VAPOR FLOW EFFECT ON FALLING FILM EVAPORATION OF R134A OUTSIDE HORIZONTAL TUBE BUNDLE

Vapor flow effect on falling film evaporation of refrigerant R134a outside horizontal tube bundle is investigated experimentally. The test space is a cube with a rectangular cross section of 0.575m (length) × 38.8mm (width). The tested tube is designed for enhancing pool boiling, which has an external fin density of 45fpi (fins per inch), and an outside diameter of 19.05mm. The transverse narrowest bundle clearance is 3.25mm where the vapor velocity can be adjusted in the range of 0 to 3.1m/s. R134a vapor is distributed uniformly from the bottom of tube array to the test section. The liquid falling film flow rate ranges from 0.07 to 0.2kg/m͌s. Experiment is firstly conducted at saturation temperature of 6Υwithout the effect of the additional vapor flow at the heat flux of 20, 60, 100 and 180kW/m²(for the first tube row). Vapor effect experiment was carried out at three heat fluxes 20, 40 and 60kW/m²For the falling film flow rate of 0.07 kg/m͌s and heat flux of 40kW/m²it is found that at the vapor velocity less than 0.5 m/s, vapor velocity has no significant effect on the falling film evaporation heat transfer. In addition, it is found that the vapor velocity effect is also dependent on the falling film flow rate: within the variation range of vapor velocity tested the variation patterns of the eight tubes are different at different flow rate.