Combining Technologies for a High-Power Solution
Liquid cooling is a time-proven approach to maintaining safe operating temperatures of high heat flux electronics. It’s effective, capable of rejecting high heat loads and well understood. Although liquid cooled approaches are active, meaning they consume power and have mechanical components, they are often the best solution when power and temperature requirements demand it. In some cases, the liquid cooling lines need to be routed close to the heat-generating components to maintain thermal goals. This can, however, present a significant risk to the system in the event of a leak. If there were to be a fluid leak in the system, it would result in catastrophic failure to the electronics system.
Risk Mitigation to mitigate fluid leaks
One way to mitigate the fluid leak scenario is by decoupling the ultimate heat sink (liquid) from the heat-generating devices. By adding heat pipes to transfer heat at low thermal resistance, you can increase reliability with a minimal performance impact. The added distance between electronics and coolant could lead to a thermal penalty.
In the example below, ACT designed, developed, and tested a thermal solution that is capable of transferring heat from within an electronics chassis to an external liquid cooling loop. This included the heat pipe heat spreader design as well as the series/parallel flow channel liquid cold plate that acted as the ultimate heat sink. The electronics assembly, referred as a slat, has a fixed thermal resistance that is set by the customer’s electronics design. This slat is mounted to a heat pipe embedded, or HiK™ heat spreader, which transfers the heat through a series of interfaces and aluminum conduction paths to the external liquid cold plates which are mounted on the top and bottom of the electronics chassis.
Each HiK™ heat spreader in this assembly transfers 180W. The power is split between the two cooling loops with each handing 90W of power. This is important to note since heat pipes work both gravity assisted, and against gravity. Since the heat pipes were designed to work in both of orientations, and have a fixed thermal resistance, the electronics were able to maintain symmetric thermal gradients using both the upper and lower HiK™ heat spreaders. There is a combination of 16 HiK™ plates within this system for a total heat load of 2,880W.
Customer Collaboration- What does the design process look like?
ACT began the effort with a detailed thermal analysis that mapped the thermal resistance network from the heat-generating components to the chilled water supply. Several design iterations were required to determine a cold plate design that met both temperature uniformity and pressure drop goals. The final solution was a serpentine tube-in-plate design to ultimately meet necessary thermal requirements. Each serpentine removed heat from four of the HiK™ plates, four serpentines were used in parallel to reject heat from the 16 HiK™ plates within the system as seen in the Figure 1 CAD drawing below.
Following the thermal analysis of the proposed system and the customers’ approval, the ACT team was ready to move forward with manufacturing the prototype. The first step in production was manufacturing the heat pipes, aluminum components and the manifold assembly. Commercial off-the-shelf cold plates were used in lieu of an all-aluminum assembly in order to simplify and expedite the prototype lead time. Following assembly of the prototype, heat loads were simulated using film heaters that matched the heat input footprint provided by the customer and chilled water was supplied using a commercial chiller. An image of the test setup is shown in Figure 2.
Results of the testing in comparison to the thermal predictions are shown in Figure 3. The max junction temperature was predicted to be 118.0°C whereas the assembly measured 118.2°C. The temperature drop highlighted in the red boxes on the left of Figure 3 represents the junction-to-case temperature drop within the electronics slot. This information was provided by the customer and is fixed in the comparison. There are minor differences between the predicted and measured interface materials. The predicted temperature drop of the cold plate was 11.7°C and measured a 14.5°C. One key temperature drop to highlight is for the HiK™ plate. The prediction estimated a 4.1°C temperature drop, whereas the assembly measured only 2.9°C.
It’s not uncommon for large chassis to have such high waste heat that it requires liquid cooling. While this is less reliable than conduction or air-cooled solution, there are methods to increase reliability by separating the liquid flow and potential leak points from the high-value electronics. By using heat pipes to transfer the heat from the source to the liquid-cooled sink, you can vastly reduce thermal resistance in this conduction path and keep the liquid loop a safe distance from the electronics. This hybrid approach can provide both high thermal performance and high reliability when compared to alternative solutions.
Discuss your application with ACT engineers today!
Having the Right Technology at the Right Time
As a leading innovator and manufacturer of new thermal management technologies, ACT is frequently sought out by our military customers to work on thermal challenges for their next generation applications. In this case, ACT worked with Lockheed Martin to help develop the Long Range Anti-Ship Missile (LRASM). LRASM, featuring a variety of highly sophisticated technologies that allow it to execute long-range missions with high accuracy and reliability, is designed to meet the needs of the U.S. Navy and the U.S. Air Force for advanced precision-guided missile systems. It aims to reduce dependence on external platforms and network links in dense electronic-warfare environments.
Our Role Solving Military Thermal Challenges
ACT supported the effort by developing an advanced passive thermal management system to replace the actively pumped cooling system. This advanced thermal management system reduces the number of parts and system complexity, which in turn creates higher system reliability. The passive operation provides critical mission capability by cooling the electronics responsible for dynamic targeting. This advanced design was the result of ACT’s commitment to consistently investing in new and emerging thermal technologies. It once again demonstrated the many benefits of having the right technology at the right time.
Hot Surface Ignition (HSI) Test System to Evaluate Flammability
Leaking flammable liquids that come in contact with hot surfaces aboard aircraft can create flammability hazards that can pose risks to the aircraft and personnel on-board. To properly assess these risks, research on the operating parameters that influence the hot surface ignition (HSI) process is required. HSI events occur when a flammable liquid impinges on a hot surface, evaporates/boils, and mixes with the surrounding air. If this mixture is between the upper flammable limit (UFL) and lower flammable limit (LFL) of the fuel and the temperature of the mixture is above the auto-ignition temperature (AIT), this mixture can ignite and, if sustained, cause damage to the aircraft and its occupants, as shown in Figure 1.
The probability of ignition by a hot surface is dependent on several factors that include but are not limited to:
- Fluid temperature
- Fluid volume injected onto the hot surface
- Injection method (e. contacts the hot surface through a stream, spray, or discrete droplets)
- Air current over the hot surface
- Temperature of the surrounding air
- Equivalence ratio of the air/fuel mixture due to ambient air pressure and high altitude conditions
- Vapor pressure and density of the flammable liquid
As shown in Figure 2, there is a significant deviation in the experimentally determined minimum hot surface ignition temperature (MHSIT) (i.e. the minimum temperature at which an HSI event will occur) across multiple research groups which is attributed to the lack of reproducibility and control of influencing environmental conditions. Furthermore, the American Petroleum Industry (API) recommends that a hot surface ignition event is not to be considered as a hazard until the hot surface temperature is 182°C above the AIT of the flammable liquid. As shown in Figure 2, HSI events can occur even when the recommendation by API is followed (see MIL-H-83282 in Figure 2).
In order to provide the repeatable test conditions that are required to properly assess HSI risks aboard aircraft, Advanced Cooling Technologies, Inc. (ACT) developed an HSI test apparatus, shown in Figure 3, under an Air Force funded Phase I Small Business Innovative Research (SBIR) program. This test apparatus was developed with a chamber that isolates the hot surface from the environment. A vapor chamber designed to operate in a temperature range from 500-700°C provides the isothermal hot surface. The isothermality of the vapor chamber compared to an equivalent metal block and the temperature response of both surfaces to a steady stream of injected fluid on the surface is presented in Figure 4 and Figure 5, respectively. The vapor chamber was designed to be removable from the test apparatus such that vapor chambers with various geometries can be integrated into the chamber to evaluate the dependence of hot surface geometry on the HSI process. Additionally, the test apparatus was developed with a fluid injection system that can deliver flammable liquids to the hot surface in either a stream, spray, or droplet.
Figure 5. Videos of a 0.5 mL injection of water onto the surface of a (top) metal block and (bottom) vapor chamber exhibiting isothermal performance of the vapor chamber throughout the duration of the fluid injection
This test apparatus was outfitted with a photodiode and an exposed thermocouple to provide response to the ignition process and the time to ignition (i.e. the time required for ignition to occur after delivery of the flammable liquid) of various types of fuel at surface temperatures from 500-700°C were evaluated in static air conditions. Single components fuels (n-decane, n-heptane, and isopropyl alcohol (IPA), were evaluated in addition to multicomponent JP-8 fuel. The probability of ignition given a length of exposure of the flammable liquid to the hot surface is presented in Figure 6.
As shown in Figure 6, the probability of ignition of n-decane, n-heptane, and JP-8 are very similar given the amount of exposure to a given surface temperature and is related to the similar physical properties of the fuel, such as normal boiling point, AIT, and vapor density (Figure 7). However, IPA exhibited markedly different HSI behavior showing almost zero probability of ignition at surface temperatures less than 600°C. This is attributed to the higher evaporation rates of the fuel from the hot surface thereby producing a lower probability of achieving an ignitable mixture.
The repeatability of this test apparatus was evaluated by developing a standardization curve for the time to ignition of n-decane at surface temperatures varying from 500-700°C and re-evaluating the time to ignition of n-decane before and after JP-8 trials (standardization checks) to ensure the HSI test apparatus provided repeatable results.
As shown in Figure 8, the time to ignition evaluated in ACT’s HSI test apparatus provided repeatable results across multiple trials at the same surface temperature. Thus, indicating the test apparatus provided a repeatable environment for evaluating the influencing parameters of HSI events.
In the Phase II effort, ACT has expanded the capabilities of the HSI test apparatus by incorporating a wind tunnel style test chamber that enables various air flow conditions to be evaluated over hot surfaces with varying geometries; see Figures 9 and 10. Swappable vapor chambers were fabricated to enable testing on various shapes of the isothermal hot surface, shown in Figure 11.
 Colwell, Jeff D., and Ali Reza. “Hot Surface Ignition of Automotive and Aviation Fuels.” Fire Technology. 2005.
 American Petroleum Institute, “Ignition Risk of Hydrocarbon Liquids and Vapors by Hot Surfaces in the Open Air.” API Recommended Practice 2216. Third Edition. Washington, D.C. December 2003.
C.T.E. Matched Vapor Chambers
There are a growing number of high power applications across industries that require highly efficient cooling solutions. Some of these products have very high localized heat fluxes, greater than 300 W/cm2, but must maintain a tight temperature range so as not to disrupt sensors or optical devices. Direct die attachment can lead to mechanical stresses at the interface if the coefficient of thermal expansion (C.T.E.) is mismatched between the die and the substrate. The standard method to overcome this is to add an interface material, such as thermal gap pads or thermal pastes, to accommodate the mismatch. Unfortunately the presence of this thermal interface layer increases the thermal resistance and likewise increases the temperature on the device itself.
Vapor Chambers are an important tool in the thermal management toolbox, since they act as flux transformers, spreading the high input heat flux over the entire surface of the vapor chamber. This allows the heat to be removed from the vapor chamber by conventional cooling methods. Most vapor chambers are limited to an input heat flux of about 75 W/cm2, however, ACT has developed a C.T.E matched vapor chamber that allows for direct bonding of the heat source; see Figure 1. This unique device has been demonstrated to dissipate heat fluxes as high as 700 W/cm2 and 2kW overall. The evaporator thermal resistance of vapor chambers with this wick design is only 0.05 K-cm2/W.
The overall envelope structure is aluminum nitride with a direct bond copper exterior. The copper on the inside of the vapor chamber ensures that the well-known water/copper performance is maintained. In areas where the heat source is to be attached, the copper layer is removed exposing Aluminum nitride. Aluminum nitride has a CTE of ~5.5 ppm/⁰C, which is close to many common semiconductor materials. The devices can be directly attached to the vapor chamber, eliminating the need for a thermal interface layer.
Return to Vapor Chamber Assemblies…
HiK™ Plates to Improve Size, Weight, and Power (SWaP)
High Conductivity (HiK™) heat sinks can also improve the Size, Weight, and Power (SWAP) compared to standard heat sinks. Placing a discreet heat source on a large metal heat sink will produce large thermal gradients as the heat slowly conducts through the aluminum to the fins. Embedding heat pipes in a HiK™ heat sink can increase the thermal conductivity from around 180 W/m K to 500-1,200 W/m K, providing an opportunity to reduce heat sink plate thickness and fin area. This approach has been proven in a variety of weight/volume sensitive applications including: Ruggedized Electronics, UAVs, Handheld/Portable Devices, LEDs and optical devices.
Embedded heat pipes can improve performance and reduce the mass of forced and natural convection heat sinks. ACT fabricated a HiK™ heat sink and an all-aluminum heat sink with the same performance; see Figure 1. The total heat dissipation is 150W in both cases. The conventional aluminum heat sink is 12 inches (30.5 cm) long, weighs 9.6 lbs. (4.4 kg) and has a base thickness of 0.6 inch (1.5 cm). Introduction of 5 heat pipes, 3 in close proximity to the heat source and another two a little further out for improved spreading reduced the length to 10 inches (25.4 cm), reduced the thickness to 0.28 in (0.7 cm), and reduced the mass to 6.3 lbs. (2.9 kg) for an overall material reduction of over 34%. Thermal images that demonstrate the improvement are shown in Figure 2. The Hi-K heat sink seen on the right maintains the same source temperature, even though the heat sink is shorter, lighter, and thinner. The improvement is directly attributable to the addition of heat pipes which can be seen as red lines in the picture on the right.
Learn more about thermal solutions for power electronics here…