Stratification and heat transfer characteristics of partially miscible R32-lubricating oil mixtures during nucleate boiling

Applying partially miscible lubricating oil instead of completely miscible lubricating oil in room air conditioners is a promising way to avoid refrigerant insufficiency. The application of partially miscible lubricating oil to air conditioners requires the clarity of the effect of partially miscible lubricating oil on the nucleate boiling characteristics. In the study, an experiment rig for R32 with a visualization feature is built to test the heat transfer performance of the mixture, including the observation of stratified layers and the measurement of heat transfer coefficients. The test results indicate that, with the increase of oil mass fraction, partially miscible mixtures firstly are homogeneous, then stratify into two layers, finally become homogeneous. The heat transfer coefficients firstly keep steady and then decrease sharply. The moment of decreasing is advanced with the increase of heat flux and initial oil mass fraction, or the decrease of the initial height. The average heat transfer coefficient increases with the increase of the heat flux, and the effects of the initial oil mass fraction and the initial height on the average heat transfer coefficient can be ignored. The average heat transfer coefficient increases by 118.3% as the heat flux increases from 5 kW/m2 to 15 kW/m2.