An experimental investigation of the flow instability characteristics in a radial vaneless diffuser of a centrifugal blower was carried out. The mass flow rate of air through the diffuser could be gradually reduced to zero value. The flowfield in the diffuser was measured using dynamic and static pressure transducers and the results are reported for steady and stall conditions. The objectives of this experimental investigation are to investigate, understand, and document the effects of diffuser geometry on the inception and fundamental characteristics of rotating stall, and also to provide a benchmark set of experimental data to aid stall model development. The results of this investigation showed that the maximum number of stall cells was two. The rotating pressure pattern with one stall cell was dominant; especially, if the mass flow rate is lower than the critical value for stall inception. The speeds of stall cells increase linearly with the impeller speed and they decrease with the increase of the diameter ratio. On the other hand, the effect of diffuser width ratio on the pressure pattern frequency is insignificant; therefore, it is not presented.

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