The Onset of Aerodynamic Instability in a 3-Stage Transonic Compressor

Abstract : The actual research activities concerning the aerodynamic instability of compressors aim at an improvement of the usable range of the compressor performance map. Usually there must he a safety margin between operating point and stability limit to avoid stall and surge. in this paper we give a survey of the pre- and the post-instability behaviour of a 3-stage transonic axial aero-engine compressor. The measurement results of fluctuations of static and total pressure, temperature and velocity under the influence of stall and surge are presented. For that were used piezo-resistive transducers, cold wires and triple-split-film sensors. The pressure signals show, that the investigated compressor flow goes into instability by a spike-type stall. The rotating stall frequency corresponds to nearly half of the shaft speed. The stall cell spreads at once over all three stages of the compressor and, after an oscillation at the beginning, extends approximately over a range of 1/3 to 1/2 of the circumference. Depending on the shaft speed different forms of instability occur like rotating stall, classic surge (local backflow) and deep surge (complete flow reversal). In the second part of this paper we analyse results using fast fourier transform and artificial neural networks. It is shown, that by using the fast fourier transform peripheral disturbances can be identified, but that an artificial neural network is the most useful tool to indicate an approaching instability in case of spike-type stalling.