Rotating Instability in a Centrifugal Pump Impeller

The objective of the study was the experimental and computational investigation of the unsteady flow in the centrifugal pumps. This paper analysed the effect of the vaneless stator interference on the exit flow field of a radial pump operated in the DIM facility. High-response pressure transducers were used to determine unsteady pressure field at three planes at the pump and at diffuser inflows. The experimental data showed that unsteady pressure disturbances modes change when the flow was reduced. Detailed analysis showed that disturbances occur at distinct frequencies and that these rotated in the circumferential direction. Comparison of the pressure signals measured at two circumferential locations on the casing confirmed the characteristic frequency pattern to be a so called "rotating instability". This unsteady phenomenon was highlighted both at design flow rate and at low flow rates. The azimuthal distributions exhibited significant nonuniformities. The amplitude of this non-uniformity was sensitive to the flow rate. A simple model showed that, contrary to the common belief, the transport of the vane wake and secondary flows across the rotor was not enough to explain the magnitude of the variations. In this paper numerical investigations of the unsteady three-dimensional flow through the pump stage were also presented. Turbulence was modelled both by the k-ω transport equations model, and Reynolds Stress Model based on the coequation. The effects of the tip leakage flow were considered by meshing the tip clearance between rotor blade and casing. Results showed the jet-wake flow pattern induced an unstable vortex, which influenced flow discharging from the adjacent passage and destabilised jet-wake flow in the passage. Both calculations and measurements detected the periodic fluctuations at impeller discharge which were found to be coherent from blade to blade and possessed a rich harmonic content.