The design of centrifugal stages with an impeller and a downstream diffuser has generally been based on the assumption of axisymmetric flow at the impeller discharge. Flow entering the diffuser has customarily been assumed to also be axisymmetric, at least on a time-averaged basis, while laying out the diffuser vanes, establishing the preferred incidence, and sizing the throat areas. Recent stage studies have shown that the flow is often not fully axisymmetric, that not all diffuser passages perform the same way even at the design or best efficiency points, and that the actual time-averaged diffuser inlet flow conditions (distortion) may be changing from one operating point to another.

In this study, comprehensive time-averaged (steady) results from one test rig with a large array of impeller exit pressure taps is examined. Supporting results from five other test rigs are reviewed to broaden the picture of possible flow states. Hypotheses, suitable for future evaluation, are given to begin the explanation of the actual flow states and to guide further research. The current status of CFD to understand these phenomena is discussed.

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