Investigation on Cavitating Flow and Energy Performance of Mixed-Flow Pump with Varied Tip Clearances

The blade tip clearance in a semi-open mixed-flow pump induces tip leakage flow (TLF). The TLF is capable of inducing tip clearance cavitation and perturbing the primary flow field, which leads to the formation of a tip leakage vortex (TLV) and consequently adversely affects the pump performance. The influence of different tip clearances on the cavitation performance in the mixed-flow pump is systematically examined through a combination of numerical simulation and experimentation. The best efficiency point of the pump moves towards the low flow region as the tip clearance increases from 0 mm to 1.7 mm under non-cavitation conditions, with a corresponding reduction in head and efficiency of 35.56% and 22.98%, respectively. When the net positive suction head available (NPSHa) falls below the critical cavitation point, the vapor volume within the impeller passage increases rapidly. Under fully developed cavitation conditions, the vapor volume in the first-stage impeller of the closed pump (MT0) is 1.46 times that of the pump with a 1.7 mm tip clearance (MT1.7), indicating that TLF has a certain inhibitory effect on the development of cavitation bubbles in the impeller. Energy characteristic analysis shows that under non-cavitation conditions, the work capability in the blade leading edge (LE) region of the MT1.7 dropped by 50% compared to the MT0. After the NPSHa drops to a critical point, the work capability of the impeller LE region decreases sharply due to the influence of cavitation bubbles. Moreover, the reduction in the MT0 is 1.6 times that of the MT1.7, indicating that TLF can break up cavitation bubble clusters at the impeller inlet, thereby increasing the effective contact area between the impeller and the liquid, and enhancing pressurization capacity of the impeller. However, TLF can also induce TLV, leading to an increase in entropy production and vorticity within the impeller region, which causes additional energy dissipation and ultimately compromises the impeller's pressurization performance.