Experimental and numerical study of tip injection in a subsonic axial flow compressor

Parametric study of tip injection was implemented experimentally on a subsonic axial flow compressor to understand the underlying flow mechanisms of stability improvement of the compressor with discrete tip injection. Injector throat height varied from 2 to 6 times the height of rotor tip clearance, and circumferential coverage percentage ranged from 8.3% to 25% of the annulus. Static pressure fluctuations over the rotor tip were measured with fast-response pressure transducers. Whole-passage time-accurate simulations were also carried out to help us understand the flow details. The combinations of tip injection with traditional casing treatments were experimentally studied to generate an engineering-acceptable method of compressor stall control. The results indicate that the maximum stability improvement is achieved when injectors are choked despite their different sizes. The effect of circumferential coverage percentage on compressor stability depends on the value of injector throat height for un-choked injectors, and vice versa. Tip blockage in the blade passage is greatly reduced by the choked injectors, which is the primary reason for stability enhancement. The accomplishment of blockage diminishment is maintained in the circumferential direction with the unsteady effect of tip injection, which manifests as a hysteresis between the recovery of tip blockage and the recovery of tip leakage vortex. The unsteady effect is primarily responsible for the effectiveness of tip injection with a partial circumferential coverage. Tip injection cannot enhance the stability of the rotor with axial slots significantly, but it can improve the stability of the rotor with circumferential grooves further. The combined structure of tip injection with circumferential grooves is an alternative for engineering application.