Oil Droplet vs. Film Flow at Discharge and Suction after Shutdown of Automotive Compressor

The compressor oil is retained at different locations of the vapor compression system during operation. After shutdown, retained oil absorbs the vapor refrigerant and mixes with the liquid refrigerant gradually. Oil retention can largely affect the heat transfer and cause insufficient oil return. In this paper, the liquid mixture behaviors at the compressor suction and discharge are observed by flow visualization. Liquid mixture property variations are estimated by existing models according to the temperature and pressure variations. At the suction, the vapor refrigerant solubility with oil rapidly increases due to the pressure increase. The viscosity and surface tension decrease quickly with temperature and refrigerant mass fraction increase in the liquid layer. Flow visualization shows that the mixture film breaks from the top of the tube wall and flows down. The oil-refrigerant mixture accumulates at the bottom of the tube within seconds. The liquid level increases then decreases. At the discharge, the refrigerant solubility fluctuates within a small range. Viscosity increases because of temperature decrease then decreases because of refrigerant mass fraction increase. After shutdown, the oil flow in contact with the inner wall of the discharge stops immediately while oil droplets in the vapor core flow slow down gradually. In the end, oil droplets flow down, coalesce with the oil film, and accumulate at the bottom of the tube. The vapor refrigerant, liquid refrigerant and oil relationship is investigated and estimated after shutdown to better understand oil retention.