颗粒聚团结构对曳力特性影响的数值模拟

黏性颗粒多以聚团形式存在于气固两相系统中，流体施加于聚团的曳力对两相流动及传热传质起着至关重要的作用，而聚团的不规则、分形结构增加了曳力特性的复杂性。基于黏性离散单元方法生成不同分形结构的聚团，利用计算流体力学方法（CFD）直接求解分形聚团内部多孔结构的气流流动，得到了气体流过聚团时的周围与内部流场，研究了低Reynolds数（Re=0.1～10）条件下聚团结构特征对曳力的影响。结果表明：聚团的疏密程度显著影响聚团整体流场分布，多孔疏松结构增强了聚团的渗透性，使其与流体接触面积增加，所受曳力增加。分析不同结构聚团的曳力系数发现：除了聚团孔隙率、分形维数等结构参数的影响，气体流经聚团的方向也影响聚团曳力系数。在此基础上，综合考虑聚团分形维数、聚团与气流的夹角方向、Reynolds数拟合得到聚团曳力系数关联式。     

双液滴碰撞行为及调控机制的研究进展

双液滴碰撞行为广泛存在于雨滴形成、燃油喷雾、喷雾冷却、喷墨印刷、农药喷洒等自然现象与工业应用过程中,其碰撞结果会受到液滴参数及气相环境等因素的综合影响,研究双液滴的碰撞行为规律及调控机制一直是该领域的热点。结合目前双液滴碰撞的实验进展和数值模型,将围绕着碰撞行为的主控因素与调控机制展开综述,具体介绍了碰撞参数、液滴理化性质、气相环境等因素对液滴碰撞行为的影响规律与调控结果,并展望了液滴碰撞理论及应用的发展趋势和方向。     

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。     

水滴与聚氧化乙烯液滴撞击荷叶表面的实验对比分析

为探究荷叶表面的液滴撞击行为规律,本文利用高速摄像机以14000帧/秒的帧率分别记录水滴和4种不同相对分子质量的聚氧化乙烯（polyethylene oxide,PEO）水溶液液滴竖直撞击荷叶表面的动力过程,其撞击速度为0.3~3m/s。实验结果表明,水滴与低相对分子质量（5×10⁴）的PEO液滴撞击荷叶表面的行为现象相似,两者随撞击速度增加依次有规则反弹、向上发射卫星液滴、不规则反弹（或部分反弹）、液滴破碎和液指断裂分离小液滴等现象发生,但水滴的接触时间更短,最大铺展系数也更小。中等相对分子质量（3×10⁵）PEO液滴在低速和高速撞击时分别为振荡弹起模态和振荡模态,临界速度为1.13m/s。高相对分子质量（1×10⁶、4×10⁶）的PEO液滴,其高分子长链与表面交互作用显著增强,表现出很强的黏性,撞击后反弹完全被抑制,均黏附沉积于荷叶表面;液滴发生沉积的临界Oh数为0.0544,且Oh数越大,液滴越难发生反弹。速度一定时,相对分子质量3×10⁵以上的3种PEO液滴的最大铺展系数均小于水滴;三者的上升系数随速度增加先减小后保持基本稳定或略微增加。     

电场作用下液滴分裂动力学行为的格子Boltzmann模拟

采用格子Boltzmann方法（LBM）伪势模型,耦合流场和电场控制方程,研究了电场作用下油水共存体系中水滴分裂的动力学行为及特性,借助形变率衡量液滴的形变大小,展现了液滴从形变至分裂的动态演变过程,分析了外加电场大小和液滴内外介电常数比对液滴分裂行为的影响。结果表明：外加电场能促使液滴发生振荡形变,且存在临界电毛细数和临界介电常数比决定液滴是否发生分裂：高于临界值,液滴形变率振荡幅度随时间不断增长,最终发生分裂;低于临界值,则液滴形变率振荡幅度不断衰减,并最终趋于一稳定值。在此基础上,综合考虑电场强度与介电常数比的影响,提出了基于现有电毛细数的修正电毛细数唯一地表征电场作用下液滴分裂与否。     

电场作用下液滴分裂动力学行为的格子Boltzmann模拟

采用格子Boltzmann方法(LBM)伪势模型,耦合流场和电场控制方程,研究了电场作用下油水共存体系中水滴分裂的动力学行为及特性,借助形变率衡量液滴的形变大小,展现了液滴从形变至分裂的动态演变过程,分析了外加电场大小和液滴内外介电常数比对液滴分裂行为的影响。结果表明:外加电场能促使液滴发生振荡形变,且存在临界电毛细数和临界介电常数比决定液滴是否发生分裂:高于临界值,液滴形变率振荡幅度随时间不断增长,最终发生分裂;低于临界值,则液滴形变率振荡幅度不断衰减,并最终趋于一稳定值。在此基础上,综合考虑电场强度与介电常数比的影响,提出了基于现有电毛细数的修正电毛细数唯一地表征电场作用下液滴分裂与否。     

液滴间相互碰撞融合与破碎的实验研究

液滴间的相互碰撞对发动机液体燃料雾化、燃烧效率及颗粒物排放的减少均有重要影响;为了探究液滴间相互碰撞后的液滴融合与破碎过程,采用高速摄影技术对液滴间相互碰撞的融合过程、融合振荡无量纲宽长比变化历程、破碎过程以及液体物性参数对碰撞破碎的影响进行了分析。结果表明,液滴间相互碰撞后主要呈现融合振荡与破碎两种运动形态;对融合振荡形态而言,两液滴间的碰撞速度及初始无量纲尺寸比越大,融合后液滴振荡的宽长比的最大值越大;对破碎形态而言,两液滴间的碰撞速度越大、初始无量纲尺寸比越小以及大液滴的表面张力越小,液滴发生破碎的时刻越小。研究结果可以为改善发动机缸内雾化,增大气/液间的接触面积,强化气/液间传热传质提供理论依据。     

液滴碰撞球形凹曲面复合level set-VOF法的数值分析

采用复合水平集法和流体体积法建立液滴冲击球形凹曲面的数值模型,通过分析计算结果揭示了液滴撞壁流动及破碎机制。研究表明:液滴的撞壁特性与液滴碰撞速度密切相关;液滴凹曲面撞壁与平面撞壁相比,铺展系数较小,回弹射流出现时间超前,回弹射流速度较大。量纲1分析得出:液滴的最大铺展系数和相对最大铺展速度与Reynolds数近似幂递增,液滴的相对最大射流长度与Reynolds数近似对数递增,液滴的相对最大射流速度与Reynolds数近似幂递减。对比分析现有液滴撞壁最大铺展系数理论解析模型,提出了液滴凹曲面撞壁最大铺展系数解析模型的发展方向。     

直流脉冲电场下液滴-界面聚并行为

为了深入探究直流脉冲电场下液滴-界面聚并行为,针对去离子水作为分散相、葵花油作为连续相的体系,分别改变电场参数（电场强度、频率、波形）和物性参数（界面张力、电导率、液滴粒径、固体颗粒）进行显微实验研究,得到了液滴-界面聚并机制及各参数的影响规律。实验结果表明,液滴-界面存在完全聚并和不完全聚并两种机制,决定因素是泵吸和颈缩过程的相互作用。电场强度增大,不完全聚并程度增大,而电场频率的作用则相反,这与电场力大小和液滴稳定程度有关。随表面活性剂浓度增大,二次液滴急剧增大,超过临界胶束浓度后,小幅减小。随电导率和SiO₂浓度增大,不完全聚并程度均先增大后减小,而随液滴粒径增大,不完全聚并程度持续增大。大部分工况下,液滴在直流稳恒电场下不完全聚并程度高于直流脉冲电场。为脉冲静电破乳机理的深入探讨及高效紧凑脉冲电脱盐脱水设备的研发奠定了理论基础。     

直流脉冲电场下液滴-界面聚并行为

为了深入探究直流脉冲电场下液滴-界面聚并行为,针对去离子水作为分散相、葵花油作为连续相的体系,分别改变电场参数(电场强度、频率、波形)和物性参数(界面张力、电导率、液滴粒径、固体颗粒)进行显微实验研究,得到了液滴-界面聚并机制及各参数的影响规律。实验结果表明,液滴-界面存在完全聚并和不完全聚并两种机制,决定因素是泵吸和颈缩过程的相互作用。电场强度增大,不完全聚并程度增大,而电场频率的作用则相反,这与电场力大小和液滴稳定程度有关。随表面活性剂浓度增大,二次液滴急剧增大,超过临界胶束浓度后,小幅减小。随电导率和SiO_2浓度增大,不完全聚并程度均先增大后减小,而随液滴粒径增大,不完全聚并程度持续增大。大部分工况下,液滴在直流稳恒电场下不完全聚并程度高于直流脉冲电场。为脉冲静电破乳机理的深入探讨及高效紧凑脉冲电脱盐脱水设备的研发奠定了理论基础。     

Hydrodynamics and morphologies of droplets coalescence on fiber

Fiber coalescence is an effective oil–water separation technology. In this article, the micro-coalescence process of droplets on fiber was studied through high-speed camera technology, and the hydrodynamics and morphology evolution in the process of droplet-fiber coalescence were explored and compared with non-fiber system. The results show that the change of the droplet surface morphology is local at the initial stage of the coalescence process. The droplet morphology changes obviously near the neck. The growth rate of the liquid bridge and the propagation speed of capillary wave in droplet-fiber system are higher than those in non-fiber system. At 0.8 ms, the capillary waves' polar angle difference between the two conditions reaches 10.44°. There is an obvious periodicity and damping attenuation mechanism in the oscillation process. In the droplet-fiber system, the oscillation process attenuates faster and the average oscillation period is shorter. The droplet relaxes to a stable state more quickly.     

非均匀电场下乳化油中液滴变形动力学行为

外加电场下液滴的变形动力学行为是乳化液电脱水机理研究的重要内容。基于Cahn-Hilliard方程的相场方法,建立了液滴在非均匀电场下的仿真模型,研究了电场作用下乳化液中液滴在形变、移动和聚结过程中电荷密度和电场力的分布规律,以及流场和电场的耦合作用。仿真分析了液滴粒径、电场强度以及电场非均匀系数对液滴运动行为的影响。利用实验室小型脱水系统开展了乳化液脱水实验,并通过高速摄像机对乳化液中液滴的运动行为进行了观测与分析。研究结果表明,在非均匀电场中液滴表面的极化电荷分布不均,由液滴中部向两端逐渐增大,在靠近电场集中方向处的电荷密度和Maxwell应力值最大;在一定范围内增大电场强度、电场非均匀系数或液滴粒径,可使液滴形变量增大,液滴向电场集中区域的移动速度以及液滴间的聚结速度增加。     

疏水表面液滴合并弹跳过程的数值模拟

液滴合并弹跳对强化热泵空调系统中的凝结传热及防结霜、除霜等方面均有良好的应用前景。在综合考虑固-液、气-固和气-液表面自由能,重力势能,液滴内部黏性耗散功及表面黏附功的基础上建立了液滴合并及弹跳的分阶段能量模型,并进行了超疏水表面不同半径液滴合并弹跳时的模型模拟与实验验证,得到较好的吻合。基于该模型研究了液滴数量、半径均匀性及不同表面状态对液滴合并弹跳过程的影响规律。结果表明,液滴数量增加时,合并阶段临界接触角由120°减小至105°,半径尺寸均匀性增加时,弹跳阶段临界接触角从140°减小至130°。当表面接触角大于140°时,固液接触系数影响微乎其微。可见,液滴数量的增多及液滴尺寸均匀性的提升有利于合并弹跳过程的发生,固液接触系数对合并弹跳过程的影响程度随表面接触角的增大而减小。     

导叶式旋流器内油滴的聚结破碎及影响因素

利用流场测试实验和模拟计算对导叶式旋流器的内部流场和湍流性能进行研究,得到导致旋流场中油滴发生聚结破碎现象的内因为时均速度梯度引起的黏性剪切力和湍流流动引起的高剪切应力及湍动能,并分析了油滴破碎的主要发生部位。同时为宏观考察导叶式旋流器内油滴聚结与破碎相对作用强弱,验证了旋流器的分离性能作为表征量的可行性,并在此基础上,利用导叶式旋流器的分离性能实验结果对油滴聚结破碎发生的外因进行研究,结果表明导向叶片的结构参数（叶片数和叶片出口角）和旋流器的操作参数（入口流速、溢流率、分散相入口浓度和操作温度）均会影响到油滴在旋流器中的存在和运动形态。     

电场作用下双液滴聚合特性

乳状液破乳分离是目前高含水期油田开采过程中难以解决的技术问题，电场破乳方法具有高效清洁等优点，是解决该问题的有效手段。采用数值模拟与试验验证相结合的方法研究电脱水过程中阶跃、斜坡电场诱导下双液滴的聚合与分离特性。结果表明，在斜坡电场作用下，界面张力引起的泵吸作用大于电场力引起的颈缩作用，有利于液滴聚并，且液滴发生二次乳化现象的概率降低。而施加阶跃电场时，一定范围内能够达到液滴破乳的目的，但液滴在聚并过程中易发生二次乳化现象。从电场对连续相影响的角度分析发现，阶跃电场不仅对液滴具有驱动作用，对连续相的影响也较为明显，阶跃电场会增大连续相内湍流作用，不利于电脱水过程。因此，采用斜坡信号诱导液滴聚合能够降低二次乳化现象发生的概率。     

Experimental study on fusion and break-up motion after droplet collision

The interactions between droplets have an important influence on the atomization of liquid fuel, the combustion efficiency, and the reduction of particulate matter emissions for an engine. For this reason, this paper presents results from an experimental study on the coalescence and break-up of droplets after collision. According to the shape and parameters of the droplets at different times after the collision of the droplets was captured by a high speed camera, analysis was done for the following effects of droplet collisions: the collision-coalescence motion for the collision between the droplets, the change history of the dimensionless length-to-width ratio of the oscillation motion, the critical size ratio of the breakup motion, and the liquid physical properties of the particles. The results show that the droplets collide and exhibit two forms of coalescence oscillation and break-up: for oscillating motion, at higher droplet collision velocities and dimensionless size ratios, there will be a larger dimensionless length-to-width ratio for the droplet oscillation; for the break-up motion, at higher collision velocities, there will be lower dimensionless size ratios, and lower liquid surface tension, shorter times over which the droplet breaks, and facilitated droplet break-up. The research results presented here can be used for atomization in engine cylinder, increasing the gas/liquid contact area and enhancing the combustion efficiency of gas/liquid heat transfer to improve the combustion efficiency of the engine.     

Experimental verification of the additivity between the barrel droplet's drag force and the fiber number

Research on the single fiber scale has an essential application value in the coalescence field. The study of the droplets' behavior on the fiber junctions can provide support data for optimizing the coalescer's drainage performance on the macroscopic scale. However, relevant research focusing on the force change of droplets during the transition from a single fiber to multifiber is relatively scarce. The droplet migration platform was built using the Lavision particle image velocimetry system, which can observe the behavior of droplets on fibers. The force and fiber number relationship were analyzed when the droplets were separated from the fiber junctions. Two detachment modes were observed during separation. Furthermore, the drag force satisfied the additivity theorem by increasing the number of fibers under the same detachment mode. In addition, the minimum Reynolds number prediction model was established under two separation forms, which had good prediction compared to the experimental data.     

Drag Correlation of Drop Motion on Fibers

The objective of this article is to correlate a drag coefficient to the Reynolds number for axial motion of barrel drops on fibers. This work includes effects of vibration-induced motion of droplets and coalescence. The study of motion of drops is important to understand the drainage behavior of droplets. Drainage of liquid helps to eliminate moisture from media samples before applying thermal energy and hence reducing the drying cost. A significant amount of literature describes the mechanisms of droplet capture, coalescence, and drainage from filter media and models are developed at a scale that accounts for the liquid held in the filter through averaged parameters such as saturation. Few papers discuss the motion of individual drops attached to fibers.

The study of drop motion on fibers is of scientific and economic interest for many possible applications like printing, coatings, drug delivery and release, and filters to remove or neutralize harmful chemicals or particulates from air streams. Gas convection–induced drop motion in fibrous materials occurs in coalescing filters, clothes dryers, textile manufacturing, convection ovens, and dewatering of filter cakes. Droplet removal can significantly reduce drying costs by reducing the free moisture contained in fibrous materials prior to applying thermal drying techniques.

In this article, the experimental drag coefficient versus Reynolds number data are compared for 1-D and 3-D cylindrical drop models. The results show that 1-D models are inadequate to predict the drag coefficient but do show the same general trends.     

Drag Correlation of Drop Motion on Fibers

The objective of this article is to correlate a drag coefficient to the Reynolds number for axial motion of barrel drops on fibers. This work includes effects of vibration-induced motion of droplets and coalescence. The study of motion of drops is important to understand the drainage behavior of droplets. Drainage of liquid helps to eliminate moisture from media samples before applying thermal energy and hence reducing the drying cost. A significant amount of literature describes the mechanisms of droplet capture, coalescence, and drainage from filter media and models are developed at a scale that accounts for the liquid held in the filter through averaged parameters such as saturation. Few papers discuss the motion of individual drops attached to fibers.

The study of drop motion on fibers is of scientific and economic interest for many possible applications like printing, coatings, drug delivery and release, and filters to remove or neutralize harmful chemicals or particulates from air streams. Gas convection-induced drop motion in fibrous materials occurs in coalescing filters, clothes dryers, textile manufacturing, convection ovens, and dewatering of filter cakes. Droplet removal can significantly reduce drying costs by reducing the free moisture contained in fibrous materials prior to applying thermal drying techniques.

In this article, the experimental drag coefficient versus Reynolds number data are compared for 1-D and 3-D cylindrical drop models. The results show that 1-D models are inadequate to predict the drag coefficient but do show the same general trends.     

Analysis of Non‐Isothermal Viscous Flow Coalescence at Micro Scale

The non‐isothermal coalescence of two spherical bodies caused by capillary‐induced viscous flow was analyzed. Based on this analysis, a new dimensionless number ( K number) was introduced for defining thermal coalescence regimes. Based on the value of this number, coalescence may or may not be affected by thermal effects in different cases. To make this clearer, the conventional coalescence models of Frenkel‐Eshelby and Pokluda were modified by assuming viscosity as a temperature dependent variable. This was conducted by considering the effects of temperature on the viscosity of the involved material through evaluating different expressions including linear and Reynolds and Williams‐Landel‐Ferry (WLF) equations. The results of the modified models for the bridge growth rate show that temperature changes significantly affect the kinetics of coalescence, particularly when the characteristic times of coalescence and heat conduction are in the same order, i.e., moderate K numbers. This analysis is applicable for diverse situations since viscous flow coalescence occurs in various physical and industrial applications of particles or droplets.