The unsteady flow in a single stage axial flow compressor at different operating conditions has been investigated with hot wire and hot film probes to find out the influence of the aerodynamic compressor load onto the periodic fluctuations. These results are compared with measurements in the last stages of a multi stage high pressure compressor of a gas turbine for normal operation and under stall conditions.

From the patterns of the frequency spectra of the measuring signals a parameter for the detection of the approach to the stability line of a compressor is derived. A method for the on line monitoring of the aerodynamic load is presented. Based on these results a monitoring system has been developed. First experiences with this system, applied to two multi stage compressors are reported.

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