分离式CO_2热管传热性能分析

用CO_2替代R22应用于分离式热管系统对于环境保护很有意义。从目前已有的实验结果来看,CO_2的流动沸腾和凝结传热系数要明显高于常规制冷剂,说明其在提高热管系统传热性能方面具有潜力。但考虑到热管内工质的沸腾和凝结换热系数较高,相对来说,热管系统的主要热阻集中在管外空气侧或水侧的对流换热热阻。因此,尽管CO_2替代常规制冷剂时管内沸腾凝结换热系数可以成倍提高,但热管系统整体传热性能的提高可能较为有限。本文通过实验对比了CO_2热管和R22热管的传热性能,并结合相关的传热模型,分析了分离式热管中的各部分热阻,结果显示,由于CO_2的管内沸腾凝结换热热阻小于R22,使得CO_2热管的整体传热性能优于R22热管,其总热阻比R22热管降低22%~25%。

Heat Transfer Performance Analysis on the CO2 Two-phase Thermosyphon Loop

In terms of environmental conservation, it is very meaningful to use CO2 as working fluid in the two-phase thermosyphon loop (TPTL) to replace R22. According to the experimental results in some research, the flow boiling or condensation heat transfer coefficient of CO2 is much higher than that of the conventional refrigerants, which means CO2 has the potential in improving the heat transfer performance of the TPTL system. Considering the high heat transfer coefficient of the refrigerant, the convective thermal resistance of the air/water outside is the main restriction of the performance. Thus, the improvement of the heat transfer performance by using CO2 may be limited. The present study compares the heat transfer performance of CO2 and R22 TPTL through experiment. Combined with some heat transfer model, each part of the thermal resistance is analyzed. The result shows that the total thermal resistance of CO2 TPTL is 22%-25% lower than that of R22 TPTL due to the reduction of the boiling and condensation thermal resistance.