Testing of a Loop Heat Pipe with Two Evaporators and Two Condensers

Abstract

Most existing LHPs consist of one single evaporator and one single condenser. LHPs with multiple evaporators will be very desirable for cooling multiple heat sources or a heat source with large thermal footprints. Extending the LHP technology to include multiple evaporators and multiple condensers faces some challenges, including the interaction between individual compensation chambers, operating temperature stability, and adaptability to rapid power and sink temperature transients. This paper describes extensive testing of an LHP with two evaporators and two condensers. Tests performed include start-up, power cycle, sink temperature cycle, reservoir temperature cycle, and capillary limit. Test results showed that the loop could operate successfully under various heat load and sink conditions. The loop operating temperature is a function of the total heat load, the heat load distribution between the two evaporators, and temperatures of the two condenser sinks. Under most conditions, only one reservoir contained two-phase fluid and the other reservoir was completely liquid filled. Moreover, control of the loop operating temperature could shift from one reservoir to the other as the test condition changed. Introduction Loop heat pipes (LHPs) are versatile heat transfer devices which have recently gained increasing acceptaalce for spacecraft thermal control. Most existing LHPs consist of one single evaporator and one single condenser. One of the major advantages offered by the LHP is its robust operation which steins from a specific physical construction. In LHP design, the evaporator and the compensation chamber form an integral part with a secondary wick connecting the two elements. The secondary wick can continuously draw liquid from the compensation chamber and the evaporator will always be replenished with liquid even when vapor bubbles are present inside the evaporator core. Thus, the evaporator will not be vapor locked and dry out. However, the physical proximity of the evaporator and compensation does impose some constraints in the LHP operation. First, the temperature of the compensation chamber, which controls the loop operating temperature, is directly affected by the operating conditions such as the heat load, the condenser sink temperature and the ambient temperature. Second, the governing thermodynamic relation requires that the temperature difference between the evaporator ad the compensation chamber match the total system pressure drop minus the pressure drop across the primary wick. Third, minimization of the volume and weight, required by most spacecraft, places stringent requirements on the sizing of the compensation chamber and the fluid inventory. Thus, the evaporator and compensation chamber are usually designed for a specific loop, and are not easily adaptable when volumes of other components change significantly. When multiple heat sources or a heat source with large thermal footprints needs to be cooled, an LHP with multiple evaporators will be very desirable. Extending the LHP technology to include multiple evaporators and multiple condensers faces some challenges. A simple thermodynamic analysis shows that, under most cases, only one of the compensation chambers will contains two-phase fluid and controls the loop operating temperature. AII other compensation chambers will be completely filled. Therefore, there may be sizing limitations on the number of evaporators that can be integrated into a single loop. There are also operating issues that require further investigation, including the interaction between individual https://ntrs.nasa.gov/search.jsp?R=20010069275 2017-09-13T20:12:54+00:00Z

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Cite this paper

@inproceedings{KuTestingOA, title={Testing of a Loop Heat Pipe with Two Evaporators and Two Condensers}, author={Jentung Ku and N Goddard and Gaj Birur} }