蒸发液面热对流与温度不连续现象的实验研究

实验研究了存在蒸发界面的水平液层中的热毛细对流对汽/液界面处温度不连续的影响.对于单纯的热毛细对流从理论和实验已有深入研究,但目前国际对蒸发与热毛细对流的耦合尚缺乏研究.特别是近期C.A.Ward等人研究发现了蒸发汽液界面处的温度不连续现象.本文以存在蒸发界面的水平薄液层为研究对象,测量了蒸发界面处的温度分布,研究了普...     

Coupling of Evaporation and Thermocapillary Convection in a Liquid Layer with Mass and Heat Exchanging Interface

We propose and analyse a new model of thermocapillary convection with evaporation in a cavity subjected to horizontal temperature gradient, rather than the previously studied model without evaporation. The pure liquid layer with a top free surface in contact with its own vapour is considered in microgravity condition. The computing programme developed for simulating this model integrates the two-dimensional, time-dependent Navier-Stokes equations and energy equation by a second-order accurate projection method. We focus on the coupling of evaporation and thermocapillary convection by investigating the influence of evaporation Biot number and Marangoni number on the interracial mass and heat transfer. Three different regimes of the coupling mechanisms are found and explained from our numerical results.     

Steady parallel flow in an evaporating fluid heated from sidewalls

Evaporation is ubiquitous in nature, but very few attempts have been made in the past to couple the effects of evaporation with fluid flow behavior. In this theoretical paper we have discussed the effects of evaporation on the dynamics of steady state thermocapillary convection in a two-dimensional rectangular container. The liquid is heated by differentially heated sidewalls and mass loss from the interface due to evaporation is compensated by the liquid entering into the container through a lower inlet, thus keeping the thickness of the liquid layer constant. We show that for an evaporating liquid one can obtain a plane parallel base state profile which depends on the evaporative mass flux.     

EXPERIMENTAL STUDY OF MARANGONI-BENARD CONVECTION IN A LIQUID LAYER INDUCED BY EVAPORATION

Marangoni-Bénard instability and convection in evaporating liquid layers have been studied experimentally through flow visualization and temperature profile measurement. Bénard cells have been observed in an evaporating thin liquid layer whether it is heated, adiabatic, or cooled from below. This experimental study has revealed a different mechanism from the traditional Rayleigh-Bénard and Marangoni-Bénard instabilities and convections, which require a negative temperature gradient in the thin liquid layer. Evaporation rate and enthalpy of evaporation have been found to be important parameters of instability and convection in an evaporating liquid layer. A modified form of Marangoni number, Ma^*, is proposed and its critical values, Ma^* _c, for alcohol and Freon-113 evaporating layers are determined experimentally. A quantitative comparison between Ma^* and the traditional Marangoni number, Ma, shows that Ma^* is an adequate indicator of the stability status in evaporating liquid layers.     

Experimental Study on Liquid Free Surface in Buoyant-Thermocapillary Convection

We investigate the surface deformations of buoyant-thermocapillary convection in a rectangular cavity due to gravity and temperature gradient between the two sidewalls. The cavity is 52mm×42 mm in horizontal cross section, the thickness of liquid layer h is changed from 2.5 mm to 6.5 mm. Surface deformations of h = 3.5 mm and 6.0mm are discussed and compared. Temperature difference is increased gradually, and the flow in the liquid layer will change from stable convection to unstable convection. Two kinds of optical diagnostic system with image processor are developed for study of the kinetics of buoyant-thermocapillary convection, they give out the information of liquid free surface. The quantitative results are calculated by Fourier transform and correlation analysis, respectively. With the increasing temperature gradient, surface deformations calculated are more declining. It is interesting phenomenon that the inclining directions of the convections in thin and thick liquid layers are different. For a thin layer, the convection is mainly controlled by thermocapillary effect. However, for a thick layer, the convection is mainly controlled by buoyancy effect. The surface deformation theoretically analysed is consistent with our experimental results. The present experiment proves that surface deformation is related to temperature gradient and thickness of the liquid layer. In other words, surface deformation lies on capillary convection and buoyancy convection.     

Numerical simulation of natural convection in a sessile liquid droplet

Heat transfer in a sessile liquid droplet was studied with numerical methods. A computer code was developed for solving the problem of convection in an axisymmetric hemispherical droplet and in a spherical layer as well. The problem of establishing an equilibrium state in a droplet was solved using several variables: temperature, stream function, and vorticity. Simulation was performed for droplets of water, ethyl alcohol, and model liquids. Variable parameters: intensity of heat transfer from droplet surface, Rayleigh and Marangoni dimensionless criteria, and the characteristic temperature difference. It was revealed that the curve of convective flow intensity versus heat transfer intensity at droplet surface has a maximum. A dual-vortex structure was obtained in a stationary hemispherical profile of liquid droplet for the case of close values for thermocapillary and thermogravitational forces. Either thermocapillary or thermogravitational vortex might be dominating phenomena in the flow structure.     

Energy transport by thermocapillary convection during Sessile-Water-droplet evaporation

The energy transport mechanisms of a sessile-water droplet evaporating steadily while maintained on a Cu substrate are compared. Buoyancy-driven convection is eliminated, but thermal conduction and thermocapillary convection are active. The dominant mode varies along the interface. Although neglected in previous studies, near the three-phase line, thermocapillary convection is by far the larger mode of energy transport, and this is the region where most of the droplet evaporation occurs.     

Thermocapillary deformation of a thin locally heated horizontal liquid layer

In this paper, steady thermocapillary flow in a thin horizontal layer of a viscous incompressible liquid with a free surface is considered. An axially symmetric steady problem with a localized thermal action on a horizontal liquid layer with a deformable free surface is solved in a thin-layer approximation. In addition to the thermocapillary effect, the model takes into account the capillary pressure caused by the free surface variable curvature and the convective mechanism of heat transfer in the liquid. Analytical expressions for the velocity vector components as functions of the liquid layer thickness and surface temperature are obtained. The free surface and velocity profiles caused by various kinds of heating are calculated. The influence of convective heat transfer on the flow pattern is analyzed.     

Study of dynamics of thin liquid layer breakdown under conditions of spot heating and formation of a droplet cluster

Breakdown dynamics was studied experimentally for the horizontal layers of various liquids (ethanol, water) with the thickness of 300 μm under the conditions of spot heating from the substrate. The main stages of the process of liquid layer breakdown were determined, and time of dry spot formation was measured. Time of dry spot formation for ethanol at the heat flux of 12.6 W/cm² was 7.85 s, and for water at the heat flux of 117 W/cm², it was 0.13 s. It was found that for both working liquids, a residual layer appears in the region of spot heating before liquid layer breakdown. It is shown that together with the thermocapillary effect, evaporation is one of the main factors affecting dynamics of liquid layer breakdown and dry spot formation.     

Thermographic investigation of the temperature field dynamics at the liquid-air interface in drops of water solutions drying on a glass substrate

The temperature field distribution at the liquid-air interface in drops of water and water solutions drying on a glass substrate is studied with thermal imaging means. It is shown that irrespective of the liquid composition, the circumferential temperature of the drops (along the boundary line) is always higher than the temperature on the top. The temperature field on the surface of the drops is nonstationary and varies chaotically during water evaporation. It is found that the dynamics of histograms for albumin-containing and albumin-free liquids differ. Mechanisms behind the origination of thermocapillary liquid flows and their directivity in the drying drops are discussed.     

分离结晶过程中熔体热毛细对流的转变

为了了解微重力条件下新型分离结晶生长过程中熔体热毛细对流的基本特征,利用有限差分法进行了数值模拟,熔体深径比A取1和2,自由界面无因次宽度B分别取0.05、0.075和0.1.当熔体上表面为自由表面时,得到了分离结晶Bridgman生长过程中熔体热毛细对流的流函数和温度分布.计算结果表明:当Ma数较小时,在上下两个自由表面的表面张力的驱动下,熔体内部产生了两个流动方向相反的流胞,流动为稳态流动,随着Ma数的增加,上下自由表面速度增大,温度分布的非线性增加;当Ma数超过某一临界值后,流动将转化为非稳态流动;与熔体上表面为固壁时相比,A=1时的临界Ma数减小,而A=2时的临界Ma数增大;流动失稳的物理机制是流速的变化和阻力的变化之间存在滞后.     

Evaporation rate of PTFE liquid marbles

Liquid marbles are hydrophilic liquid drops encapsulated with a hydrophobic powder. They behave as micro-reservoirs of liquids able to move rapidly without any leakage and are promising candidates to be applied in genetic analysis where 2D microfluidics and lab-on-a-chip methods are used. The manipulation of liquid marbles using gravitational, electrostatic and magnetic fields were recently investigated. In this work, we determined the evaporation rates of PTFE marbles formed by encapsulating PTFE micropowder on a water droplet in a closed chamber where relative humidity and temperature was kept constant. Evaporation rates of PTFE marbles were compared with the rates of pure water droplets in terms of evaporation resistance, ? parameter and it was found that PTFE marbles have longer life-time than water droplets so that ? values were found to increase regularly from 0.365 to 0.627 with the increase of RH of the evaporating medium. The barrier effect of PTFE microparticles at the water-air interface was more effective when water was evaporating slowly. PTFE water marbles have life-time of 26-60 min to retain their spherical shape under normal atmospheric conditions which is suitable for many promising applications in microfluidics, genetic analysis, electromagnetic actuators and valves.     

Large Eddy Simulation of a swirl stabilized spray flame

Large Eddy Simulations (LES) of kerosene spray combustion in an axial-swirl combustor have been carried out focusing on the effect of the evaporating droplets on the flame temperature and species concentrations. The LES-PDF methodology is used for both dispersed (liquid) and gas phases. The liquid phase is described using a Lagrangian formulation whilst an Eulerian approach is employed for the gas phase. The predictive capability of LES with sub-grid scale models for spray dispersion and evaporation is assessed placing emphasis on the effect of the unresolved velocity and temperature fields on the droplet evaporation rate. The results of the fully coupled LES formulation exhibit good agreement between the measured and simulated mean velocity fields. The global behaviour of the spray combustion, such as droplet dispersion and evaporation, are captured reasonably well in the simulations. It was found that the large velocity fluctuations observed in the shear layer strongly affect the evaporation rate and thus the temperature distributions. The present work also demonstrated the feasibility of LES to study complex flow features which are typical of gas-turbine combustion chambers.     

微重力下液封对液桥内热毛细对流影响的研究

本文建立了具有液封的液桥（不相溶混的双层同轴液柱）内热毛细对流的物理模型和数学模型。采用涡量－流函数法对微重力条件下具有液封的液桥内热毛细对流进行了数值模拟，得到了双层液柱主流区的温度场和流场，证实了液封能够削弱液桥内热毛细对流，从而提高浮区晶体生长质量，并得到液封厚度对液桥内热毛细对流的影响规律。     

Steady and oscillatory flame spread over liquid fuels

The flow induced in a layer of liquid fuel at sub-flash temperature by the thermocapillary forces associated with the spreading of a flame that heats and vaporizes the liquid is analysed numerically and asymptotically, for large values of the Marangoni number and of the Reynolds number based on the propagation speed. Upstream heat convection in a recirculating region moving with the flame front is described for a steady model problem and for uniform and pulsating flame spread. A possible mechanism triggering flow oscillations entirely dependent on the liquid phase is identified and discussed.     

存在液膜的毛细蒸发过程研究

采用水浴恒温蒸发方法研究了液膜存在时的毛细蒸发过程. 研究结果表明：毛细蒸发过程中液/气界面符合黏性指延,且不受蒸发速度及液膜状态的影响;蒸发过程可分为剧烈减速、匀速蒸发、线性减速蒸发和边界效应四个阶段,分别对应不同的液膜状态;毛细蒸发的主要区域在毛细管端口液膜处,工质由液体区流向端口蒸发区的过程中液膜起到了通道桥梁作用. 关键词： 毛细蒸发 液/气界面 蒸发阶段 液膜     

Rheological flow from a die and painting on a moving solid wall

1IntroductionThemanufacturesoffluidfilmandpolymerrequireanunderstandingofthehydrodynamicprocess[1,2].Theenlargedcross-sectionofthefluidjetwasobservedinthepolymerprocessing,wherethedieswellisanimportantphenomenon.TheSwellorDieSwelleffectisexplainedusuallybyrheologicalpropertiesoftheliquidmedium.TannersuggestedatheoryofDie-Swell,andassumedthatthevelocityvectorhasonlyonecomponentalongthejet[3].Becauseofthenon-uniformityofthecross-section,theflowfieldintheDie-Swelltheoryshouldbeatleasttwo-dime…     

磁场对水平温度梯度下双层流体热对流的影响

在水平温度梯度下,双层流体交界面的表面张力会出现梯度,驱动热毛细对流运动,造成热剪切层内的扰动.本文数值模拟了不同重力条件下,双层流体内的对流现象,得出了在微重力时,对流运动将引起热剪切层内强烈的扰动.为了减弱这种扰动,我们利用磁场对流体的运动进行控制.为此,又对微重力条件下,不同方向应用磁场下的热剪切层内扰动行为进行了数值研究,结果显示,磁场对热剪切层稳定性有促进作用,加入法向的应用磁场最为有效.     

Combined action of different mechanisms of convective instability in multilayer systems

The nonlinear regimes of convection in a system of three immiscible viscous fluids are investigated by the finite-difference method. We study new phenomena caused by direct and indirect interaction of thermocapillary and buoyancy (Rayleigh and anticonvective) instability mechanisms. Two variants of heating-from below and from above-are considered. The interfaces are assumed to be flat. We focus on nonlinear evolution of steady and oscillatory motions and selection of stable convective structures depending on the parameters of systems. The influence of the lateral boundary conditions is also investigated. A classification of different variants of interaction between Rayleigh and thermocapillary instability mechanisms is presented, and several typical examples are studied. Specifically, we considered six different configurations where the Rayleigh convection arises mainly in a definite layer, and the thermocapillary convection appears mainly near the definite interface. Also, the case where both interfaces are active and alternatively play a dominant role is investigated. Some configurations of interaction between anticonvective and thermocapillary instability mechanisms are considered.     

大尺度环形液池内双层热毛细对流不稳定性

假设双层流体的交界面不发生变形,热毛细力作用于此交界面,三维数值研究了大尺度环形液池中双层流体系统在内外壁面温差加热下的热毛细对流不稳定性,其中外壁面维持高温,内壁面维持低温。计算结果显示,上下层流体的流动特性受Marangoni效应和浮力效应的影响;热毛细对流的振荡产生于内壁面附近,并沿着温度梯度的方向传播;随着温差的增大,热毛细对流的振荡逐渐增强,温度振荡波数增大。