Effect of oil concentration on falling film vaporization on horizontal tubes in R-245fa/oil mixture

This study investigates the evaporation heat transfer performance of R-245fa and R-245fa with lubricant RL-150S mixture (1%, 2% and 5% lubricant mass fraction) in pool boiling and falling film. Two kinds of tubes, a smooth tube and a fin tube of 0.4mm fin height, 60FPI(Fins Per Inch) were tested. Experiments were performed at saturation temperature of 20°C. The liquid flowed through a liquid feeder with a row of circular holes at a rate of 0.012 ∼ 0.04kg/ms, and heat fluxes varied from 6 to 50kW/m2. The experimental results showed that the heat transfer coefficient of smooth tube decreases with increasing oil concentration. For both the smooth and fin tubes, the heat transfer coefficient increases with increasing oil concentration because that the bubble nucleation is enhanced by the foam. Exceptionally, the heat transfer coefficient of the fin tube at the center decreases with oil concentration for heat flux > 20 kW/m², because oil increases the viscosity and results in more liquid on the top tube.