Rotating stall characteristics in a radial vaneless diffuser model was investigated experimentally. Measurements were made using hot-wires and pressure transducers (static and dynamic). The mass flowrate through the blower, at constant impeller speed, was gradually reduced until flow instability occurred in the diffuser. This enabled the onset and propagation of rotating stall to be fully described. The blower was operated without the vaneless diffuser in order to ascertain the cause of the flow instability. It was discovered that the impeller did not stall at the flow rates at which the blower was operated with the diffuser. The critical flow angles measured at the diffuser inlet, and midway between the diffuser walls, were in good agreement with earlier reported values in the open literature. The maximum number of rotating stall cells found in this study was two. The single-stall cell structure was found to be dominant over the two-stall cell structure at flow coefficients much lower than the critical flow coefficient.

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