Unsteady measurements of the flow and performance of a high speed 5-stage HP compressor have been carried out at different speeds under undistorted conditions and with swirl and total pressure inlet distortions. Distributions of incidence and diffusion factor have been derived from the test data which, together with hot-wire measurements of stall inception, provide new insights into the onset of stall with inlet distortion.

A stall cell initiates as a disturbance in the distorted flow sector, which may decay as it passes through the undistorted sector. Stall inception occurs only when the damping of the disturbance in the undistorted sector is insufficient to prevail its growth. As this damping depends on the size of the disturbance, the Parallel Compressor model, based on the linear stability properties of the undistorted compressor alone, is unable to predict the stall inception with inlet distortion.

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