Understanding Unsteady Flow Behaviour in a Low Aspect Ratio Contra-Rotating Axial Fan Under Radially Distorted Inflow

Abstract Contra-rotation has several advantages like swirl-free discharge, high pressure-rise per stage, and possibility of operating both the rotors at different speeds. With these merits, contra-rotating fan emerges as a competent technology for future gas turbine engines. During operation, gas turbine engines undergo situations like high angle of attack manoeuvres, large cross-winds, bird-hits, etc. which distort the flow at the inlet of the engine. A thorough understanding of the effect of distortion on low aspect ratio contra-rotating fans is missing in literature. This paper reports the consequences of radial distortion on the performance of a low aspect ratio contra-rotating fan. The uniform inlet flow is distorted radially using wire mesh screens. The unsteady data obtained from high response pressure sensors are analysed using Discrete Spatial Fourier Series (DSFS) and Morlet wavelet transform. Both Long Length Scale Disturbances (LLSD) or modal waves and Short Length Scale Disturbances (SLSD) or spikes are observed for different inflow conditions. The stage stalls primarily due to the instabilities arising at the tip region of rotor-1. Rotor-2 shows poor coherence in the disturbances prior to stall compared to that of rotor-1. Tip-distorted flow is dominated with SLSDs in the pre-stall region and hence a stall precursor is not observed whereas clean and hub-distorted flows show prominent LLSDs prior to stall. The radial distortions get redistributed at the exit of rotor-1 and hence, the distorted inflows do not severely lead to instabilities on rotor-2. In summary, this work explains in detail the development of unsteady flow phenomena occurring in a low-aspect ratio contra-rotating fan stage leading to stall and the way in which the system responds to it.