A study of pool boiling heat transfer on horizontal tubes in R-245fa/oil mixture.

The study investigates the boiling heat transfer performance of pure R-245fa and mixtures with lubricant RL-150S (approximate 1%, 2% and 5% lubricant mass fraction). 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 temperatures of 5 and 20oC, and heat fluxes varied from 6 to slightly above 50kW/m2. Different effects of oil are shown in the pool boiling heat transfer for the smooth and fin tubes. The bubble size and the heat transfer coefficient of smooth tube decreases with increasing oil concentration. For the fin tube, the heat transfer coefficient increases with increasing oil concentration. The nucleation site density increases with the oil concentration for both the smooth tube and the fin tube. The oil concentration is measured by a refractive index sensor. The oil concentration 15 mm under the boiling surface remains steady with varying heat flux for both smooth and fin tube.