Operation of an LHP with Multiple Heat Sources

Loop Heat Pipes (LHP) are widely used in space applications: deployable radiators, instrument cooling and precise temperature control. These applications are utilizing classical LHP designs with single evaporator and single condenser. Traditionally, the LHP is considered as an ideal solution for heat sources with concentrated power. The rule of thumb for a thermal control system based on the LHP is one heat source – one evaporator. It has been demonstrated several times that LHPs with dual evaporators are feasible, however, the volume of the compensation chamber grows dramatically with the number of evaporators. Therefore, LHPs with more than three evaporators are not practical. These restrictions restrict the area of LHP applications.

The LHP evaporator generates vapor, which travels through the vapor line toward the condenser. In principle, this vapor could be used to absorb some additional power from heat sources other than the heat source feeding the evaporator. But single-phase heat exchange between these secondary heat sources and the vapor is not very effective. However, if the vapor would be condensed prior to entering the secondary heat source, this condensed vapor with a quality close to zero would absorb significant amounts of heat from the secondary source. Ineffective single-phase heat exchange would be substituted by highly effective two-phase heat exchange. The chain of the secondary condensers and heat sources can be as long as the capillary pressure of the primary wick will permit. Its capillary pressure must support the additional hydraulic losses in the secondary condensers and heat sources. The evaporator of the driving pump must be connected to the heat source with the highest power. A system of this type was successfully tested, demonstrating reliable operation over a wide range of powers and temperatures. The paper is describes the test article and discusses the test results.