The flow in a centrifugal compressor stage is dominated by the geometry of the impeller, the diffuser and the exit collecting chamber. Asymmetries of the flow field in the annular space behind the impeller may create a circumferential pressure distortion, influencing the energy transfer by the impeller in a negative way, creating variable force on the impeller blades and a radial force on the shaft.
Experimental investigations of the circumferential static pressure variations have been carried out on a large radial compressor test stand. Measurements with various impellers, diffusers and collecting chambers show the influence of geometrical modifications on the flow and compressor characteristics.
It is shown how a concentric exit chamber results in a very large pressure distortion in the diffuser. Although the amplitude of this distortion is increasing towards the diffuser inlet, it has a negligible effect on the pressure distortion upstream of the impeller.
It is further shown how the asymmetry of the flow in the diffuser can be reduced either by using a throttling ring at the diffuser exit, a vaned diffuser or by replacing the constant cross section collector by a volute with circumferentially increasing cross section.