Analysis of entrainment and mixing in a supersonic air ejector using Large-Eddy Simulation

Abstract

The interest towards supersonic ejectors as replacement for compressors in various industrial applications has grown over the last decade. In contrast with the simplicity of their working principle, the turbulent phenomena responsible for the entrainment and mixing are still not fully understood nor well captured by numerical simulations. Indeed, mostly Reynolds-Averaged Navier–Stokes (RANS) simulations have been performed up to now, where the unsteady phenomena responsible for the entrainment are solely modeled using an effective turbulent viscosity, and that also needs calibration. This work aims at providing the tools to improve the understanding and design of those devices by analyzing the instantaneous and averaged fields from a Large-Eddy Simulation (LES) of a supersonic air ejector. First, the structure of the mixing layers is analyzed, showing a very early transition to turbulence in the shear layers and the necessity to properly capture the associated fluctuations. The impact of turbulence injection in the primary flow is also discussed. Then, the post-processing tools, using total exergy fluxes, are presented. The exergy fluxes between the streams in the mixing duct are computed, and are compared to their incomplete counterpart obtained using RANS simulation results. The observed discrepancies are shown to be due to the large differences in the turbulent shear stress and turbulent heat flux through the shear layer between the RANS and LES results. A new criterion to measure the completion of the mixing between the flows is introduced thanks to a novel decomposition of the total exergy flux. Finally, the complete mixing of both streams, as newly defined, is shown not to be a sufficient condition to find the optimum performance of the ejector, e.g. a maximum compression at the maximum flow rate of the secondary stream. This optimum is found to be much more dependent on the balance between shear stress between streams and wall-friction.