轴流泵叶顶泄漏涡云状空化脱落机理与不稳定性控制

Cavitation instability in axial flow pumps is one of the key problems which will be solved in the large-scale water diversion projects and defense equipments, and the investigation on the tip leakage vortex cavitation cloud shedding is the basic study to solve this problem. A full transparent axial flow pump model will be used in the present study with the combined methods of numerical simulation and experimental measurements including 3D-PIV, high-speed photography, pressure fluctuation and dynamic forces of impeller. The main study contents are as follows: 1) The cavitation forms of tip leakage vortex cavitation and shear layer cavitation as well as the unsteady characteristics will be analyzed. The three-dimensional structure and evolution of the tip leakage vortex cavitation cloud will be discussed, and the pressure field, vorticity field, shear stress, eddy dissipation,turbulent kinetic energy and TKE production rate of tip leakage vortex will be revealed, finally the relationship between cloud cavitation vortex shedding process and the unsteady characteristics of TLV will be clarified. 2) The cloud cavitation growth, development and collapse process will be discussed, and the unstable oscillations cloud cavitation characteristics of the pump system flow and pressure will be analysed. At the same time, the instability mechanism of TLV cavitation cloud shedding, induced blade load dump and unstable flow separation will be revealed. 3) The instability frequency of tip cloud cavitation will be calculated, and the cavitation instability control methods will be proposed and evaluated. The expected results in this project will provide theoretical foundation for the design and stable operation of the axial flow pumps.

轴流泵空化不稳定性问题是大型调水工程和国防装备急需解决的难题之一，研究叶顶泄漏涡云状空化脱落机理是解决该问题的基础研究。本项目以全透明轴流泵模型为研究对象，采用数值模拟、3D-PIV、高速摄影、压力脉动、叶轮动态力测量等实验手段开展研究。主要研究内容有：1）探明叶顶泄漏涡和剪切层空穴形态产生机理，分析叶顶泄漏涡空化云的三维结构及其时空演化规律，掌握叶顶泄漏涡的压力场、涡量场、剪切应力、涡耗散、湍动能及其生成速率等湍流特性，揭示云状空化的脱落过程与非定常涡结构的关联特性。2)基于可视化实验，分析云状空化的生长、发展与溃灭过程，阐明空化云诱导泵流量和压力的振荡特性,揭示不稳定空化云诱导叶片载荷突降、压力面流动分离及失稳机理。3)掌握叶顶泄漏涡云状空化的周期性脱落频率特性，绘制轴流泵空化不稳定性图谱，并提出不稳定空化云的主动控制方法。本项目预期成果，将为轴流泵机组的设计和稳定运行提供理论基础。

轴流泵在南水北调工程、大中型泵站中具有广泛的应用。由于轮缘与转轮室之间不可避免地存在叶顶间隙，在叶片压力面（PS）和吸力面（SS）的压差驱动下，叶顶间隙内将产生泄漏流（Tip leakage flow）,其与主流卷吸形成叶顶泄漏涡（Tip leakage vortex, TLV）。轴流泵叶顶空化不稳定性问题是大型调水工程和国防装备急需解决的难题之一，研究叶顶泄漏涡云状空化脱落机理是解决该问题的基础研究。本项目以水翼和轴流泵模型为研究对象，采用数值模拟、3D-PIV、高速摄影、压力脉动测量等实验手段开展研究。主要研究成果如下：.（1）选用不同水翼进行湍流模型和空化模型验算。针对水翼表面片状空化生长、空化云聚集、脱落和溃灭等瞬态过程，对比分析了实验数据与模拟结果，验证了湍流模型和空化模型的适用性。.（2）基于不同湍流模型，对轴流泵叶顶云状空化进行非定常数值模拟，结合3D-PIV实验研究，分析了叶顶泄漏涡和剪切层空化形态与生成机理，探究了叶顶泄漏涡空化云三维结构及其时空演化规律，掌握叶顶泄漏涡的压力场、涡量场、剪切应力、涡耗散、湍动能等湍流特性，进一步研究了云状空化尾缘脱落与附着片状空化的相干机制。.（3）基于高速摄影、压力脉动测量和非定常空化流数值模拟，观察了不同运行工况下叶顶泄漏涡空化云生成、发展及溃灭过程，分析了不稳定云状空化诱导的系统流量压力振荡特性。针对叶顶泄漏涡云状空化时空演化过程，分析了空化云脱落和溃灭周期，解释了不稳定空化云对主流和相邻叶片压力面流场扰动的物理过程，从而掌握云状空化及其诱导涡引起的堵塞流道、流动失稳机理。.（4）基于不同工况数值模拟和内流试验，提出了不同叶片数、不同叶顶形状和间隙大小等叶轮方案。分析了不同方案叶轮的云状空化不稳定振荡特性，根据叶顶泄漏涡及其诱导空化的强度、尺度及发展规律，评价了不同叶轮方案对不稳定叶顶泄漏涡云状空化的抑制效果。为轴流泵机组的设计和稳定运行提供理论基础。