Effect on Noncondensible Gas and Evaporator Mass on Loop Heat Pipe Performance

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. In order to represent realistic applications, an LHP was tested with additional mass attached to the evaporator. To further simulate potential end-of-life LHP performance, noncondensible gas was also introduced into the loop. This paper presents the test results and the effects on LHP performance when tested with both large evaporator mass and substantial noncondensible gas.