Open-Cell Foam Heat Sink for High Heat Flux Electronics Cooling, Phase II
Raytheon Missile Systems"Gen 4B W-band submodule for its Solid State Active Denial Technology system currently uses liquid coolant, specifically a water-ethylene glycol mixture, for thermal management. Future active denial systems are expected to use up to 1024 of these submodules. Because each submodule has both an inlet and an outlet, the coolant path of each active denial system will have up to 2048 O-ring seals, with the overall system design incorporating a complex set of manifolds for distributing coolant to the submodules. Every seal and coolant line junction in the electronics enclosure must be 100% reliable with no potential for leakage. An alternative, advanced thermal management system could eliminate the use of liquid coolant, thereby greatly improving reliability. Ultramet"s open-cell foam heat sink technology allows the liquid coolant to be replaced with air, eliminating the risks to the electronics from liquid leakage at the O-ring seals. In Phase I, Ultramet modeled, designed, fabricated, and tested low-cost, high-performance heat sinks for actively cooled high-power electronics. A novel graphite foam was developed based on processing methods used in long-established, ongoing commercial manufacturing of vitreous carbon foam at Ultramet. The resulting open-cell graphite foam acts as a high thermal conductivity, high surface area cooling fin, and was tested under a range of heat flux and air flow conditions. Graphite was chosen based on its high thermal conductivity and excellent corrosion resistance. The foam ligaments act as an extended surface for heat dissipation. By forcing cooling air through the foam, heat is removed very rapidly due to the high thermal conductivity and the extremely high surface area of the foam structure. Heat transfer testing was performed both with and without foam, and the data showed that for a given power dissipation and coolant flow rate, the foam-based heat sinks yielded surface temperatures 100-150°C lower than those without foam. Similarly, for a given Reynolds number, the measured heat transfer coefficients were more than 200% greater when foam was present. In Phase II, Ultramet will perform additional analysis, design, and testing to further improve heat transfer in the submodule heat sink by utilizing foam-based strategies while minimizing pressure and volume pumping requirements. Ultramet will work with Raytheon to design and fabricate hardware and perform submodule testing to fully evaluate the potential of this thermal management approach. A system-level demonstration will follow, with a complete coolant system to be designed and built for Raytheon"s active denial system and a 1000-hour test performed to determine volume and pressure capability as well as identify any erosion. Finally, the coolant system will be interfaced to a 100-W submodule setup for heating via surrogate load, packaged complete with instrumentation and any other supporting equipment, and delivered to the Army for evaluation.
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