Suppression of rotating stall in vaneless diffuser by wall roughness control

The authors have proposed a new technique to suppress successfully the rotating stall in the vaneless diffuser. By positioning the completely rough wall between R = 1.2 and 1.6 on the hub side diffuser wall alone, the flow rate of rotating stall inception was decreased by 42 % at a small pressure drop less than 1 % of the blower pressure rise. This is based on that the local reverse flow occurs firstly at the hub side in most centrifugal blowers with backswept blade impeller mainly depending on the distortion of tangential component of inlet main-flow velocity. The 3-D boundary layer calculation shows that the local reverse flow is decreased remarkably by an increase in wall friction, and it disappears if the friction coefficient is 0.01 about twice of the smooth wall. By analyzing the effect of wall friction and turbulence induced by the rough wall, it is clearly shown that the rough wall positioned in the limited region, where the wall-limiting streamline flow angle shows the negative maximum, is quite effective to suppress the rotating stall at a small drop of the diffuser pressure recovery.