Rotating stall in vaneless diffusers of centrifugal compressors

The distinction between impeller and diffuser rotating stall; the triggering mechanism of diffuser rotating stall and resulting flow patterns; and stability criteria for diffuser rotating stall and prediction of relative rotational speeds are discussed. Comparison of the unsteady flow patterns observed at rotor inlet and inside the vaneless diffusor indicates distinction should be made between impeller rotating stall and diffuser rotating stall. The unsteady flow theory of Jansen is shown to provide a valuable prediction of diffuser rotating stall, if combined with an improved boundary layer calculation method.