Centrifugal Compressors Diffuser Rotating Stall: Vaneless Vs. Vaned

Diffuser rotating stall (in both cases of a vaneless or a vaned configuration) is still one of the open questions which has never been fully understood because of the complexity of the phenomenon and the experimental difficulties to get reliable measurements in such a complex environment. Under this perspective, Computational Fluid Dynamics (CFD) is an interesting tool to "see" the flow and to provide a basic understanding of the associated physics. Several published work have shown that a simplified model of the diffuser without the upstream impeller and the downstream return channel with, some realistic boundary conditions entering the diffuser, can provide a qualitative analysis of the stall onset. Thanks to the simplified model it has been possible to change progressively the boundary conditions in such a way that the continuous reduction of flow in time, as it is the experimental procedure, has been fully simulated. Vaneless and vaned options have been compared in terms of loss coefficient and rotating stall onset characteristics.