片状颗粒间液桥力变化规律的计算研究

研究颗粒间液桥力有助于揭示非饱和土持水特性的内在机理. 为探究片状颗粒间液桥力演化规律, 从细观尺度研究非饱和土的水力特性机理, 使用Surface Evolver软件在两平行的片状颗粒间构建出三维液桥模型, 分析了液桥拉伸过程中接触角、液桥体积、分离距离以及固液接触线钉扎效应等对液桥力变化规律的影响. 基于圆弧假定, 计算相应条件下液桥力以及接触半径的大小, 并与上述模拟结果进行对比分析. 结果表明: 片状颗粒间液桥力随液桥体积增大而递增, 随分离距离的增大而递减, 随固液接触角的增大先增后减或一直递减; 液桥体积一定时, 在钉扎状态下, 其液桥力随着分离距离的增大迅速递增达到峰值, 而后逐渐降低; Surface Evolver模拟与液桥界面环形近似的计算结果相对比, 当固液接触角较大时(θ = 60°和θ = 80°), 二者相对误差在6%以内, 而当固液接触角减小到30°及以下时, 相对误差随之增大, 且颗粒间分离距离越大, 相对误差越大.      

不等径颗粒间液桥力学参数及形态的试验研究

作为一种自然界中广泛存在的力, 液桥力的研究对制药、重金属回收、颗粒分离等领域具有十分重要的意义. 利用纳米多功能拉伸试验机测量不等径颗粒间液桥拉伸过程中的液桥力?位移曲线, 同时配合CCD工业相机记录拉伸全过程液桥形态的变化. 首先分析了液桥力?位移曲线形态、最大液桥力、断裂距离随粒径比及液桥体积的变化规律, 其次基于圆环假设及Y-L方程对本文试验结果的合理性进行验算, 最后针对圆环假设在液桥力计算中存在的不足分析了其原因, 并结合重力对液桥形态的影响对液桥拉伸全过程的形态变化进行了具体分析. 结果表明: 最大液桥力受粒径比的影响较大而受液桥体积的影响较小, 与最大液桥力相反, 断裂距离受液桥体积的影响较大而受粒径比的影响较小; 圆环假设可以较好地预测最大液桥力大小但对拉伸过程中的液桥力预测不准, 这是由于当液桥力达到最大值后液桥的外轮廓已不能用圆环表示; 根据重力对液桥形态的影响, 将拉伸过程液桥外轮廓的变化简化为重力影响可以忽略时的圆环形?抛物线形, 重力影响处于过渡阶段或影响较小时的长轴与短轴之比不断增大的椭圆形, 以及重力影响不可忽略时的“冷却塔形”?双曲线形.      

重力影响下板间液桥断裂距离研究

运用实验测量和数值模拟两种方法对两平板间特征尺度为毫米量级的液桥断裂距离进行了研究,定量地给出了在有重力和无重力两种条件下,液桥断裂距离随着液桥体积和固体表面润湿性质变化的规律。结果显示,重力对液桥断裂距离的影响随着液桥特征尺寸的增大而增大,当特征尺寸达到毛细长度时,重力对亲水表面间液桥断裂距离的影响可达11%以上,对于疏水表面间的断裂距离影响更增加到20%以上。通过Surface Evolver软件可以很好地模拟准静态液桥拉伸-断裂过程,得到准确的液桥断裂距离。结果表明,即使在液桥特征尺寸小于毛细长度的条件下,液桥断裂距离也不能完全忽略重力的影响。     

两异径湿球颗粒间静态液桥力的近似解析公式及数值评测

针对异径球形湿颗粒间液桥力，以Young-Laplace公式为基础，结合环形近似法并引入等效半径，在宽松设定的条件下推导出了简化的近似解析公式。此公式形式简洁，且当两球半径相等时，可回归到Pitois等提出的径球液桥力公式。本文对此近似解析公式进行参数敏感度分析，发现随着颗粒间距的增大，液桥力对颗粒半径的敏感度降低。此外，本文近似解析公式与其他实验和理论结果进行对比发现，该近似解析公式与上述实验和数值结果相当吻合，表明该近似解析公式可以较准确地计算接触角较小时的异径球颗粒间的液桥力，因而适于湿颗粒系统的数值模拟。     

基于介观结构的饱和与非饱和多孔介质有效应力

基于描述含液颗粒材料介观结构的Voronoi 胞元模型和离散颗粒集合体与多孔连续体间的介-宏观均匀化过程, 定义饱和与非饱和多孔介质有效应力. 导出了计及孔隙液压引起之颗粒体积变形的饱和多孔介质广义有效应力. 用以定义广义有效应力的Biot 系数不仅依赖于颗粒材料的多孔连续体固体骨架及单个固体颗粒的体积模量(材料参数),同时与固体骨架当前平均广义有效应力及单个固体颗粒的体积应变(状态量) 有关. 提出了描述非饱和多孔介质中非混和固体颗粒、孔隙液体和气体等三相相互作用的具介观结构的Voronoi 胞元模型.具体考虑在低饱和度下双联(binary bond) 模式的摆动(pendular) 液桥系统介观结构. 导出了基于介观水力-力学模型的非饱和多孔介质的各向异性有效应力张量与有效压力张量. 考虑非饱和多孔介质Voronoi 胞元模型介观结构的各向同性情况,得到了与非饱和多孔连续体理论中唯象地假定的标量有效压力相同的有效压力形式.但本文定义的与确定非饱和多孔介质有效应力和有效压力相关联的Bishop 参数由基于三相介观水力-力学模型, 作为饱和度、孔隙度和介观结构参数的函数导出,而非唯象假定.      

阿姆斯特朗电解液液桥的实验现象及断裂机理研究

阿姆斯特朗液桥是电流体力学领域中的一种有趣现象,它悬在空中抵抗重力流动的神奇状态引起了人们的广泛关注．近十几年来,去离子水液桥已经通过理论和实验得到深入研究,但是对于电解液液桥的研究依然十分有限．本文以Na2SO4电解液作为形成液桥的实验液体,利用高速摄相机和红外热像仪研究了电解液液桥的形成过程、焦耳效应、流动中的热气泡产生及其破碎导致的液桥断裂等现象,提出了不同于去离子水液桥的断裂机理,有助于进一步加深人们对液桥这一复杂现象的理解．     

Marangoni效应下的液桥自由面

改进Ｏｓｈｅｒ的ＬｅｖｅｌＳｅｔ方法用来计算考虑Ｍａｒａｎｇｏｎｉ效应的液桥自由面及内部的涡流．提出以双调和函数来作为Ｌｅｖｅｌ函数,从而避免了它们的奇性．求出的自由面的波纹形状以及二次涡远比简化算法所得要小．     

球形和方形湿颗粒团的碰撞模拟研究

本研究利用颗粒离散元方法,并结合接触力学理论,对微米级颗粒组成的湿颗粒聚合体碰撞破损的细观力学机理进行了模拟研究。模拟的颗粒聚合体分成球形和方形。将两种形状的颗粒聚合体碰撞结果做比较,颗粒之间的接触采用干、湿接触两种模型并存的方式来模拟湿颗粒团碰撞中的不可恢复变形过程。在碰撞速度保持不变的情况,考察了不同液体黏度对颗粒聚合体碰撞的影响;在液体黏度保持不变的情况,考察了不同碰撞速度对碰撞结果的影响。研究结果表明,湿颗粒团表现出了与干颗粒团完全不同的损伤模式,没有产生类似于弹性颗粒团的裂解损伤。湿颗粒团在碰撞中,它的内部液桥数量会在与平板碰撞接触的初期快速减少,然后慢慢累计,或会超过湿颗粒团在碰撞前的液桥数量。这一现象在此仿真研究中首次被发现。比如,液体黏度为50mPa·s的方形湿颗粒团以2.0m/s撞击平板时,其内部液桥数可由约9400降低至约8700后,逐渐增加至11300左右;由低黏度液体组成的湿颗粒团在碰撞后会发生反弹现象。而随着液体黏度的增加,恢复系数会趋近于0。     

滴状模式下液桥形成及断裂的电流体动力学特性研究

对电场作用下微通道荷电液滴脱落过程中液桥形成及断裂的显微演变特征进行了可视化实验研究.借助时空分辨率较高的高速摄像技术精确捕捉了电场作用下液桥形成及断裂的界面演化过程,研究了液桥的界面结构变化及其断裂的动力学显微演变行为,获得了时间特征数、电邦德数及半月面形成角对液桥长度及断裂顺序的作用规律.实验结果显示,液桥断裂长度取决于黏度与表面张力之比,而受荷电弛豫时间的影响甚微,低电压工况下各实验介质液桥相对长度的变化并不明显,而在较高电压工况下相对液桥长度的增长速度加快.随着电邦德数的不断增加,液桥长度的变化在较高邦德数下更为明显且存在突变区,此时伴随着雾化模式的转变,表明液桥的突变恰恰是雾化模式过渡的信号.不同物性介质的射流过渡行为由于液桥上下游形成角的变化而存在较大差异.对于无水乙醇介质,电邦德数的增加使滴状模式首先过渡到纺锤模式,而对于生物柴油,滴状模式后会首先出现脉动模式而非纺锤模式.      

半浮区液桥热毛细对流速度场特征

液桥流场的速度分布是液桥表面张力梯度驱动对流的一个重要研究内容。通过地面模拟微重力环境的液桥实验以及数值计算,研究了外加温差、液桥腰径等对流场速度分布的影响,实验结果与数值计算结果相当符合。结果还表明,当液桥腰径减小时,流场的结构发生了变化,涡心由一个转化成两个。     

Exact solution for capillary interactions between two particles with fixed liquid volume

The capillary interactions, including the capillary force and capillary suction, between two unequal-sized particles with a fixed liquid volume are investigated. The capillary interaction model is used within the Young-Laplace framework. With the profile of the meridian of the liquid bridge, the capillary suction, and the liquid volume as state variables, the governing equations with two-fixed-point boundary are first derived using a variable substitution technique, in which the gravity effects are neglected. The capillary suction and geometry of the liquid bridge with a fixed volume are solved with a shooting method. In modeling the capillary force, the Gorge method is applied. The effects of various parameters including the distance between two particles, the ratio of particle radii, and the liquid-solid contact angles are discussed.     

Capillary bridges and capillary forces between two axisymmetric power–law particles

Capillary interactions are fundamentally important in many scientific and industrial fields. However, most existing models of the capillary bridges and capillary forces between two solids with a mediated liquid, are based on extremely simple geometrical configurations, such as sphere–plate, sphere–sphere, and plate–plate. The capillary bridge and capillary force between two axisymmetric power–law profile particles with a mediated constant-volume liquid are investigated in this study. A dimensionless method is adopted to calculate the capillary bridge shape between two power–law profile particles based on the Young–Laplace equation. The critical rupture criterion of the liquid bridge is shown in four forms that produce consistent results. It was found that the dimensionless rupture distance changes little when the shape index is larger than 2. The results show that the power–law index has a significant influence on the capillary force between two power–law particles. This is directly attributed to the different shape profiles of power–law particles with different indices. Effects of various other parameters such as ratio of the particle equivalent radii, liquid contact angle, liquid volume, and interparticle distance on the capillary force between two power–law particles are also examined.     

Liquid bridge force between two unequal-sized spheres or a sphere and a plane

Liquid bridge force acting between wet particles is an important property in particle characterization. This paper deals with liquid bridge force between either two unequal-sized spherical particles or a sphere and a flat plate under conditions where gravitational effect arising from bridge distortion is negligible. In order to calculate the force of the liquid bridge efficiently and accurately, expressions of liquid configuration and liquid bridge force were derived by building a mechanical model, which assumes the liquid bridge to be circular in shape between either two unequal-sized spheres or a sphere and a plane. To assess the accuracy of the numerical results of the calculated liquid bridge forces, they were compared to the published experimental data.     

基于离散元法的干湿颗粒系统仿真

介绍了基于离散元法的干湿颗粒系统仿真软件DEMSIM。对于干颗粒系统,DEMSIM可以分析二维和三维颗粒系统的弹性和塑性接触碰撞过程;对于湿颗粒系统,DEMSIM采用传统的液桥模型;对于颗粒-流体系统,DEMSIM采用CFD-DEM细观耦合模型模拟。一系列典型算例的模拟分析,验证了干湿颗粒系统仿真软件DEMSIM的精度和有效性。     

Separation efficiency of liquid–solid undergoing vibration based on breakage of liquid bridge

Vibrating separation is a significant method for liquid–solid separation. A typical example is the vibrating screen to dewater wet granular matter. The properties of granular matter and the vibrating parameters significantly affect the separation efficiency. This study investigates the effect of vibration parameters in separation based on the breakage of large-scale liquid bridge numerically by using a calibrated simulation model. Through analysing the simulation results, the liquid bridge shape and the volume between two sphere particles for various particle sizes and particle distances were studied in the static condition under the effect of gravity. The results show a general reducing trend of liquid bridge volume when the radius ratio of two particles increases, particularly when the ratio increases to 5. Additionally, a set of vibrating motion was applied to the liquid bridge in the simulation model. A group of experiments were also performed to validate the simulation model with vibration. Then, the effect of vibrating peak acceleration, distance between spheres and radius on the separation efficiency which was reflected by the residual water were investigated. It is found that separation efficiency increased obviously with the peak acceleration and the increase slowed down after the peak acceleration over 1 m/s².     

Analysis of liquid bridge between spherical particles

A pair of central moving spherical particles connected by a pendular liquid bridge with interstitial Newtonlan fluid is otten encountered in particulate coalescence process. In this paper, by assuming perfect-wet condition, the effects of liquid volume and separation distance on static liquid bridge are analyzed, and the relation between rupture energy and liquid bridge volume is also studied. These points would be of significance in industrial processes related to adhesive particles.     

Lattice Boltzmann study of the interaction between a single solid particle and a thin liquid film

The isothermal single-component multi-phase lattice Boltzmann method(LBM) combined with the particle motion model is used to simulate the detailed process of liquid film rupture induced by a single spherical particle.The entire process of the liquid film rupture can be divided into two stages.In Stage 1,the particle contacts with the liquid film and moves into it due to the interfacial force and finally penetrates the liquid film.Then in Stage 2,the upper and lower liquid surfaces of the thin film are driven by the capillary force and approach to each other along the surface of the particle,resulting in a complete rupture.It is found that a hydrophobic particle with a contact angle of 106.7° shows the shortest rupture duration when the liquid film thickness is less than the particle radius.When the thickness of the liquid film is greater than the immersed depth of the particle at equilibrium,the time of liquid film rupture caused by a hydrophobic particle will be increased.On the other hand,a moderately hydrophilic particle can form a bridge in the middle of the liquid film to enhance the stability of the thin liquid film.