双气泡聚并的流体力学行为研究
发布时间：2018-12-07 09:42
【摘要】：气泡之间的聚并行为能够显著改变流场结构和气-液两相界面,进而深刻影响整个体系的特性,本文结合实验研究和数值模拟方法,对高黏体系中共轴线上升的低雷诺数双气泡接近及聚并的全过程进行了系统的研究,并重点对聚并过程中所形成的连接型气泡的动力学行为及其对聚并的影响进行了深入分析。在实验方面,本文主要采用高速摄影技术对六种实验体系(三种不同温度的纯甘油溶液、一种95%的甘油-水溶液及两种加入了表面活性剂CDEA的甘油溶液)中当量直径Dc为6～10.5 mm的气泡所形成的共轴线自由上升的双气泡体系的流体力学行为进行了研究,液相莫顿数Mo为1.69～661.83,气泡雷诺数Re范围是0.190～2.998,涵盖了气泡生成、上升、碰撞、连接、液膜破碎实现聚并的全过程。研究中将连接聚并过程划分为接触阶段、连接阶段、排液阶段三个阶段,观测双气泡在多种黏度和表面张力系数的粘性流体体系中的聚并行为以及聚并后的运动状况,进一步分析双气泡上升接触过程中的速度和形变规律以及气泡之间的相互作用规律,并对聚并条件和聚并时间进行讨论。此外,对连接型气泡的形成、形态、动力学行为也进行了系统的研究。结果表明：连接型气泡在排液阶段以恒定外形做匀速上升运动直到发生聚并或滑移,不仅与单气泡具有类似的运动特征且能够用以描述聚并瞬间的气泡状态。同时,分析得到了单气泡和连接型气泡各自最适合的曳力系数模型和上升速度模型,并用以计算连接型气泡所受曳力和气泡间的相互作用力。连接时间主要依赖于实验体系的黏度,而排液时间则呈现出较强的随机性。随着体系黏度的降低或表面活性剂的逐渐加入气泡聚并愈发困难而是倾向于滑移。在模拟方面,本文使用最小网格尺寸为50μm的结构化网格,采用VOF模型对等径双气泡的直接聚并过程进行了二维瞬态模拟,获得双气泡的流场结构和液膜厚度随时间的变化规律。结果表明,本文的模拟结果无论是双气泡的形态还是聚并各阶段的气泡动力学行为均与实验拍摄记录相符。对两气泡相互靠近过程中气泡四周的特别是两气泡间的流场进行了深入分析,此过程中,气泡两侧由四个小漩涡造成的低速区域的范围逐渐缩小,并与外围整体循环流动融为一体；同时,气泡周围的液相速度逐渐增大且分布趋于均匀,在中心轴线处存在最大值,并沿水平方向向两侧延伸下降至漩涡中心低速区,之后又在漩涡外侧的整体循环区域得到发展。两气泡接触后的液膜厚度随时间的推移呈现出先迅速变薄后缓慢变薄直至破碎的规律。
[Abstract]:The coalescence behavior between bubbles can significantly change the structure of the flow field and the gas-liquid interface, thus profoundly affecting the characteristics of the whole system. In this paper, the whole process of approaching and coalescence of low Reynolds number double bubbles in high viscosity system with coaxial rise is systematically studied, and the dynamic behavior of the connected bubbles formed in the process of coalescence and its influence on coalescence are analyzed in detail. In the aspect of experiment, this paper mainly uses high-speed photography technology to study six experimental systems (three kinds of pure glycerine solution at different temperature), A 95% glycerol-aqueous solution and two glycerol solutions with surfactant CDEA) were used to study the hydrodynamic behavior of a two-bubble system with a free-rising coaxial line formed by a bubble with an equivalent diameter of 610.5 mm. The liquid Morton number (Mo) is 1.69 ~ 661.83, and the bubble Reynolds number (Re) is 0.190 ~ (2.998), which covers the whole process of bubble formation, rising, collision, connection and film fragmentation. In the study, the coalescence process was divided into three stages: contact stage, connection stage and liquid discharge stage. The coalescence behavior and motion of the two bubbles in viscous fluid systems with various viscosity and surface tension coefficients were observed. The law of velocity and deformation and the interaction between bubbles in the process of double bubble ascending contact are analyzed, and the coalescence conditions and coalescence time are discussed. In addition, the formation, morphology and kinetic behavior of the connected bubbles are also systematically studied. The results show that the contiguous bubbles rise at a constant velocity in the discharge stage until the coalescence or slip occurs, which is not only similar to that of the single bubble, but also can be used to describe the bubble state at the moment of coalescence. At the same time, the drag coefficient model and the rising velocity model of the single bubble and the connected bubble are obtained, and the drag force and the interaction force between the bubbles are calculated. The connection time mainly depends on the viscosity of the experimental system, while the drainage time shows a strong randomness. With the decrease of the viscosity of the system or the gradual addition of surfactant, it is more difficult to agglomerate but tend to slip. In the aspect of simulation, a structured grid with the minimum mesh size of 50 渭 m is used in this paper, and the VOF model is used to simulate the direct coalescence of two bubbles with equal diameter. The flow field structure and the thickness of liquid film of the double bubble are obtained. The results show that both the morphology of the double bubbles and the dynamic behavior of the bubbles in each phase of coalescence are in agreement with the experimental records. The flow field around the bubble, especially between the two bubbles, during the process of two bubbles approaching each other is deeply analyzed. In this process, the range of the low velocity region caused by the four small swirls on both sides of the bubble is gradually reduced. And integrated with the peripheral circulation flow; At the same time, the velocity of liquid phase around the bubble increases gradually and tends to be uniform. There is a maximum at the center axis, and the velocity decreases along the horizontal direction to the low velocity region of the vortex center. Then the whole circulation area outside the vortex is developed. The thickness of liquid film after contact with two bubbles shows the law of thinning quickly and then thinning slowly and then breaking with time.
【学位授予单位】：北京化工大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TQ021.1
本文编号：2367000
[Abstract]:The coalescence behavior between bubbles can significantly change the structure of the flow field and the gas-liquid interface, thus profoundly affecting the characteristics of the whole system. In this paper, the whole process of approaching and coalescence of low Reynolds number double bubbles in high viscosity system with coaxial rise is systematically studied, and the dynamic behavior of the connected bubbles formed in the process of coalescence and its influence on coalescence are analyzed in detail. In the aspect of experiment, this paper mainly uses high-speed photography technology to study six experimental systems (three kinds of pure glycerine solution at different temperature), A 95% glycerol-aqueous solution and two glycerol solutions with surfactant CDEA) were used to study the hydrodynamic behavior of a two-bubble system with a free-rising coaxial line formed by a bubble with an equivalent diameter of 610.5 mm. The liquid Morton number (Mo) is 1.69 ~ 661.83, and the bubble Reynolds number (Re) is 0.190 ~ (2.998), which covers the whole process of bubble formation, rising, collision, connection and film fragmentation. In the study, the coalescence process was divided into three stages: contact stage, connection stage and liquid discharge stage. The coalescence behavior and motion of the two bubbles in viscous fluid systems with various viscosity and surface tension coefficients were observed. The law of velocity and deformation and the interaction between bubbles in the process of double bubble ascending contact are analyzed, and the coalescence conditions and coalescence time are discussed. In addition, the formation, morphology and kinetic behavior of the connected bubbles are also systematically studied. The results show that the contiguous bubbles rise at a constant velocity in the discharge stage until the coalescence or slip occurs, which is not only similar to that of the single bubble, but also can be used to describe the bubble state at the moment of coalescence. At the same time, the drag coefficient model and the rising velocity model of the single bubble and the connected bubble are obtained, and the drag force and the interaction force between the bubbles are calculated. The connection time mainly depends on the viscosity of the experimental system, while the drainage time shows a strong randomness. With the decrease of the viscosity of the system or the gradual addition of surfactant, it is more difficult to agglomerate but tend to slip. In the aspect of simulation, a structured grid with the minimum mesh size of 50 渭 m is used in this paper, and the VOF model is used to simulate the direct coalescence of two bubbles with equal diameter. The flow field structure and the thickness of liquid film of the double bubble are obtained. The results show that both the morphology of the double bubbles and the dynamic behavior of the bubbles in each phase of coalescence are in agreement with the experimental records. The flow field around the bubble, especially between the two bubbles, during the process of two bubbles approaching each other is deeply analyzed. In this process, the range of the low velocity region caused by the four small swirls on both sides of the bubble is gradually reduced. And integrated with the peripheral circulation flow; At the same time, the velocity of liquid phase around the bubble increases gradually and tends to be uniform. There is a maximum at the center axis, and the velocity decreases along the horizontal direction to the low velocity region of the vortex center. Then the whole circulation area outside the vortex is developed. The thickness of liquid film after contact with two bubbles shows the law of thinning quickly and then thinning slowly and then breaking with time.
【学位授予单位】：北京化工大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TQ021.1
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