Impeller instability based on the autocorrelation method in a centrifugal compressor

Instability behaviors significantly influence the performance and stable operating range in a centrifugal compressor, which is investigated by both experimental and numerical methods. To elucidate impeller instability characteristics, unsteady pressure signals were measured by high-response static-pressure transducers during the throttling process at various rotational speeds. Then, both the autocorrelation and spectrum methods were employed to analyze the instability process and behaviors. The results showed that during stall onset, the mean values of the autocorrelation coefficient (AC) decreased dramatically at all circumferential positions, and the root mean square values of the AC increased remarkably. The steep change in the mean value curves of the AC can be used to predict impeller stall effectively. Compressor stall was embodied by a rotating instability in the preliminary stage. The periodicity number of the AC curves in one rotor revolution varies from 2-5, corresponding to a broadband hump with 2-5 times of impeller frequency and indicating the rotating instability. The features and mechanisms of instability flow field were further discussed by unsteady simulation. Based on the clarification of relationship among the time domain periodicity behaviors, frequency spectra, and detailed flow field, the deep understanding and effective prediction for impeller instability are acquired.