三维孔喉结构微通道内液滴的破裂行为研究

利用高速摄像仪研究了三维孔喉结构微通道内液滴的破裂行为。采用不同黏度的甘油-水溶液作为分散相,含4%(质量)表面活性剂(Span 20)的矿物油作为连续相。液滴通过孔喉结构后,观察到了三种流型:球形破裂、非球形破裂和不破裂。除极低连续相毛细数的情形外,分散相黏度和两相流量的增加不利于液滴破裂,液滴的破裂位置均接近于喉道出口。研究了液滴的球形破裂,结果表明,球形破裂中子液滴平均尺寸随分散相黏度和连续相流量的增加而降低,且与两相总毛细数呈幂律关系,模型预测值与实验结果吻合良好。     

Y聚焦型微通道内磁流体液滴的生成与调控

利用高速摄像仪研究了截面为400 μm×400 μm Y聚焦型微通道内磁流体液滴在矿物油中的生成过程。以水基磁流体EMG 807为分散相,含4%表面活性剂Span-20的矿物油为连续相。实验观察到了3种流型：弹状流、滴状流和喷射流。分别考察了两相流量、连续相毛细数及磁感应强度对液滴尺寸及生成过程的影响。结果表明：可通过改变两相流量及磁场调控液滴尺寸。当分散相流量不变时,液滴尺寸随着两相流量比的增加而减小。液滴尺寸随着连续相毛细数及磁感应强度的增加而减小,随着分散相流量的增加而增加。以两相流量比、连续相毛细数和磁Bond数为参数提出了一个液滴尺寸的关联式,预测值与实验值吻合良好。     

Y聚焦型微通道内磁流体液滴的生成与调控

利用高速摄像仪研究了截面为400μm×400μm Y聚焦型微通道内磁流体液滴在矿物油中的生成过程。以水基磁流体EMG 807为分散相,含4%表面活性剂Span-20的矿物油为连续相。实验观察到了3种流型:弹状流、滴状流和喷射流。分别考察了两相流量、连续相毛细数及磁感应强度对液滴尺寸及生成过程的影响。结果表明:可通过改变两相流量及磁场调控液滴尺寸。当分散相流量不变时,液滴尺寸随着两相流量比的增加而减小。液滴尺寸随着连续相毛细数及磁感应强度的增加而减小,随着分散相流量的增加而增加。以两相流量比、连续相毛细数和磁Bond数为参数提出了一个液滴尺寸的关联式,预测值与实验值吻合良好。     

同轴微通道内管结构对液滴生成的影响规律研究

研究了同轴微通道内水-硅油体系的液滴生成过程。在不同的连续毛细数Ca_c和分散相Weber数We_d下,观察到滴出流和喷射流两种不同的流型。实验考察了两相流量、黏度和内管结构对液滴尺寸和液滴生成频率的影响。结果表明液滴尺寸随着分散相流量增加而增大,而随着连续相流量和黏度的增加而降低。此外,随着We_d的增加,流型会从滴出流过渡到喷射流,而更大尺寸的内管会更早过渡到喷射流,从而得到更大的液滴尺寸。对于液滴生成频率,其随Ca_c和We_d的增加均呈现先快速增加后变缓的趋势。当内管通道尺寸基本相同时,不同结构的内管通道在固定Ca_c时其液滴生成频率相差不大,且随着通道尺寸的减小,液滴生成频率逐渐增加。基于实验结果,建立了液滴尺寸预测模型,预测值与实验值吻合较好。     

十字聚焦型微通道内弹状液滴在黏弹性流体中的生成与尺寸预测

利用高速摄像仪对十字聚焦微通道内液滴在黏弹性流体中的生成过程进行了实验研究。微通道截面为600μm×600 μm 的正方形结构,采用硅油作为分散相,含0.3%表面活性剂十二烷基硫酸钠(SDS)的聚环氧乙烷(PEO)水溶液(质量分数分别为0.1%,0.3%,0.6%)为连续相。实验观察到了弹状流、滴状流和喷射流3 种流型。对弹状流型下液滴生成过程的颈部动力学进行了研究,考察了两相流率、连续相毛细数及弹性数对液滴尺寸的影响。结果表明:弹状液滴尺寸随连续相流率、毛细数及弹性数的增加而减小,随分散相流率的增加而增加,连续相弹性对液滴尺寸的影响相对较小。以油水两相流率比和连续相的毛细数及Reynolds 数为变量建立了弹状液滴尺寸的预测关联式,预测值与实验值吻合良好。     

T型微流控芯片中微液滴破裂的数值模拟

利用VOF模型对T型结构微流控芯片中微液滴的三维破裂过程进行了数值模拟,获得了液滴发生破裂和不会破裂两种流型。一定轴向长度的微液滴对应着一个临界毛细数,当主流流体的毛细数大于此临界毛细数时,微液滴发生破裂并分别流向T型结构两侧;否则不会发生破裂,微液滴流向任意一侧。通过多个工况的计算,拟合了临界毛细数与微液滴相对轴向长度的关系,探讨了黏度比对微液滴破裂的影响。发现黏度比越小,微液滴越易发生破裂。     

液-液两相流体黏度对微通道内流型的影响

为了研究流体黏度对液-液两相流流型的影响，采用实验和数值模拟相结合的方法,研究十字型微通道内液-液两相流流型变化。结果表明，当两相流体系中存在高黏度流体时，会加剧两相界面的不稳定性，两相流流型极易向不规则流和环状流转换，且当连续相流体黏度较高时，液滴的形状更易为子弹流。通过引入毛细数和韦伯数，提出两相流流型转换关系。当连续相流体和分散相流体分别由水平通道和垂直通道流入时，通过合理调节两相流体流速，可在微通道下游实现大小液滴的融合。这一方式将为高黏度流体流动操控提供新思路，通过控制两相流速，可以实现不依赖于复杂微通道结构的液滴被动融合。     

不对称T型微通道内液滴的无阻塞破裂动力学

利用高速摄像机研究了不对称T型微通道内液滴的无阻塞破裂过程。以甘油-水溶液为分散相,含4%（质量分数）表面活性剂Span-20的矿物油为连续相。液滴的无阻塞破裂过程可分为三个阶段：进入阶段、形变阶段和破裂阶段。其中破裂阶段又可分为快速破裂阶段和细丝破裂阶段两个子阶段。考察了表观流速、无量纲液滴长度和两相黏度比对破裂阶段的影响。结果表明：快速破裂阶段是一个自相似过程,无量纲颈部最小宽度与无量纲剩余时间呈幂律关系,幂律指数约为1.35。细丝破裂阶段无量纲最小颈部宽度与无量纲剩余时间呈线性关系。斜率随着表观速度和无量纲液滴长度的增大而增大,随两相黏度比的增大而减小。     

T型微通道内液滴尺寸的实验测定与关联

利用高速摄像仪对不同深宽比的T型微通道内液滴尺寸进行了实验研究。分别采用3种不同尺寸(深度×宽度)的微通道:400 μm×400 μm、400 μm×600 μm、400 μm×800 μm。以环己烷为分散油相,含0.3%表面活性剂十二烷基硫酸钠(SDS)的蒸馏水-甘油(质量分数分别为20%、40%、60%)溶液为连续相。考察了弹状流、过渡流和滴状流流型下微通道尺寸、两相流率、物性对液滴尺寸的影响。结果表明:液滴尺寸随微通道深宽比、连续相流率、黏度和毛细数的增加而减小,随分散相流率的增加而增加。用毛细数、两相流量比和通道深宽比对微通道内液滴尺寸进行了关联和预测,预测值与实验结果吻合良好。     

非对称Y型分岔微通道内气泡破裂与分配规律

利用高速摄像仪对气泡在非对称Y型微通道分岔口的破裂行为和分配规律进行了实验研究。采用氮气（N₂）作为分散相,含0.3%表面活性剂十二烷基硫酸钠（SDS）的蒸馏水-甘油（质量分数分别为20%、40%、50%）溶液为连续相。在分岔口处观察到了3种不同的气泡行为：无间隙的不对称破裂、有间隙的不对称破裂以及不破裂。考察了气泡破裂和不破裂行为之间的转变,并与文献进行了比较。考察了两相流率及物性对破裂气泡分配规律的影响。结果表明：破裂后两个子气泡的长度均随气相流量与气泡长度的增大而增大,随液相流量和黏度的增大而减小。随液相速度和黏度的增大,气泡破裂的不对称程度减弱。     

Liquid Jet Stability in a Laminar Flow Field

The breakup of a Newtonian liquid jet into droplets injected horizontally into another flowing immiscible Newtonian fluid was studied experimentally under creeping flow conditions. Different breakup mechanisms take place in different flow regions. No filament is generated at very low velocities of the continuous phase when the droplets peel off directly at the nozzle tip. As soon as the flow rate of the continuous phase exceeds a critical value, a filament of a characteristic length begins to grow. The filament breaks up due to instabilities in terms of developing interfacial waves. The laminar breakup length of the filament is found to correlate with the flow rates of both phases and their viscosity ratio. The impact of the capillary diameter, through which the disperse phase is injected, on the filament length was investigated and the maximum droplet size was estimated.     

Transient and steady-state deformations and breakup of dispersed-phase droplets of immiscible polymer blends in steady shear flow

Transient and steady-state deformations and breakup of viscoelastic polystyrene droplets dispersed in viscoelastic high-density polyethylene matrices were observed in a simple steady shear flow between two transparent parallel disks. By separately varying the elasticities of the individual blend components, the matrix shear viscosity, and the viscosity ratio, their effects on the transient deformation, steady-state droplet size, and the breakup sequence were determined. After the startup of a steady shear flow, the viscoelastic droplet initially exhibits oscillations of its length in the flow direction, but eventually stretches preferentially in the vorticity direction. We find that at fixed capillary number, the oscillation amplitude decreases with increasing droplet elasticity, while the oscillation period depends primarily on, and increases with, the viscosity ratio. At steady-state, the droplet length along the vorticity direction increases with increasing capillary number, viscosity ratio, and droplet elasticity. Remarkably, at a viscosity ratio of unity, the droplets remain in a nearly undeformed state as the capillary number is varied between 2 and 8, apparently because under these conditions a tendency for the droplets to widen in the vorticity direction counteracts their tendency to stretch in the flow direction. When a critical capillary number, Ca_c, is exceeded, the droplet finally stretches in the vorticity direction and forms a string which becomes thinner and finally breaks up, provided that the droplet elasticity is sufficiently high. For a fixed matrix shear stress and droplet elasticity, the steady-state deformation along the vorticity direction and the critical capillary number for breakup both increase with increasing viscosity ratio.     

Theory of competition between breakup and coalescence of droplets in flowing polymer blends

The Smoluchowski equation for the breakup and coalescence of dispersed droplets has been solved for flowing polymer blends. A scaling form for the distribution of droplet sized derived and published for a system of clusters with fragmentation and coagualation was used in our dervation. Equations are developed here for the average droplet size and for the characteristic time of transition to steady state flow of blends with a high content of the dispersed phase. Expressions reasonably describing the average size of droplets for all concentrations were obtained by a theory modification. Measured dependences of droplet size on the blend composition can be matched only if simultaneous collisions of three and more droplets are considered. The results of the theory indicate that the mechanism of droplet breakup (formation of pieces with the same or different volumes) has only a small effect on their average size in concentrated systems. The dependence of droplet size on the shear rate in flow is determined by properties of the blend components, and is generally nonmonotonic.     

A novel theoretical breakup kernel function for bubbles/droplets in a turbulent flow

Several models for the daughter bubble/droplet size distribution are reviewed and a detailed discussion is given to get a better understanding of the daughter size distribution. A novel theoretical breakup kernel function for bubbles/droplets in a turbulent flow, based on an eddy-bubble/droplet collision method, is developed. It takes into account the energy distribution of turbulent eddies, effect of capillary pressure and surface energy increase during bubble/droplet breakup. An increase in the mother bubble/droplet size and energy dissipation rate increases the probability of unequal breakup. The model prediction is in good agreement with experimental results and the underlying physical situation.     

Development of polymer blend morphology during compounding in a twin-screw extruder. Part I: Droplet dispersion and coalescence—a review

The theoretical and experimental data on the breakup of droplets are reviewed. Several factors influence development of droplets: flow type and its intensity, viscosity ratio, elasticity of polymers, composition, thermodynamic interactions, time, etc. For Newtonian systems undergoing small, linear deformation, both the viscosity ratio and the capillary number control deformability of drops. On the other hand, the breakup process can be described by the dimensionless breakup time and the critical capillary number. Drops are more efficiently broken in elongational flow than in shear, especially when the viscosity ratio λ ? 3. The drop deformation and breakup seems to be more difficult in viscoelastic systems than in Newtonian ones. There is no theory able to describe the deformability of viscoelastic droplet suspended in a viscoelastic or even Newtonian medium. The effect of droplets coalescence on the final morphology ought to be considered, even at low concentration of the dispersed phase, ?_d ? 0.005. Several drop breakup and coalescence theories were briefly reviewed. However, they are of little direct use for quantitative prediction of the polymer blend morphology during compounding in a twin-screw extruder. Their value is limited to serving as general guides to the process modeling.     

正弦型微通道内液-液两相流型及流动特性实验研究

采用实验的方法对不混溶的液液两相流体在不同入口结构下的正弦微通道(直通道正弦、波峰正弦和波中正弦)内液滴的流动特性进行了分析。硅油作为离散相,含有0.5% SDS的蒸馏水作为连续相,观测到弹状流、滴状流和射状流。分析了两相流动参数及不同的微通道入口结构对流型和液滴长度的影响。流型受微通道入口结构影响较大,波峰正弦微通道能够生成最大范围的稳定的流型。液滴长度随离散相体积流量和离散相与连续相体积流量之比的增大而增大,随连续相的体积流量和毛细数的增大而降低。微通道入口结构对液滴长度有影响,直通道的正弦微通道内液滴长度最短,更有利于液滴的形成。三种通道生成的液滴中,最大的液滴尺寸是最小的液滴尺寸的1.15~1.39倍,但正弦流动段对液滴速度几乎没有影响。