Experimental investigation on stability-enhancement mechanism of tip air injection in an axial-flow compressor with circumferential distortions

We conduct experimental investigations of the effect of the tip air injection as a stability control method on a low-speed axial-flow compressor experiencing severe circumferential total pressure distortion. Eight Coanda-shaped injectors, uniformly distributed upstream of the rotor blade leading edge, were employed to counteract stall margin degradation caused by a flat-baffle circumferential distortion. Unsteady pressure data from the tip clearance and rotor wake regions were captured using time-resolved sensors and probes and analyzed through time-frequency transformation and proper orthogonal decomposition (POD) techniques. POD investigation under distorted inflow conditions reveals that the third-order POD mode, which characterizes unsteady tip leakage flow (TLF), plays a dominant role in triggering stall. Based on this insight, tip air injection with varying momentum ratios was applied to enhance the stall margin. The positive correlation between the injected momentum ratio and stall margin improvement was established, demonstrating the stabilizing effect of tip air injection under the circumferential distortion. The results indicate that tip air injection primarily acts on the rotor blade tip region to suppress the unsteady TLF at the center and downstream areas of the distortion, leading to an increase in tip blade load and a reduction in both the scale and number of pre-stall disturbances. However, the injection has a minimal impact on the unsteady TLF frequency band in the rotor wake region. Additionally, the POD analysis confirms that as the injection rate is increased, the energy percentage, flow field intensity, and power spectral density amplitude of the third-order POD mode are decreased, further reflecting the mitigation of unsteady TLF. The present work provides important insight into the stability control mechanisms of a tip air injection under circumferential distortion, offering design guidance for the implementation of active stability control strategies in axial-flow compressors.