Paper reports a numerical study on vaneless diffuser flow instability performed for the purpose of better understanding of rotating stall mechanism in radial vaneless diffusers. This analysis is restricted to the two-dimensional flow where effect of wall boundary layers is neglected. Numerical results reveal that a two-dimensional rotating flow instability similar to rotating stall occurs when critical flow angle is exceeded. They also show that the stability limit and the structure of a two dimensional rotating instability are influenced by the configuration geometry and inlet and outlet flow conditions. Good agreement with data from the literature is found for the stability limit and number and speed of propagating cells. Number of cells and their speed is somewhat higher than observed in experiments from literature. This might imply that inception point is caused by the core flow instability and that wall boundary layers are more determinative for the structure of rotating instability.

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