Heat transfer performance of loop heat pipe for space vehicle thermal control under bypass line operation

•A bypass line was installed between the liquid reservoir and vapor channel in loop heat pipe.•Start-up characteristics were presented for a loop heat pipe with a bypass line.•The effect of the bypass line on the start-up performance of the loop heat pipe was experimentally analyzed.•The thermal performance of the LHP was able to be enhanced by the bypass line.•The local maximum wall temperature was significantly reduced by the bypass line.•Bypass line is a passive means for temperature control without sacrificing thermal resistance. This study analyzed the experimental results regarding the heat transfer performance of a loop heat pipe (LHP) with a vapor bypass line and a polypropylene fiber capillary wick. The LHP with the bypass tube between the vapor channel and liquid chamber was fabricated, tested, and analyzed. A control valve was attached to the inlet of the bypass line to manually switch between the bypass line operating mode and the normal operating mode. The polypropylene capillary wick generated the capillary pressure to operate the LHP. The container and tubing system of the LHP were of stainless steel, and methanol was used as the working fluid. The planar dimensions of the LHP with a flat evaporator were 40 mm × 50 mm. The study investigated the effect of the auxiliary loop on the LHP. The effects of the bypass line were investigated in terms of the minimum start-up thermal load, start-up thermal performance, and steady-state performance. The minimum start-up thermal load of the LHP was reduced by up to 63% under the bypass line operation compared with the normal operation. The temperature overshoot in the evaporator outer wall was eliminated during the start-up under the bypass line operation. Furthermore, the steady-state heat transfer performance was favorably enhanced at high thermal loads under the bypass line operation.