【作者】 刘莹；

【导师】 刘联胜；

【作者基本信息】 河北工业大学 ， 热能工程， 2018， 硕士

【摘要】 气泡的发生、膨胀、凝并和破碎,在生活中随处可见,也普遍存在于蒸发器、两相反应器、介质雾化器等涉及气液两相流动的工业设备中。在两相流体系中,气体通常以气泡形式分散到液体中,气泡的形成、发展及破碎过程是影响气液两相流的重要参数。如气泡雾化过程中,气泡的破碎有利于减小颗粒平均直径、强化液雾蒸发,而锅炉汽包内的气泡破碎可使蒸汽带水量提高,降低工作效率。因此对气泡破碎的相关研究,具有一定的实际意义和工程价值。本文首先利用实验方法,对自由空间内悬挂气泡的破碎过程及行为现象进行了探究,利用高速摄像对气泡破碎过程进行拍摄。自由空间内气泡破碎过程可分为排液、成孔、液膜卷曲、射流四个过程。排液过程所需时间定义为气泡自然破碎的“气泡寿命”,与液膜质量和气泡尺度有关;成孔过程,自由空间内气泡形成初始破裂点的位置随机出现,且顶部概率最大;液膜卷曲过程,曲面液膜整体呈现径向收缩现象,且随着气泡曲率半径的减小,收缩更为明显;射流过程,液膜收缩成一团,聚焦效应造成剩余液膜向初始破裂点方向喷射。气泡的破碎过程一个复杂的非线性不稳定发展过程,在不同的环境条件和液膜物性下表现出不同的规律,受溶液表面张力系数、粘度、气泡尺度、液膜厚度等多个因素影响。吹泡溶液表面张力系数越大,排液时间越短,液膜厚度越均匀性,破碎末期射流越明显;溶液粘度越大,越不易成泡,破碎卷曲过程中膜液滴数量越少,越不易形成射流;气泡初始直径越大,卷曲平均速率越小,形成的膜液滴数量越多,膜液滴空间分布越广;随着液膜厚度增加,边缘液膜卷曲持续时间变长,液线断裂形成的膜液滴和喷射液滴也越多。利用理论方法探究了气泡破碎前后的整体受力情况,气泡破碎过程中半封闭曲面液膜整体形变是受力情况发生变化的结果。基于质量守恒、动量守恒和能量守恒,研究了曲面液膜破碎过程各个阶段的边缘液膜卷曲特性。悬挂气泡排液过程导致的液膜厚度局部减薄,是造成气泡破碎初始阶段边缘液膜卷曲角速度增大的主要原因;在扰动波延迟阶段,边缘液膜堆积、质量增大导致卷曲角速度呈现逐渐降低的趋势;在膜液滴分离阶段,边缘液膜卷曲角速度基本维持不变,破碎液膜表面势能主要转化为膜液滴分离动能和边缘液膜切向/径向动能。本文研究成果丰富了对气泡破碎行为规律及机理的理解,对解决相关工程中实际问题具有一定指导意义。更多还原

【Abstract】 The occurrence,expansion,coagulation and fragmentation of bubbles can be seen everywhere in life,and they are also commonly found in industrial equipment involving gas-liquid two-phase flow such as evaporators,two-phase reactors,and medium atomizers.In a two-phase flow system,the gas is usually dispersed into the liquid in the form of bubbles.The formation,development and fragmentation of the bubbles are important parameters affecting the gas-liquid two-phase flow.For example,in the bubble atomization process,the breakage of the bubbles is beneficial to reduce the average diameter of the particles and enhance the evaporation of the liquid mist,and the bubble breakage in the boiler steam drum can increase the water volume of the steam and reduce the working efficiency.Therefore,the related research on bubble breaking has certain practical significance and engineering value.In this paper,the experimental method is used to explore the breaking process and behavioral phenomena of suspended bubbles in free space.The high-speed camera is used to capture the bubble breaking process.Bubble breakage in free space is divided into four processes: draining,forming holes,liquid film curling,and jetting.The time required for the draining process is defined as the "bubble life" of the natural collapse of the bubble,which is related to the mass and size of bubbles.In the hole forming process,the position of the initial break point of the bubble in the free space appears randomly,and the top probability is the largest.In the process of liquid film curled,the surface liquid film exhibits a radial shrinkage phenomenon,and the shrinkage is more obvious as the radius of curvature of the bubble decreases.During the jet process,the liquid film shrinks into a mass,and the focusing effect causes the remaining liquid film to be ejected toward the initial fracture point.The process of bubble breaking is a complex non-stable and unstable development process,which shows different laws under different environmental conditions and liquid film properties,and it is affected by many factors such as solution surface tension coefficient,viscosity,bubble size and liquid film thickness.The larger the surface tension coefficient of the bubble blowing solution is,the shorter the liquid discharge time is,the more uniform the liquid film thickness is,and the more obvious the jet flow at the end ofthe crushing is.the larger the viscosity of the solution is,the less likely the bubble is formed,and the smaller the number of film droplets during the crimping process is,the more difficult to form a jet.the larger the initial diameter of the bubble is,the smaller the average curl rate is,the more the number of film droplets formed,the wider the spatial distribution of the film droplets is.As the thickness of the liquid film increases,the edge liquid film curls for a longer period of time,and more droplets are formed by the liquid line breakage.The theoretical method is used to investigate the overall stress before and after the bubble is broken.During the bubble breaking process,the overall deformation of the semi-closed curved liquid film is the result of the change of the force.Based on mass conservation,momentum conservation and energy conservation,we study the edge liquid film curling characteristics of liquid film.The local thinning of the liquid film thickness caused by the discharge process of suspended bubbles is the main reason for the increase of the curling angular velocity of the liquid film in the initial stage of bubble breaking.In the stage of disturbance wave delay,the edge liquid film accumulation and mass increase lead to a gradual decrease of the curl angular velocity.In the membrane droplet separation stage,the edge liquid film curl angular velocity remains basically unchanged,and the surface potential energy of broken liquid film is mainly converted into separation kinetic energy of droplets and tangential/radial kinetic energy of edge liquid film.In this paper the research results not only can enrich the understanding of the law and mechanism of bubble breaking behavior,but also have certain guiding significance for solving practical problems in related projects.更多还原