Proceedings of ISROMAC2006: The Eleventh International Symposium on Transport Phenomena and Dynamics of Rotating Machinery February 26-March 2, 2006, Honolulu, Hawaii, USA ISROMAC2006 EXPERIMENTAL AND COMPUTATION INVESTIGATION OF THE ROTATING INSTABILITY IN A CENTRIFUGAL PUMP IMPELLER Giorgio Pavesi Department of Mechanical Engineering - University of Padova Via Venezia, 1, 35131 Padova – Italy Tel. +39 049 8276768 Fax. +39 049 8276785 giorgio.pavesi@unipd.it Guido Ardizzon Department of Mechanical Engineering - University of Padova Via Venezia, 1, 35131 Padova – Italy Tel. +39 049 8276763 Fax. +39 049 8276785 guido.ardizzon@unipd.it Giovanna Cavazzini Department of Mechanical Engineering - University of Padova Via Venezia, 1, 35131 Padova – Italy Tel. +39 049 8276700 Fax. +39 049 8276785 giovanna.cavazzin@unipd.it ABSTRACT The paper reports on experimental and numerical investigations aimed at understanding the mechanisms of rotating instabilities in a centrifugal pump. The phenomena of rotating instabilities in the impeller were first identified with an experimental study. Afterward an unsteady numerical analysis was applied to confirm the phenomena and to detail the mechanisms behind them. The experimental study was conducted with high-response pressure measurements at three planes, at the pump, and at diffuser inflows. The numerical investigation, for the entire pump, was performed with an unsteady 3-D Navier-Stokes method. Turbulence was modelled both by the k-ω transport equations model and Reynolds Stress Model. The effects of the tip leakage flow were considered by meshing the tip clearance between rotor blade and casing The current study reveals that a vortex structure forms near the leading edge plane. The formation and movement of this vortex seem to be the main causes of unsteadiness when rotating instability develops. This unsteady phenomenon was highlighted both at design flow rate and at low flow rates. The azimuthal distributions exhibited no significant uniformities and the amplitude of this non-uniformity was sensitive to the flow rate. INTRODUCTION During the last decades several research groups were involved in the aero investigation of unsteady flows in single and multi-stage turbomachine. The unsteadiness generated by stator-rotor interactions were often classified into potential inviscid interactions, viscous effects and shock wave passing. Different approaches were used to investigate rotating instability such as the wall boundary layer approach and the two-dimensional diffuser flow approach where the wall boundary layers were excluded from the analysis. Several theories that explain the vaneless diffuser rotating instability mechanism were presented, but also two or maybe more different flow mechanisms were assumed to exist that could cause the rotating instability. Mostly one mechanism was associated with the core flow or the jet-wake instability occurring when the critical flow angle was reached and the other was associated with the three-dimensional wall boundary layer instability. Jansen [1] found that unsteady flows would occur when a three-dimensional flow separation exists and that the location of flow separation depends on the flow angle, the inlet Reynolds number, the Mach number and the diffuser geometry. According to Frigne and Van Den Braembussche [2] a transient perturbation of the static pressure distribution at the diffuser outlet induce the rotating flow oscillation if the absolute inlet flow angle reaches a critical value and the periodicity of the perturbation corresponds to the experimentally observed value. Dou and Mizuki, S [3] found for wide vaneless diffusers that when reverse flow zone in the wall boundary layer appeared at the rear part of the diffuser no rotating stall was generated and that rotating stall was only generated when the reverse flow extended close to the entry region. Besides the analytical analyses a lot of experimental techniques were used to measure rotating stall within the vaneless radial diffusers. Measurements of the rotor unsteady pressure field in short duration facilities were carried out by several researchers, e.g. Guenette et al. [4], Rao et al. [5], Moss et al. [6], Hilditch et al. [7], Dénos et al. [8]. The details of wake transport across the rotor could be tracked in continuously running facilities thanks to Laser Two-Focus (Kost et al., [9]) and stereoscopic Particle 1 Copyright © 2006 by ASME