Monitoring the Approach to the Surge Line of a 4-Stage Transonic Compressor Based on Cepstral-Analysis

Current research concerned with the aerodynamic instability of compressors aims at an extension of the operating range of the compressor towards decreased mass flow. In practice a safety margin is maintained between operating point and surge line to prevent the compressor from going into stall and surge. In this paper the behavior of a 4-stage transonic axial compressor before entering aerodynamic instability is investigated using the well-known Fourier-Transform as well as the Cepstral-Analysis. All results are based on measurements of the unsteady static wall pressure of only one sensor in front of the first rotor. The analysis of the static pressure signals using the Fast Fourier Transform shows for this compressor, that peripheral disturbances (modal waves) prior to aerodynamic instability can only be identified in a small speed range close to nominal speed (at 95%). At lower speeds (50% to 80% of nominal speed) the investigated compressor flow enters instability by spike-type stall. The intensity of rotating disturbances did not indicate unambiguously the approach to the surge line. As modal waves occurred only in the range of high compressor speed before entering aerodynamic instability, the intensity of rotating disturbances increased only in the said speed range. The Cepstral-Analysis shows that the quefrency band (computed cepstral time range) corresponding to the blade passing frequency and their higher harmonics decreases when approaching the surge line. On the basis of these results a parameter based on high-speed measurements is presented which is an indicator for the current compressor operating point. A compressor monitoring based on this parameter is able to cover the entire speed range as well as the entire mass flow range of the compressor. The method works reliably and independent of the mechanism of stall inception i.e. for spike-type stall as well as for modal waves.