Flow Separation Control in an Annular to Conical Diffuser Using Two Dimensional and Three Dimensional Wall Steps

Abstract

Conical diffusers are often installed downstream of a turbomachine with a central hub. Previous studies showed that nonstreamlined hubs had extended separated wakes that reduced the adverse pressure gradient in the diffuser. Active flow control techniques can rapidly close the central separation bubble, but this restores the adverse pressure gradient, which can cause the outer wall boundary layer to separate. The present study focuses on the use of a stepwall diffuser to stabilize the wall boundary layer separation in the presence of core flow control. Threecomponent mean velocity data for a set of conical diffusers were acquired using magnetic resonance velocimetry. The results showed the stepwall diffuser stabilized the wall boundary layer separation by fixing its location. An axisymmetric step separation bubble was formed. A step with a periodically varying height reduced the reattachment length of the step separation and allowed the diffuser to be shortened. The stepwall diffuser was found to be robust in a range of core flow velocity profiles. The minimum distance between the core flow control mechanism and the stepwall diffuser as well as the minimum length of the step were determined.