Three-Dimensional Separation in Complex Internal Flow Fields

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Evolution and structure of rotating stall cells in an axial compressor rotor have been investigated. To capture the flow field phase-locked to both of the rotor and the stall cell, a double phase-locked averaging technique has been developed. Time-dependent ensemble averages of three-dimensional velocity components upstream and downstream of the rotor have been obtained with a slanted hot-wire, and pressure distributions on the casing wall with high response pressure transducers.The short length-scale stall cell is characterized by a concentrated vortex spanning from the casing wall ahead of the rotor to the blade suction surface. In the long length-scale stall cell, the separation vortices go upstream irregularly when blade separation develops in the front half of the cell, and reenter the rotor on the hub side in the rear half of it. Behavior of casing wall pressure disturbances corresponding to the short and long length-scale stall cells is different depending on the stator-rotor gap. For the large and middle gap, the stall inception is detected by a spiky short length-scale disturbance, and the number of spiky waves increases to generate the high frequency waves. They becomes the short length-scale part-span stall cells at the mild stall for the large gap, while they turn into a big stall cell with growth or a long length-scale disturbance for the middle gap. In the latter case, therefore, the stalling process was identified with 'high frequency stall inception'. For the small stator-rotor gap, the stalling process is identified with 'long wave-length stall inception', and supported the recent computational model for the short wave-length stall inception by showing that closing the rotor-stator gaps suppressed the growth of short length-scale disturbances. It is found that the short length-scale disturbance results from a separation vortex from a blade surface to reduce circulation.