Stall inception and its control in an axial flow fan under dynamic inflow distortion

The present experimental study demonstrates the effects of clean and distorted inflow conditions on the performance and nature of stall inception in a single stage axial flow fan. A 90° distortion screen located upstream of the rotor leading edge was rotated up to 40 per cent of the rotor speed in the clockwise (co-rotation) and counterclockwise (counter-rotation) directions to generate dynamic inflow distortion. It was observed that the stall margin deteriorated substantially under co-rotating inflow distortion as compared to counter-rotating inflow distortion. The degradation in stall margin was about 15 per cent and 1.98 per cent under co- and counter-rotating inflow distortions, respectively. Tip injection was used as a means of enhancing the fan performance under distorted inflow. With tip injection under co-rotating inflow distortion, about 2.8 per cent improvement in stall margin was observed. The improvement in stall margin under counter-rotating inflow distortion with tip injection was 2.98 per cent. The unsteady static pressure traces show clear differences in the nature of stall inception under co- and counter-rotating inflow distortions. The stall inception occurs through long-length-scale disturbances under co-rotating inflow distortion, while the mode of stall inception under clean flow and counter-rotating inflow distortions was through short-length-scale disturbances. The nature of stall inception under dynamic inflow distortion with tip injection remains the same as without tip injection.