Short and long length-scale disturbances leading to rotating stall in an axial compressor stage with different stator/rotor gaps

The transient processes of rotating stall evolution have been investigated experimentally, in a low-speed axial compressor stage with three stator-rotor gaps, e pressure traces at eight circumferential locations oil the casing wall near the rotor leading edge have been analyzed by the wavelet transforms. With the appropriate mother wavelets, the evolution Of short and long length-scale disturbances leading to the stall call he captured clearly. Behavior of these disturbances is different depending on the stator-rotor gap. For the large and middle gap, the stall inception is detected by it spiky, short length-scale disturbance, and the number of spiky waves increases to generate, the high frequency waves. They become the short length-scale part-span stall cells tit the mild stall for the large gap, while they turn into a big stall cell with growth of a long length-scale disturbance for the middle gap. In the latter case, therefore, the stalling process vias identified with "high-frequency stall inception." For the small stator-rotor gap, the stalling process is identified with "long wavelength stall inception" and supported the recent computational model for the short wavelength stall inception by showing that closing the rotor-stator gaps suppressed the growth of short length-scale disturbances. From the measurement of the pressure field traces oil the casing all, a hypothesis has been developed that the short length-scale disturbance should result from a separation vortex from a blade surface to reduce circulation. The processes of the stall evolution are discussed oil this hypothesis.