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In response to the identified needs of emerging high power spacecraft applications, a multiple evaporator Hybrid Loop Heat Pipe (H-LHP) was developed and tested as part of a Dual Use Science and Technology (DUS&T) program co-sponsored by ATK and AFRL/PRP. During the course of the DUS&T program, a two-kilowatt system with three evaporators was developed and tested to identify viable system architectures and characterize system performance capabilities as a function of heat load profiles and spatial distribution of the evaporators. Following the successful development of the two-kilowatt system, a 10-kilowatt system with six evaporators was fabricated and tested. Tests were performed with the system operating in a totally passive, capillary-pumped mode, where applying a small amount of power to a sweepage evaporator provides the auxiliary flow through the primary evaporators, and as a self-regulating, capillary-controlled mechanically pumped system.

Loop Heat Pipes (LHPs) are passive two-phase heat transport devices that have been baselined for many spacecraft thermal management applications. The design life of a spacecraft can extend to 15 years or longer, thus requiring a robust thermal management system. Based on conventional aluminum/ammonia heat pipe experience, there exists a potential for the generation of noncondensible gas in LHPs over the spacecraft lifetime. In addition, some applications would have the LHP evaporator attached directly to spacecraft equipment having large thermal mass. To address the potential issues associated with LHP operation with noncondensible gas and large thermal mass attached to the evaporator, a test program was implemented to examine the effect of mass and gas on ammonia LHP performance. Many laboratory test programs for LHPs have heat delivered to the evaporator through light-weight aluminum heater blocks.

Loop Heat Pipes have been shown to be reliable, self-starting, high capacity heat transport devices which offer significant performance improvements over conventional heat pipes. They are also capable of both constant and variable conductance operation, providing both temperature control and heat transport functions. Two different Loop Heat Pipes were tested to characterize their temperature control capabilities. Operation in both a passive, autoregulating mode and an active control mode were investigated. The test results demonstrate that temperature regulation in each mode is attainable. The effects of the operational environment on temperature control characteristics were found to be important and are discussed.

Loop Heat Pipes combine the advantages of both heat pipes and Capillary Pumped Loops, while overcoming the limitations of each. Loop Heat Pipes provide very high thermal transport capacities; they can transport heat over long distances, through small cross-sectional tubes and have the capillary pumping capacity to overcome high gravitational heads. Most of these features are available from capillary pumped loops (CPLs), but unlike CPLs, Loop Heat Pipes are inherently self priming and totally passive in operation. This paper describes the operating principles of Loop Heat Pipes, provides performance data from hardware tests, describes some areas of ongoing development, and discusses applications, terrestial as well as space, where Loop Heat Pipes could confer major benefits.

Heat-Pipe reflux receivers have been identified as a desirable interface to couple a Stirling engine with a parabolic dish solar concentrator. The reflux receiver provides power uniformly and nearly isothermally to the engine heater heads while de-coupling the heater head design from the solar absorber surface design. Therefore, the heat pipe reflux receiver allows the receiver and heater head to be independently thermally optimized, leading to high receiver thermal transport efficiency. Dynatherm Corporation designed and fabricated a screen-wick heat-pipe receiver for possible application to the Cummins Power Generation, Inc. first-generation 4kWe free-piston dish-Stirling system, which required up to 30 kWt. The receiver features a composite absorber wick and a homogeneous sponge-wick on the aft dome to provide sodium to the absorber during hot restarts. The screen wick is attached to the absorber dome by spot welds.

The Capillary Pumped Loop (CAPL) Flight Experiment has undergone numerous design modifications to reflect recent changes in the thermal control system baselined for Earth Observation System (EOS) spacecraft. The experiment redesign has also allowed technological advances in two-phase fluid loop components to be incorporated. The experiment's reservoir is one of the components targeted for redesign. The design and development of a new reservoir for the CAPL Flight Experiment is discussed in this paper. A prototype reservoir is described, and a hydrodynamic analysis of its wick structure is included. Testing of the prototype reservoir is also discussed.