工质对180～230 K低温环路热管工作特性的影响

摘 要：低温环路热管（Loop heat pipe, LHP）是一种高效的两相传热装置，常被用在航天热控系统中。为探究用于空间探测项目中180～230 K环路热管的合适工质，采用乙烯、乙烷和丙烯为工质对LHP在不同热沉温度下的启动特性、工作温度对热负荷增大的响应变化及稳态传热热阻进行了实验研究。实验和分析结果表明启动前蒸发器和补偿器温度低于工质临界温度至少10 K时，LHP均能以5 W的热负荷成功启动。启动前蒸发器温度相近时，乙烷LHP和丙烯LHP的启动温升均在2 K以内，乙烷LHP启动时间短于丙烯LHP，工质汽化潜热和液体粘度差异是造成LHP启动时间差异的重要因素。工质对LHP工作温度随热负荷增大的响应影响不明显：蒸发器温度均随热负荷的增加而先减小后增大，最低工作温度出现于20～30 W，LHP达到稳定状态的时间随热负荷增大而缩短。工质能明显的影响LHP的稳态传热热阻，工质在外环路的压降是其影响LHP稳态传热热阻的重要因素，而工质的气相压降占外环路总压降的绝大部分：小的气相压降对应小的稳态传热热阻，同热负荷下的乙烯LHP气相压降小于乙烷LHP和丙烯LHP，且气相压降均随温度升高而减小，对应的乙烯LHP稳态传热热阻小于乙烷LHP和丙烯LHP，且传热热阻都随温度升高而减小，实验得到的LHP最小稳态传热热阻达到0.21 K / W。

Effects of working fluids on operating characteristics of the cryogenic loop heat pipe in 180~230 K

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.