轴流泵尾缘垂直空化涡初生及其诱导流场失稳的机理研究

Vortex cavitation in axial flow pumps is one of the key problems which are not be well solved in the large-scale water diversion projects and defense equipments for a long time. A full transparent axial flow pump model in this project will be used in the present study with the combined methods of numerical simulation and experimental measurements including 3D-PIV, high-speed photography etc to investigate the blade trailing edge perpendicular cavitating vortex(PCV) and evolution. The main study contents are as followed: 1) The mechanism of interaction between perpendicular cavitating vortex and tip leakage vortex, as well as suction side sheet cavitation will be discussed, and turbulent shear stress, turbulent kinetic energy and its production rate, transport and dissipation in the tip region will be analyzed. Then, the PCV inception mechanism will be revealed. 2) The evolution of three-dimensional space-time characteristics of PCV inception, development and collapse will be analyzed. The PCV generation and development mechanisms which are related to the complex cavitation patterns in the tip region will be revealed. 3) Based on the high-speed photography, the PCV interference process with adjacent blade passage flow at different flow rate and cavitation number conditions will be observed. The blockage effects of the PCV cloud which may result in the head drop in the flow passages will be analyzed, and the mechanisms of PCV which induces cavitation instability will be revealed. Finally, the active control methods of PCV-induced cavitation instability will be put forward. The expected results in this project will provide theoretical foundation for the design and stable operation of the axial flow pumps.

轴流泵漩涡空化是大型调水工程和国防装备长期未能解决的难题之一，本项目以轴流泵模型为研究对象，采用大涡模拟、3D-PIV、高速摄影等实验手段，深入研究轴流泵叶片尾缘垂直空化涡（PCV）及其诱导流动失稳机理问题。主要研究内容有：1）阐明尾缘垂直空化涡与叶顶泄漏涡、吸力面片状空化的相干机制，掌握叶顶区剪切应力、湍动能及其生成速率、输运、扩散和耗散等湍流特性，揭示PCV的初生机理。2）分析尾缘垂直空化涡初生位置和漩涡矢量变化规律，掌握PCV产生、发展和溃灭的三维时空演化特性，揭示叶顶区多种空化形态诱导PCV初生和发展机理。3）基于高速摄影观察PCV在不同工况条件下与相邻叶片的干涉过程，掌握垂直空化涡堵塞流道、诱导叶片载荷突降的根本原因，揭示PCV发展和溃灭过程诱导流动失稳的机理，并提出PCV的空化不稳定性控制方法和策略。本项目预期成果，将为轴流泵机组设计和稳定运行提供理论基础。

轴流泵叶顶区泄漏流及多尺度漩涡结构是轴流泵内部复杂的漩涡流动现象，易产生复杂不稳定漩涡空化结构。本项目系统研究了叶顶泄漏涡复杂云状空化脱落诱导水力振荡机理，揭示了垂直空化涡（SSPCV）初生、发展、溃灭过程及其诱导流场失稳机理，并提出了抑制方法，主要研究结论如下：.1.通过试验研究发现，在小流量工况下轴流泵内流漩涡更易产生初生空化，空化数较低时，叶顶区形成三角形空化云，SSPCV自叶顶空化云尾部生成；大流量工况下，叶顶泄漏涡涡带为狭长的低压区；随着流量和空化数的减小，低压区范围扩大。SSPCV向相邻叶片压力面移动过程中与叶顶泄漏涡尾迹混合，其导致强烈的压力波动。SSPCV在迁移中更容易被相邻叶片切断，引起相邻流道内空化不稳定，通过数值和实验揭示了垂直空化涡的初生过程。同时，基于无量纲流量和空化数，确定了不同叶栅稠密度下叶顶空化形态特征图谱。.2.通过改进空化流数值模型，实现了轴流泵漩涡空化流准确数值模拟，揭示了垂直空化涡初生位置与漩涡矢量变化规律。研究结果表明，SSPCV的时空演化主要分为生成、脱落和耗散三个阶段；叶顶空化云尾部由主流、回射流和径向射流的共同作用产生断裂点，导致SSPCV周期性脱落，SSPCV处涡量主要由圆周方向与轴向涡量分量贡献；脱落的SSPCV在流道中迁移逐渐扭曲形成垂直于叶片形态，并与相邻叶片干涉，改变入口液流角，导致载荷突降。.3.提出了叶顶圆角、叶顶双曲孔结构和蜣螂仿生结构等抑制叶顶泄漏涡及其空化的几何结构。发现倒圆叶顶可消除叶顶角涡，斜切叶顶则降低了泄漏涡影响；双曲孔的方案对TLV低压区分布及空化有明显的抑制作用，同时翼型升力无明显下降。水翼端面布置仿生球形凹坑，涡强度降低且涡中心的负压值减小，有利于抑制叶顶泄漏涡空化。.本项目系统研究了轴流泵尾缘垂直空化涡初生及其诱导的流动不稳定性问题，并提出叶顶复杂空化不稳定性的控制方法，圆满完成了任务书目标与任务，为南水北调大型轴流泵机组的设计与稳定运行提供了坚实的理论基础和技术支撑。