Suppression of Rotating Stall by Wall Roughness Control in Vaneless Diffusers of Centrifugal Blowers

Abstract

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 percent at a small pressure drop of less than 1 percent. This is based on the fact that the local reverse flow occurs first in the hub side in most centrifugal blowers with a backswept blade impeller. The three-dimensional boundary layer calculation shows that the increase in wall shear component normal to the main-flow direction markedly decreases the skewed angle of the three-dimensional boundary layer, and results in suppression of the three-dimensional separation. It is also clarified theoretically that the diffuser pressure recovery is hardly deteriorated by the rough wall positioned downstream of R = l.2 because the increase in the radial momentum change, resulting from reduction in the skewed angle of the three-dimensional boundary layer, supports the adverse pressure gradient.