Rotor-stator interactions, turbulence modeling and rotating stall in a centrifugal pump with diffuser vanes.

摘要

Particle Image Velocimetry (PIV) has been used to identify the unsteady flow structures and turbulence in a transparent centrifugal pump with a vaned diffuser. A 2K x 2K digital camera with an image-shifting feature has been used to record the images. From the measurements made at design conditions, the phase-averaged velocity fields, vorticity and turbulent parameters show that the entire flow field is dominated by wakes generated by impeller blades, diffuser vanes and unsteady separation phenomena. The boundary layer structure in the diffuser and the associated turbulence are strongly affected by the unsteadiness generated by the impeller. The cyclic variations are higher than the turbulent fluctuations both within the impeller and diffuser, but decrease below the turbulence level with increasing distance downstream of the diffuser vane trailing edge.; The passage-averaged approach and Large Eddy Simulation (LES) have been addressed for turbomachine flows. In passage averaging, unsteady effects caused by neighboring rows are averaged out over all blade orientations, but are accounted for through modeled “deterministic” stresses. The PIV data are used to calculate these stresses from the difference between the phase-averaged and passage-averaged data. The deterministic stresses are larger than the Reynolds stresses in regions close to the interface between blade rows and relatively lower in regions located far from the transition region. In the case of LES, the measured subgrid stresses determined by spatially filtering the data are compared to eddy-viscosity models and show significant discrepancies, especially in regions with separating shear layers. Backscatter of energy has also been observed.; Velocity and pressure measurements are also used to investigate the occurrence of a rotating stall in the pump. For our geometry, the measurements indicate a rotating stall that repeats itself in a vane passage at 0.93 Hz. The stalling is confined near the exit and propagates with a varying speed from one passage to another. There is substantial leakage between the impeller and the stalled passages of the diffuser. A high-speed jet and flow reversal that occurs periodically in the diffuser vane passage is observed. Also, the varying propagation speed of rotating stall indicates the influence of the outlet boundary conditions.