• Energy Research

Tip leakage vortex (TLV) in an axial flow pump was simulated by using the shear-stress transport (SST) k-ω turbulence model with a refined high-quality structured grid at different flow rate conditions. The TLV trajectories were obtained by using the swirling strength method corresponding to the cross-sections of streamlines of the TLV. High-speed photography experiments were conducted to observe the TLV trajectory based on cavitation tracing bubbles in an axial flow pump with a transparent casing. The TLV trajectories predicted by the SST k-ω turbulence model agreed well with the visualization results. The numerical and experimental results show that the starting point of the TLV is near the leading edge at part-load flow rate condition (Q/QBEP=0.85), and it moves towards the trailing edge to approximately 20% blade chord at the design flow rate condition (Q/QBEP=1.0). At large flow rate conditions (Q/QBEP=1.2), the starting point of the TLV shifts to about 40% blade chord, and the relative angle between the TLV trajectory and the blade chord is gradually reduced with the increased flow rate. Detailed statistics of the fluid dynamics of the end-wall shear layer and the TLV at design and off-design conditions were discussed based on the numerical results. The shear layer and jetting flow in the tip gap are highly affected by the pressure difference between the pressure side (PS) and suction side (SS). It was also found that the distributions of static pressure, turbulent kinetic energy (TKE) and vorticity inside the TLV core are associated with the TLV structure which is affected by blade loading and operation conditions of the axial flow pump.

The tip leakage vortex (TLV) cavitating flow in an axial flow pump was simulated based on an improved shear stress transport (SST) k-ω turbulence model and the homogeneous cavitation model. The generation and dynamics of the TLV cavitation throughout the blade cascades at different cavitation numbers were investigated by the numerical and experimental visualizations. The investigation results show that the corner vortex cavitation in the tip clearance is correlated with the reversed flow at the pressure side (PS) corner of blade, and TLV shear layer cavitation is caused by the interaction between the wall jet flow in the tip and the main flow in the impeller. The TLV cavitation patterns including TLV cavitation, tip corner vortex cavitation, shear layer cavitation, and blowing cavitation are merged into the unstable large-scale TLV cloud cavitation at critical cavitation conditions, which grows and collapses periodically near trailing edge (TE).