Variable Conductance Heat Pipes for Variable Thermal Links

William G. Anderson, John R. Hartenstine, and Christopher J. Peters
Advanced Cooling Technologies, Inc.
1045 New Holland Ave., Lancaster, PA 17601 U.S.A.
717-295-6061, 717-295-6064 Fax, Bill.Anderson@1-act.com, John.Hartenstine @1-act.com

ABSTRACT

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.

KEY WORDS: Variable Conductance Heat Pipes, VCHPs, Variable Thermal Links

 A typical VCHP with an evaporator, a single condenser, and an electrically heated reservoir at the end of the condenser.


1. INTRODUCTION 

A typical Variable Conductance Heat Pipe (VCHP), shown in Figure 1, has an evaporator, a single condenser, and an electrically heated reservoir at the end of the condenser. This system is commonly used in spacecraft thermal control to provide ±1-2°C temperature control over widely varying powers and sink temperatures.

 While the standard VCHP is used for tight temperature control, this paper will discuss VCHP applications where a variable thermal link is desired instead. In most of these applications, liquid in the condenser can freeze. VCHPs or gasloaded heat pipes simplify start-up with a frozen condenser.

2. VCHP DESIGN FOR LUNAR LANDERS AND ROVERS

The lunar environment presents a number of challenges to the design and operation of thermal management systems. The heat rejection sink can be 330 K during daytime and can drop down to 50K at night or in dark craters (Swanson and Butler, 2006). Instruments and equipment, such as batteries, will need to be maintained within -20ºC to 40ºC throughout the large diurnal temperature swings (Birur and Tsuyuki, 2009). In addition, depending upon the mission, the thermal system will be required to work both on the lunar surface after deployment, and during the transit time from the earth to the moon. Due to the wide temperature swings, future lunar landers and rovers will require a variable conductance thermal link that can reject heat during the day and passively shut off during the lunar night without requiring any electrical power. During the long lunar day, the thermal management system 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. For solar powered systems, the variable link design is complicated by the heavy mass penalties associated with providing continuous power throughout the 14-day-long lunar night: It is estimated that 5 kg of batteries, solar cells, etc., are required to supply 1 W of electricity. Therefore, designs that operate without electrical power are highly desirable. The Lunar Anchor Node design targets are shown in Table 1.

Anchor Node Design Targets.

The Anchor Node VCHP design is shown in Figure 2. The VCHP evaporator is nominally horizontal, and the condenser is nominally vertical during operation on the Lunar Surface. An important design target consideration for Lunar Landers and Rovers is that the VCHP must work both in space and in a gravity-aided mode on the lunar surface. For the Lunar Anchor Node, the VCHP needs to operate against a maximum slope. For the Anchor Node, it is ±14° (due both to the general tilt of the terrain, as well as any local rock or depression). The maximum slope for rovers can be as high as ±45°. The basic VCHP layout is shown in Figure 2 (a). The VCHP evaporator sits on the WEB enclosure base plate. The adiabatic section is tilted so that it is always gravity aided on the moon.

Due to the ±14° slope, the conventional spacecraft all-grooved wick is not suitable in the evaporator section. Typically, grooved heat pipes are tested on earth with an adverse tilt (evaporator above condenser) of 0.1 inch (2.54 mm). As shown in Figure 2 (b), the lander evaporator can have an adverse tilt of several cm. To accommodate this high adverse elevation, a hybrid wick was developed. All of the condenser and most of the adiabatic section employ axial grooves for liquid return. A small portion of the adiabatic section and all of the evaporator use wrapped screen mesh as the wicking structure. The screen mesh wick, which has higher capillary pumping pressure than grooves, enables the evaporator to operate against the tilt caused by the uneven lunar surface.

 Schematic of the VCHP layout from the WEB to the radiator

Figure 2. a. Schematic of the VCHP layout from the WEB to the radiator. b. Depending on the terrain, the evaporator can have a ±14° tilt relative to horizontal. In some cases, the evaporator wick
must work against gravity on the lunar surface, which is not possible with a conventional grooved wick.

A schematic of the VCHP is shown in Figure 3. The VCHP incorporates three novel features in order to achieve the design targets of the ILN program:

1.Hybrid-Wick, discussed above.
2.Reservoir near Evaporator, to prevent the
reservoir temperature from dropping during the
lunar night.
3.Bimetallic Adiabatic Section, to minimize heat
losses during the lunar night.

A stainless steel heat pipe section of 12.7 cm (5 inches), which replaces the original aluminum portion in the adiabatic section, acts as a thermal dam, and minimizes axial heat leak to the cold
radiator during shutdown.

Schematic of the VCHP with a hybrid wick, to allow operation at different tilts.

Figure 3. Schematic of the VCHP with a hybrid wick, to allow operation at different tilts. Placing the reservoir near the evaporator keeps the reservoir warm, minimizing the required reservoir size. Also, part of the adiabatic section is stainless steel, which minimizes heat leaks when the VCHP is turned off.

The heat pipe uses anhydrous ammonia as the working fluid, since it is the best working fluid when the heat pipe is operational. The NonCondensable Gas (NCG) has two purposes: 1. Provides a variable thermal link that turns off as the evaporator temperature drops, and 2.
Suppresses the freezing of ammonia in the condenser during the lunar night. The freezing point of ammonia is 195 K, while the radiator could conceivably cool down to 96 K. In a CCHP, ammonia would tend to freeze in the condenser. However, in a VCHP, the gas in the adiabatic and condenser sections blocks the flow of ammonia from the evaporator to the condenser. Ammonia can only slowly diffuse through the gas. The NCG also aids in starting up the heat pipe after sunrise (Ellis and Anderson, 2009).

 VCHPs typically use argon as the NCG. Due to the low temperatures, neon was selected for this VCHP instead. The reason is that the critical temperature of Argon is 151 K, so the argon would not be a perfect gas during the lunar night, and could actually condense. Neon, with a critical temperature of 44 K, will behave like a perfect gas.

2.1 Reservoir Location
Placing the NCG reservoir near the evaporator, as opposed to the traditional location near the condenser, keeps the gas reservoir warm and minimizes the reservoir size. A conventional VCHP, shown in Figure 1, has the reservoir located next to the condenser. The VCHP reservoir temperature is controlled by tying it thermally to another portion of the spacecraft to cold bias the VCHP, then adding heaters on the reservoir to control power.

In contrast, no electrical heating is available to control the VCHP temperature in a lunar lander or rover. If the VCHP reservoir was located at the top of the condenser in Figure 2 (a), then the VCHP reservoir would operate near the sink temperature of 96 K during the lunar night. Anderson, Ellis, and Walker (2009) developed the equations to size this type of radiator, and showed that with a wide variation in sink temperature, there is a minimum ΔTVCHP, even with an infinite reservoir.

Placing the NCG reservoir near the condenser would necessitate a very large reservoir that can only provide coarse temperature control (ΔTVCHP ≥ 30 °C). ΔTVCHP is defined as the difference between the operating and shutdown temperatures of the evaporator (ΔTVCHP = TOn – TOff). A very precisely controlled VCHP would have a very small, but finite ΔTVCHP. Such temperature control is only possible with a reservoir near the evaporator. Figure 4 illustrates these principles.

Comparison between NCG Reservoir Locations.

In Figure 4, items colored red correspond to a warm reservoir placed near the evaporator and items colored in blue correspond to a cold reservoir located near the condenser. The two dashed
vertical lines represent the asymptotes for either the warm or cold reservoir. In both reservoir types, the mass of the system trends exponentially towards infinity as the degree of control increases
and ΔTVCHP decreases. Observe that an infinitely large cold reservoir provides ΔTVCHP ≈ 30 °C and an infinitely large warm reservoir provides ΔTVCHP = 0 °C. For this reason, the reservoir is located in the WEB, next to the evaporator, where it can be kept warm by the heat pipes used to transfer heat in the WEB. A small NCG tube passes through the entire length of the heat pipe to pneumatically connect the NCG reservoir to the condenser (see Figure 3). This NCG tube allows the NCG reservoir to be located near the evaporator rather than the condenser. While this heat pipe design has been theoretically discussed as far back as Marcus (1971), to the best of our knowledge this is the first time that this type of VCHP has been fabricated.

3. VCHP TEST SETUP AND TESTING

3.1 Test Setup
The VCHP had the following specifications:
• 30.5 cm (12 inch) Condenser / ≈ 48.3 cm (19 inch) Adiabatic Section – Grooved aluminum extrusion (6063-T6 Al)
• Bimetallic Transition – 1.25 inch 6061-T6 Al × 5 inch 304 SS × 1.25 inch 6061-T6 Al
• 22.9 cm (9 inch) Evaporator – Nickel 200 50×50 screen mesh
• NCG Tube (304 SS) – 0.32 cm (0.125 inch) outer diameter
• NCG Reservoir (304 SS) – 73.7 cm3 (4.5 inch3 ) internal volume
• Working Fluid (Ammonia) – 20.8 grams

• Non-Condensable Gas (Neon) – ≈0.65 grams

completed hybrid wick VCHP

Figure 5. Hybrid wick VCHP with reservoir adjacent to the evaporator.

The completed hybrid wick VCHP is shown in Figure 5. In addition to an intrusive thermocouple in the reservoir, thermocouples were attached every 5 cm to the evaporator, adiabatic, and condenser sections. , Heat was supplied to the evaporator with electric cartridge heaters embedded in an aluminum block. Heat was removed from the condenser with a cold plate, cooled with liquid nitrogen to a fixed temperature.

3.2 Test Objectives
The first major test objective was a simulated Lunar Freeze/Thaw test: Demonstrate the ability of the VCHP to act as a variable thermal link on the moon, and minimize the heat transfer as the
condenser temperature dropped below the freezing point of ammonia. In addition, verify that the VCHP can operate for short periods with a cold condenser (which can occur in some lander and
rover scenarios). The second major test objective was to demonstrate that the VCHP will act as a diode in space, preventing heat from solar insolation from heating the WEB.

3.3 Variable Thermal Link and Freeze/Thaw Testing
For the simulated lunar tests, the VCHP was mounted in a test fixture on an optical table that set the condenser vertical and allowed the evaporator to be configured for gravity aided (+2.3°), gravity neutral (0°) and gravity adverse (-2.3°) inclinations (2.3° on earth is equivalent to a 14° inclination on the moon).

 VCHP Temperature Profile as a Function of Time (-2.3° Orientation).

Figure 6 plots VCHP temperatures as a function of time during the lunar freeze/thaw test. TC1 corresponds to the gas temperature in the NCG reservoir. TC10 measures the vapor temperature of the evaporator. TC23, TC26, TC27 and TC30 detect the vapor temperature of four locations within condenser, with TC23 at the entrance of the condenser and TC30 close to the tip of the condenser. The power curve shows the electrical power input into the heater block of the evaporator.

 Initially, the pipe is operating at a nominal 25 °C and 50 W. At about 6000 seconds the pipe temperature and power input are reduced to -60 °C and 0.2 W, respectively. The purpose of 0.2 W of heat input was to maintain the evaporator above – 10°C. At around 9000 seconds, power is temporarily increased to the full 95 W, to simulate a brief period of activity during the lunar night. After this power increase, the pipe was returned to the -60 °C shutdown state. Next, the sink temperature is further reduced to -177 °C (96 K, ammonia freezes at 195 K). The pipe reaches a steady-state shutdown at -177 °C and 0.1 W. At approximately 17,500 seconds, the power is briefly increased to 25 W and the transient response of the frozen pipe was observed. With no indication of problems, the pipe is returned to -177 °C shutdown. Power is then increased to a full 95 W for a short duration. After the full power increase, the pipe is returned to a state of shutoff until around 21000 seconds when the power is gradually increased and heat pipe startup begins. Finally, the VCHP is brought to nominal steady-state operation at 95 W and 25 °C. Overall conductances for the heat pipe during normal operation, and with -60 and -177°C sinks, given in Table 2, show that the heat pipe operates as a variable thermal link.

Heat Pipe Overall Conductances for Freeze/Thaw, -2.3° Inclination.

3.4 Diode Operation in Space
During transit, at some times the radiator will be hotter than the WEB due to solar insolation. During these times, it is desirable for the VCHP to act as a diode, preventing overheating of the WEB.
A conventional VCHP would behave like a gasloaded diode in this situation. Tests were conducted to verify that a VCHP with the reservoir near the evaporator would also act as a diode. During these tests, the ILN VCHP was kept in a horizontal orientation; however, the adverse elevation and heat input/output were reversed compared to the space thermal performance test. The goal of this test was to verify that the pipe inhibited heat transfer in the reverse direction; therefore, the test intentionally attempted to operate the heat pipe backwards. Heat was input into the condenser and rejected from the evaporator. Since the capillary flow would travel from the evaporator to the condenser, the adverse elevation was defined as the condenser being 0.1, 0.2, and 0.3 inch (2.54mm, 5.08mm and 7.62mm) above the evaporator.

During these tests, the power into the nominal condenser was adjusted until a steady-state temperature difference of 20 °C was observed between the evaporator and condenser. Once this temperature difference was achieved, the input power was recorded as the reverse heat transfer rate.

Table 3 lists the results of the thermal diode experiments. All of the reverse powers are low (less than 4 % of the 117 W target for space) and the conductances are minimal (two orders of magnitude less than the values of the space thermal performance test). The conductances are negative because the condenser is hotter than the evaporator, which is the opposite of normal operation.

 VCHP Results for Space Thermal Diode

3. CONCLUSIONS

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.
Differences from a conventional spacecraft VCHP include 1. A hybrid wick, to allow the evaporator to operate when tilted at adverse orientations of up to 14°. 2. The reservoir was located next to the evaporator, to minimize the reservoir size and mass, while using no electrical heaters, and 3. The addition of a bimetallic adiabatic section, with a length of grooved stainless steel to minimize heat leaks during the lunar night.

The simulated lunar performance testing demonstrated that the VCHP shut off as the condenser temperature was lowered, so the system acted as a variable 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 shutdowns. Startup of the VCHP with a frozen condenser was also demonstrated. As expected, the VCHP behaves as a gas diode heat pipe when the condenser is heated in a simulated space environment.

ACKNOWLEDGEMENT

This program was sponsored by NASA Marshall Space Flight Center under Purchase Order No. NAS802060. We would like to thank the technical monitor, Jeffery Farmer of NASA Marshall, for many helpful technical discussions.

REFERENCES

W. G. Anderson, M. C. Ellis, and K. Walker, “Variable Conductance Heat Pipe Radiators for Lunar and Martian Environments,” SPESIF 2009, Huntsville, AL, February 24 – 27, 2009.
Birur, G., Tsuyuki, G., “JPL Advanced Thermal Control Roadmap – 2009”, presented at the Spacecraft Thermal Control Workshop, March 10-12, 2009.
M. C. Ellis and W. G. Anderson, “Variable Conductance Heat Pipe Performance after Extended Periods of Freezing,” SPESIF 2009, Huntsville, AL, February 24 – 27, 2009.
Hartenstine, J. R., Walker, K. L., and Anderson, W. G., “Loop Heat Pipe with Thermal Control Valve for Variable Thermal Conductance,” 41st International Conference on Environmental Systems (ICES 2011), Portland, OR, July 17-21, 2011.
Marcus, B. D., “Theory and Design of Variable Conductance Heat Pipes: Hydrodynamics and Heat Transfer,” NASA Report No. NASA-CR-146195, April 1971.
Swanson, T., and Butler, D., “NASA/Goddard Thermal Control Technology Roadmap-2006”, 17th Aerospace Spacecraft Thermal Control Workshop, Los Angeles, CA, March 14-16, 2006.