Visualization study on start-up characteristics of a loop heat pipe with a carbon fiber capillary wick

Abstract

A flat loop heat pipe (LHP) with visible evaporator and condenser was designed and observed during the start-up stages. A novel porous material, the carbon fiber felt, was used as the capillary wick. To enhance the heat conduction and the flow fluency, the wick was modified into comb shape and pin fins were fabricated in the evaporator. To directly observe the flow and the boiling condition inside LHP, silica glasses were fixed on the evaporator and condenser. During the slow evaporation stage, small working fluid droplets condensed on the inner surface of the silica glass and no boiling phenomenon can be observed inside the evaporator. During the boiling appearance stage, large amount of vapor generated in the evaporator chamber. Under different operating conditions, temperatures at six measure points were monitored and recorded. When the heating power changed from 30 W to 200 W, the stable temperature of the heating sheet linearly increased from 55.57 °C to 111.9 °C. A large filling ratio always results in a long start-up process and a large temperature fluctuation. Among LHP with three different working fluids, LHP with acetone-ethanol mixture has the quickest start-up, and LHP with the pure acetone has the lowest stable temperature.