Study of Flashing Flows Entering Evacuated Sampling Cylinders for Oil Circulation Rate Measurement in an Automotive Air Conditioning System

The oil circulation rate (OCR) is a deciding factor for the performance of automotive air conditioning systems at both the system and component levels. OCR is defined as the percentage by mass of oil present in a representative sample of oil-refrigerant mixture drawn from the system at steady state. In recent years, many industries are opting for low-OCR compressors, and so the OCR values are getting smaller, making it even more important to be able to accurately measure the OCR in the system. All the different OCR measurement techniques rely on the ASHRAE Standard 41.4 for proper calibration. This standard describes OCR measurement using a sampling technique which involves connecting an evacuated sampling cylinder at the liquid line of the system at steady state to draw a sample containing the liquid mixture of refrigerant and oil. However, several factors such as orientation of sampling cylinder and valve opening speed can affect the OCR results. The flow entering the sampling cylinder initially consists of both liquid refrigerant and liquid oil. As the valve on the sampling cylinder is opened, the refrigerant undergoes flashing followed by recompression and re-condensation of vapor refrigerant, and simultaneous filling up with liquid mixture until pressure equalization between the system supply line and evacuated sampling cylinder is achieved. This two-phase, two-component, phase changing flow is complex, and for the present study, flow visualization using a high-speed camera and analytical tools are used to study the flashing flow of pure refrigerant entering an evacuated sampling cylinder. The obtained results are used to compare with those obtained with oil-refrigerant mixture to shed light on how parameters such as sampling cylinder orientation and valve opening speed can affect the measured OCR values.