Experimental and numerical studies were performed to investigate influences of the return channel flow on the surge margin of a multistage centrifugal compressor. Two return channels, which were named RCH-A and RCH-B, were designed and evaluated in a two-stage centrifugal compressor. The measured performance of the compressor suggested that the surge margin of this compressor was dominated by the operating limit of the second stage and that the surge margin of RCH-B was 5% larger than that of RCH-A. The outlet flow of RCH-A and RCH-B swirled in a counter-rotating direction near the shroud region, and the flow angle at the outlet of RCH-A was larger than that of RCH-B. CFD was conducted to investigate the internal flow in the return channel. The CFD results of both RCH-A and RCH-B showed that the flow separation occurred on the suction surface of the return vanes near the operating limit. This separation induced the velocity difference between the suction and pressure sides, and the swirl flow in the counter-rotating direction was generated by this velocity difference. The swirl flow in the counter-rotating direction increased the blade loading of the second stage impeller at the operating limit. It was considered that the blade loading of RCH-B was lower than that of RCH-A at the operating limit, because the swirl flow in the counter-rotating direction of RCH-B was weaker than that of RCH-A. Therefore, the surge margin of the second impeller with RCH-B seemed to be larger than that with RCH-A. It was conclude from the experimental and numerical results that the locally swirl flow in the counter-rotating direction at the outlet of the return channels near the shroud side influenced the surge margins of the downstream impeller and the multi-stage centrifugal compressor.

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