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This paper describes a study on the capillary limit of a loop heat pipe (LHP) at low powers. The slow thermal response of the loop at low powers makes it possible to observe interactions among various components after the capillary limit is exceeded. The capillary limit at low powers is achieved by imposing an additional pressure drop on the vapor line through the use of a metering valve. A differential pressure transducer is also used to measure the pressure drop across the evaporator and the compensation chamber (CC). Test results show that when the capillary limit is exceeded, vapor will penetrate the primary wick, resulting in an increase of the CC temperature. Because the evaporator can tolerate vapor bubbles, the LHP will continue to function and may reach a new steady state at a higher operating temperature. Thus, the LHP will exhibit a graceful degradation in performance rather than a complete failure.

This paper presents test results of an experimental study of low power operation in a loop heat pipe. The main objective was to demonstrate how changes in the vapor void fraction inside the evaporator core would affect the loop behavior. The fluid inventory and the relative tilt between the evaporator and the compensation chamber were varied so as to create different vapor void fractions in the evaporator core. The effect on the loop start-up, operating temperature, and capillary limit was investigated. Test results indicate that the vapor void fraction inside the evaporator core is the single most important factor in determining the loop operation at low powers.

The success of CAPL 2 flight experiment has stirred many interests in using capillary pumped loop (CPL) devices for spacecraft thermal control. With only one evaporator in the loop, CAPL 2 was considered a point design for the Earth Observing System (EOS-AM). To realize the full benefits of CPLs, a reliable system with multiple evaporators must be developed and successfully demonstrated in space. The Capillary Pumped Loop (CAPL 3) Flight Experiment was designed to flight demonstrate a multiple evaporator CPL in a space environment. New hardware and concepts were developed for CAPL 3 to enable reliable start-up, constant conductance operation, and heat load sharing. A rigorous ground test program was developed and extensive characterization tests were conducted. All performance requirements were met, and the loop demonstrated very reliable operation.