Whole annulus unsteady simulations are performed by CFD with the whole flow passage model from inlet guide vanes to volute of an unshrouded centrifugal compressor. Characteristics and development mechanism of rotating stall are analyzed including the flow field and the impeller blade load in time and frequency domain. Rotating stall with three cells is observed in both two actual operation conditions but the cell rotating speed and the forming process is different. Leading edge tip clearance leakage is a criterion to predict the formation of a spike stall in centrifugal compressors. Tip clearance flow also plays an important role in the moving of rotating instabilities and the propagation of stall cells. It can effectively slow down the stall forming and decrease the pressure load on blade by reduced the tip clearance size at the leading edge.

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