Development of boiling flow pattern map and heat transfer correlation of R32-oil mixture inside a horizontal micro-fin tube

R32 has been widely applied in room air-conditioners (ACs) because of its low GWP, and its heat transfer characteristics in refrigeration cycles may be affected by the lubricating oil mixed in R32. To quantitatively evaluate the effect of oil on the flow boiling heat transfer characteristics of R32, the flow pattern map and heat transfer correlation of R32-oil mixture are needed. In this study, an experimental rig for R32-oil mixture is established, in which the boiling flow patterns are observed and the heat transfer coefficients are measured. The experimental conditions cover the evaporation temperature ranging from -5 to 15 °C, mass flux ranging from 100 to 400 kg m−2 s−1, vapor quality ranging from 0.1 to 0.9 and oil concentration ranging from 0% to 5%. A total of 80 flow patterns and 336 heat transfer coefficients have been obtained under the above experimental conditions. The results show that the flow patterns of R32-oil mixture contain stratified wavy flow, intermittent flow, annular flow and dryout flow, and the presence of the oil could promote the transformation from stratified wavy flow to annular flow; the heat transfer would be enhanced by the oil under lower vapor qualities and be deteriorated under higher vapor qualities. The boiling flow pattern map is developed based on observation results, which could well predict the flow patterns under the experimental conditions. The heat transfer correlation is developed based on the flow patterns and could predict 82% of the experimental results within a deviation of ± 20%.