Analysis of reversible ejectors and definition of an ejector efficiency

Second Law analyses of ejector performance have rarely been conducted in literature. Measures of ejector efficiency have not always been clearly defined and the rationale underlying and justifying current performance metrics is often unclear. One common means of assessing performance is to define a thermodynamically reversible reference process against which real processes may be benchmarked. These reversible processes represent the thermodynamic limit of real ejector performance. In this paper, parameters from real and reversible processes are compared and performance metrics are defined. In particular, the entrainment ratio of real devices is compared to the reversible entrainment ratio and denoted the reversible entrainment ratio efficiency. An efficiency comparing the ejector performance to that of a turbine-compressor system is also investigated, as is an exergetic efficiency. A rigorous analysis of performance metrics reported in the literature is undertaken. Graphical illustrations are provided to support intuitive understanding of these metrics. Analytical equations are also formulated for ideal-gas models. The performance metrics are then applied to existing experimental data to illustrate the difference in their numerical values. The reversible entrainment ratio efficiency ηRER is shown to always be lower than the turbine-compressor efficiency ηTER. For general air–air and steam–steam ejectors, the exergetic efficiency ηX is very close in numerical value to the reversible entrainment ratio efficiency ηRER. ► A reversible entrainment ratio may be calculated for any steady flow ejector. ► The reversible entrainment ratio of an ideal gas ejector may be formulated analytically. ► Where the motive fluid is saturated steam, an optimal pressure exists that maximizes the reversible entrainment ratio. ► The reversible vapor entrainment ratio for steam-moist-carrier gas ejectors is insensitive to the carrier gas. ► The reversible entrainment ratio ejector efficiency approximately equals the exergetic efficiency and is always less than the turbine-compressor efficiency.