Capillary Limit in a Loop Heat Pipe with a Single Evaporator

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. The loop can recover from degraded performance due to previous vapor penetration by reducing the heat load without the need of a re-start. Moreover, the ground test results show that the fluid inventory and the relative tilt between the evaporator and the CC have profound effects on the loop operating temperature due to their effects on the vapor content inside the evaporator core. The operating temperature in turn affects the wick’s capillary capability and the total pressure drop. This makes the concept of the LHP heat transport limit somewhat ambiguous.