Periodic wake impingement and stall driving loss in axial compressors

Axial compressors are used in power generation and aircraft engines. In compressor design, three dimensional blading is already common. Since the flow structure in axial compressors is inherently unsteady, it is of interest to study salient features of axial compressor flows by time-resolved approaches. This paper shows how flow separation can be affected by the periodic interaction between the wake of a preceding blade row and the vane boundary layer, and how stall inception in a highly loaded axial compressor develops. Laminar flow separation on a flat plate under periodic wake impingement is studied in the closed-loop low-speed wind tunnel of the University of Leicester. Static pressure measurements by flush-mounted taps and microphones visualize the flow separation on the flat plate. These measurements also show that the naturally occurring transitional flow separations can be calmed by periodic forcing the inflow using small gusts produced by leading-edge mounted synthetic jets. The unsteady flow developed during stall inception in the NASA rotor 37 is investigated by time-dependent RANS simulations. An improvement in the rotor stall margin is obtained by a new casing treatment concept, based on a recirculation channel in the casing. The numerical simulations predict that the recirculation channel extends the rotor operating range significantly without any penalty to the rotor adiabatic efficiency over the rotor operating line.