Investigation of flow instability characteristics in a low specific speed centrifugal pump using a modified partially averaged Navier–Stokes model

Abstract

In this study, a modified partially averaged Navier–Stokes (MPANS) model is applied to investigate the flow instability characteristics in a low specific centrifugal pump. In MPANS model, the unresolved-to-total ratio of kinetic energy fk is determined according to the local grid size and turbulence length scale. The numerical results by MPANS model are compared with that simulated by SST k-ω model and the available experimental data. It is noted that MPANS model shows better performance for investigating the unstable flow in the current pump under part-load operation conditions. The time-averaged internal flow and flow incidence in the pump impeller depicts that with the decreasing flow coefficient, flow separation develops in the impeller. Owing to the strong separation flow as well as vortex evolution, incidence angle is large and varies remarkably at the entrance of blade-to-blade passage in the pump impeller. The evolution dynamics of rotating stall is further discussed in detail based on vorticity transport equation. During the evolution of rotating stall, the vortex stretching term has an important effect on vorticity transport under the part-load conditions. The analysis of the pressure fluctuations excited by periodic evolution of rotating stall shows that the rotating stall cell propagates along the rotational direction, and identifies the rotating stall frequency ( fstall), which is much lower than the rotational frequency of the impeller, fn ( fstall = 8.76% fn). Finally, two-dimensional Lagrangian coherent structure (LCS) is used to reveal the separation flow in blade-to-blade passages of the pump by monitoring the trajectory of the particles. Both LCS and vortex structure by λ2 can clearly demonstrates the passage blockage and flow separation under the part-load operation conditions, depicting that the separation flow occurs at blade suction side and develops from the leading edge to the main passage in the impeller.