均匀电场中液滴变形特性的耗散粒子动力学模拟

基于耗散粒子动力学方法,建立了电场作用下近似的液滴粒子力学模型,对两相不相溶液体中液滴在电场作用下的变形特性进行了模拟。模拟结果与他人的实验结果比较表明,模拟结果对液滴形状随时间的演化预测基本符合实际,仅在液滴变形较大时有一定偏差。模拟结果还表明,当外加场强较小时,液滴变形度随时间呈现振荡状态,变形度不会随时间继续增大。增大外加场强,液滴变形幅度增大,振荡频率变慢。当外加场强增大到一定程度时,液滴变形度不再振荡,而是随时间急剧增大,以至液滴最终破碎。场强越大,液滴破碎所需的时间也越短。     

液滴撞击平板的铺展特征

采用高速摄像仪研究了液滴在不同韦伯数和撞击角下撞击干燥平板时的铺展行为.结果表明,当撞击角相同时,韦伯数越小,液滴最大横向铺展直径也越小;韦伯数相同,撞击角越大,液滴横向铺展直径越大;在铺展和回缩过程中,环形液层外圈相对于内圈在达到横向最大铺展直径时具有滞后现象.     

振动微液滴的传质机理与模型

提出了微液滴振动传质的机理模型,基于微液滴振动引起的周围流体介质周期性变化流场和基本输运方程,得到简化的传质控制方程及其数值解,模拟结果表明传质组分的浓度分布具有明显的非均匀特征,其迎流面的浓度梯度显著大于背流面,导出的平均Sherwood数和传质速率随振动频率增加而增大.采用电动力学天平中作一维拟简谐振动的十二烷醇微液滴在氮气介质中的蒸发传质实验数据对模型进行验证,结果表明本文模型所预测的平均Sherwood数变化趋势与实验结果一致,平均Sherwood数预测误差为39.5%,说明本文的传质模型能描述此类微尺度下具有周期性运动特征的单个液滴表面传质现象.     

单向温度梯度下异质液滴的热毛细迁移

石蜡油与去离子水按多种比例混合形成异质液滴,放置在金属基板表面,在外加不均匀温度场作用下进行热毛细迁移,采用高速摄像机记录温度梯度驱动液滴从高温区迁移至低温区过程中液滴的形态,建立了理论模型. 结果表明,异质液滴的接触角随温度升高而减小,润湿性增强;异质液滴的迁移速度随粘度增加而减小,随温度梯度增加而增加,不同粘度下的速度差随温度梯度增大而增大,迁移速度随时间增加迅速降低,后缓慢减小到趋于0. 推导了迁移速度与Marangoni数Ma的关系式并进行了实验验证,迁移速度随Ma增加而显著增加,Ma越大,实验结果与模拟结果的匹配程度越高.     

甘露醇喷雾干燥过程中液滴粒度分布变化的群体粒数衡算模拟和实验研究

利用群体粒数衡算（population balance，PB）计算机模拟和实验研究了甘露醇水溶液的喷雾干燥过程中液滴的粒度分布的变化规律。液滴干燥过程中的颗粒粒度的萎缩速率，在群体粒数衡算模型中描述为液滴的逆（或负）生长项，通过单个液滴反应动力学方法（reaction engineering approach，REA）获得。基于单个液滴干燥的反应工程方法模型REA和群体粒数衡算模型PB集成建立了PBREA模型。PBREA 模型的求解是通过高分辨率数值方法。本文模拟研究了不同工况下，不同粒径液滴的干燥时间、液滴平均含湿量以及液滴粒度分布随时间的变化。结果显示，液滴粒径越大，干燥时间越长，模型预测的颗粒平均粒径为实验值的1.0~1.5倍，粒度分布跨度是实验值的0.61~0.89倍。模拟误差主要来源于液滴及颗粒粒径分布统计精度、单个静止液滴与群体运动液滴干燥的差异、热导率及扩散系数是经验值3个方面。在使用Buchi 290 小型喷雾干燥仪进行的实验中，使用了图像采集和分析方法得到了液滴及颗粒的数密度分布，并和模拟结果做了对比。结果表明该模型可以有效地预测喷雾干燥过程中干燥颗粒的平均粒度及分布跨度。     

液体弹珠翻越固着液滴的动态特性及运动机理

对体积在10—40μL之间的液体弹珠翻越液滴的过程进行可视化实验，研究液体弹珠翻越水平面上的固着液滴的动态行为。实验结果表明：弹珠以一定初速度撞击固着液滴后会发生反弹、翻越、弹飞和破裂4种运动状态，小体积弹珠只会产生前3种状态，弹珠体积越大越趋向于破裂。不同运动状态主要受弹珠初速度、弹珠体积、韦伯数(We)、雷诺数(Re)和邦德数(Bo)控制，在弹珠体积为10μL时，能够翻越同体积液滴的临界韦伯数在14—20之间，当体积达到40μL时，临界韦伯数为45—120。弹珠撞击液滴后受挤压，两者都会持续振荡变形，体积越大，弹珠的变形系数极值越大且液滴的振荡周期越长。该研究可为液体弹珠在微反应器方面的应用提供参考。     

基于图像法的环雾状流液滴参数测量与分析

环雾状流广泛存在于石油化工领域,其内部流场测量具有重要意义。本文结合光学图像法和高速摄影技术对撞针式喷嘴的雾化特性进行了测量分析,以此为基础对基于雾化混合的环雾状流中夹带液滴特性开展了实验研究。利用高速摄影技术对喷雾进行可视化,采用单帧单曝光法对液滴尺寸和速度信息进行提取。研究发现,液滴速度随轴向距离增大呈衰减趋势,且相同轴向距离（约在径向位置10mm处）条件下,速度达到峰值;液滴索泰尔平均直径（SMD）随喷嘴孔径d₀的增大而增大,并与液相质量流量m_l和喷嘴上下游压差?p均呈负相关;另外,在环雾状流环境中,相同气压条件下液滴SMD随气相体积流量Q_g增大而减小,而相同气相体积流量条件下SMD随气压p_g增大而增大。基于实验测量结果,以气相韦伯数We_g和液相雷诺数Re_l为主影响参数,引入相间滑移和压力系数建立了基于量纲分析的环雾状流液滴SMD预测模型,平均绝对百分比误差MAPE为11.4672%。     

匀强电场作用下分散相液滴的变形和破裂

基于Cahn-Hilliard方程的相场方法,建立了在匀强电场作用下液滴的变形和破裂行为模型。从微观角度研究分散相液滴变形过程中电荷密度、电场强度和电场力的分布规律以及流场和电场分布,探讨了微观液滴变形机理;采用数值模拟方法研究了电场强度、液滴直径和界面张力对液滴变形的影响,结果表明电场强度越强,液滴直径越大,界面张力越小,液滴变形量越大;分析了液滴的两种主要破裂方式,其破裂主要取决于连续相和分散相物性条件,为电破乳技术提供了理论基础。     

匀强电场作用下分散相液滴的变形和破裂

基于Cahn-Hilliard方程的相场方法,建立了在匀强电场作用下液滴的变形和破裂行为模型。从微观角度研究分散相液滴变形过程中电荷密度、电场强度和电场力的分布规律以及流场和电场分布,探讨了微观液滴变形机理;采用数值模拟方法研究了电场强度、液滴直径和界面张力对液滴变形的影响,结果表明电场强度越强,液滴直径越大,界面张力越小,液滴变形量越大;分析了液滴的两种主要破裂方式,其破裂主要取决于连续相和分散相物性条件,为电破乳技术提供了理论基础。     

喷嘴雾化特性及脱硫废水蒸发数值模拟

利用相位多普勒粒度分析仪（PDA）实验台测量了双流体喷嘴出口速度与粒径分布,利用得到的速度与粒径数据对江苏某生物质电厂进行小水量脱硫废水蒸发的数值模拟,着重研究了液滴粒径以及烟气中水蒸气的体积分数对液滴蒸发过程的影响。PDA实验结果表明,该双流体喷嘴在特定气液比条件下出口粒径均小于100μm。应用离散相模型与随机轨道模型,利用Rosin-Rammler分布模拟喷雾液滴分布范围（0～100μm）。模拟结果表明,粒径低于100μm的液滴能够完全蒸发,液滴粒径越小,完全蒸发时间越短,液滴经历的平稳吸热时间越短。随着粒径的增加,液滴完全蒸发时间增幅变大。随着烟气中水蒸气体积分数增加,液滴蒸发速率变缓,液滴开始蒸发的时间延长,且体积分数越大,出口未蒸发完全的液滴直径越大,但出口液滴粒径增大的幅度在减小。     

电场作用下液滴的变形及库仑分裂模式

基于高速成像技术,本文对电场作用下甲醇液滴的显微形貌特征进行了可视化研究,精确捕捉了两相流体系中不同生长阶段的荷电液滴基于时间分辨特性的变形及库仑分裂演变行为,得到不同工况下荷电液滴的变形分裂过程及行为演化细节。基于液滴所受库仑力和介电泳力与周围流域的耦合作用,揭示了电场作用下不同生长阶段的液滴库仑分裂形成机理。结果表明,电场强度和液滴粒径是决定液滴变形及库仑分裂模式的主要因素,荷电液滴的变形及库仑分裂模式可以分为推压变形、顶部破碎、顶部-边端破碎、伞状破碎。结合量纲为1参数对液滴的变形及破碎特征进行了定量分析,随着电场强度的增大及液滴粒径的减小,液滴变形及顶部破碎的程度更加剧烈,液滴临界伞状破碎长度减小。     

Characterizations of Cloud Droplet Shatter Artifacts in Two Airborne Aerosol Inlets

Aircraft-based aerosol sampling in clouds is complicated by the generation of shatter artifact particles from aerodynamic or impaction breakup of cloud droplets and ice particles in and around the aerosol inlet. Aerodynamic breakup occurs when the Weber number of a droplet, which primarily depends on the droplet size and the magnitude of the relative motion of the droplet and the local air mass, exceeds a critical value. Impaction breakup of a droplet occurs when the droplet's impaction breakup parameter, K, which is a combination of Weber and Ohnesorge numbers, exceeds a critical value. Considering these two mechanisms, the critical breakup diameters are estimated for two aerosol inlets of different designs—a conventional forward-facing solid diffuser inlet (SDI) and a cross-flow sampling sub-micron aerosol inlet (SMAI). From numerical simulations, it is determined that cloud droplets of all sizes will experience impaction breakup in SDI, while only droplets larger than ～16 μm will experience impaction breakup in SMAI. The relatively better in-cloud sampling performance of SMAI is because of its cone design that slows the flow just upstream of the sample tube. The slowing upstream flow, however, causes aerodynamic breakup of drops larger than ～100 μm. The critical breakup diameters determined from analysis of field data largely validate numerical predictions. The cross-flow sampling design of SMAI is seen to ensure that shatter artifacts in the inlet are minimal even when there are a significant number of particles larger that the critical breakup size. The study results, thus, suggest that the SMAI design presents an effective approach to sample interstitial particles from aircraft.

© 2013 American Association for Aerosol Research     

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。     

SIMULATION OF BUBBLE BREAKUP DYNAMICS IN HOMOGENEOUS TURBULENCE

This article presents numerical simulation results for the deformation and breakup of bubbles in homogeneous turbulence under zero gravity conditions. The lattice Boltzmann method was used in the simulations. Homogeneous turbulence was generated by a random stirring force that acted on the fluid in a three-dimensional periodic box. The grid size was sufficiently small that the smallest scales of motion could be simulated for the underlying bubble-free flow. The minimum Weber number for bubble breakup was found to be about 3. Bubble breakup was stochastic, and the average time needed for breakup was much larger for small Weber numbers than for larger Weber numbers. For small Weber numbers, breakup was preceded by a long period of oscillatory behavior during which the largest linear dimension of the bubble gradually increased. For all Weber numbers, breakup was preceded by a sudden increase in the largest linear dimension of the bubble. When the Weber number exceeded the minimum value, the average surface area increased by as much as 80%.     

液-液水力旋流器中的液滴破碎

在总结前人研究成果的基础上,分析了液-液水力旋流嚣中液滴破碎产生的原因,指出流场的湍流特性是产生液滴破碎的主要原因。对水力旋流器中的湍流度、雷诺切应力及颗粒的湍动能进行了分析,给出了水力旋流器中液滴破碎可能性较大的几个部位,并对旋流器边壁液滴破碎的可能性进行了讨论。分析了水力旋流器中液滴破碎的机理,阐明了液滴破碎判据——临界Weber数的表达式,并对理想球形液滴的破碎进行了分析。     

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.     

Primary breakup of shear-thickening suspension jet by an annular air jet

An experimental investigation is conducted to study the influence of shear-thickening behavior on the primary breakup process of suspension jets. The primary breakup morphologies of cornstarch–water suspension are observed via a high-speed camera. The unique hardened breakup mode only occurs when the suspension exhibits discontinuous shear thickening (DST). During hardened breakup mode, the oscillating portion of the suspension jet becomes perpendicular to the air direction and keeps cylindrical instead of deforming into a thin sheet or membrane structure. The suspension jets break off into large pieces rather than tiny droplets. The dimensionless number N is established to describe the relative magnitude of the increment of the viscous force and aerodynamic force during the primary breakup process. The primary breakup regime map of Newtonian fluids and shear-thickening suspensions is also promoted based on the dimensionless number N and the Weber number We.     

同轴双通道喷嘴雾化滴径沿轴向分布特性

Experiments were performed on the breakup of water droplets in a coaxial twin-fluid air-blast atomizer by a high-speed gas stream.With the measurements of Sauter mean diameter（SMD）distributions at a downstream distance of the atomizer,the droplet size was found first to decrease,reaching a minimum,and then to increase monotonically with the downstream distance.Correlation of L_c where the minimum droplet size appeared as a function of aerodynamic Weber number and liquid Reynolds number was analyzed.The growth rate of droplet size passes by L_c downstream increased with increasing water velocity,and the relationship between SMD and x/D_g accurately reflected the increasing trend of droplet size.