The interaction between impeller and diffuser in a high-pressure ratio centrifugal compressor is considered to have a strong influence on the unsteady flow field, the impeller response and the performance of the compressor. A computational study was performed to investigate the interactions between a backswept impeller and its downstream vaned diffuser with emphasis on the impeller response at 2 different vane settings. The unsteady computations were conducted using two different modelling levels of increasing fidelity. The computational domain included an impeller with 15 main and 15 splitter blades and 22-vane wedge diffuser. A steady-state stage calculation with a mixing-plane interface between the impeller trailing edge and the vane leading edge was conducted first to assess the performance. A whole-annulus unsteady stage calculation was conducted to study the response of the impeller. The effect of radial gap between the impeller trailing edge and the vane leading edge on the performance of the impeller was investigated in some detail. In agreement with other similar studies, the results suggest that there is an optimum value of the radius ratio for best performance.

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