Unsteady Flow Characteristics in a Centrifugal Compressor With Vaned Diffuser

Self-excited flow oscillations in a high performance centrifugal compressor with vaned diffuser have been experimentally investigated over a wide range of operating conditions. The space and time characteristics of the flow oscillations in the compressor from inlet to outlet were measured using fast response dynamic pressure transducers on the shroud wall and blade mounted straingages. Multi-channel signal analysis techniques in the frequency domain clearly identified the onset of the oscillations and its type. Rotating stall was found to exist in certain regimes of the compressor map but did not necessarily preceed the occurrence of the surge phenomena. At compressor speeds below 13600 rpm the rotating non-uniform flow when it occurred was composed of three lobes and rotated at approximately 5–6 % of the impeller speed. Above 13600 rpm the rotating pattern changed to two lobes and rotated at approximately 16–20 % of the impeller speed. The direction of rotation of both patterns was opposite to that of the impeller. Analysis of the performance characteristics of the compressor components prior to and during flow oscillations indicates that the relative magnitude of the flow fluctuations in the semi-vaneless space downstream of the impeller are the largest which points towards the close relationship between the conditions leading to the onset of the oscillations and the flow details in this region of the compressor. Additional confirmation of this relationship is obtained from comparison between the results obtained in this study and those obtained when the same compressor was operated with a vaneless diffuser.