Unsteady Flow in a Centrifugal Compressor With Different Types of Vaned Diffusers

Experiments were conducted to investigate the characteristics of self-excited flow oscillations in a high-performance centrifugal compressor system with a straight channel radial vaned diffuser. Fast response dynamic pressure transducers on the shroud wall and blade-mounted strain gages were used to identify the onset of the oscillations and their characteristics in space and time. In addition, flow characteristics near the shroud wall were visualized by an oil injection method, showing the extent of upstream directed reverse flow in the impeller range during significant unsteady flow compressor operation. Rotating nonuniform flow patterns were found in a wide range of operating speeds before the occurrence of surge. The number of lobes in the nonuniform flow patterns was dependent on the operating conditions and varied from two to four. Results of this experimental investigation were compared with those obtained from a previous investigation of the same compressor but with a cambered vane diffuser. Considerable similarity between the two configurations was found in the spatial distribution of the unsteady pressure field and in the frequencies of the fluctuations. The stability margin before the occurrence of surge and the operating regimes in which very intense pressure fluctuations were found were however different. In both cases, flow visualization techniques revealed the occurrence of reversed flow near the shroud wall of the impeller. Reverse flow extent up to the leading edge of the splitter blades systematically correlated with the occurrence of a nonuniform pressure pattern rotating with relatively high speed. Low rotational speed pressure patterns were observed when the extent of the reverse flow was up to the leading edge of the long blade. These different flow characteristics can be related to the occurrence of distinct rotating stall cell numbers. This result could be confirmed by unsteady pressure and blade vibration measurements.