In this paper, an unsteady tip leakage flow instability is identified and investigated for an axial compressor at near surge. We describe the results of experimental verification of a new compressor developed by improving the blade geometry based on the criterion for the occurrence of unsteady phenomenon. In a high-speed multistage axial compressor, a surge test was carried out by changing the tip clearance. Under a large clearance condition, a drastic decrease in static pressure rise near surge was observed. Large, unsteady pressure fluctuation at the blade tip was confirmed, and the occurrence of tip leakage vortex breakdown that was clarified by unsteady pressure measurement and numerical simulations. It is found that the vortex breakdown propagated circumferentially and caused the rotating instability. In order to investigate the relationship between unsteady flow phenomenon, tip clearance and flow pattern, unsteady calculation was conducted by changing the blade tip stagger and clearance. A new concept of staggered pitch-based clearance is proposed, which makes it possible to include the blade loading effect on the clearance and clarifies that there exists a threshold at which vortex breakdown occurs. On the basis of the above results, an improved compressor was designed and manufactured to prevent vortex breakdown. A clearance change test was conducted and increase in the static pressure rise near surge was confirmed for each clearance. The design concept of the improved blade which suppressed the vortex breakdown was verified, and the effectiveness of the design guideline in actual gas turbine was confirmed.

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