Optimized Casing Treatment to Improve Stall Margin Under Circumferential Pressure Distortion in a High-Speed Axial Flow Fan

Abstract An optimized axial slot casing treatment (OPCT) was designed that experimentally improves the stall margin by 7.31% with an efficiency gain of 1.94% at a design speed of 100% in a high-speed axial flow fan. Subsequently, the OPCT was tested under the distorted inflow with a 9% composite distortion index; it can still improve the stall margin by 6.19% with an efficiency gain of 1.63% at the 100% design speed. The unsteady measurement results indicate that the location of the shock wave obviously moves forward toward the leading edge of the blade tip as the rotor blade rotates out of the distorted region and induces the stall in advance. When the OPCT is applied, the strength of the interaction between the shock wave and tip leakage flow can be suppressed, and the location of the shock wave at the distorted region is pushed backward. Thus, the trigger of the stall inception generated in the downstream of the distortion region is postponed. Meanwhile, a new phenomenon was discovered that the OPCT does not alter the instability mode of the fan, and the fan eventually surges, which is consistent with the B parameter value of 2.942. However, under the distorted inflow, the B parameter can be reduced from 2.942 to 0.863, thereby the instability mode of the fan is changed from surge to stall. It can guide the instability prediction when the boundary condition (distortion or casing treatment) of the fan is changed.