Analysis of two-phase ejector performance metrics and comparison of R134a and CO₂ ejector performance

Two-phase ejector studies in recent years have shown that the performance of an ejector cycle is strongly dependent on the performance of the ejector. However, measuring the performance of a two-phase ejector is not always straightforward. In the present study, several differentmetrics that have been proposed formeasuring the performance of ejectors are presented and analyzed. Performance metrics originally proposed for single-phase ejectors as well as those proposed specifically for two-phase ejectors are both considered. A simple numerical ejector model is used to simulate ejector operation and calculate the performance of the ejector based on the various performance metrics. The various ejector performance metrics are then compared based on the numerical results. Experimental data from previous studies for R134a and CO₂ two-phase ejectors is also presented, and the ejector performance metrics are compared based on the available experimental data as well. It is seen that for a given ejector operating point, the different performance metrics can yield very different values. Based on the data from available ejector studies, it seems that CO₂ ejectors can achieve somewhat better performance than R134a ejectors. It is also seen that better ejector performance does not necessarily correlate to better ejector cycle performance.