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

Abstract

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.

Unsteady Behavior of Leading Edge Vortex and Diffuser Stall Inception in a Centrifugal Compressor With Vaned Diffuser

“…Yoshinaga et al (1980Yoshinaga et al ( , 1983 took detailed measurements to show the effect of impeller and diffuser geometries on the compressor performance and unsteady flow fields in the diffuser passage. Haupt, et al (1988) investigated pressure fluctuation that occurs between the impeller inlet and the diffuser outlet, and found that it was due to the periodic changes in the flow conditions near the shroud side of the impeller inlet (Haupt et al, 1988). The authors previously reported on the unsteady behavior and evolution of the leadingedge vortex (LEV) in off-design operation, which was proposed to be a possible cause of diffuser stall in a centrifugal compressor (Ohta et al, 2010(Ohta et al, , 2013.…”

Stall Inception Behavior in a Centrifugal Compressor

“…Tested impeller and diffuser. There are a number of impellers and diffusers tested in the past in this rig for their rotating stall and surge characteristics ( Haupt et al, 1987b( Haupt et al, , 1988. The one used in present study is an unshrouded radially-bladed impeller which has 20 blades, but with every second blade cut back at the inlet till x/s = 0.27 (Fig.…”

Numerical Study of Pump-Turbine Instabilities Under Pumping Mode Off-Design Conditions

“…The rotating stall phenomena have been initially investigated for axial and centrifugal compressors [1][2][3]. First detailed explanations were given by [4] in 1959, but the rotating stall investigations on the centrifugal geometry were performed much later.…”