In this work the numerical analysis of a small flow coefficient stage vaneless diffuser is described. The main purpose of this analysis is to go inside the rotating stall physics with particular attention to the onset of the rotating cells and their properties. The diffuser aspect ratio (width divided radial extension) is low (0.03) and the effect of the wall (hub and shroud boundary layers) cannot be neglected. For this reason the model is a 3D model including the hub, the shroud walls and the downstream 180° cross- over bend. The numerical analysis has been carried out using an incompressible, viscous, unsteady commercial solver. Radial and tangential typical velocity profiles as produced by a hypotetical upstream impeller have been applied as boundary conditions and they have been changed in time to simulate the actual throttling valve closure as it is done during a test. Under these circumstances a rotating phenomenon is found to exist similar to what is found experimentally on similar geometries. The instability starts as a complex 3D flow involving both hub-to-shroud flow pulsations as well as radial non-uniformities and it stabilizes in the form of four rotating cells.
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