Liquid-vapor phase displacement in the evaporator of a loop heat pipe at the start-up involving nucleate boiling

Abstract

Liquid-vapor phase distribution and displacement in the capillary evaporator of loop heat pipes (LHP) is a key phenomenon to understand the steady state and transient behavior. However, the phenomenon has not been investigated sufficiently thus far. In this study, the liquid-vapor phase behavior on the contact surface between the wick and evaporator case at start-up is observed with a transparent evaporator case coated with an ITO heater. Visualization experiments are conducted during the LHP operation at 0.59 - 5.6 W/cm² of heat flux applied to the evaporator. Nucleate boiling and temperature drop on the evaporator wall by superheating are observed when the grooves and vapor line inlet are initially saturated with liquid. Right after nucleate boiling, the whole surface of the wick is dried instantaneously, then liquid in the compensation chamber (CC) wets the wick. In the imbibition, two regimes where the liquid filling rate into the wick is different are found. At a high heat flux, vapor pockets are observed at the contact surface between the wick and evaporator case even in a steady state. Thermo-fluid simulations which take into account disordered porous media and effective thermal conductivity of the wick obtained from directly solving a heat conduction equation in reconstructed porous media using X-ray computed tomography are conducted. The simulation results of evaporator wall temperature and saturation at the contact surface agree with the observation at 5.6 W/cm² of heat flux. Both simulation and observation results show residual liquid phase along the three-phase contact lines within the case, wick, and grooves.