Placement of Recirculation Channel Casing Treatment in Transonic Fans: A Flow Physics Approach

This study investigates the effects of recirculation channel casing treatment positioning in a transonic fan. Recirculation channels are a passive casing treatment, consisting of an injection and extraction port connected by a secondary flowpath. Casing treatments typically extend stall margin at the cost of efficiency. However, in a transonic flowfield, strong pressure gradients can introduce an important unsteady component to the recirculation channel flow, giving these channels the potential to reduce this tradeoff by timing the injection and removal of the recirculated flow for maximum effectiveness. This timing aspect of the recirculation channel can be designed using the circumferential positioning (clocking) of the injection port relative to the extraction port. The presented results show two stability-enhancing mechanisms. First, the oscillation-attenuation mechanism improves flow stability by increasing the mean axial velocity at the tip and attenuating the oscillation of the shock and tip leakage vortex. Second, the tip leakage vortex (TLV) energization mechanism improves flow stability by injecting a jet that locally energizes the TLV. This study demonstrates that the stability-enhancing mechanism of the recirculation channel is determined by effective clocking.