Abstract: Based on an established three-dimensional closed-loop model including impeller, guide vane, volute, valve, surge tank and pipeline，the numerical simulation for the start-up of mixed flow pumps was carried out. The generation reasons for form of reverse flow were analysed. 
The disributions of the axial length, axial and circumferential velocities of reverse flow, the variations of reverse flow rates and reverse flow areas at different axial positions were studied. The results show that the numerical computation results are reliable according to the comparison between test and numerical results. The separation flow of suction surfaces and leakage flow of tip clearances are mainly as the reasons for reverse flows. The each reverse parameter increases first and then decreases with time, the axial velocities of main flow and reverse flow present symmetrical distribution in the early stage of the start-up. After 0.8 s, the axial velocities of the reverse flow appear asymmetric distribution. Additional, the farther away from the impeller inlet, the more obvious the asymmetric distribution is. The axial velocity distribution of the main flow presents different changing tendency at different sections. The circumferential velocity gradually presents asymmetric distribution and the velocity direction at the circular section of the inlet pipe gradually deviates from the tangential direction of the circumference. The tendency of reverse flow rate and area at each axial section are basically consistent, and each section gradually presents a maximum return area. 

Key words: mixed-flow pump, start-up, closed-loop, transient characteristic, inlet reverse flow

摘要： 通过建立包括叶轮、导叶、蜗壳、阀门、稳压罐及管路在内的全三维闭合回路模型对混流泵启动过程进行数值模拟，分析了启动过程进口回流的形态及形成原因，研究了回流的轴向长度、轴向速度和圆周速度的分布以及轴向截面回流量、回流面积的变化。研究结果表明，数值模拟与试验结果基本一致，数值计算方法较为可靠。叶片吸力面的流动分离和叶顶间隙泄漏是造成进口回流的主要因素。启动过程中的回流参数均随时间呈现先增大后减小的趋势，主流与回流轴向速度在启动初期呈现对称分布，0.8 s后的回流轴向速度出现非对称分布且离叶轮进口越远非对称分布越明显；受回流排挤，主流轴向速度分布在不同截面呈现不同变化趋势，圆周速度逐渐呈非对称分布，进口管圆截面处平面速度方向逐渐偏离圆周切向，各轴向截面处回流量、回流面积的变化趋势基本一致，每个截面相继出现一个回流面积极大值。

关键词: 混流泵, 启动, 闭合回路, 瞬态特性, 进口回流