the high heat flux waste heat at the CPV cell, and reject the heat to ambient through natural convection. With a heat flux of 40 W/cm2, the heat pipe heat sink rejected the heat to the environment by natural convection, with a total cell-toambient temperature rise of only 40°C. In contrast, the ΔT between the cell and ambient would be over 110°C using natural convection from the backplate.

Startup Characteristics and Gravity Effects on a Medium/High-Lift Heat Pump Using Advanced Hybrid Loop Technology, Eric Sunada, et al., 38th SAE International Conference on Environmental Systems, San Francisco, California, June 2008

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Start-Up Characteristics and Gravity Effects on a Medium/High-Lift Heat Pump using Advanced Hybrid Loop Technology

Thermal characterization was performed on a vapor compression heat pump using a novel, hybrid two phase loop design. Previous work on this technology has demonstrated its ability to provide passive phase separation and flow control based on capillary action. This provides high quality vapor to the compressor without relying on gravity-based phase separation or other active devices. This paper describes the subsequent work done to characterize evaporator performance under various startup scenarios, tilt angles, and heat loads. The use of a thermal expansion valve as a method to regulate operation was investigated. The effect of past history of use on startup behavior was also studied.

Testing under various tilt angles showed evaporator performance to be affected by both adverse and favorable tilts for the given compressor. And depending on the distribution of liquid in the system upon startup, markedly different performance can result for the same system settings and heat loads. In this sense, the specific configuration and settings of the system are not mutually exclusive to a given performance. In general, four basic states of operation were identified which can result. It was also shown that active control of a thermal expansion valve may be used to recover from a nonoptimal state.

Recommendations for future work include optimization of the evaporator port geometry and wick structure to better mitigate against detrimental effects of compressor suction. The compressor itself was identified as an area in need of technology development to better match compression ratios, suction pressure, and throughput to the specific mission requirements. It is recommended that future prototypes consider the use of an actively controlled thermal expansion valve to mitigate against off-nominal states of operation.

High Temperature and High Heat Flux Thermal Management for Electronics, David Sarraf and William Anderson, IMAPS International Conference on High Temperature Electronics Conference (HiTEC 2008), Albuquerque, New Mexico, May 2008

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High Temperature and High Heat Flux Thermal Management for Electronics

Four new cooling technologies are under development with applications in electronics cooling: 1. High temperature water heat pipes: Titanium/water and Monel/water heat pipes have been developed and life tested at temperatures up to 280°C, much higher than conventional copper/water heat pipes (150°C). The titanium heat pipes can also reduce the heat pipe mass. 2. High temperature Water Loop Heat Pipes (LHPs): These titanium/water LHPs extend the operating temperature range for LHPs from the current 60°C to 200°C. 3. Cold Plates with Oscillating Flow: A cold plate uses the oscillating flow of a single phase fluid to spread the heat. Preliminary results show effective thermal conductivities of up to 240,000 W/m K (versus 1,200 W/m K for diamond and 90,000 W/m K for heat pipes), and the ability to remove heat fluxes as large as 1,200 W/cm2. 4. Hybrid pumped/wick system with multiple evaporators: In a hybrid pumped/wick system, water flows through an artery in contact with a wick. Water evaporates from the wick, cooling the electronics. Capillary forces pull replacement water into the wick, passively adjusting the water flow rate. The use of a pump allows multiple evaporators to be used. Very high heat fluxes can be removed at very low thermal resistances (as low as 0.16°C/W/cm2).

Heat Pipe Cooling of Concentrating Photovoltaic Cells, William Anderson, et al., 33rd IEEE Photovoltaic Specialists Conference, San Diego, California, May 2008

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Heat Pipe Cooling of Concentrating Photovoltaic Cells

Concentrating photovoltaic systems (CPV) utilize low cost optical elements such as Fresnel lens or mini-reflecting mirrors to concentrate the solar intensity to 200 to 1000 suns. The concentrated solar energy is delivered to the solar cell at up to 20 to 100 W/cm2. A portion of the energy is converted to electricity, while the portion that is not converted to electricity must be dissipated as waste heat. Solar cell cooling must be an integral part of the CPV design, since lower cell temperatures result in higher conversion efficiencies. Heat pipes can be used to passively remove the high heat flux waste heat at the CPV cell level, and reject the heat to ambient through natural convection. This paper discusses a cooling design that uses a copper/water heat pipe with aluminum fins to cool a CPV cell by natural convection. With a cell level waste heat flux of 40 W/cm2, the heat pipe heat sink rejected the heat to the environment by natural convection, with a total cell-to-ambient temperature rise of only 40°C.

Local Heat Transfer Coefficient Measurements of Flat Angles Sprays Using Thermal Test Vehicle, Richard Bonner, et al., 24th IEEE Semi-Therm Symposium, San Jose, California, March 2008

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Local Heat Transfer Coefficient Measurements of Flat Angled Sprays Using Thermal Test Vehicle

Impingement cooling methods, such as spray cooling and jet impingement have demonstrated the capability of cooling high heat flux surfaces while maintaining a low thermal resistance. Most spray cooling and jet impingement experiments attempt to measure the average heat transfer coefficient, even though it is known that heat transfer coefficients are known to change as a function of distance from the impact zone. Secondly, most experiments are done on thick uniformly heated surfaces although most electronic devices are very thin (<0.2mm) and generate heat very nonuniformly with very large peak heat fluxes (>1000W/cm2) over very small areas (<0.25mm2). In this study an accurate measurement of the uniformity of the spray cooling thermal solution was attained using an Intel supplied thermal test vehicle. The heater block is a thin silicon chip (<0.25mm thick and 7cm2 in surface area) delivering a uniform heat flux to 70W/cm2. The platform also has the ability to power large peak heat fluxes (>1000W/cm2) over small areas (<0.25mm2). Experiments using jet impingement with flat spray nozzles angled to the surface were conducted with water, methanol, and HFE-7000. The axial heat transfer coefficient variation was measured under uniform heat loading. Finally, the measurements are compared to modified models from the literature with good agreement.

Pressure Controlled Heat Pipe for Precise Temperature Control, David Sarraf, et al., Space Technology and Applications International Forum (STAIF), Albuquerque, New Mexico, February 2008

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Pressure Controlled Heat Pipe for Precise Temperature Control

This paper discusses the design and test of a pressure controlled heat pipe (PCHP) for spacecraft thermal management. The PCHP combines a conventional grooved aluminum-ammonia heat pipe with a variable-volume noncondensable gas reservoir to create a heat pipe whose conductance can be precisely controlled. Testing showed that a prototype PCHP was capable of maintaining a stable evaporator temperature within 0.1K despite wide swings in heat load and heat sink temperature. A similarly-sized variable-conductance heat pipe (VCHP) yielded temperature swings of over 3.5K for the same variation of heat load and smk temperature. Using a non-optimized control system, the PCHP was capable of maintaining evaporator temperature within 0.05K over time. The PCHP had a much faster transient response than other devices such as heated-reservoir VCHPs, as well as providing a means for changing the set point temperature after assembly. The PCHP is a significant advance over other means of temperature control, even in its current non-optimized state.

Titanium Loop Heat Pipes for Space Nuclear Power Systems, John Hartenstine, et al., Space Technology and Applications International Forum (STAIF), Albuquerque, New Mexico, February 2008

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Titanium Loop Heat Pipes for Space Nuclear Power Systems

Space nuclear power systems require a radiator to dissipate the waste heat generated during the thermal-to-electric conversion process. A previously conducted radiator trade study showed that radiators with titanium/water Loop Heat Pipes (LHP) have the highest specific power (ratio of heat dissipation to radiator mass) in the temperature range from 300 K to 550 K. A prototype titanium/water LHP was designed and fabricated to operate within this temperature range. The LHP was all titanium, to eliminate incompatibility problems between water and dissimilar metals. The LHP had a 2.54 cm (1 inch) O.D., 20 cm (8 in.) long evaporator wick, and was designed to carry 500 W of heat load. The liquid and vapor lines were roughly 2 m long, typical of the requirements for a spacecraft radiator. The LHP was tested to more than 550 W, at an adverse elevation of 5 cm and an operating temperature of 413 K. This paper describes the details of the titanium/water LHP design, wick development, and titanium LHP fabrication and tests.

Variable Conductance Heat Pipes for Radioisotope Stirling Systems, William Anderson and Calin Tarau, Space Technology and Applications International Forum (STAIF), Albuquerque, New Mexico, February 2008

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Variable Conductance Heat Pipes for Radioisotope Stirling Systems

In a Stirling radioisotope system, heat must continually be removed from the GPHS modules, to maintain the GPHS modules and surrounding insulation at acceptable temperatures. Normally, the Stirling convertor provides this cooling. If the Stirling engine stops in the current system, the insulation is designed to spoil, preventing damage to the GPHS, but also ending the mission. An alkali-metal Variable Conductance Heat Pipe (VCHP) was designed to allow multiple stops and restarts of the Stirling engine. A VCHP was designed for the Advanced Stirling Radioisotope Generator, with a 850°C heater head temperature. The VCHP turns on with a ΔT of 30°C, which is high enough to not risk standard ASRG operation but low enough to save most heater head life. This VCHP has a low mass, and low thermal losses for normal operation. In addition to the design, a proof-of-concept NaK VCHP was fabricated and tested. While NaK is normally not used in heat pipes, it has an advantage in that it is liquid at the reservoir operating temperature, while Na or K alone would freeze. The VCHP had two condensers, one simulating the heater head, and the other simulating the radiator. The experiments successfully demonstrated operation with the simulated heater head condenser off and on, while allowing the reservoir temperature to vary over 40 to 120°C, the maximum range expected. In agreement with previous NaK heat pipe tests, the evaporator ΔT was roughly 70°C, due to distillation of the NaK in the evaporator.

Vapor Compression Hybrid Two-Phase Loop Technology for Lunar Surface Applications, Chanwoo Park and Eric Sunada, Space Technology and Applications International Forum (STAIF), Albuquerque, New Mexico, February 2008

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Vapor Compression Hybrid Two-Phase Loop Technology for Lunar Surface Applications

NASA’s vision for Space Exploration that would return humans to the Moon by 2020 in preparation for human explorations of Mars. This requires innovative technical advances. The lunar mission requires a temperature-lift (heat pump) technology to reject waste heat to hot lunar surface (heat sink) environments during lunar daytime. The lunar outpost and Lunar Surface Access Module (LSAM) to operate anywhere during the hot lunar daytime require a high performance and energy-efficient, yet reliable refrigeration technology. A vapor compressor-driven hybrid twophase loop was developed for such hgh temperature-lift applications. The vapor compression loop used an advanced porous wick evaporator capable of gravity-insensitive capillary phase separation and excess liquid management to achieve high temperature-lift, large-area, isothermal and high heat flux cooling capability and efficient compression. The high temperature lift will allow the lunar surface systems use compact radiators by increased heat rejection temperature.

Experimental Study of Oscillating Flow Heat Transfer, Angie Fan, et al., Micro/Nanoscale Heat Transfer International Conference, Tainan, Taiwan, January 2008

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Experimental Study of Oscillating Flow Heat Transfer

This paper presents an experimental study of the heat transfer characteristics of mechanically driven oscillating flows inside small diameter channels and at various frequencies and stroke lengths. Two important parameters were studied: the effective thermal conductivity between the heat source and sink, and the heat transfer coefficients in the heating and cooling regions. The test data were compared to theoretical correlations in the literature to assess their validity in the operating range of interest. Kurzweg’s correlation agreed reasonably well with the test data at low frequencies (1 Hz) and small amplitudes (7.6 cm). The highest effective thermal conductivity achieved during this study was more than 210,000 W/m-K. As a reference, pure copper and diamond materials have thermal conductivities around 400 W/m-K and 1,200 W/m-K, respectively. At low oscillating frequencies, the measured heat transfer coefficients in the heating region agreed reasonably well to Shin (1998)’s correlation. The correlation tends to under predict the heat transfer coefficient at higher frequencies. The experimental study investigated the effects of various frequencies and stroke length and demonstrated heat transfer coefficients in the heating region in excess of 34,000W/m2-K

Metal Hydride Heat Storage Technology for Directed Energy Weapon Systems, Chanwoo Park, et al., 2007 ASME International Mechanical Engineering Congress & Exhibition, Seattle, Washington, November 2007

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Metal Hydride Heat Storage Technology For Directed Energy Weapon Systems

Directed Energy Weapon (DEW) systems in a pulse operation mode dissipate excessively large, transient waste heat because of their inherent inefficiencies. The heat storage system can store such a pulsed heat load not relying on oversized systems and dissipate the stored heat over time after the pulse operation. A compressor-driven metal hydride heat storage system was developed for efficient, compact heat storage and dissipation of the transient heat from the DEW systems. The greater volumetric heat storage capacity of metal hydride material was realized into more compact design than conventional Phase Change Material (PCM) systems. Other exclusive advantages of the metal hydride system were fast thermal response time and active heat pumping capability required for precision temperature control and on-demand cooling. This paper presented the operating principle and heat storage performance results of the compressor-driven metal hydride heat storage system through system modeling and prototype testing. The modeling and test results showed that the metal hydride system can store the average heat of 4.4kW during the heat storage period of 250 seconds and release the stored heat during the subsequent regeneration period of 900 seconds.

Electronics Thermal Management Using Advanced Hybrid Two-Phase Loop Technology, Chanwoo Park, et al.,, 2007 ASME-JSME Thermal Engineering Summer Heat Transfer Conference, Vancouver, Canada, July 2007.

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Electronics Thermal Management Using Advanced Hybrid Two-Phase Loop Technology

The paper discusses an advanced Hybrid Two-Phase Loop (HTPL) technology for electronics thermal management. The HTPL combined active mechanical pumping with passive capillary pumping realizing a reliable yet high performance cooling system. The evaporator developed for the HTPL used 3-dimensional metallic wick structures to enhance boiling heat transfer by passive capillary separation of liquid and vapor phases. Through the testing using various prototype hybrid loops, it was demonstrated that the hybrid loops were capable of removing high heat fluxes from multiple heat sources with large surface areas up to 135cm2 and 10kW heat load. Because of the passive capillary phase separation, the hybrid loop operation didn’t require any active flow control of the liquid in the evaporator, even at highly transient and asymmetrical heat inputs between the evaporators. These results represent the significant advance over state-of-the-art heat pipes, loop heat pipes and evaporative spray cooling devices in terms of performance, robustness and simplicity.

Loop Thermosyphon Design for Cooling of Large Area, High Heat Flux Sources, John Hartenstine, et al., InterPACK 2007, Vancouver, Canada, July 2007.

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Loop Thermosyphon Design For Cooling Of Large Area, High Heat Flux Sources

Two-phase flow loop technologies capable of acquiring high heat fluxes (>1kW/cm2) from large area heat sources (10cm2) are being considered for the next generation naval thermal requirements. A loop thermosyphon device (~1 meter tall) was fabricated and tested that included several copper porous wick structures in cylindrical evaporators. The first two were standard annular monoporous and biporous wick designs. The third wick consists of an annular evaporator wick and an integral secondary slab wick for improved liquid transport. In this configuration a circular array of cylindrical vapor vents are formed integral to the primary and secondary transport wick composite. Critical heat fluxes using these wick structures were measured between 240W/cm2 and 465W/cm2 over a 10cm2 area with water as the working fluid at 70°C saturation temperature. A thermosyphon model capable of predicting flow rate at various operating conditions based on a separated flow model is presented.

Heat Pipes for High Temperature Thermal Management, David Sarraf and William Anderson, InterPACK 2007, Vancouver, Canada, July 2007.

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Heat Pipes for High Temperature Thermal Management

Copper water heat pipes are a well-established solution for many conventional electronics cooling applications; however they have several problems when applied to high temperature electronics. The high vapor pressure of the working fluid combined with the decreasing strength of an already soft material leads to excessive wall thickness, high mass, and an inability to make thermally useful structures such as planar heat pipes (vapor chambers) or heat pipes with flat input surfaces. Titanium/water and Monel/water heat pipes can overcome the disadvantages of copper/water heat pipes and produce a viable thermal management solution for high temperature electronics. Water remains the fluid of choice at temperature up to about 280°C due to its favorable transport properties. Life tests have shown compatibility at high temperature. At temperatures above roughly 300°C, water is no longer a suitable fluid, due to high vapor pressure and low surface tension as the critical point is approached. At higher temperatures, another working fluid/envelope combination is required, either an organic or halide working fluid. Preliminary halide life test results are presented, giving fluids that can operate at temperatures as high as 425°C. At higher temperatures, alkali metal heat pipes are suitable. Water and the higher temperature working fluids can offer solutions for cooling high-temperature electronics, or those working at or above 150°C.

Intermediate Temperature Fluids for Heat Pipes and Loop Heat Pipes, William Anderson, 2007 International Energy Conversion Engineering Conference, St. Louis, MO, June 2007.

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Intermediate Temperature Fluids for Heat Pipes and Loop Heat Pipes

There are a number of applications that could use heat pipes or loop heat pipes (LHPs) in the intermediate temperature range of 450 to 750 K, including space nuclear power system radiators, fuel cells, geothermal power, waste heat recovery systems, and high temperature electronics cooling. Potential working fluids include organic fluids, elements, and halides. The paper reviews previous life tests conducted with 30 different intermediate temperature working fluids, and over 60 different working fluid/envelope combinations. Life tests have been run with three elemental working fluids: sulfur, sulfur-iodine mixtures, and mercury. Other fluids offer benefits over these three liquids in this temperature range. Life tests have been conducted with 19 different organic working fluids. As the temperature is increased, all of the organics start to decompose. Typically they generate non-condensable gas, and often the viscosity increases. The maximum operating temperature is a function of how much NCG can be tolerated, and the heat pipe operating lifetime. The highest long term life tests were run at 623 K (350°C), with short term tests at temperatures up to 653 K (380°C). Three sets of organic fluids stand out as good intermediate temperature fluids: (1) Diphenyl, Diphenyl Oxide, and Eutectic Diphenyl/Diphenyl Oxide, (2) Naphthalene, and (3) Toluene. While fluorinating organic compounds is believed to make them more stable, this has not yet been demonstrated during heat pipe life tests. Ongoing life tests suggest that the halides may be suitable for temperatures up to 673 K (400°C). However, property data for the halides is incomplete.

Intermediate Temperature Fluids Life Tests – Experiments, William Anderson, et al., 2007 International Energy Conversion Engineering Conference, St. Louis, MO, June 2007.

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Intermediate Temperature Fluids Life Tests – Experiments

There are a number of different applications that could use heat pipes or loop heat pipes (LHPs) in the intermediate temperature range of 450 to 725 K (170 to 450°C), including space nuclear power system radiators, fuel cells, and high temperature electronics cooling. Historically, water has been used in heat pipes at temperatures up to about 425 K (150°C). Recent life tests, updated below, demonstrate that titanium/water and Monel/water heat pipes can be used at temperatures up to 550 K (277°C), due to water’s favorable transport properties. At temperatures above roughly 570 K (300°C), water is no longer a suitable fluid, due to high vapor pressure and low surface tension as the critical point is approached. At higher temperatures, another working fluid/envelope combination is required, either an organic or halide working fluid. An electromotive force method was used to predict the compatibility of halide working fluids with envelope materials. This procedure was used to reject aluminum and aluminum alloys as envelope materials, due to their high decomposition potential. Titanium and three corrosion resistant superalloys were chosen as envelope materials. Life tests were conducted with these envelopes and six different working fluids: AlBr3, GaCl3, SnCl4, TiCl4, TiBr4, and eutectic diphenyl/diphenyl oxide (Therminol VP-1/Dowtherm A). All of the life tests except for the GaCl3 are ongoing; the GaCl3 was incompatible. As the temperature approaches 725 K (450°C), cesium is a potential heat pipe working fluid. Life tests results are also presented for cesium/Monel 400 and cesium/70-30 copper/nickel heat pipes operating near 750 K (477°C). These materials are not suitable for long term operation, due to copper transport from the condenser to the evaporator.

Intermediate Temperature Fluids Life Tests – Theory, Calin Tarau, et al., Space Technology and Applications International Forum (STAIF), Albuquerque, NM, February 11 - 15, 2007.

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Intermediate Temperature Fluids Life Tests – Theory

There are a number of different applications that could use heat pipes or loop heat pipes (LHPs) in the intermediate temperature range of 450 to 750 K, including space nuclear power system radiators, and high temperature electronics cooling. Potential working fluids include organic fluids, elements, and halides, with halides being the leastunderstood, with only a few life tests conducted. Potential envelope materials for halide working fluids include pure aluminum, aluminum alloys, commercially pure (CP) titanium, titanium alloys, and corrosion resistant superalloys. Life tests were conducted with three halides (AlBr3, SbBr3, and TiCl4) and water in three different envelopes: two aluminum alloys (Al-5052, Al-6061) and CP-2 titanium. The AlBr3 attacked the grain boundaries in the aluminum envelopes, and formed TiAl compounds in the titanium. The SbBr3 was incompatible with the only envelope material that it was tested with, Al-6061. TiCl4 and water were both compatible with CP2-titanium. A theoretical model was developed that uses electromotive force differences to predict the compatibility of halide working fluids with envelope materials. This theory predicts that iron, nickel, and molybdenum are good envelope materials, while aluminum and titanium halides are good working fluids. The model is in good agreement with results from previous life tests, as well as the current life tests.

Spacecraft Thermal Management Using Advanced Hybrid Two-Phase Loop Technology, Chanwoo Park, et al., Space Technology and Applications International Forum (STAIF), Albuquerque, NM, February 11 - 15, 2007.

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Spacecraft Thermal Management using Advanced Hybrid Two-Phase Loop Technology

The paper discusses an advanced hybrid two-phase loop technology for spacecraft thermal management. The hybrid loop integrates active mechanical pumping with passive capillary pumping promising a reliable yet high performance cooling system. The advanced evaporator design using porous wick structures was developed for the hybrid loop to enhance boiling heat transfer by passive phase separation. The prototype testing using various hybrid loops and components demonstrated that the hybrid loop was capable of removing high heat fluxes from multiple heat sources with large surface areas up to 135 cm2. Because of the passive capillary phase separation, the hybrid loop operation doesn’t require any active flow control of excess liquid in the evaporator, even at highly transient and asymmetrical heat inputs. These performance results represent significant improvements over state-of-the-art heat pipes, loop heat pipes and evaporative spray cooling devices in terms of performance, robustness and simplicity.

Advanced Hybrid Cooling Loop Technology for High Performance Thermal Management, Chanwoo Park, et al., 2006 International Energy Conversion Engineering Conference, San Diego, CA, June 2006.

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Advanced Hybrid Cooling Loop Technology for High Performance Thermal Management

Advanced hybrid cooling loop technology has been developed for the high performance cooling systems for such as U.S. Army next generation vehicles, “Future Combat System (FCS)” and Directed Energy Weapons (DEW), high power Solid State Laser Systems. The hybrid cooling loop combines the active liquid pumping with the passive capillary liquid management in the sintered wick structure of the evaporator and its liquid/vapor separation. The prototype hybrid cooling loop using planar evaporator design was tested to be capable of managing up to 4kW cooling load which is equivalent to the heat flux up to 30W/cm2 over the cooling surface area of 135cm2 (=7.6cm×17.8cm). From the temperature results, however, much higher heat flux conditions are very likely to be achieved. The measured boiling thermal resistance was as low as 0.16°C-cm2/W and remained relatively constant during heat load variations except cold start conditions. This paper discusses the operating principle of the hybrid cooling loop with single evaporator and presents the test results under various power cycles. The results represent major improvements over the state-of-art heat pipes, loop heat pipes and two-phase spray and jet impingement cooling devices in terms of heat flux, cooling surface area and design simplicity.

Heat Pipe Heat Exchanger with Two Levels of Isolation for Environmental Control of Manned Spacecraft Crew Compartment, David Sarraf, 37th International Conference on Environmental Systems, Norfolk, VA, July 17-20, 2006.

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Heat Pipe Heat Exchanger with Two Levels of Isolation for Environmental Control of Manned Spacecraft Crew Compartment

Current environmental control thermal management systems use a two-loop, two-exchanger arrangement to isolate the crew from potentially hazardous ammonia or Freon in the final heat reject loop. Although that method does provide isolation, it is not fault-tolerant and does not provide advance warning of an exchanger leak. A heat pipe heat exchanger under development improves the existing system by providing two levels of isolation between the fluid streams. The new heat exchanger is fault-tolerant and can provide advance warning of a leak. Described are the heat exchanger design, test results from a sub-scale exchanger segment, and the results of a design optimization and trade study.

Passive Thermal Management for a Fuel Cell Reforming Process, David Sarraf, et al., 2006 International Energy Conversion Engineering Conference, San Diego, CA, June 2006.

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Passive Thermal Management for a Fuel Cell Reforming Process

The US Navy is investigating hydrogen fuel cells powered by reformed naval logistic diesel fuel as a means of providing distributed ship service electrical power. Operation on diesel fuel requires a reformer system to remove sulfur and convert the synthesis gas into a hydrogen rich stream. Temperature control of the reformer system is made difficult because rapid changes in the fuel cell electrical load require rapid changes in the reactant flow rate. The current valve-based control system has several drawbacks, including increased system volume and pressure drop and decreased reliability. A heat exchanger based on Variable Conductance Heat Pipes (VCHP-HX) is currently under development. The VCHP-HX can passively regulate reactant temperature without a control valve and its attendant disadvantages. This paper presents the results to date of a VCHP-HX development program, including compatibility testing of candidate working fluid and wall materials, test data from a single VCHP, and test results from an array of VCHPs operating at reduced temperature. Suitable working fluids have been identified and the test results show good agreement with model predictions.

High Temperature Water-Titanium Heat Pipe Radiator, William Anderson, et al., 2006 International Energy Conversion Engineering Conference, San Diego, CA, June 2006.

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High Temperature Water-Titanium Heat Pipe Radiator

Space nuclear systems require large area radiators to reject the unconverted heat to space. System optimizations with Brayton cycles lead to radiators with radiator temperatures in the 400 to 550 K range. To date, nearly all space radiator systems have used aluminum/ammonia heat pipes but these components cannot function at the required temperatures. A Graphite Fiber Reinforced Composites (GFRC) radiator with high temperature titanium-water heat pipes is currently under development. Three candidate fin materials have been evaluated: K13D2U fibers with 5250-4, EX1551, and HPFE resin. Titanium was selected over Monel as the baseline envelope material, due to its lower mass and previous experience with bonding titanium into honeycomb panels. Graphite foam saddles are used to bond the heat pipes to the radiator fins. In addition to providing a heat transfer path between the round heat pipes and flat fins, the graphite saddle also provides micrometeroid protection, and reduces the effects of the coefficient of thermal expansion difference between the heat pipe and the fin. This paper also discusses mechanical and thermal tests of the laminate material, as well as a series of test panels.

High Temperature Titanium-Water and Monel-Water Heat Pipes, William Anderson, et al., 2006 International Energy Conversion Engineering Conference, San Diego, CA, June 2006.

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High Temperature Titanium-Water and Monel-Water Heat Pipes

Space nuclear systems require large area radiators to reject the unconverted heat to space. System optimizations with Brayton cycles lead to radiators with radiator temperatures in the 400 to 550 K range. To date, nearly all space radiator systems have used aluminum/ammonia heat pipes but these components cannot function at the required temperatures. Titanium-water and Monel-water heat pipes will operate in the temperature range, but titanium and Monel cannot be extruded in the same way as aluminum to form grooved heat pipes for space radiators. A method has been developed to form heat pipes in these materials. Grooves are machined into a flat plate, than the plate is bent and welded to form a heat pipe with grooves. Titanium-water heat pipes with a 1.3 cm O.D. and a length of roughly one meter have been fabricated with 3 different groove designs. The heat pipes carried 300-400 W at temperatures of 425 and 475 K. Water life test pipes have been fabricated with commercially pure (CP) titanium, Monel K-500, Monel 400, and various titanium alloys. CP-Ti and Monel pipes now have 17,400 hours of operation. These pipes continue to operate successfully, with a small amount of gas generation in the CP-Ti pipes. Life test pipes with titanium alloys are also underway, with between 4,000 and 9,000 hours of operation.

High-Temperature Water Heat Pipes, David Sarraf and William Anderson, IMAPS International Conference on High Temperature Electronics, Santa Fe, NM, May 15 - 18, 2006

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High-Temperature Water Heat Pipes

High Temperature Electronics require innovative thermal management devices. Copper water heat pipes are a well-established solution for many conventional electronics cooling applications; however they have several problems when applied to high temperature electronics. The high vapor pressure of the working fluid combined with the decreasing strength of an already soft material leads to excessive wall thickness, high mass, and an inability to make thermally useful structures such as planar heat pipes (vapor chambers) or heat pipes with flat input surfaces. Recent work has shown that titanium/water and Monel/water heat pipes can overcome the disadvantages of copper/water heat pipes and produce a viable thermal management solution for high temperature electronics. A study has shown that water remains the fluid of choice due to its favorable transport properties. Monel and titanium offer much higher strength and result in reasonable wall thickness and mass. Testing has shown compatibility at high temperature. Presented in this paper are a survey of potential replacement fluids, results from high-temperature life testing of water in Monel and titanium envelopes, and comparison of mass and performance with competing approaches such as copper/water.

High Performance Heat Storage and Dissipation Technology, Chanwoo Park, et, al., 2005 ASME International Mechanical Engineering Congress & Exposition (IMECE), Orlando, FL, November 5 - 11, 2005.

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High Performance Heat Storage And Dissipation Technology

High power solid state laser systems operating in a pulse mode dissipate the transient and excessively large waste heat from the laser diode arrays and gain material. The heat storage option using Phase Change Materials (PCMs) has been considered to manage such peak heat loads not relying on oversized systems for real-time cooling. However, the PCM heat storage systems suffer from the low heat storage densities and poor thermal conductivities of the conventional PCMs, consequently requiring large PCM volumes housed in thermal conductors such as aluminum or graphite foams.

We developed a high performance metal hydride heat storage system for efficient and passive acquisition, storage, transport and dissipation of the transient, high heat flux heat from the high power solid state laser systems. The greater volumetric heat storage capacity of metal hydrides than the conventional PCMs can be translated into very compact systems with shorter heat transfer paths and therefore less thermal resistance. Other exclusive properties of the metal hydride materials consist of fast thermal response and active cooling capability required for the precision temperature control and transient high heat flux cooling.

This paper discusses the operating principle and heat storage performance results of the metal hydride heat storage system through system analysis and prototype testing. The results revealed the superior heat storage performance of the metal hydride system to a conventional PCM system in terms of temperature excursion and system volume requirement.

Design and Testing of Titanium/Cesium and Titanium/Potassium Heat Pipes, Peter Dussinger, et, al., 2005 International Energy Conversion Engineering Conference (IECEC), San Francisco, CA, August 2005.

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Design and Testing of Titanium/Cesium and Titanium/Potassium Heat Pipes

Alkali metal heat pipes are ideal radiator elements for large area, high temperature, waste heat rejection radiator panels. Proposed low mass designs include a thin walled titanium (foil) / cesium or potassium heat pipe encased in a structural carbon-carbon shell. In this paper , the results of a preliminary design study are presented that explored heat pipe performance as a function of operating temperature, physical dimensions, working fluid (Cesium or Potassium), and wick structure selection. At the lower end of the operating temperature range 550K to 740K, in general cesium/titanium heat pipes can be designed to transfer more power than potassium. For heat pipes in the 50 to 75mm diameter range, cesium heat pipes can be designed to transfer 1 to 5 kW. Above 740K, potassium is a significantly better working fluid than cesium, with a capacity to transfer 5 to 15kW. The study also included the fabrication and initial testing of two heat pipes: one titanium/cesium (Ti/Cs) and one titanium/potassium (Ti/K). Long term compatibility data for standard wall thickness titanium/cesium and titanium/potassium heat pipes is scarce. Two life test pipes were fabricated and tested for 48 hours with no sign of degradation. This study provides further evidence that Ti/Cs and /or Ti/K heat pipes are capable of meeting the mass and performance goals for radiator systems of future nuclear-electric propulsion missions.

High Temperature Lightweight Heat Pipe Panel Technology Development, Ted Stern and William Anderson, Space Nuclear Conference 2005, San Diego, CA, June 2005.

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High Temperature Lightweight Heat Pipe Panel Technology Development

Lightweight, high temperature capable heat pipe panels are needed for the radiator element of a space nuclear power system. An approach was developed to provide technology for space heat pipe radiators that are lightweight and can reject heat at temperatures in the range from 300 – 550K. The approach resulted from a trade-off of alternative heat pipe and radiator panel materials and configurations. A high conductivity, high temperature capable, organic matrix graphite fiber reinforced composite material was developed to provide the fin for radiating heat from titanium / water heat pipes. A face sheet having a thickness of less than 0.2mm was demonstrated. Graphite foam saddles were used to bridge the round heat pipe configuration to a flat panel design having optimal structural properties. A panel coupon was fabricated from representative materials, assembled and tested at design temperatures in a vacuum to evaluate the durability of organic resins as both the matrix and adhesive material for the sandwich panel. The test showed the ability of GFRC to provide a suitable, lightweight radiator panel at these temperatures.

Loop Heat Pipe Radiator Trade Study for the 300-550K Temperature Range, William Anderson and Walter Bienert, Space Technology and Applications International Forum (STAIF), Albuquerque, New Mexico, February 2005

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Loop Heat Pipe Radiator Trade Study for the 300-550 K Temperature Range

A loop heat pipe (LHP) radiator trade study has been completed for radiators in the 300-550 K temperature range. Initially, a thorough component level study was completed to determine LHP operating properties over the temperature range, particularly at the high end where the Merit Number starts to fall off. Water was found to be the optimum fluid for the high end, while ammonia was a better working fluid for temperatures below 350 K. A trade study was then conducted that varied condenser O.D., temperature, panel area, fin width, fin thickness, and fin thermal conductivity. A comparison with comparable heat pipe radiators shows that the specific power of LHP radiators can be as much as 50 percent higher at temperatures above 500 K. This is offset by the fact that LHP radiator technology is much less mature.

Hybrid Loop Thermal Bus Technology for Vehicle Thermal Management, Chanwoo Park, et, al., 24th Army Science Conference, Orlando, FL, November 29 - December 2, 2004

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Hybrid Loop Thermal Bus Technology for Vehicle Thermal Management

Army’s next generation vehicles require more electric and electronic devices with increasing power density for improved multi-functionality. The increasing waste heat from these devices will present great challenges to the capabilities of conventional air/liquid cooling systems in cooling multiple, high heat flux sources dispersed over the entire vehicle. In this paper, a high performance hybrid loop thermal bus technology for vehicle thermal management is presented. The technology combines the robust operation of pumped two-phase flow cooling with the simplicity of capillary flow management. The test results show that the hybrid loop thermal bus can manage multiple high heat flux heat sources during the startup and transient heat input operation with no coolant flow control.