This study experimentally investigated compressor instability characteristics of two transonic axial compressors. From the stall precursors when their operating points are approaching the stall line, stall initiation and its development and its transition to compressor surge were measured with high frequency pressure transducers, step by step. First part of this paper describes three-stage transonic axial compressor instability characteristics. The compressor has been designed to have positive incidence angle in order from third stage to first stage as compressor outlet throttle valve closes at the design speed. Then periodic modal signal appeared from the third stage before it spread toward the first stage as a precursor of the compressor stall. However, the actual compressor stall took place from the first stage and developed toward the third stage. Interestingly, the stall cell spread toward downstream by spiral direction, which means stall cell moves with axial and circumferential velocity components. When the stall cell reached the collector through the third stage, sudden strong one-dimensional fluctuation, system surge, appeared. Then operating point revolved along the hysteresis loop on the compressor map repeatedly before anti-surge valve opened. Second part of this paper describes a single stage transonic compressor instability characteristics. Different from the three-stage compressor, spike type stall precursor was detected just ahead of rotating stall cell appearance. Before spike initiation there was no warning signal of compressor stall. When the spike was fully developed to the rotating stall, the operating points deviated from the operating line and settled at deteriorated operating points until the anti-surge valve opened. Also a weak compressor system Helmholtz frequency was detected regardless of the rotating speed. To identify the overall compressor instability behavior and difference between two axial compressors, Greizter's B parameter was found to be very useful. B parameter of the three stage compressor was about 0.83, it means classic surge could occur at the final stage of instability. On the other hand, B parameter of the single stage compressor was 0.62. According to this B parameter, the single stage compressor stall would no longer develop to compressor surge and it well agrees with experimental results.
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