Flow Structures and Turbulence in the Rotor Passage of an Axial Waterjet Pump at Off-Design Conditions

摘要

Two-dimensional and stereoscopic particle image velocimetry (PIV) are used for investigating the effect of working conditions on the morphology of flow structures in the rotor passage tip region of an axial waterjet pump. The measurements are performed in an optically index matched facility that enables unobstructed access to the entire flow field from any desired orientation. Focusing on the evolution of tip leakage vortices, the PIV data are acquired in meridional planes. Datasets are obtained both above and below the best efficiency point (BEP). The results include distributions of instantaneous and phase averaged velocity, circumferential vorticity and turbulent kinetic energy. Both the topology of flow structures in the passage as well as the magnitudes of velocity, vorticity and turbulence change substantially as the flow rate is reduced from above to below BEP. With decreasing flow, the blade loading increases, causing an increase in tip leakage backflow, a shift in the tip leakage vortex (TLV) location further away from the blade, and earlier vortex breakdown. Consequently, in the same midchord plane, slightly above BEP, the TLV appears as a clearly defined compact structure located close to the blade. Conversely, at 70% of BEP flowrate, the backward leakage flow extends over the entire blade passage, and the TLV appears as a broad and fragmented ensemble of vortex filaments that occupy the entire tip region. Instantaneous realizations of the flow are highly variable, resulting in a high turbulence level, and disappearance of the distinct phase averaged vortex core. Other major contributors to turbulence involve substantial spatial non-uniformities in all three velocity components associated with the leakage flow and high blade loading.