Pressure fluctuations at various locations on the flow path of two centrifugal compressor stages have been recorded and analyzed in the time and frequency domains. Two distinct types of unsteady phenomena were measured: a rotating pressure pattern in the diffuser and compressor system surge. The rotating pressure pattern was generated at a much higher mass flow rate than the one leading to surge. At the onset of the diffuser instability, the pressure fluctuations were sinusoidal and lines of equal phase were radial. For the tests conducted in the present investigation, two nodal diameters existed in the pattern. Both amplitude and rotational speed of the pressure pattern gradually increased as the mass flow rate was gradually decreased. It is shown that the measured pressure fluctuations should not be attributed to rotating zones of separated boundary layers at the diffuser walls. This does not mean, however, that a stationary separation zone in the diffuser is not necessary to generate the measured diffuser instability.

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