Stratification and heat transfer characteristics of partially miscible lubricating oil-refrigerant mixtures during pool boiling

•Partially miscible oil-refrigerant mixture stratifies into four layers at boiling.•Newly proposed nucleate boiling model reflects effects of mixture stratifications.•Bubble dynamics and concentration diffusion determine mass transfer among layers.•Predicted thicknesses of separated layers agree well with observation results.•Mean deviation between predicted and measured heat transfer coefficients is 15.3% Applying partially miscible lubricating oil-refrigerant mixtures instead of completely miscible ones in refrigeration systems is a promising way to avoid the refrigerant insufficiency, and the application of partially miscible oil requires the clarity of nucleate pool boiling characteristics of partially miscible mixtures. As partially miscible mixtures at boiling conditions show an entirely different stratification process compared to completely miscible mixtures or partially miscible mixtures at non-boiling conditions studied by former researchers, the existing researches cannot be directly applied to evaluate pool boiling characteristics of the partially miscible mixture. The purpose of this study is to experimentally investigate and model both stratification and heat transfer performance of nucleate pool boiling of the partially miscible oil-refrigerant mixture. The stratification phenomenon at boiling of partially miscible mixtures is observed, and up to four layers (i.e., an oil-rich micro-layer, a middle oil-poor layer, a top oil-rich layer, and a gas phase layer) are found. The boiling process of partially miscible mixtures stratifying from a single layer into four layers is modelled, and calculation methods of mass transfer rates among layers are proposed. The model validation shows that predicted thicknesses of layers accord with experimental images, which covers the conditions of heat flux ranging from 5 to 15 kW/m2, initial oil mass fraction ranging from 5 to 15%, and initial height ranging from 80 to 120 mm. Predicted heat transfer coefficients agree with 80% of the experimental data within the deviation of ± 25%, and the mean deviation is 15.3%.