CFD Study on the Influence of Geometric Parameters on the Aerodynamics Within an Ejector Injection System for Compressor Stabilization

Abstract

Injection of high-momentum air into the tip-gap region of rotor stages is a measure of active aerodynamic stabilization of turbo compression systems. The Institute of Jet Propulsion at the University of the German Federal Armed Forces Munich advanced the concept of conventional tip air injection by deliberately deploying the ejector effect in order to increase the mass flow rate of the air injected. A novel Ejector Injection System (EIS) has been developed for the Larzac 04 jet engine and its intended ejector performance was proven in experimental pre-investigations. In addition to that, the corresponding CFD setup has been validated and an approach for highly efficient CFD simulations of the EIS ejector aerodynamics (node number reduction > 90%) was developed, verified, and validated. Thus, optimization of the ejector geometry in order to enhance the ejector aerodynamics and subsequently the stabilization performance of the EIS comes into focus now. Within this paper, a parametric CFD study is conducted to determine the influence of three main geometry parameters of the EIS ejector design on the ejector’s performance. The parameters, namely the injection nozzle spacing, the mixing duct length, and the ejector nozzle height, are introduced in the context of the overall EIS design and functionality. For efficiency purposes, a script-based procedure which deploys ANSYS ICEM CFD and ANSYS CFX has been developed in order to conduct the CFD parameter study covering 205 simulations fully automated. Each ejector geometry is thereby simulated with five different primary air mass flow rates supplied to the EIS covering a range from low-speed to transonic operation. It is revealed that all three geometry parameters investigated show partially significant impact on the ejector performance in terms of the entrainment ratio μ. In order to get a detailed insight into the inner EIS aerodynamics, also primary air Mach and Reynolds numbers, the state of mixing between primary and secondary air, and velocity profiles in the LPC’s tip region are subjects of investigation. Based on these findings and the general aerodynamic coherences discovered, recommendations for optimizing the current EIS ejector design are presented.