Experimental Investigation On Rotor Tip Wake Of Axial Compressors With Stability Controls

Rotor blade tip flow especially the tip leakage flow has close relationship with the compressor stability, and intervention of the blade tip flow is an effective stability enhancement strategy. The mechanisms of flow instability and stability control around blade tip flow are a hotspot in the aerodynamic research area.Rotor wake, which is a complex combination of blade boundary layer, tip leakage flow, trailing edge corner vortex, and some other secondary flows, plays a significant role in the performance and stability of multistage compressors. In this paper, the rotor wake was experimental studied as a breakthrough point to investigate the mechanisms of rotating stall and stability enhancement.Firstly, a time-resolved total pressure probe was designed to measure the compressor rotor wake flow field. This probe was structured with a Kiel head and a semi-infinite pressure tube. The Kiel head was able to accurately measure the total pressure of flows within45degree deflection, which enabled the probe to measure the highly three dimensional rotor wake flow. The semi-infinite pressure tube could decrease the pressure signal attenuation and the cavity effect in the probe. The dynamic transfer function of the probe was calibrated using the pulse pressure signals over a compressor rotor, and it was used to correct the input time domain pressure signal so as to improve the accuracy and frequency response of the probe.Using the probe, steady and unsteady features of the rotor wake of axial compressors were experimental investigated under smooth casing condition. The discipline of the rotor wake averaged total pressure dissipation along axial direction was clarified. The total pressure contours of the rotor wake shown that the interaction between main flow and wake flow along circumferential direction contribute the most to the total pressure decrease. As the mass flow rate of the compressor decreasing, the rotor blade loading increased much more at the tip span than that at the lower span. As the compressor operating toward the rotating stall, the power of the frequency band around0.5blade passing frequency increased and the power of frequency band around the1blade passing frequency decreased at the same time.Based on the understanding of the rotor wake characteristics, the stability and rotor tip wake of a low-speed axial compressor were experimental investigated with circumferential grooved casing treatment. The influence of the chordwise location of the circumferential groove on the stall margin improvement was studied and the results shown that the casing with the groove covering the50%chord produced the largest stability enhancement of the compressor. The rotor tip wake flow shown that when the blade loading at about95%and above span moved to the80%-95%span, and the power of the frequency band around0.5blade passing frequency decreased and the power of the frequency band around1blade passing frequency increased at the same time, the circumferential grooved casing treatment would improve the compressor stabilily.Then, the stability and the rotor tip wake of the low-speed axial compressor were experimental investigated with blade tip air injection. Influences of the flow momentum and angle of the tip air injection on the compressor stall margin improvement were studied and the results shown that smaller injection angle and larger air momentum produced wider stall margin of the compressor. Compared with the rotor wake at the near stall point of the compressor under smooth casing condition, the rotor wake with tip air injection shown the same steady and unsteady phenomena with that with circumferential grooved casing treatment. So these two kinds of stability control techniques might have the same mechanism of stability enhancement.