An experimental study of falling film evaporation on horizontal tubes using R-134a

This study investigates evaporation heat transfer performance of refrigerant R-134a falling film on three horizontal copper tubes in a vertical column. Experiments were performed at saturation temperatures of 10 and 26.7C. The liquid flows through a liquid feeder with a row of circular holes at a rate of 0.0075 0.0363kg/ms, while heat fluxes varied from 4.5 to 48.5kW/m ². A smooth tube, a fin tube of 0.4mm fin height, 60FPI (Fins Per Inch), and a new boiling enhanced tube (mesh tube) were tested. The test results show that heat transfer coefficient of the smooth tube increases with increasing heat flux and fluid temperature, and increases slightly with increasing flow rate before dry-out occurs. At low flow rates (less than 0.015kg/ms) or when Ref (255), the fin tube is in thin film evaporation mode and results in a large heat transfer coefficient. At high flow rates (0.0225, 0.03, and 0.0375kg/ms) the falling film evaporation curves are similar to those in pool boiling. For all tubes, the fluid temperature and the flow rate have only minor influences on heat transfer coefficient before dry-out occurs. The 60 FPI tube and the mesh tube enhance the falling film evaporation heat transfer coefficient 6.3 8.29 fold and 1.9 5.0 fold, respectively, as compared with the smooth tube. A new correlation of falling film evaporation, accounting for contributions of nucleate boiling and spray convection, is proposed. It predicts h-values of the falling film evaporation data of the smooth surface within 30%.