Mechanism for end-wall slots to improve hump in an axial flow pump

Axial-flow pump is widely used in farmland irrigation, chemical industry, and water transfer engineering, due to its a high specific speed with a large flow rate and a low water head. A hump area normally appears on the flow-head curve in the presence of the backflow and vortex, when the axial-flow pump works at lower mass flow rates. The hump area can cause increased vibration and noise in the axial flow pump, which endangers the safe and stable operation of a pump unit. This study aims to find a feasible way that can effectively depress the hump in an axial-flow pump using systematic numerical simulations for the axial-flow pump with axial slots. An AYSYS Turbo Grid was used to build high-precision hexahedral structured O grids for the impeller and guide vane. H grids were selected for the inlet and outlet pipes using AYSYS ICEM CFD module. An ANSYS CFX software was used to solve the three-dimensional flow fields inside the axial-flow pump. An SST k-ω turbulence model was adopted to predict flow separation caused by reverse pressure gradients. The maximum error was less than 2.6% compared with the pump head in experimental and numerical data, verifying the reliability of numerical simulation. A parametric analysis was conducted to explore the effects of slot numbers, slot length, and slot angle on the pump performance. A mechanism was proposed to improve hump area for the optimal axial slots using unsteady simulations. The results show that the hump area of the axial-flow pump can be effectively suppressed by the axial slots. The pump head and efficiency increased by 83.5% and 8.13% in the hump area, respectively, while the pump efficiency reduced by 4.3% at the design condition. The ability of the axial slots in depressing hump enhanced, when increasing the slot numbers and the slot length. However, the efficiency at the design condition decreased significantly when the slots were too long. Moreover, the increase in the radial skewed angle of the slots was beneficial to the improvement of the hump area. In the pump without slots, there was no obvious vortex trajectory of tip leakage at lower mass flow rates, whereas, the blade tip area was covered by a large area of reversed flow regions where the tip leakage flow rolls up with the main flow, resulting in a tip leakage vortex. The tip leakage vortex moved towards upstream direction at lower mass flow rates, and thereby blocked the blade tip passage, leading to the increased flow losses and decreased of pump head near the blade tip. The flow recirculation of injection and suction was found to be established in the slots. Under the effect of flow recirculation, the relative flow angle above 0.9 times relative blade height was significantly reduced under the stall condition. The tip leakage vortex was controlled successfully by the slots, and the average leakage intensity was reduced by 41.4%. The mixing of the injection and the main flow caused the increased flow losses at the tip of the impeller at the design condition, resulting in the decrease of the pump efficiency. Further research can be needed to improve the pump efficiency at the design condition. At the stall condition, the tip flow losses were reduced because of the effect of the flow recirculation on the tip leakage vortex, and the pump efficiency increased. In addition, the pressure fluctuations induced by the tip leakage vortex near the blade tip was remarkably weakened by the slots. Consequently, the axial slots have a great potential to improve the hump area and the efficiency under the stall condition for the axial-flow pump.