Effects of component polytropic efficiencies on the dimensions of monophasic ejectors

An inappropriate choice of isentropic efficiencies for the sizing of the component parts of monophasic ejectors is one of the greatest sources of error in their design. The present study shows that the replacement of isentropic efficiencies with polytropic efficiencies within 1D ejector models provides more accurate results. Polytropic efficiencies access more precisely the effects of the pressure ratio variation on the irreversibilities of the acceleration and deceleration processes. It is demonstrated that the deviation in the constant area duct length, calculated by using an isentropic efficiency from the real length is three times greater than the one calculated by using a polytropic efficiency. The polytropic efficiencies are extracted from the CFD modeling of two types of ejectors, a constant area mixing (CAM) ejector and a constant pressure mixing (CPM) ejector. A 1D thermodynamic model for CAM and CPM ejectors, based on the polytropic efficiency concept, is proposed and validated against experimental data. Parametric studies based on this model were completed. They reveal that the variation in polytropic efficiency of the primary nozzle has the most important impact on the constant area duct length. New empirical correlations to estimate the polytropic efficiencies are also provided.