Centrifugal compressors with vaneless diffusers have been widely utilised in turbochargers for internal combustion engines, particularly in the automotive industry, due to their compact size and ability to deliver high pressure ratios over a wide flow range. Methods to improve compressor efficiency near the surge line without compromising surge margin has received considerable interest. Changes to the impeller blade design and/or diffuser are typically limited in order to maintain performance around the engine rated power operating condition. Such competing demands have encouraged designers to develop variable geometry compressors with the aim of improving performance near surge without sacrificing rated power performance. The on/off type vaned diffuser is a variable geometry device capable of delivering a pressure ratio and efficiency benefit near surge by utilizing a vaned diffuser rather than a vaneless diffuser. Diffuser vane shaped clearances in the hub or the shroud end wall, termed side clearances, enable axial sliding variability of the vanes into and out of the diffuser passage. It is not known how side clearance geometry and position affect compressor performance i.e., pressure ratio, efficiency, and stability. Hence, a numerical and experimental investigation was conducted to determine the impact of clearance geometry and end wall position on compressor performance. It was found that side clearance positioned at the shroud end wall reduced the compressor surge margin for both clearance geometries considered. In contrast to this, a clearance positioned at the hub end wall improved the compressor surge margin but achieved the lowest efficiency benefit.

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