端壁开缝改善轴流泵驼峰的机理

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

为了探索可有效抑制轴流泵特性曲线驼峰区的方法,该研究针对某轴流泵开展端壁沿轴向开缝的数值模拟研究,分析缝数目、缝长度和缝角度对轴流泵性能的影响规律,结合全通道非定常模拟揭示端壁开缝对轴流泵驼峰区的改善机理。研究结果表明,端壁开缝能够有效抑制轴流泵的驼峰现象,失速工况的扬程和效率分别提高了83.5%和8.13%。增加缝的数目和缝长可提高开缝抑制驼峰的能力,但缝过长会降低设计工况的效率,在一定范围内增加缝的径向倾角有利于驼峰区的改善,但不宜超过45°。在驼峰工况区,叶顶泄漏流呈旋涡状向叶轮进口方向发展,与来流共同作用堵塞叶顶通道,导致叶顶区域扬程突降。在叶片正背面压差作用下,缝内建立的喷射与抽吸的流动循环可使相对液流角在0.9倍相对叶高处以上部分明显降低,最大降幅62°,平均泄漏强度降低41.4%,叶顶中部附面层厚度降低18 mm,有效抑制由叶顶泄漏涡与主流相互作用造成的堵塞,并可削弱叶顶部位由叶顶泄漏涡等二次流诱发的压力脉动,是改善轴流泵驼峰区以及提升小流量工况效率的原因。端壁开缝具有改善轴流泵驼峰的巨大潜力。

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.