Comparison of a two-dimensional viscid and inviscid model for rotating stall analysis

A two-dimensional numerical analysis of the vaneless diffuser core flow, where the influence of the wall boundary layers is neglected, is performed to investigate the rotating stall instability. A commercial code with the standard incompressible viscous flow solver is applied to model the vaneless diffuser core flow in the plane parallel to the diffuser walls. At the diffuser inlet a rotating jet-wake velocity pattern is prescribed, and at the diffuser outlet a constant static pressure is assumed. With this model a two-dimensional rotating instability was obtained, which is associated with the rotating stall instability in wide vaneless diffusers. In this paper it is shown that the number of rotating cells is dependent of the diffuser geometry, and that the maximum number of rotating cells observed in the vaneless diffuser space can be estimated. This twodimensional numerical model is compared with the two-dimensional inviscid flow model of the vaneless diffuser rotating stall based on instability analysis. Similar results are obtained with both models for the critical flow angle, number of rotating cells and their propagation speed