Experimental Investigation of Stall Mechanism and Warning Method in a Two-Stage Transonic Fan Under Inlet Distortion With Axial Slot Casing Treatment

Abstract A stall warning method has been employed to predict the level of disturbances in active control systems. Significant advancements have been made by identifying the pre-stall disturbances. Regardless of the propagation, disturbances almost initiate simultaneously at a specific circumferential location under uniform inflow. When the compressor suffers from inlet distortion, the disturbances might disappear in the undistorted region. The differences between the distorted and undistorted regions pose challenges for developing an accurate and robust warning method. Revealing the characteristics of pre-stall disturbances under the distorted inflow is a key to realize engineering applications. For this purpose, the stalling process and stall warning method are investigated in a two-stage transonic fan. The work on this project has concluded that the pre-stall disturbances initially arise in the region where the rotor exits the distorted region, diminish in the process of circumferential propagation, and ultimately disappear in the position where the rotor enters the distorted region. Due to the impact of circumferential secondary flow, the incidence is the largest and the fluctuation of the tip leakage flow is the strongest in the position where the rotor exits the distorted region. In the pursuit of an effective stall warning system under inlet distortion, a novel warning index associated with the characteristic frequency band is proposed. This index, coupled with a variable threshold, enables stall warning with only one transducer. The efficiency of this method is further validated in low-speed and transonic compressors.