A theoretical model of the impedance at blade tip clearance in aeroengine compressor

Abstract

Acoustic impedance model of slits plays a crucial role in addressing noise reduction challenges in aircraft engines. To gain further insights into the sound absorption mechanisms of slits and to develop acoustic impedance model, this study investigates the bias flow effect on the acoustic impedance of compressor blade tip slit. The evolution of the blade tip leakage flow is calculated by the combination of two-dimensional discrete vortex model and one-dimensional acoustic propagation model. In this manner, the bias flow effects on the acoustic characteristics of the blade tip slits, such as slit impedance and sound absorption coefficient, are investigated. The model is validated through the experiment of bias flow effect on a circular orifice. It is further extended to calculate the flow field response of slits with different blade height and different aspect ratios. The results show that the acoustic impedance of equal area slits aligns closer with circular orifice experimental results than the acoustic impedance of equal width slits. Larger hub to shroud distances causes less influence on the blade slits of the same width. Increasing hub to shroud distance reduces the Ma of the maximum absorption coefficient. As aspect ratio increases, the acoustic reactance component corresponding to the acoustic mass of the slit decreases. Increasing the hub to shroud distance and increasing the aspect ratio of blade chord length to slit width can both improve the sound absorption under feasible conditions.