Instability of the compressor operation is manifested in two forms: by mild or deep surge. Both of them should be avoided as they lead to reduced performance or serious damage to the machine and its driving system. One of the pre-surge symptoms is rotating stall that appears mainly in the impeller and transfers to other parts of the compressor. This phenomenon can be described as a rotating pressure wave arising and decaying intermittently. Such an irregularly changing object needs an analyzing tool that allows for a spectrum fluctuation study. The authors used the signal time-frequency representation with progressive wavelets to analyze the static pressure signals acquired during a few deep surge cycles. The rotating stall was identified with the cross-correlation of the continuous wavelet transform. The same method allowed for calculation of the time lag between oscillatory pressure signals coming from the diffuser and the inlet of the compressor, which estimates the reverse flow velocity.

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