环形掺气坎下部水气两相流特性及其作用机理研究

The use of an annular aerator placing in the vertical shaft of flood discharge tunnel is a new engineering idea, and is different from the traditional ones. It not only has the aeration effect, but also can improve the flow pattern and increase the energy dissipation ratio. However, the hydraulic characteristics of water-air two-phase flow in the lower zone and the working mechanism of annular aerator have not been researched, which is a key to the popularization and application of this new type of technology. Based on the previous experimental researches, the hydraulic characteristics of water-air two-phase flow in the lower zone of the annular aerator, such as flow pattern, pressure, ventilation, aeration, cavity length and energy dissipation in two-phase flow, and etc., were studied by numerical simulation, model test and theoretical analysis. The mixing mechanism of water and air, mechanism of energy dissipation, fluctuation characteristics of the flow and reduction of cavitation erosion on the tunnel wall were reveled. The model scale effect of ventilation, aeration and cavity length was analyzed. On the basis of those studies the assumption of applying the annular aerator to large drop height water transfer project was posed, and the effect of energy dissipation, decompression and flow optimization were estimated from the point view of ecological environment and economic benefit. The project research results are beneficial to enrich air entrainment technology and can provid the theoretical basis for application of annular aerator in discharge tunnel with high water head and water transfer project in case of large drop height, and have great significance and value to the basic application research in academic.

泄洪洞竖井内的环形掺气坎是不同于传统掺气技术的新创意，其不仅具有掺气减蚀的作用，而且可改善水流流态并增加水流的消能率，但对其下部水气两相流特性及其作用机理等问题缺乏研究，这些问题是该类掺气技术得到推广应用需要解决的关键技术问题。在前期试验研究基础上，项目拟采用数值模拟、试验研究和理论分析相结合的方法，深入研究环形掺气坎下部水气两相流的流态、压力、掺气浓度、空腔长度、消能率等水力特性，揭示水气混掺机理、能量耗散机理、水流脉动特性和壁面掺气减蚀特性，分析模型通风量、掺气浓度、空腔长度、空腔负压等水力参数的缩尺效应；在此基础上提出将环形掺气坎应用至大落差调水工程竖井中的设想，并从工程的安全性和经济性角度评估其消能、减压、流态优化效果。本项目的研究有利于丰富掺气减蚀技术，可为环形掺气坎在高水头泄洪建筑物和大落差调水工程等工程中的应用提供理论依据，具有重要的科学意义与应用基础研究的价值。

泄洪洞竖井内的环形掺气坎是不同于传统掺气技术的新创意，但对其下部水气两相流特性及其作用机理等问题缺乏研究。项目基于公伯峡水平旋流洞竖井段的环形掺气体型，采用试验研究、数值模拟、理论分析相结合的方法，研究了环形掺气坎下部竖井段水气两相流特性，揭示了水气混掺机理、能量耗散机理、水流脉动特性，分析了模型水力参数的缩尺效应。结果表明，环形掺气坎孔口存在自由流、过渡流、淹没流的基本流态序列，相对堰上水头头H/D=1.76是流态由自由流向淹没流转变的临界条件。竖井段脉动压强由竖井边壁小尺度旋涡紊动和环形掺气坎孔口淹没射流而致的大尺度旋涡扰动而引起，脉动压强均方根介于7.6～27.6 kPa之间。环形通气孔通风由通气孔进出口的压差产生；在几何相似和重力相似的条件下，要保证原模型通风量相似，空腔内真空度比尺应等于几何比尺。设置环形掺气坎后竖井内水流的掺气效果明显，掺气由环形掺气坎孔口射流冲击竖井水体产生大尺度旋涡并挟带气泡所致；掺气浓度沿高程呈乘幂规律衰减。对比了原型观测、模型试验和数值模拟3种水力学研究方法对水平旋流消能技术的适宜性，3种方法对宏观水流流态、旋流洞壁面压强、消能率的研究结果基本一致；模型试验和数值模拟的环状空腔长度与原型观测存在不同程度的差异，进而影响竖井内其他参数的准确性；数值模拟和模型试验是研究旋流空腔形态和旋流洞内流速的可行方法，能有效克服原型量测技术的局限；1：40及以上比尺的模型试验是研究起旋室通风井通风特性的有效方法。环形掺气坎孔口射流冲击竖井水体后的强烈紊动和水气混掺产生了显著的消能效果，竖井段的能量损失约占总水头的17.7 %。提出了环形掺气坎的设计原则、设计方法及适用范围。此外，基于切向流速的组合涡分布规律，还对环形掺气坎下部水平段空腔旋流的流场特性进行了分析。项目的研究成果可为环形掺气坎在高水头泄洪建筑物等工程中的应用提供理论依据。