Stall inception in the compressor system of a turbofan engine

Compressor flow instabilities have been the subject of a great number of investigations during the past decade. While most of this research work was done on isolated test-rig compressors, this paper presents stall inception measurements in the compressor system of a two-spool turbofan engine at various 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 wavelet 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, while long wavy pressure fluctuations corresponding to modal waves were observed prior to stall at midspeed for undistorted inlet flow. At high speed, the rotor shaft unbalancing dominates the stall inception process as an external forcing function. In the case of distorted inlet flow spike-type stall inception behavior dominates throughout the speed range. While filtering and the Fourier spectra give a good insight into the physical background of the stall inception process (but with a very short warning time), the wavelet transform indicates the approach of the stalling process a few hundred rotor 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.