Effect of impedance boundary-controlled casing treatment on performance of a fan subjected to inlet swirls

An experimental investigation is conducted to evaluate the performance and the stalling process of a fan subjected to inlet swirls, as well as the effectiveness of an Impedance Boundary-Controlled (IBC) Casing Treatment (CT) on the stall margin recovery. An operating cycle is proposed based on the hysteresis effect of harmonic flap oscillation of airfoils and parallel compressor theory to explain the pressure characteristic of the fan under twin swirl inlets. Twin swirls are observed to reduce the stall margin of the fan, and the circumferential location where the spike is detected turns to the intersection area of the twin swirl. The IBC CT is proven to extend the stall margin of the fan for 12.7%–22.3% when subjected to inlet swirls with an efficiency loss of around 1%. The IBC CT helps to reduce the size of the operating cycle of the fan by redistributing the blade loading and adding the system damping to dissipate the perturbation energy.