Experimental setup, measurement and analysis of the onset of compressor flow instabilities in an aeroengine

In the past decade great efforts have been undertaken to understand and control the flow structure in a compressor working close to its stability limit. Most of this research work was done at low-speed compressors in isolated test rig facilities. This paper presents the experimental setup, measurement and different analysing techniques of highly unsteady pressure signals during the onset process of compressor flow instabilities in the two-spool bypass aeroengine LARZAC 04 at various power settings. Several analysing techniques such as temporal low-pass and band-pass filtering, temporal and spatial Fourier transform and a wavelet analysing technique are applied. While filtering and the Fourier spectrum gives a good insight into the physical background of the stall inception process-but with very little warning time-the wavelet frequency spectrum indicates the approach of the stalling process a few hundred rotor revolutions (corresponding to 1-2 seconds in time) in advance independently of the type of precursor. Depending on the rotor speed three different types of stall inception processes were observed within the low pressure compressor. While a low-speed stall originates from spike-type precursors, long wavy pressure fluctuations corresponding to modal waves were observed prior to stall at mid-speed. At high-speed the rotor shaft unbalancing dominates the stall inception process as an external forcing function.