Results of high-response measurements in a high-pressure-ratio centrifugal compressor during surge operation are presented. The interpretation of the data provides improved understanding of the flow mechanism of surge and the phenomena which apparently trigger surge. The mass flow throughout the compressor was observed to oscillate in synchronism during supposedly “stable” operation. Surge of the test compressor occurred when a flow perturbation, during “stable” operation, exceeded an apparent limit. When the instantaneous value of diffuser-inlet pressure recovery factor, Cp2*–4, reached approximately 0.40–0.45, the diffuser inlet flow was badly deteriorated and surge occurred. However, gross separation of the diffuser inlet boundary layer was not observed before surge commencement. It is concluded that instantaneous operating states must be recognized for competent surge prediction. Further, doing so will probably require analysis of the entire compressor system including piping, vessels, flow resistances, etc.

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