Stall Inception Mechanism in an Axial Flow Fan Under Clean and Distorted Inflows

The present paper describes the use of Morlet wavelet transform in understanding the stall inception mechanism in a single stage axial flow fan. Unsteady pressure data from wall mounted sensors were used in the wavelet transforms. This paper was carried out under undistorted and distorted inflow conditions as well as for slow throttle closure and throttle ramping. It was observed from the wavelet transforms that the stall inception under clean inflow (undistorted) and counter-rotating inflow distortions (in the direction opposing the rotor rotation) incur through short length-scale disturbances and through long length-scale disturbances under static and co-rotating inflow distortions (in the same direction of rotor rotation). Modal activity was observed to be insignificant under clean inflow while under static inflow distortion, long length-scale disturbances evolved due to interaction between rotor blades and the distorted sector, especially near the trailing edge of the distortion screen. The presence of a strong mode was observed under both co- and counter-rotating inflow distortions. With throttle ramping, stall inception occurs through long and short length-scale disturbances under co- and counter-rotating inflow distortions, respectively. Some preliminary flow characteristics were studied using a seven hole probe. A significant increase in flow angle and decrease in axial flow coefficient close to the rotor tip were observed under co-rotating inflow distortion as compared with counter-rotating inflow distortion.