Abstract: Based on a refrigeration cycle using R134a as a working fluid,the falling film evaporation on horizontal tubes was used in the evaporator.The heat transfer characteristics of the falling film evaporation were investigated experimentally under different conditions.The heat fluxes varied from 8 to 40 kW·m^-2 and the mass flow rate of the sprayed liquid ranged from 0.005 to 0.04 kg·m^-1·s^-1.The performance of a smooth tube and two enhanced tubes(a TJX tube and an EX tube)were tested in the experiment.The two enhanced tubes were designed for the falling film evaporation specifically.The experimental data were processed by using the modified Wilson method.The test results show that the heat transfer characteristics of the falling film evaporation are mainly associated with heat flux and mass flow rate.For all tubes,the heat transfer coefficient increases before the occurrence of the critical heat flux.The performance of enhanced tubes are better than that of smooth tube,especially the EX tube with grill channels on the surface.With the oil in the refrigeration system,the presence of oil in the experimental loop deteriorates the heat transfer coefficient significantly.Finally,a new heat transfer correlation is used to predict the heat transfer coefficient of the smooth tube,and correction factors for the heat transfer prediction of enhanced tubes are proposed.

Key words: falling film evaporation, enhanced surface, heat transfer, prediction

摘要： 以R134a制冷循环为基础,将水平管降膜蒸发应用于蒸发器中,通过调节不同工况来研究其换热特性。其中热通量8～40 kW·m^-2,工质的喷淋量0.005～0.04 kg·m^-1·s^-1,以光滑管为基础,对针对降膜蒸发而设计的TJX、EX两种强化管进行了实验研究,并利用修正威尔逊法进行数据处理。分析表明,降膜蒸发换热特性主要与热通量和喷淋量有关,而随着热通量的增加,每根管型传热系数在遇到临界热通量点后呈现下降趋势。此外强化表面的传热系数要明显优于光滑管,特别是具有网格槽道设计的EX管。由于实验系统是一个制冷系统,含有润滑油,发现润滑油的存在明显降低了传热系数。最后在实验基础上对光滑管换热特性进行预测,并在此基础上提出强化管的修正系数。

关键词: 降膜蒸发, 强化表面, 传热, 预测