Ported shroud is a cost effective casing treatment that can greatly improve stability of centrifugal compressors. It is widely used in turbochargers and other applications where compressors with wide flow range are required. This paper reviews the development of the ported shroud concept from its first conception in the 1980s to its current various configurations, explores the underline mechanisms that deliver the performance improvement. It is explained that by removing stagnant fluid from impeller inducer shroud end wall boundary-layer region and recirculating it to the impeller inlet, blade loading near the inducer shroud is increased with improved inlet suction. For transonic flow, ported shroud weakens the shock wave and reduces flow separation on the inducer suction surface. It is argued that the effectiveness of the ported shroud is a balance of blade loading and the flow loss inside the ported shroud cavity. The loss needs to be minimised if ported shroud is to be more effective. Blade loading may be increased by various methods such as using high inducer blade turning and using full bladed impellers. The blade loading can also be improved by removing flow swirl in ported shroud flow by vanes, or imposing negative swirl by vanes in ported shroud. Circumferential flow variation caused by volute housing can be taken into account by variable pitch vanes or by variable port position.

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