非对称微通道内气泡破裂行为的研究

发布时间：2018-05-25 05:35

本文选题：非对称微通道 + 气泡　；参考：《天津大学》2015年硕士论文

【摘要】：近年来,微化工技术发展迅速,作为微化工技术的重要基础,气泡在微流控设备中的破裂行为受到了中外学者的广泛重视。本文采用实验和数值模拟的方法研究了气泡在非对称微通道内的破裂行为。采用高速摄像仪对不等宽T型分岔微通道中气泡的破裂和分配规律进行了研究。实验观测到完全阻塞破裂(POB),有空隙破裂(TB),不破裂(NB)三种流型。对于破裂的流型,由于微通道下游子气泡对分岔口处气泡破裂的反馈效应,两个子气泡分配比V2/V1随气液流量比的增大呈先增大后减小的非线性变化趋势。随着无量纲母气泡体积V0/w02h的增大,V2/V1也呈先增大后减小的变化趋势,并且存在一个临界无量纲母气泡体积(V0/w02h)c。在实验范围内,气液流量比对(V0/w02h)c的影响较大,而液相黏度对(V0/w02h)c的影响可以忽略。提出了子气泡分配比的预测式,预测值和实验结果吻合良好。气泡完全阻塞破裂过程可以分为两个阶段:挤压阶段和快速夹断阶段。挤压阶段又可分为快速挤压阶段和慢速挤压阶段。快速挤压阶段尚未发现普遍的变化规律。慢速挤压阶段无量纲气泡颈部最小宽度与时间存在指数关系:(1-wm/w0)∝t0.62。表观流速和液相黏度的增加可使颈部变细速率加快,而气泡初始长度对气泡颈部变化的影响可以忽略。在快速夹断阶段,无量纲气泡颈部最小宽度与剩余时间存在指数关系:wm/w0∝(T-t)0.32。实验研究了气泡在T型不等长微通道内的流型及分配规律。实验观察到完全阻塞破裂(POB)、有空隙破裂(TB)、不破裂三种流型(NB)。气泡是否破裂的转变规律可用无量纲初始气泡长度和Ca数的幂律关系表示。气泡破裂后的分配比随无量纲初始长度的增加而减小。不同初始长度气泡破裂后的分配行为,其机理不同。对于较短的初始气泡,子气泡分配比由局部界面变化和下游支通道阻力共同控制。对于较长的初始气泡,子气泡分配比主要由下游支通道阻力决定。采用流体体积法(Volume of Fluid,VOF)模拟了气泡在T型旁路微通道内的阻塞破裂过程。在研究范围内,液相平均来流速度较低时(um=0.062-0.20m·s-1),气泡在分岔口处接近对称破裂,表面张力和气泡初始长度的变化对子气泡长度比几乎没有影响。液相平均来流速度较高时(um=0.25-0.40 m·s-1),破裂后子气泡的不对称性随平均来流速度增大而增大,随表面张力和气泡初始长度的增大而减小。提出了一个子气泡长度比的预测模型,预测值和模拟结果吻合良好。
[Abstract]:In recent years, micro-chemical technology has developed rapidly. As an important foundation of micro-chemical technology, bubble fracture behavior in microfluidic equipment has been widely paid attention to by domestic and foreign scholars. In this paper, the fracture behavior of bubbles in asymmetric microchannels has been studied by means of experiments and numerical simulations. The rupture and distribution of bubbles in a T-type bifurcation microchannel with different widths were studied by using a high-speed camera. Three flow patterns have been observed experimentally, namely, complete blocking rupture (POB), void rupture (TB), and no rupture (NB). For the flow pattern of the rupture, because of the feedback effect of the wandering bubble on the bubble rupture at the bifurcation under the microchannel, the distribution ratio of the two sub-bubbles increases first and then decreases with the increase of the gas-liquid flow ratio. With the increase of the volume of dimensionless mother bubble V0/w02h, the volume of V _ 2 / V _ 1 increases first and then decreases, and there exists a critical dimensionless bubble volume of V _ 0 / W _ (02) h ~ (-1). In the experimental range, the effect of gas-liquid flow ratio on V _ 0 / w _ (02) h ~ (-1) is significant, while the effect of liquid viscosity on V _ (0 / W _ (02) h ~ (-1) can be neglected. The prediction formula of sub-bubble partition ratio is proposed, and the predicted value is in good agreement with the experimental results. The process of complete bubble blocking and rupture can be divided into two stages: extrusion stage and fast clamping stage. The extrusion stage can be divided into fast extrusion stage and slow extrusion stage. The general rule of change has not been found in the rapid extrusion stage. There is an exponential relationship between the minimum neck width of dimensionless bubble and time in slow extrusion stage. The increase of apparent velocity of flow and viscosity of liquid phase can accelerate the rate of neck thinning, while the effect of initial bubble length on the change of bubble neck can be neglected. At the stage of fast clamping, there is an exponential relationship between the minimum width of dimensionless bubble neck and the remaining time. The flow pattern and distribution of bubbles in T-shaped microchannels are experimentally studied. It has been observed that there are three types of flow patterns, I. e., complete blocking rupture (POB), void rupture (TBN), and no rupture (NBN). The transformation law of bubble rupture can be expressed by the power law of the dimensionless initial bubble length and Ca number. The distribution ratio decreases with the increase of dimensionless initial length. The distribution behavior of bubbles with different initial lengths is different. For the short initial bubble, the sub-bubble distribution ratio is controlled by the variation of the local interface and the downstream branch channel resistance. For long initial bubbles, the sub-bubble distribution ratio is mainly determined by the downstream branch channel resistance. The flow volume of Fluido VOF was used to simulate the blocking and rupture process of bubbles in T-shaped bypass microchannels. In the range of study, when the average flow velocity of liquid phase is low, the bubble is nearly symmetrically ruptured at the bifurcation, and the change of surface tension and initial bubble length have little effect on the ratio of sub-bubble length. When the average flow velocity of liquid phase is higher, the asymmetry of the sub-bubble increases with the increase of the average velocity of flow and decreases with the increase of the surface tension and the initial length of the bubble. A prediction model of sub-bubble length ratio is proposed. The predicted value is in good agreement with the simulation results.
【学位授予单位】：天津大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TQ021

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