Behavior of short and long length-scale disturbances in the process of rotating stall evolution in an axial compressor

The transient processes of rotating stall evolution were investigated experimentally in a low-speed axial compressor stage with three stator-rotor gaps. The pressure traces at 8 circumferential locations on the casing wall near the rotor leading edge were analyzed by the wavelet transforms, which is useful to look into the behavior of disturbances with various length scales. In addition to this, the pressure field traces on the casing wall covering the rotor region were examined to imagine the transient event in the stalling process. The experimental results show that the evolution process of these disturbances depends on the stator-rotor gap. For the large and middle gap, the stall is initiated by a spiky short length-scale disturbance, and the number of spiky waves increases to generate the high frequency waves. These high frequency waves become the short length-scale cells for large gap, while they turn into a big stall cell with growth of a long length-scale disturbance for the middle gap. For the small stator-rotor gap, the stalling process is identified with 'long wave-length stall inception'.