Analysis and Comparison of Two-Phase Ejector Performance Metrics for R134a and CO2 Ejectors

Two-phase ejectors have been gaining increased attention in recent years due to their ability to directly improve the COP of the cycle. Of common interest in two-phase ejector studies is how the ejector improves cycle COP. However, less emphasis is often given to the performance of the two-phase ejector itself. The amount of COP improvement offered by an ejector cycle is very strongly influenced by the performance of the two-phase ejector; thus, it is important to understand the operation and performance of the two-phase ejector. Defining the performance of a two-phase ejector is not as straightforward as for an isentropic expander because there are multiple fluid streams in an ejector and because it is difficult to obtain flow properties at some locations in the ejector. As a result, there are a variety of performance metrics that have been proposed for use with ejectors in general and specifically for two-phase ejectors. In the present study, several different metrics that have been proposed for measuring the performance of ejectors are presented and analyzed. Performance metrics that were 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 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 R134a and CO2 offer somewhat similar ejector performance, though the CO2 ejector does seem to have noticeably better performance. CO2 has significantly higher throttling loss than R134a, meaning that there is more work available for the two-phase ejector to recover with CO2 and larger potential COP improvement for a CO2 cycle.