A rotating stall associated with massive flow separation on the pressure surface (p-stall) of a centrifugal fan blade is identified. The stall cell rotates in the same sense as the rotation of the impeller. The frequency of pressure pulsation is greater than the running frequency and approaches 4/3 the running frequency. It is proposed that the mechanism of p-stall propagation is similar to that of the classical rotating stall caused by flow separation on the suction surface (s-stall). However, it is emphasized that the cause of p-stall is different from s-stall and any corrective treatment applied should take its nature into consideration. In the present work, pressure surface stall was induced by prerotating the inlet flow using inlet guide vanes and operating the fan against a low system resistance. Heuristic models are proposed for the inception and propagation of p-stall, and computational predictions of the flow fields, along with experimental results, are presented to support the model.

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