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 analysing 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 analysing 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 mid-speed 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 independendy of the type of precursor. Setting up a reliable stall avoidance control based on this analysis scheme seems to be promising.