Transient Oil Migration and Flow Behavior during Automotive Compressor Startup

Oil migration has a great impact on vapor compression systems, especially for automobile air conditioning systems which require frequent on-off cycling without sufficient oil management inside the compressor. Excessive amounts of oil retained in the system and lack of oil returned to the compressor can cause low system efficiency and potential compressor failure. This paper explores and quantifies the transient effects in oil migration and property changes at the compressor suction and the discharge. Oil flow behavior and oil migration are quantified and analyzed by the high-speed camera recording and optical method at the compressor discharge and the liquid line under different compressor startup speeds and on-off frequencies. The flow under cold and warm startup conditions is studied and compared. The oil-refrigerant mixture flow at the suction and discharge during cold startup transitions from two-phase flow to vapor refrigerant flow and oil annular mist flow. Severe oil foaming happens inside the compressor and at the discharge. Plug flow, foam flow, bubbly flow and annular mist flow were observed within three minutes after compressor cold startup before steady state is reached. In contrast, after a short idle time, the oil flow develops to annular mist flow in steady state within one minute without foam happening and phase transition. The difference between oil flow behaviors is explained by mixture property variations such as oil-refrigerant solubility, viscosity, and saturation pressure. The characterization helps to reduce oil migration and improves further oil management strategies of the system.