微重力下汽泡底部微层的对流和蒸发

本文数值研究了微重力下沸腾时汽泡底部微层区的多维传热及流动，不仅考虑由温度梯度引起的汽泡表面张力梯度而诱发的Marangoni流动，而且考虑汽泡表面的凝结和蒸发．微层厚度自0．01μm到10μm的计算结果表明，微层中流动很小，温度分布与纯导热几乎相同．在界面传热小于理想传热的情况下，Marangoni对流对传热增加几个百分比，微层中的流动与薄平界面的滑移近似相吻合．     

液滴碰撞对液膜内汽泡运动与换热影响

采用数值模拟的方法,研究了沸腾雾化喷射过程中热壁面薄液膜层受到液滴碰撞扰动时液膜层内汽泡运动,相界面变化和由此引起的壁面换热特性.模拟结果显示汽泡生长初期相界面变化与液膜层内二次核化特征与文献结果吻合良好,汽泡生长后期相界面变化存在滞后.讨论了液滴下降速度.液滴直径与初始位置,多液滴碰撞对液膜层内流动与壁面换热的影响.     

一个新的核态沸腾汽泡动力学模型

本文在经典汽泡动力学理论基础上，提出了描述汽泡生长过程的综合界面模型．本模型的核心在于汽泡内部的热力学过程的详细分析及汽液界面的传热、传质过程的详细描述．并对汽泡生长过程进行了模拟计算，给出了动力学控制阶段的时间范围．本模型对汽泡生长、汽膜发展的理论分析及数值模拟提供了良好的基础．     

溶液中球形晶体溶解的分析

从物质溶液浓度变化角度考虑了球形晶体在溶液中溶解随表面张力的变化,利用渐近方法求出了在溶液中球形晶体溶解的浓度和界面的近似解析解,能够计算出溶解过程中球形晶体溶解的浓度、界面演化形态.研究了表面张力对于溶液中溶质浓度分布、球晶界面形态和溶解速度的影响.结果表明,表面张力促进了球形晶体在溶液中的溶解.随着表面张力参数增大,溶液中在界面前沿的溶质浓度升高,球形晶体的溶解速度增大;随着时间的增加,溶解速度逐渐变大,球形晶体半径逐渐变小,直至溶解结束.     

微纳复合结构表面汽泡动力学行为可视化研究

本文利用高速摄像运动分析技术对竖直放置的微纳复合结构表面热沉中的汽泡动力学行为进行了可视化实验研究。本文对微纳复合结构热沉表面的汽泡生长过程进行了描述;得到了纳米涂层的厚度变化对汽泡生长当量直径和生命周期的影响的实验结果。实验结果表明:随着微纳复合结构表面的纳米层厚度增加,汽泡生命周期缩短,汽泡的三个生长阶段之间的界限逐渐不明显,汽泡破裂时的汽泡当量直径减小。     

流体力学平衡条件的补充形式与球面系统表面张力定义

本文以锥形球面两相系统体积变化时的功为基础，在球面迁移层取分割面后导出流体力学平衡条件补充形式．并对两相球界面复元系统自由能变化进行了分析，导出球表界的表面张力定义，讨论了与过去定义的关系．     

微通道中凝结换热的流型及多通道效应

本文在空气自然冷却条件下,对MEMS硅基并联微通道中蒸汽凝结换热的流型和多通道效应进行了可视化研究,发现在低入口饱和蒸汽压下,通道入口为准静止状态的汽弹、汽弹前端周期性脱离汽泡、通道下游为泡状流;在高入口饱和蒸汽压下,通道内为环状流.汽泡脱离存在单汽丝断裂、双汽丝同步断裂以及双汽丝非同步断裂三种不同模式.汽液界面上表面张力不均匀引起Marangoni对流,使得两侧通道中的汽泡一旦形成,便被推向通道的高温侧.     

气泡微液底层蒸干特性分析

本文从气泡微液底层的流动特性出发，利用Ｈａｌｌｉｎａｎ和Ｅｒｖｉｎ模型对微层蒸干的Ｓｔｒａｕｂ判据进行了理论分析，证实了临界蒸干半径与湿润性有关，同时指出微层蒸发中起关键作用的是膨胀压力而不是表面张力梯度。     

微槽群内汽泡动力学行为对接触线的影响

针对沸腾情形下毛细微槽群热沉内汽泡对汽液固三相接触线的影响,进行了可视化实验研究。研究结果表明：汽泡动力学行为会直接导致微槽群内三相接触线的形状变化,从而引起蒸发薄液膜的厚度、面积以及汽液界面曲率等对薄液膜的蒸发换热特性有重要影响的物理量的变化。实验证实了在开放式微细尺度槽群结构热沉中,固有弯月面区域里的沸腾与扩展弯月...     

振动工况下过冷流动沸腾界面参数实验研究

过冷流动沸腾中局部时均界面参数对发展两流体模型十分重要,而振动工况下过冷流动沸腾的界面参数变化很少有文献报道。本研究采用四探头电导探针分别对静止工况和低频水平振动工况下环管内过冷流动沸腾的时均空泡份额、界面面积浓度和汽泡平均直径进行了测量,并且研究了不同流动工况对局部参数在径向位置处分布的影响。实验发现,低频水平振动对局部时均参数的影响比较有限,局部时均空泡份额和界面面积浓度略有下降,汽泡直径分布出现轻微波动。     

耦合界面张力的三维流体界面不稳定性的格子Boltzmann模拟

采用介观格子Boltzmann方法模拟界面张力作用下三维流体界面的Rayleigh-Taylor (RT)不稳定性的增长过程,主要分析表面张力对流体界面动力学行为及尖钉和气泡后期增长的影响机制.首先发现三维RT不稳定性的发生存在临界表面张力(σ_c),其值随着流体Atwood数的增大而增大,且数值预测值与理论分析结果σ_c=(ρ_h-ρ₁)g/k~2一致.另外,随着表面张力的增大,不稳定性演化过程中界面卷吸程度和结构复杂性逐渐减弱,系统中界面破裂形成离散液滴的数目也显著减少.相界面的后期动力学行为也从非对称发展转向始终保持关于中轴线对称.尖钉与气泡振幅在表面张力较小时对其变化不显著,当表面张力增大到一定值后,可以有效地抑制尖钉与气泡振幅的增长.进一步发现,高雷诺数三维RT不稳定性在不同表面张力下均经历4个不同的发展阶段:线性阶段、饱和速度阶段、重加速和混沌混合阶段.尖钉与气泡在饱和速度阶段以近似恒定的速度增长,其渐进速度的值与修正的势流理论模型结果一致.受非线性Kelvin-Helmholtz旋涡的剪切作...     

Heterogeneous diffuse interfaces: A new mechanism for arrested coarsening in binary mixtures Heterogeneous diffuse interfaces

We discuss the dynamics of binary fluid mixtures in which surface tension density is allowed to become locally negative within the interface, while still preserving positivity of the overall surface tension (heterogeneous diffuse interface). Numerical simulations of two-dimensional Ginzburg-Landau phase field equations implementing such mechanism and including hydrodynamic motion, show evidence of dynamically arrested domain coarsening. Under specific conditions on the functional form of the surface tension density, dynamical arrest can be interpreted in terms of the collective dynamics of metastable, non-linear excitations of the density field, named compactons, as they are localized to finite-size regions of configuration space and strictly zero elsewhere. Aside from compactons, the heterogeneous diffuse interface scenario appears to provide a robust mechanism for the interpretation of many aspects of soft-glassy behaviour in binary fluid mixtures.     

Heterogeneous diffuse interfaces: a new mechanism for arrested coarsening in binary mixtures. Heterogeneous diffuse interfaces

We discuss the dynamics of binary fluid mixtures in which surface tension density is allowed to become locally negative within the interface, while still preserving positivity of the overall surface tension (heterogeneous diffuse interface). Numerical simulations of two-dimensional Ginzburg-Landau phase field equations implementing such mechanism and including hydrodynamic motion, show evidence of dynamically arrested domain coarsening. Under specific conditions on the functional form of the surface tension density, dynamical arrest can be interpreted in terms of the collective dynamics of metastable, non-linear excitations of the density field, named compactons, as they are localized to finite-size regions of configuration space and strictly zero elsewhere. Aside from compactons, the heterogeneous diffuse interface scenario appears to provide a robust mechanism for the interpretation of many aspects of soft-glassy behaviour in binary fluid mixtures.     

Effects of finite thickness on interfacial widths in confined thin films of coexisting phases

The capillary broadening of a 2-phase interface is investigated both experimentally and theoretically. When a binary mixture in a thin film with thickness D segregates into two coexisting phases the interface between the two phases may form parallel to the substrate due to preferential surface attraction of one of the components. We show that the interfacial profile (of intrinsic width w₀) is broadened due to capillary waves, which lead to fluctuations, of correlation length of the local interface positions in the directions parallel to the confining walls. We postulate that acts as an upper cutoff for the spectrum of capillary waves on the interface, so that the effective mean square interfacial width w varies as . In the limit of large D this yields or respectively for the case of short- or long-range forces between walls and the interface. We used the Nuclear Reaction Analysis depth profiling technique, to investigate this broadening effect directly in two binary polymer mixtures. Our results reveal that the interfacial width indeed increases with film thickness D, though the observed interfacial width is lower than the predicted w. This is probably due to surface tension effects imposed by the confining surfaces which are not taken into account in our model. Received: 19 February 1998 / Received in final form: 2 September 1998 / Accepted: 8 September 1998     

Numerical Simulation of Vapor Bubble Growth and Heat Transfer in a Thin Liquid Film

A mathematical model is developed to investigate the dynamics of vapor bubble growth in a thin fiquid film, movement of the interface between two fluids and the surface heat transfer characteristics. The model takes into account the effects of phase change between the vapor and liquid, gravity, surface tension and viscosity. The details of the multiphase flow and heat transfer are discussed for two cases： （1） when a water micro-droplet impacts a thin liquid film with a vapor bubble growing and （2） when the vapor bubble grows and merges with the vapor layer above the liquid film without the droplet impacting. The development trend of the interface between the vapor and liquid is coincident qualitatively with the available literature, mostly at the first stage. We also provide an important method to better understand the mechanism of nucleate spray cooling.     

Curvature-dependent surface tension of a growing droplet

A ghost interface simulation technique is developed and applied to supersaturated Lennard-Jones liquid-vapor interfaces. It is shown that the surface tension decreases approximately linearly with the supersaturation ratio and that it vanishes at the spinodal. The effect leads to a curvature-dependent surface tension since, it is argued, the local supersaturation of the vapor above a droplet is greater than in the bulk due to slow diffusion in the vapor phase. An analytic approximation is given for the local supersaturation ratio, and an analytic expression for this contribution to Tolman's length is derived. The theory gives a smaller critical radius and reduces the free energy barrier to nucleation compared to classical homogeneous nucleation theory, which have important implications for the kinetics of droplet and bubble formation.     

Theory of the liquid-vapour interface of a binary mixture of Lennard-Jones fluids

We present results of calculations of the equilibrium surface tension and density profiles for the liquid-vapour interface of a binary mixture of Lennard-Jones 12-6 fluids. The calculations are based on a density-functional theory for the Helmholtz free energy of the inhomogeneous mixture. This is a ‘microscopic’ generalization of the van der Waals-Cahn-Hilliard theory for the interface of a binary mixture.

Our calculations cover the full range of liquid-vapour coexistence and the whole range of concentration. We find a correlation between the excess surface tension of the mixture and the surface segregation (adsorption) of the species with the lower surface tension. The ways in which segregation and excess surface tension depend on the Lennard-Jones parameters of the pure components are briefly discussed. Our results for the excess surface tension of mixtures of Ar and N₂ and Ar and CH₄ are compared with experiment; the agreement is reasonable.     

Viscoelastic theory for nematic interfaces

A complete macroscopic theory for compressible nematic-viscous fluid interfaces is developed and used to characterize the interfacial elastic, viscous, and viscoelastic material properties. The derived expression for the interfacial stress tensor includes elastic and viscous components. Surface gradients of the interfacial elastic stress tensor generates tangential Marangoni forces as well as normal forces. The latter may be present even in planar surfaces, implying that in principle static planar interfaces may accommodate pressure jumps. The asymmetric interfacial viscous stress tensor takes into account the surface nematic ordering and is given in terms of the interfacial rate of deformation and interfacial Jaumann derivative. The material function that describes the anisotropic viscoelasticity is the dynamic interfacial tension, which includes the interfacial tension and dilational viscosities. Viscous dissipation due to interfacial compressibility is described by the anisotropic dilational viscosity, and it is shown to describe the Boussinesq surface fluid appropriate for Newtonian interfaces when the director is homeotropic. Three characteristic interfacial shear viscosities are defined according to whether the surface orientation is along the velocity direction, the velocity gradient, or the unit normal. In the last case the expression reduces to the interfacial shear viscosity of the Boussinesq surface fluid. The theory provides a theoretical framework to study interfacial stability, thin liquid film stability and hydrodynamics, and any other interfacial rheology phenomena.     

表面活性剂浓度对球形气泡界面和尾流的影响

发展了一种研究气泡界面污染程度的数值模型,并用其对流场中不同表面活性剂浓度下、上浮气泡的界面参量和周围流场进行了模拟研究。该模型假设吸附于气泡界面的表面活性剂分布在毗邻气液界面的薄吸附层中,且气泡界面上表面活性剂的吸附与解吸过程也发生于此;界面切应力为界面浓度的函数。研究发现:气泡界面的流动性会因表面活性剂的吸附而降低,该现象会增大气泡周围流域中切向速度在界面法向上的变化量,从而对界面性质和周围流场产生影响;由于对流的作用和吸附-解吸动态平衡的存在,气泡前部界面不完全干净,且受污染界面的流动性也不完全为零。     

Laser-induced force for bubble-trapping in liquids

The laser-induced thermal force that traps a bubble in an absorbing liquid is studied theoretically and experimentally. The force origins from the thermal gradients of the surface tension induced by a laser beam over the bubble surface. It is shown that this force is proportional to the square of the bubble radius and to the thermal gradient of the surface tension. A simple experimental method based on comparison with the Archimedes' flotation force, is used for the measurement of this force. The experimental results, performed on ethanol solutions of iodine, confirm the theoretical predictions.