Effect of lubricating oil on flow boiling characteristics of R-600a/oil inside a horizontal smooth tube

The aim of the present study is to experimentally investigate the effect of oil on flow boiling of R-600a as a hydrocarbon refrigerant inside a horizontal smooth tube. In order to study the effect of oil on the flow boiling of R-600a, a well instrumented apparatus was designed, fabricated and installed. The experimental conditions of this study include nominal oil concentrations from 0% to 2.5%, mass velocities in range of 130–380 kg/m2s, inlet vapor qualities from 0.05 to 0.77 and heat fluxes from 10 to 28 kW/m2which were conducted in a copper test tube with the inner diameter of 8.7 mm. Several parameters affecting the heat transfer coefficient and pressure drop of refrigerant–oil mixture, such as oil concentration, mass flux and vapor quality were investigated. The comparison between two-phase pressure drop for pure refrigerant and the refrigerant oil mixture reveals that pressure drop increases with the increase of oil concentration in all ranges of vapor quality and mass velocities. However, heat transfer coefficients tend to increase at low vapor qualities and decrease at the middle and high vapor qualities due to presence of oil.