Liquid–vapor phase behavior and operating characteristics of the capillary evaporator of a loop heat pipe at start-up

The liquid–vapor phase distribution and displacement in the capillary evaporator of a loop heat pipe (LHP) are key phenomena affecting the steady-state and transient operating behavior. This study conducts optical observation of liquid–vapor phase behavior in the evaporator using a transparent glass tube heated by far-infrared radiation during start-up of the LHP. A quartz wick-acetone LHP system is designed, fabricated, and operated successfully with a heat flux of 1 W/cm2 transported by the cylindrical evaporator. To investigate effect of the liquid–vapor phase distribution in the evaporator at the initial state, start-up experiments with eight phase patterns—which are either liquid-saturated or two-phase in each of the inlet of the vapor line, grooves, and compensation chamber (CC)—are performed. The phase distribution affects the temperature profile although it has no effect upon the steady-state characteristics, and nucleate boiling occurs in the wick or groove simultaneously with evaporator-temperature drop when both the grooves and the inlet of the vapor line are completely filled with liquid. Liquid–vapor displacements from start-up to the steady-state are classified into four transient behaviors, which include boiling in the CC and meniscus oscillation at the three phase contact line within the case, wick, and groove.