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Investigation of Capillary Limit in a Loop Heat Pipe

Jentung Ku
NASA Goddard Space Flight Center, Greenbelt, Maryland, USA

Laura Ottenstein
NASA Goddard Space Flight Center, Greenbelt, Maryland, USA

Paul Rogers
U.S. Army TARDEC, Warren, USA

Kwok Cheung
Naval Research Laboratory, Washington, D.C., USA


This paper presents an experimental study on the capillary limit of a loop heat pipe (LHP) at low powers. The slow thermal response of the LHP 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 usedto measure the pressure drop across the evaporator andthe compensation chamber (CC). Test results show that when the capillary limit is exceeded, vapor will penetrate the primary wick, resulting in a partial dry-out of the evaporator anda rapidincrease of the CC temperature. Because the evaporator can tolerate vapor bubbles, the LHP will continue to function andm ay reach a new steady state at a higher temperature. Thus, the LHP will exhibit a graceful degradation in performance rather than a complete failure. Moreover, the LHP can recover from a partial dry-out by reducing the heat load without the need of a re-start.

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