In the paper the effect of negative pre-swirl on the performance and the flow in a centrifugal compressor stage are investigated by both the experimental and numerical methods. The results show that, at a negative pre-swirl, whether the pressure ratio of compressor is increased or not is determined by the contest between the increased energy transfer and flow losses. At a large negative pre-swirl, the flow losses in the IGV and the impeller are increased obviously, which makes the pressure ratio and efficiency reduced dramatically. The further flow analysis show that, the enhanced separation area in the IGV and the depravation of the inlet flow condition of the impeller are the main sources of the losses, while there is almost no influence of the pre-swirl on the diffuser losses. Additionally, at a large negative pre-swirl there exist vortexes shedding from the separation region at impeller leading edge, which causes severe excited force on the impeller leading edge.

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