Core Flow Instability in Wide Vaneless Diffusers for the Purpose of Rotating Stall Investigation

A study of the core flow instability in the radial vaneless diffusers is performed for the purpose of better understanding of the rotating stall mechanism. In this paper the numerical results are compared with the experimental results. 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. Since the analysis is restricted to the core flow region, the influence of the wall boundary layers is neglected. Therefore, this model is only applicable to the wide vaneless radial diffusers. Using this numerical model a twodimensional rotating flow instability, similar to rotating stall, is found to exist. The numerical results are compared with the Particle Image Velocimetry (PIV) measurements performed in the vaneless diffuser behind a radial flow pump impeller. The measurements are performed at low mass flow rates, when the unsteady flow phenomena like rotating stall occur. In the PIV measurements similar flow structures are observed as obtained by the numerical model. In both cases, clockwise and counterclockwise rotating vortex structures are observed near the diffuser outlet. Comparison of these flow structures is made and discussed in this paper.