两相流中微尺度界面现象的研究

发布时间：2018-03-21 04:51

  本文选题：微流体　切入点：液滴　出处：《天津大学》2015年博士论文　论文类型：学位论文

【摘要】：多相流广泛存在于多种工业过程中,其通常伴随着界面现象与流体力学相互耦合。近年来,将多相流与微流控技术结合已逐渐成为一种前沿技术,具有广泛的应用前景。微通道内两相流是该领域的重要研究内容。本文利用高速摄像仪研究了微通道内液滴(气泡)的生成,破裂及聚并过程。具体研究内容包括:第一部分主要研究了磁场调控下聚焦型微通道内磁流体液滴的生成过程。重点关注了磁流体液滴在通道内的扩张及破裂动力学行为。考察了流体流速、磁场强度及磁场方向对扩张及破裂的影响。研究并比较了无磁场(NM)、径向磁场(RM)和轴向磁场(AM)下的液滴形成过程。结果发现,磁流体液滴的体积可通过所施加的磁场来进行有效调控。径向磁场和轴向磁场分别通过影响液滴的扩张阶段和破裂阶段来影响液滴的尺寸。磁流体破裂过程的颈部最小宽度与剩余时间呈幂指数关系。第二部分研究了磁场对磁流体液滴破裂的影响以及反馈作用对气泡破裂的影响。首先研究了匀强磁场及非匀强磁场下T型微通道分叉处磁流体液滴破裂动力学。研究发现在匀强磁场下,可以通过改变磁场强度来调控液滴破裂方式以及液滴破裂频率。在非匀强磁场下,通过改变磁场强度可以调控液滴破裂的不对称度从而使液滴的临界破裂点发生迁移。此外,还研究了气泡在T型微通道分叉处的不对称破裂行为。气泡不对称破裂行为主要归因于破裂所形成的子气泡在下游通道内动力学行为的反馈作用。考察了下游汇聚口处子气泡碰撞以及交替通过出口时的反馈作用对破裂的影响。结果表明,当流速较小且子气泡不发生碰撞时,反馈作用可以忽略。当子气泡在下游汇聚口出发生碰撞时反馈作用较为明显。此外,在高流速下通道瑕疵所带来的影响亦不可忽略。第三部分研究了卫星液滴(气泡)的生成过程。本章前半部分主要关注聚焦型和T型微通道内液-液界面收缩破裂过程中形成的卫星液滴。实验结果表明,在颈部夹断后期,由于粘性力和表面张力的平衡和强弱转换,液滴的颈部不可避免从快速夹断区进入慢速断裂区。研究发现在慢速夹断区颈部的体积几乎保持不变,最细颈部位置也由中间过渡到颈部细丝的两侧,最后在颈部细丝两侧发生断裂并形成卫星液滴。在实验范围内卫星液滴尺寸随连续相毛细数的增长而增长。其后研究了直通道内剪切力引起的液滴(气泡)尾部断裂过程。重点关注了液滴(气泡)尺寸及流速对尾部尖端流的影响。结果表明,随着液滴(气泡)的尺寸或毛细数的增长,其形变愈发显著。存在一个临界毛细数,当大于这一数值,液滴(气泡)尾部将发生破裂并产生尖端流。由尖端流所生成的卫星液滴的尺寸通常比主液滴小三个数量级。论文的最后部分研究了液滴(气泡)的聚并过程。首先研究了气泡在T型微通道汇聚处的聚并行为。在不同的毛细数及气泡尺寸下,实验观测到了三种主要行为:碰撞式聚并、挤压式聚并及不聚并。实验结果表明,中等粘度下不论是碰撞式聚并还是挤压式聚并,其聚并效率都随两相表观流速的增加而降低。两相表观流速的增加将致使碰撞式聚并过渡到挤压式聚并,同时聚并效率小幅增加。此外,连续相粘度的增加或者气泡尺寸的减小都会降低聚并效率。此外,研究了磁流体液滴在不同磁场强度下的聚并过程。两个同轴的磁流体液滴在磁场作用下相互吸引并变形为圆锥体。实验重点关注了两个圆椎体液滴相互靠近相继发生聚并及断裂的过程。通过高速摄像仪发现当液滴之间的距离在10微米左右时液滴相互靠近的界面会形成柱状突起并在两个液滴间形成搭桥。实验结果表明,磁流体液滴聚并的推动力为惯性力而不是表面张力,磁流体液滴聚并后会形成液柱。存在一个临界磁场强度,高于临界磁场强度磁流体液柱将变得不稳定甚至发生破裂,液柱破裂的形式与磁场强度有关。
[Abstract]:Multi phase flow widely exists in many industrial processes, it is often accompanied by interfacial phenomena and hydrodynamic coupling. In recent years, the multiphase flow and combination of microfluidic technology has gradually become a new technology, has a wide application prospect. In the micro channel flow is an important research in this field. This paper use high speed camera is studied in micro channel drops (bubble) generation, rupture and coalescence. The research contents include: the first part is to study the magnetic field generating process of magnetic fluid droplet focusing control in micro channel. Focusing on the expansion and rupture of magnetic fluid droplets inside the channel dynamics effects of fluid velocity, magnetic field intensity and magnetic field influence on expansion and rupture. Study and compare the magnetic field (NM), radial magnetic field (RM) and axial magnetic field (AM) of the droplet formation process. The results show that the magnetic fluid The volume of the droplet by the applied magnetic field can effectively control the radial magnetic field and axial magnetic field respectively. Through the expansion phase of droplet impact and crack stage to affect the droplet size. The magnetic fluid rupture exponent and the minimum width of the neck of the remaining time. The second part studied the influence of magnetic field on magnetic rupture the fluid droplet and the influence of feedback effects on the bubble rupture. Firstly, study the T type magnetic field and nonuniform magnetic fields under the micro channel bifurcation of magnetic fluid droplet rupture dynamics. The study found in the magnetic field, can change the intensity of magnetic field to control the droplet breakup and droplet breakup in frequency. Non uniform magnetic fields, by changing the magnetic field strength can control the droplet rupture asymmetry so that the nanotest droplet transfer. In addition, also study the bubbles in the T type micro channel bifurcation Asymmetric rupture behavior. Feedback dynamics in the downstream channel rupture behavior of bubbles is mainly due to asymmetry formed by rupture of bubbles was studied. The convergence of downstream export debut bubble collision and alternating through feedback effect on the export effect of rupture. The results show that when the flow rate is small and the bubbles do not collide, feedback effect when the bubble can be ignored. The more obvious role in the convergence of downstream feedback mouth collision. In addition, effects in high speed channel defects can not be ignored. The third part studies the satellite droplets (bubble) formation process. The first part of this chapter, the main focus and T in micro channel the liquid-liquid interface contraction of the satellite droplets formed in the process of rupture. The experimental results show that in the neck pinch off late, due to the viscous force and surface tension balance and strength of the conversion, drop the neck The Ministry of the inevitable from fast pinch off region into slow rupture zone. The study found that the size of the neck pinch off region in the slow almost unchanged, the most narrow neck position from both sides of the intermediate filaments to the neck, finally in the neck fracture and the formation of filaments on both sides of the satellite droplets. In the experimental range of satellite droplet size with continuous phase the breakdown of hair growth and growth. Then study the droplet in straight channel shear stress (bubble) tail fracture process. Focus on the droplet size and velocity (bubble) effect on the tail tip flow. The results show that, with the droplet size (bubble) or capillary growth in the number, the more deformation significant. There is a critical capillary number, when greater than this value, the droplet (bubble) will rupture and tail tip flow. Satellite droplet sizes generated by tip flow is usually higher than the main drop orders. The mistress The last part of the droplet (bubble) coalescence process. Firstly, the bubble in the T type micro channel sink at the coalescence behavior. The capillary number and bubble size under different experimental observation to three kinds of behavior: collision coalescence, extrusion and coalescence coalescence experiments. The results show that medium viscosity both collision coalescence or extrusion coalescence, the coalescence efficiency of the two-phase flow and reduce the increase of apparent increase. Two apparent velocity will cause collision coalescence and transition to squeeze together and at the same time, the efficiency of coalescence increased slightly. In addition, even. Continuous phase viscosity increase or bubble size will reduce the coalescence efficiency. In addition, the Research on magnetic fluid droplet under different magnetic field intensity of the coalescence process. The magnetic fluid droplet two coaxial in magnetic field and attract each other is a cone. The experiment focuses on the deformation of two cone The droplet coalescence process occurred near each other and fracture. By the high speed camera found that when the distance between the droplets in the 10 micron droplet interface will be close to each other to form columnar protrusions and formation of bypass in two drops. The experimental results show that the magnetic fluid droplet coalescence and impetus for inertia instead of surface tension force, magnetic fluid droplet coalescence will form the liquid column. There exists a critical magnetic field strength is higher than the critical magnetic field strength magnetic fluid column will become unstable or even rupture, rupture of the liquid column form associated with the magnetic field strength.

【学位授予单位】：天津大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TQ021.1
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本文编号：1642341