Influence of Oil on Heat Transfer Characteristics of R410A Flow Boiling in Conventional and Small Size Microfin Tubes

In order to extend the existing heat transfer coefficient correlation of R410A-oil mixture flow boiling in conventional size (7.0 mm) microfin tube to be suitable for widely used small diameter tubes (4.0~5.0 mm), the experiments of R410A-oil mixture flow boiling inside three small diameter microfin tubes with different outside diameters of 4.0~5.0 mm and different microfin structures were performed. For the tested tubes with different diameter, the decrease of tube diameter may weaken the deterioration effect of oil on heat transfer at intermediate and high vapor qualities. For the fixed outside diameter microfin tubes with different microfin structures, larger fin height and contact area of liquid with tube wall may enhance the heat transfer for oil-free R410A, but result in smaller enhancement effect of oil at low vapor qualities and smaller deterioration effect of oil at intermediate and high vapor qualities for R410A-oil mixture. A general correlation to predict the heat transfer coefficients of R410A-oil mixture flow boiling inside conventional size and small diameter microfin tubes was developed, and it agrees with 94% of the experimental data of R410A-oil mixture in 4.0 mm ~ 7.0 mm microfin tubes within a deviation of ±30%.