混流泵内部流动不稳定特性的数值模拟

为了研究小流量工况下混流泵内存在的不稳定流动特性,基于大涡模拟亚格子尺度模型与滑移网格技术, 对包括进口管和出口弯管的混流泵全流场进行三维非定常湍流计算．外特性试验结果表明,在60％～85％最优工况范围内,扬程-流量特性曲线呈正斜率特性．数值模拟结果与试验结果误差控制在4％内,表明大涡模拟可以准确预估混流泵存在的扬程-流量正斜率不稳定特性．在此基础上,分析了混流泵产生正斜率不稳定特性的内流机理．分析结果表明,在小流量工况下叶轮入口切向速度呈明显的非对称性,叶轮与导叶流道内液流的失速效应使叶轮叶片表面和导叶叶片入口轮毂侧存在大尺度的旋涡结构．导叶流道内旋涡尺度较大,压力脉动沿导叶轴向呈明显的周期性波动,使旋涡区域从吸力面侧逐渐扩展到导叶流道,旋涡结构的涡核附着在压力脉动最小值的导叶吸力面中间叶高区,且涡核旋向与叶轮旋向相同．

Numerical simulation of unstable characteristics in head curve of mixed flow pump

The three dimensional unsteady flow field in a mixed flow pump was calculated by means of the large eddy simulation (LES), sub grid scale model and sliding mesh technique in order to analyze the instability in head curve and flow characteristics caused from the interaction between impeller and guide vane in the pump under low flow rate conditions. The experimental results showed the head capacity curve exhibits a positive slope at 60%Qd and 85%Qd operating conditions, and the error between predicted and experimental results is less than 4%, suggesting LES can predict the positive slope characteristic accurately. On this basis, the flow mechanism for the unstable characteristic was analyzed. The simulated results showed that the fluid tangential velocity is distributed non axisymme trically in the impeller inlet evidently. Because of the stall of flow in the impeller and guide vane passages, there is a large scale vortex structure on the blade surface and the hub surface at the leading ledge of guide vane. The scale of the vortices in the guide vane passages is larger than that in the impeller. It is apparent that the pressure fluctuates periodically along the axial direction, allowing the vortex zone extends into the middle of guide vane passage from the suction side of guide vane. The core of large scale vortex is attached to the suction side at the middle span of the guide vane where pressure fluctuation is minimal; moreover, the rotational direction of the core is the same as the impeller.