EXPERIMENTAL INVESTIGATION AND CORRELATION OF TWO-PHASE HEAT TRANSFER OF R410A/OIL MIXTURE FLOW BOILING IN A 5-MM MICROFIN TUBE

Two-phase heat transfer characteristics of R410A/oil mixture flow boiling inside a horizontal microfin tube with the outside diameter of 5.0 mm were investigated experimentally at nominal oil concentrations from 0 to 5%. The test results show that the presence of oil enhances the heat transfer by a maximum of 30% at vapor qualities lower than 0.8. However, at vapor qualities near 0.8, the local heat transfer coefficients increase at low nominal oil concentration, reach the maximum at 3-5% nominal oil concentrations under the present experimental conditions, and then fall off with the increase of nominal oil concentration. At high vapor qualities from 0.85 to 0.9, the maximum of local heat transfer coefficient shifts to lower nominal oil concentration with the increase of vapor quality and mass flux, and the local heat transfer coefficient falls off rapidly with the increase of nominal oil concentration and vapor quality. A correlation to predict the local heat transfer coefficients of R410A/oil mixture flow boiling inside microfin tubes was developed based on local properties of refrigerant-oil mixture, and it agrees with 91% of the experimental data within a deviation of +/- 35%.