Hybrid heat transfer with falling film and pool boiling using R-134a outside a vertical tube

Abstract

In this study, falling liquid film, pool boiling and hybrid heat transfer with refrigerant R-134a outside a vertical tube were experimentally investigated. A smooth tube and an enhanced tube specially designed for falling film were adopted for the tests. The results revealed that the heat transfer ratio of falling film to pool boiling gradually decreased as the heat flux increased for the two tubes. The heat transfer coefficients (HTCs) of falling film were smaller than those of pool boiling at a heat flux of 25 and 40 kW m−2 for the smooth and enhanced tubes, respectively. In the hybrid heat transfer mode analysis, it was observed that at a low heat flux, the HTCs of a completed falling film were higher than those of other heat transfer modes; moreover, the proportion increase of pool boiling led to a slight decrease in performance. However, at a high heat flux, a modest increase in the submerged height of the tubes in the refrigerant was beneficial for improving the performance, and the heat transfer had an optimal effect at the height of 800 mm and 600 mm for the smooth and enhanced tubes, respectively. Based on the experimental data, the heat transfer correlations of falling film combined with pool boiling at different submerged heights were obtained according to a dimensional analysis. The findings of the study serve as a guide for the design of vertical evaporators.