Experimental investigation and theoretical analysis of oil circulation rates in ejector cooling cycles

In this study, the influence of compressor speed, ejector motive nozzle needle position and evaporator inlet metering valve opening on the oil circulation rates (OCRs) of an R744 transcritical standard ejector cycle was experimentally investigated. Significantly higher OCR (∼10%) was observed at the evaporator inlet of the ejector cycle than that at the high pressure side (∼1%) measured in the same cycle under the same conditions. It has been observed that evaporator OCR was increased with increasing compressor speed. When the motive nozzle needle moved towards the nozzle throat, both compressor discharge flow rate and evaporator OCR were observed to be significantly lowered. As the evaporator inlet metering valve opening was adjusted, the compressor mass flow rate did not vary significantly while the evaporator mass flow rate decreased with decreasing metering valve opening. The evaporator OCR decreased from 6.5% to 2.2% as the metering valve opening varied from 86% to 27%. High evaporator OCR results in large evaporator pressure drop and low heat transfer coefficient. In addition to the standard ejector cycle, several alternative ejector cycles were theoretically analyzed to see if there is similar problem of high OCR in the evaporator. In ejector liquid recirculation cycle and multi-stage multi-ejector supermarket refrigeration cycle, similar high OCR problem in the evaporator may exist, while in two evaporator ejector cycle, evaporator OCR is equal to compressor OCR at steady state.