Stall inception in the compressor system of a turbofan engine

Compressor flow instabilities have been the subject of a great number of in vestigations during the past decade. While most of this research work was d one on isolated test-rig compressors, this paper presents stall inception m easurements in the compressor system of a two-spool turbofan engine at vari ous power settings, Several analyzing techniques such as temporal low-pass and band-pass filtering, temporal and spatial Fourier transforms including power-spectral-density calculations of the spatial coefficients, and a wave let analyzing technique are applied. For the low-pressure compressor three different types of stall inception processes were observed depending on the rotor speed. At low speed stall originates from spike-type precursors, whi le long wavy pressure fluctuations corresponding to modal waves were observ ed prior to stall at midspeed for undistorted inlet flow. At high speed, th e rotor shaft unbalancing dominates the stall inception process as an exter nal forcing function. In the case of distorted inlet flow spike-type stall inception behavior dominates throughout the speed range. While filtering an d the Fourier spectra give a good insight into the physical background of t he stall inception process (but with a very short warning time), the wavele t transform indicates the approach of the stalling process a few hundred ro tor revolutions in advance independently of the type of precursor. Setting rip a reliable stall avoidance control based on this analysis scheme seems to be promising.