Overshoot elimination of the evaporator wall temperature of a loop heat pipe through a bypass line

During the startup period, a loop heat pipe (LHP) may experience a temperature overshoot, which often leads to a startup failure. As a passive means to alleviate the temperature overshoot in the evaporator, a bypass line was proposed that connects the evaporator vapor exit and the liquid reservoir, to facilitate quick escape of the vapor. The bypass line was incorporated into an LHP having a sintered metal wick and a flat evaporator, of which the dimensions were 40 mm x 50 mm. The wall and tubing system were made of stainless steel, and distilled water was used as the working fluid. An experiment was conducted to identify the effect of the bypass line on the LHP performance, in terms of the temperatures at the representative points and the thermal resistance. The characteristics were compared between the LHPs with and without the bypass line up to an input thermal load of 160 W. The experimental results for the LHP with the bypass line showed cases where the overshoot temperature was suppressed as much as 92.5 degrees C, and the startup time was reduced by 5.2 min, without considerable sacrifice in thermal resistance.