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The present paper aims at providing an experimental analysis of the path to surge of a centrifugal compressor stage designed and built by Safran Helicopter Engines. Depending on the rotation speed of the compressor, two distinct flow patterns are observed in the radial diffuser at stabilized operating points near the surge, an asymmetric and a symmetric pattern. At medium rotation speed, the alternate pattern consisting of a two-channel pattern in the radial diffuser develops. One passage over two is stalled, the adjacent passage is free and this pattern replicates over the whole circumference while pulsing at a frequency of roughly 12 Hz which is close to the Helmholtz frequency of the test rig. By lowering the rotation speed, the two-channel pattern fades away and gives way to a periodical behavior of the radial diffuser passages called symmetric mode. The flow in each channel is identical presenting a stalled behavior pulsating in phase at a higher frequency of roughly 42 Hz. The two 12 Hz and 42 Hz modes are described and their existences are imputed to a lock-in of the natural frequencies of the instabilities with the acoustic modes of the test rig.

Low frequency stall modes of a radial vaned diffuser flow