TED-AJ03-154 Effect of Sink Temperature on LHP System Performance : Preliminary Results
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概要
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A Loop Heat Pipe (LHP) is a passive two-phase heat transfer device developed and successfully employed to cool spacecraft (satellites). Capillary action in a porous, sintered metal wick provides pumping pressure. Vapor and liquid lines separate the evaporator and condenser sections, allowing heat rejection many meters from the energy source. Adapting LHP technology for terrestrial applications (e. g., cooling of ground vehicle systems) poses significant technical questions. Systems developed for space applications use radiation panels to dissipate energy from the condenser to a cold sink (deep space). Sink temperatures for ground applications are much higher. Since much of the LHP research focuses on space use, very little data exist for terrestrial applications at higher sink temperatures. Laboratory testing is often performed at low sink temperatures (as low as -40℃) where heat rejection is not limited by condenser surface. Condenser design and performance is important in achieving the maximum heat transfer for the LHP. It plays a direct role in determining the liquid return temperature, thus setting the compensation chamber temperature. An under sized condenser requires a larger temperature difference between the working fluid and sink to transfer the energy applied to the evaporator. The sink temperature is usually steady so the working fluid temperature must increase to achieve the necessary temperature difference. For this series of experiments high condenser effectiveness was achieved in all cases due to the self-regulating nature of the LHP. The LHP increases the loop operating temperature to dissipate the applied heat load. However the increased temperatures correspond to changes in the fluid properties in the evaporator. Fluid property changes impact the maximum capillary pressure and possible energy transport. A properly sized condenser is able to handle the maximum load and heat leak without increasing the condenser exit temperature. A sink temperature exists that limits the condenser and LHP's ability to reject the heat load without increasing the loop temperatures. Condenser-sink coupling resistance and sink temperature directly affect condenser and LHP performance despite the high condenser effectiveness values. For this particular LHP the condenser was unable to maintain a minimum condenser exit temperature for powers above 900 W at 0℃ or 200 W at 15℃. Also, higher sink temperatures demand more condenser surface area to reject the latent heat. Therefore the condenser utilization increases with higher sink temperatures. This paper reports the results of a series of experiments using a laboratory-scale test LHP with ammonia as the working fluid. Heat rejection rates of 1.1 kW were obtained at a loop operating temperature up to 35℃while rejecting heat to an ambient sink (ethylene glycol bath) ranging from -10℃ to 20℃. Results are discussed in the context of sink temperature and power dissipation effects on condenser effectiveness and utilization.
- 一般社団法人日本機械学会の論文
著者
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Nelson David
Michigan Technological University
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Rogers Paul
U. S. Army TACOM
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Ku Jentung
NASA-Goddard Space Flight Center