Behavior of multiple short length-scale stall cells propagating in an axial compressor rotor

Behavior of multiple stall cells with short length-scale in an axial compressor rotor have been investigated experimentally. In a low-speed research compressor rotor tested, the multiple short length-scale stall cells appeared under a mild stall condition. In order to capture the short length-scale cells in the mild stall state, a so-called 'double phase-locked averaging technique' has been developed, by which the flow field can be measured phase locked to both of the rotor and the stall cell rotation. Then, time-dependent ensemble averages of the 3 D velocity components upstream and downstream of the rotor have been obtained with a slanted hot-wire, and the pressure distributions on the casing wall with high response pressure transducers. From the experimental results, the flow mechanism of the short length-scale stall cell has been clarified. The distinctive feature of the stall cell structure is a tornado-like separation vortex which spans from the casing wall ahead of the rotor to the blade suction surface and travels with changing the blade in turn on which the vortex leg stands.