From a consideration of flow stability, it is shown that the onset of separation in a diffuser depends upon the local blockage factor. Using performance data for two-dimensional diffusers from the literature, the shape factor of the boundary layer at the separation point Hs is related to the blockage factor Bs at that section, and the formula Hs = 1.8+3.75Bs is deduced as the separation-limit relation. It is proved that this separation-limit relation is also applicable to conical diffusers. Furthermore, a simple theory is derived to evaluate the time-mean pressure recovery in the separated region. Using this method, it is possible to predict whether a separation occurs in a diffuser and to evaluate the pressure recovery.

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