Abstract:

As a high-efficiency heat exchange element, pulsating heat pipes (PHP) can play an important role in the cold transfer process of cryogenic refrigerators. Visualization on the flow patterns enables a comprehensive understanding of the fluid dynamic and heat transfer mechanism involved in the PHP. However, due to the complexity of conducting visual experiments in cryogenic environments, there has been a scarcity of experimental studies on flow patterns in cryogenic PHPs. In this study, we developed a visualization setup utilizing a cryocooler as the heat sink to systematically investigate the effects of condenser temperature on heat transfer performance. The results show that higher condenser temperatures decrease the required heat load for startup at low filling ratio, but the heat transfer limit occurs. At the medium filling ratio, the partial operation, which is not fully activated, leads to an increment in the thermal resistance at low condenser temperatures. The fully activated operation can decrease the thermal resistance. With a high filling ratio, the regular gas-liquid motion occurs inside the PHP. As the heat load increases, the thermal resistance initially decreases and then stabilizes at a constant value. For the three kinds of filling ratios, the condenser temperature increases, which can benefit for the fully activated operation inside the PHP, consequently resulting in lower thermal resistances.

Key words: multiphase flow, phase change, heat transfer, pulsating heat pipe, liquid nitrogen, visualization

摘要：

脉动热管作为一种高效换热元件，能够在低温制冷机的冷量传输过程中发挥重要作用。对脉动热管内流型的可视化研究能够全面掌握工质的流动与传热机理，但由于低温环境可视化实验的复杂性，目前从实验中获得的低温脉动热管流型特性是稀缺数据。通过搭建低温脉动热管可视化实验台，采用制冷机作为冷源，系统地研究了以氮为工质时冷凝段温度对装置传热性能的影响。结果表明，充液率较低时，冷凝段温度越高，启动所需的热负荷越低，但易出现干涸现象；中等充液率工况下在冷凝段温度较低时由于流道未全部启动，热阻会升高，全局运行后热阻降低稳定；高充液率下气液运动稳定，随着热负荷增大，热阻下降后保持不变；各充液率下冷凝段温度的升高皆有利于脉动热管全局运行，并获得更低的热阻。

关键词: 多相流, 相变, 传热, 脉动热管, 液氮, 可视化