Characteristics of tip leakage vortex cavitation in axial flow pump at small flow rate condition

The strong tip leakage vortex (TLV) in an axial flow pump at small flow rate condition will generate a cavitation vortex that would cause flow instability. In order to investigate tip leakage vortex (TLV) cavitation in axial flow pump in small flow rate, TJ04-ZL-02 hydraulic axial flow pump model was investigated based on the modified cavitation model and SST k-ω turbulence model. Numerical results showed that the separation vortex cavitation within the tip gap occurs around the blade pressure side corner, which was made up of a cavitation inception together with a tip leakage vortex cavitation. The gap region cavitation changes with the different blade chord ratio sections when the cavitation number is constant. With the increasing of the blade chord ratio, TLV cavitation region and bubble volume fraction increase gradually. With the decreasing of cavitation number, bubbles occur in the TLV entrainment region that emerged with the TLV cavitation vortex, and merged into bubble clouds. At small flow rate condition, owing to the large pressure difference between pressure side and suction side at the tip, the axial velocity of flow in the gap is negative, and the absolute value of axial velocity increases gradually from the pressure side to the suction side. High-speed photography experimental results show that as the cavitation number decreases gradually, leakage vortex cavitation initiates in the tip clearance at small flow rate condition, and then cavitation deteriorates severely and the bubble cloud breaks up in the trailing region of the blade, and gathers in the tip region and immensely occupies the blade passage region. While the cavitation number σ reaches 0.187~0.232, the suction side of blade is covered by bubbles, and bubbles separation and blasting occur due to the volume fraction increasing of the bubble cloud. The results from this study provide theoretical and experimental references to TLV cavitation in an axial pump.