Experimental investigation of stall inception of a low speed contra rotating axial flow fan under circumferential distorted flow condition

The present paper is aimed to understand the stall inception phenomenon and its propagation in a low speed, contra rotating axial flow fan stage, operating with circumferential inflow distortion. A 90-degree sector mesh screen is used to simulate the total pressure inflow distortion. Unsteady pressure data from casing wall-mounted sensors are used to understand the stall inception mechanism. The unsteady pressure data is analyzed using wavelet transforms and Fast Fourier Transforms (FFT). The wavelet transform is used to identify the stall inception mechanism where as FFT is also used for quantitative analysis. The rotors used in this study have relatively large tip gap and aspect ratio of 3. The study was carried out near-stall and at stall mass flow conditions for design speed and off design rotational speed combinations of the rotors. The study reveals that under circumferential inflow distortion, the stall incepts at higher mass flow compared with clean flow and the stall emerges first at the distorted sector and it propagates circumferentially based on the rotational direction of the two rotors. The wavelet transform clearly indicates that stall inception occurs mainly through long length-scale disturbances (LLSDs) for both the rotors and the spikes are observed to appear within the LLSDs. In some cases, only spike type of stall appears for rotor-1. The most interesting observation of the present study is that the casing wall pressure fluctuations and the stall frequency amplitude of rotor-2 are stronger than that of rotor-1.