Measurement and Correlation of Flow-Boiling Heat Transfer of a R-410A/Oil Mixture Inside a 4.18 mm Straight Smooth Tube

Abstract

Two-phase flow patterns and heat transfer characteristics of a R-410A/oil mixture flow boiling inside a horizontal straight smooth tube with the inside diameter of 4.18 mm were investigated experimentally. The flow pattern observations for a R-410A/oil mixture match well with the flow pattern map of Wojtan et al. (2005) by using the refrigerant-oil properties instead of pure-refrigerant properties. The test results show that the heat transfer coefficient of a R-410A/oil mixture increases with mass flux of a refrigerant-oil mixture; the presence of oil enhances the heat transfer by a maximum of 60% at the range of low and intermediate vapor qualities. There is a peak of local heat transfer coefficient at about 2%–5% nominal oil concentration at higher vapor qualities, and the peak shifts to lower nominal oil concentration with the increase of vapor qualities. Higher nominal oil concentration gives a more detrimental effect at high vapor qualities. A new correlation to predict the local heat transfer of a R-410A/oil mixture flow boiling inside the horizontal smooth tubes was developed based on flow patterns and local properties of a refrigerant-oil mixture. The new correlation agrees with 92% of the experimental data of R-410A/oil in a 4.18 mm horizontal smooth tube within a deviation of ±30%, and it agrees with 96% of the experimental data (Hu et al. 2008a) of a R-410A/oil mixture in a 6.34 mm horizontal smooth tube within a deviation of ±20%.