In order to explorer the mechanism of the "dangerous frequency"

a rotating distortion generator was designed to investigate the effects of rotating distortion on the stall inception of a two-stage low speed axial flow compressor. Experiment results show that the rotating distortion generator can output a rectangle-like total pressure distribution at different rotating speeds. The compressor exhibited significant loss in total-static pressure rise and stability margin arising from rotating distortion

especially when it rotated at a particular speed. Detailed measurements and analysis suggest that the modal wave appears firstly in the distorted area and is restrained when it spreads into un-distorted zone. When transmit frequency of modal wave is equal to the distortion rotating frequency

the modal wave needs least time to develop into rotating stall

the loss in stability margin is the maximum. The time-frequency analysis of the stall inception signal suggests that the "dangerous frequency" is equal to the transmit frequency of the modal wave.