In the present paper a computational analysis of the interactions between a backswept impeller and its downstream vaned diffuser in a high-speed centrifugal compressor is presented. Both steady and unsteady simulations are carried out at the peak efficiency point. Geometry scaling was used in the unsteady simulation in order to deal with the problem of unequal pitch. For the steady simulation, an averaging approach is used at the interface between the impeller and the diffuser. A detailed comparison between the time averaged unsteady results and the steady simulation results is performed and some unsteady phenomena are also discussed in order to advance the understanding of the flow physics involved. One important conclusion is that the unsteady simulation is important since the difference between the time averaged unsteady results and steady simulation results is quite significant especially in the velocity field and the stage efficiency. From the comparisons of the predicted results with available experimental data in terms of velocity vectors and isentropic efficiency, it can be concluded that the geometry scaling method used in the current unsteady simulation is reasonable and successful and the computational model employed for the predictions consists a reliable computational tool. In general, the contours of different flow variables for the averaged unsteady simulation results are more uniform than the steady ones, especially in the vaneless space. In the static pressure field, there are relatively small differences. The main differences occur in the region of the vaneless space and downstream of the throat area, and in general the difference is found to be quite small. In the velocity field, the differences are large compared with the ones occurring in the pressure field. The area with the largest differences extends from the vaneless space into the semi-vaneless and vaned diffuser channel. The unsteadiness also modifies the flow angle and hence the incidence angle at the leading edge of the vaned diffuser, the maximum difference can reach 3 degrees. The difference in the stage isentropic efficiency can reach above 1 per cent, which is considered to be quite high. The results also indicate that the larger differences between the time-averaged unsteady and steady state simulation results usually occurred in the area that flows are highly unsteady or nearly separated. Consequently the steady state simulation is still not very accurate to predict highly unsteady flow and separated flows.

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