Abstract: The cryogenic loop heat pipe is a high-efficiency two-phase heat transfer device that is commonly used in space thermal control systems. In order to investigate the suitable working fluids for LHP in 180~230 K for space exploration applications, the effects of working fluids on the operating characteristics including start-ups, responses of the operating temperatures to the heat load rise, and thermal resistances of the LHP at steady states were researched by employing ethylene, ethane and propylene as fluid candidates. The experimental results and analysis showed that by controlling the temperatures of the evaporator and the compensation chamber at least 10 K below the critical temperatures of the fluids, the LHP achieved successful start-up with heat loads of 5 W. With approximate evaporator temperatures before the start-up, the start-up temperature rise were both within 2 K for the ethane and propylene charged LHP, the start-up time of the ethane charged LHP was less than that of the propylene charged one, and the latent heat and liquid viscosity of working fluids are largely responsible for the differences in the start-up time. The effect of working fluids on the responses of the LHP operating temperatures to the heat load rise was not obvious: the evaporator temperatures decreased and then increased with the rising heat load. The lowest evaporator temperatures occurred with heat loads of 20~30 W, and the time for the LHP to reach the new steady-state decreased with the heat load rise. The pressure drops of the working fluids in the external loop accounted for the significant differences among the heat transfer thermal resistances of the LHP at steady states, and the vapor phase pressure drops took the majority part of the external pressure drops: the smaller vapor pressure drop leads to the smaller thermal resistance. The vapor pressure drops of the ethylene charged LHP were less than that of the ethane and then the propylene charged LHP with the same heat loads, and the vapor pressure drops for all three fluids decreased with the increasing temperature. Correspondingly, the thermal resistances of the ethylene charged LHP were less than that of the ethane and then the propylene charged one, all thermal resistances decreased with the rising temperature. The smallest thermal resistance obtained experimentally was 0.21 K / W.
