Parametric study of tip injection in an axial flow compressor stage

Tip injection upstream of the rotor blade is a well-known technique for suppressing instabilities in axial compressors and recovering from a fully developed stall. In tip critical rotors, tip injection can effectively increase the blade loading and the stable operating margin of compressors by unloading the rotor tip. Compared to fully annular injection, discrete tip injection is better able to increase the compressor stability. This paper presents a numerical study of the effects of discrete upstream air injection on the stability of an axial flow compressor. Reynolds-averaged Navier-Stokes equations have been solved using ANSYS CFX with a epsilon turbulence model for three compressor blade passages. To validate the simulations, mass flow rate, pressure ratio, and efficiency at the compressor design point have been computed and compared to experimental data and results are in good agreement. For simulation of tip injection, 10 axisymmetric, but circumferentially discrete injectors, positioned at 50% of the blade axial chord upstream of the rotor blade leading edge, have been modelled. The ports are mounted on the casing and provide high-pressure jet of air at a 15 degrees angle in the radial direction. To study the effects of injection, the compressor map at design speed has been compared for the models with and without injection. Results indicate that tip injection improves the compressor stability by unloading the rotor tip. Simulations show that by increasing the injection mass flow, the compressor stable operating margin can be improved. Simulations also predict optimum values for the injection port width-to-length ratio and the injection angle when the injection mass flow rate and area are kept constant. Further studies have been done to investigate the effect of the axial position of the injector.