Characterization of instability initiation in centrifugal compressors

On a compressor performance map, the region of unstable operation is bounded by the surge line. As operation in this unstable region can be detrimental to the integrity of the machine, a surge margin is maintained. The proximity of the surge line to the compressor maximum pressure rise makes a reduction in the required surge margin desirable. Prior to establishing a strategy for designing a compressor with reduced surge margin, an understanding of the instability initiation process must be attained. The determination of the role that rotating stall plays in surge initiation of high speed compressors is of particular interest. Toward this end, a series of experiments at both low and high speed were undertaken to characterize centrifugal compressor rotating stall. Utilizing spatial arrays of inlet microphones and impeller mounted microphones, the origin of the low speed compressor rotating stall is found to be a separation zone near the impeller midchord that propagates at the stall frequency. This disturbance grows as the compressor is destabilized. The impeller flowfield during rotating stall has also been measured using PIV. The inducer region shows no separation at midspan. A region of zero relative velocity is located during the stall cycle at 70% impeller meridional chord. The investigation of the high-speed centrifugal compressor has identified two distinct rotating stall modes, a one mode and a nine mode. Both are located at or near the diffuser. Examination of the component performance reveals that the one mode is associated with mild or deep surge cycles while the nine mode exists independently and serves as a blockage, reducing diffuser performance. Rotating stall is observed between surge cycles, similar to axial compressor results. A surge chamber diffuser is employed and produces a 1% increase in maximum stable pressure coefficient. The surge chamber reduces the magnitude of the rotating stall by 60%. Speed transients at constant throttle settings are examined and reveal three distinct instabilities, deep surge, mild surge and stable rotating stall. The compressor exhibits rotating stall at surge initiation and termination