微气泡对液相湍流和摩擦阻力调制机理的研究

Due to the lack of an accurate understanding on mechanisms of the liquid turbulence and frictional drag modulation by microbubbles, the efficient applications of the bubbly flow in industrial fields are significantly limited. And thus, it is very important and meaningful to perform the deep and systematic studies on both of them. For the present study, based on the real and emulational bubbly flow as the study object, the influences of physical parameters and move characteristics of bubbles on rheological properties of liquid will be investigated detailedly by the high-speed camera and viscometry, and the dependence of the liquid viscosity on them will be described quantitatively. By use of the high-speed camera and PIV, the bubble path and dynamic characteristics of collision and coalescence will be observed and analyzed in detail, and the influences of the bubble physical parameters, the gas-liquid interface purity and the bubble dynamic characteristics on the liquid turbulence and the frictional drag will be investigated. On the basis of experimental studies, the influence of the bubble physical properties on the bubble wake flow and the bubble injection influencing on the liquid turbulence and frictional drag will be systemically simulated under different conditions considering the effective viscosity change,buoyancy and inertia effect with DNS. On the basis of the above-mentioned experimental and numerical investigations, the mechanisms of the liquid turbulence modulation and drag reduction by microbubbles will be well understood, the hydrodynamic phenomena of the bubbly flow can be well predicted, and an evaluation criterion on the liquid turbulence modulation by bubbles will be developed. Thus，the theoretical system of the gas-liquid two-phase flow will be further added. And a theoretical principle for the industrial applications of drag reduction and energy saving by bubbles will be presented.

对微气泡引起的液相湍流调制和摩擦阻力改变理解的局限性严重限制了泡状流在工程实践中的高效应用，故对其开展深入细致的研究具有重要意义。以真实和仿真泡状流作为研究对象，利用粘度仪和高速相机相结合的方法研究气泡特征参数和运动特征对液相有效粘度的影响,并建立粘度对不同参数的依赖关系；利用高速相机和粒子成像测速仪详细研究气泡轨迹、气泡碰撞和聚合等运动特征，并详细调查气泡特征参数、界面洁净程度以及气泡运动行为对液相湍流结构和摩擦阻力的影响。结合实验研究，利用直接数值模拟方法详细研究气泡自身物理特性对气泡尾流的影响，以及气泡不同行为和不同工况下分别考虑有效粘度、浮力和惯性等效应,气泡加入对液相湍流结构和湍流摩擦阻力的影响，准确理解和预测气泡注入对液相湍流结构和摩擦阻力的影响规律，并建立液相湍流调制和气泡减阻评判准则。进一步补充和完善气液两相流水动力学理论体系和为指导气液两相流高效的工业应用提供理论基础。

与其它添加剂减阻节能技术相比，微气泡减阻技术具有减阻率高、不带来二次污染的优点，所以深入研究微气泡减阻技术对其向工程实践中的推广应用具有重要意义。首先，利用实验测试和数值模拟（直接数值模拟和混合相模型）方法，详细研究微气泡对液相湍流摩擦阻力和湍流结构的影响，研究了气泡直径、体积分数、液相雷诺数、重力水平、液相黏度变化、气泡聚并等对液相摩擦阻力和湍流结构的影响。研究发现，微气泡减阻率可高达80%，减阻发生与微气泡注入对液相湍流调制和液相黏度改变直接相关。借助相间作用力，微气泡抑制了涡结构的衰减、限制了高低速区流体的动量传递，减小了壁面附近湍流猝发事件的强度和频率，导致液相速度脉动强度和雷诺应力减小、平均速度提高，引发了减阻的发生。微气泡减阻同时取决于气泡直径、体积分数、重力方向和液相雷诺数等，是一个多影响因素、复杂的物理现象。上述物理因素存在最佳匹配关系，仅在最佳匹配条件下，才能获得高减阻率。其次，利用VOF方法和流变仪分析了气泡注入对液相表观黏度的影响，研究了体积分数、毛细数及气泡聚并等对液相表观黏度的影响。研究表明，毛细数较低时，气泡近似为球形、液相的表观黏度增大；毛细数较高时，气泡变形严重、液相的表观黏度减小；相同毛细数下，体积分数越大，对液相表观黏度的影响越大。低毛细数下，气泡合并过程会引起相对黏度的增加；高毛细数下，会引起相对的黏度减小；当气液相黏度比小于1时，气泡破碎使相对黏度减小。再次，研究了气泡界面污染对气泡尾流的影响，气泡在幂律流体中的运动行为，以及聚合物和表面活性剂的联合减阻。研究表明，气泡界面污染越轻，尾涡面积越小、阻力系数越小；流体剪切稀化越显著，气泡变形越剧烈、尾涡结构越复杂，气泡终端速越大；聚合物和表面活性剂同时加入提高了减阻率、扩大了减阻有效雷诺数范围。本研究分析了微气泡减阻规律、揭示了减阻机理，丰富了气液两相流相关理论体系，对工程实践应用具有指导意义。