Abstract:

A visualization study of the evaporator and compensator for the loop heat pipe (LHP) was conducted to explore the influence of different heating methods on the startup and stability characteristics of the LHP. The performance of an R245fa charged LHP with 50% liquid filling rate under three methods of heat load application was investigated: heating at the top of evaporator, heating at the bottom of evaporator, and heating both the top and bottom of evaporator at the same time. According to the main phase change modes in the cavity of the evaporator during the startup process, three startup modes were characterized: evaporation startup mode, evaporation-boiling mixed startup mode, and boiling startup mode. The results showed that the startup process in the latter two startup modes was faster when 5W of heat load was applied, and there were bubbles overflowing from capillary vapor channels of the evaporator cavity. The startup time was 760s and 1180s, respectively, far less than 2370s for the evaporation startup mode. The startup time of LHP was closely related to initial liquid level and the average liquid disappearance rate in the evaporation cavity. Additionally, the bubble growth in the cavity under different starting modes was explored. With the same heat load, the thermal resistances of the LHPs and the liquid height in the compensator under different heating methods were different. The LHP with the bottom heating method had the smallest thermal resistance. It is found that different heating methods affect the evaporation efficiency of liquid working in the evaporator. Meanwhile, it also could change the liquid height in the compensator and the heat leakage from the evaporator to the compensator, thus affecting the performance of the LHP.

Key words: loop heat pipe, evaporation, visualization, startup mode, phase change, heat transfer

摘要：

为了探索不同加热方式对环路热管的启动及稳定运行性能的影响，对环路热管的蒸发器补偿器进行了可视化试验研究。环路热管以R245fa为工质，充液率为50%，分别采用了三种不同的热负载施加方法：蒸发器顶部加热、蒸发器上下同时加热、蒸发器底部加热。根据启动过程中蒸发器空腔内的主要相变模式不同，对应地分为3种启动模式：蒸发启动模式、蒸发沸腾混合启动模式、沸腾启动模式。结果发现：在5W启动过程中，蒸发沸腾混合模式和沸腾模式下的启动速度最快，并在蒸发器空腔气体槽道出口处伴有气泡溢出，分别历时760s、1180s，远小于蒸发启动模式的2370s。分析可知，环路热管的启动速度与蒸发器空腔内的初始液面及其平均液体消失速度密切相关。另外，为研究蒸发器空腔内液相工质的沸腾，对不同的启动模式下空腔内的气泡生长进行了探索。在稳定工况中，同热负载下不同加热方式的环路热管的热阻及补偿器液面高度都不相同，其中底部加热方式的环路热管热阻最小。经分析发现，同热负载下不同加热方式会影响蒸发器内液相工质的蒸发效率，同时也会改变补偿器液面高度和蒸发器向补偿器的漏热，进而影响环路热管性能。

关键词: 环路热管, 蒸发, 可视化, 启动模式, 相变, 传热