Comparison of rotating stall characteristics between the viscid and inviscid two-dimensional model

To study the rotating stall phenomenon, a two-dimensional numerical analysis of the vaneless diffuser core flow is performed, where the influence of the wall boundary layers is neglected. Therefore, a commercial code with standard incompressible viscous flow solver is applied to model the vaneless diffuser core flow in the plane parallel to the diffuser walls. By assuming the rotating jet-wake velocity pattern at the diffuser inlet, and constant static pressure at the diffuser outlet, a two-dimensional rotating instability was obtained, which is associated with the rotating stall phenomenon in wide vaneless radial diffusers. In this paper, results of the two-dimensional numerical model are compared with those of the two-dimensional inviscid flow analysis, which is performed to study the vaneless diffuser core flow instability. Similar relations are obtained with both models for the critical flow angle, number of rotating cells and their propagation speed versus the diffuser radius ratio. Besides comparing the rotating stall characteristics, also the velocity and pressure fluctuation fields were compared. Again, good agreement is found between the two models.