A study of falling film evaporation on finned tubes

This study investigates the evaporation heat transfer performance of R-134a falling film on three horizontal tubes in a vertical column. This study tested three kinds of tubes, a smooth tube, and a fin tube of 0.4mm fin height, 60FPI (Fins Per Inch). Experiments were performed at saturation temperatures of 10, 20 and 26.7°C. The liquid flows through a liquid feeder with a row of circular holes at a rate of 0.0075~0.0363kg/ms, and heat fluxes varied from 4.5 to 48.5kW/m². The test results show that the heat transfer coefficient of the smooth tube increases with increasing heat flux, fluid temperature, and it increases slightly with increasing flow rate before dry-out occurs. At low flow rates (less than 0.015kg/ms) or Ref (≤ 255), the fin tube is in thin film evaporation mode, which is revealed by its high heat transfer coefficient, and at high flow rates (0.0225, 0.03, and 0.0375kg/ms) the falling film evaporation curves are similar to those in pool boiling. The fluid temperature and the flow rate have only minor influence on heat transfer coefficient before dry-out occurs. The fin tube enhances the heat transfer coefficient by 6.3~8.29 fold.