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.