The effect of oil contamination on the nucleate pool-boiling performance of R-114 from a porous-coated surface

The external, nucleate pool-boiling heat-transfer coefficient in R-114 with up to 10% oil (by mass) was measured on a smooth copper tube and on a porous-coated copper-nickel tube. The heat flux was varied from 160 to 30,000 Btu/hr . ft/sup 2/ (0.5 to 95 kW/m/sup 2/) at boiling temperatures of 28 F and 44 F (-2.20C and 6.70C). The porous-coated surface was found to improve the heat-transfer coefficient over the smooth surface by a factor of 7 to 10 in oil-free R-114. The presence of up to 3% oil caused about a 35% reduction in the heat-transfer coefficient of the porous-coated surface at all heat fluxes. With 6% or more oil, the boiling performance of this surface declined drastically at heat fluxes in excess of 9500 Btu/hr . ft/sup 2/ (30 kW/m/sup 2/). Some plausible mechanisms to explain the observed behavior are described