By positioning the completely rough wall locally on the hub side diffuser wall alone in the vaneless diffuser, the flow rate of rotating stall inception was decreased by 42% at a small pressure drop less than 1%. This is based on the fact that the local reverse flow occurs firstly in the hub side in most centrifugal blowers with the backswept blade impeller. The 3-D boundary layer calculation shows that the increase in wall shear component normal to the main-flow direction decreases markedly the skewed angle of the 3-D boundary layer, and results in suppression of the 3-D separation. It is also clarified theoretically that the diffuser pressure recovery is hardly deteriorated by the rough wall positioned downstream of R = 1.2 because the increase in the radial momentum change, resulting from reduction in the skewed angle of the 3-D boundary layer, supports the adverse pressure gradient.

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