A sharp-interface level-set method for analysis of Marangoni effect on microdroplet evaporation

A level-set method is presented for computation of microdroplet evaporation including not only the effects of heat and mass transfer, phase change and contact line dynamics but also the Marangoni effect, which is a key parameter affecting the internal flow of the droplet and the particle deposition pattern. A sharp-interface formulation of the Marangoni force is derived and tested for two-phase Marangoni convection in a cavity. The computed results show good convergence in both the liquid and gas regions and are in excellent agreement with the analytical solutions. The level-set formulation is applied to microdroplet evaporation on a solid surface to investigate the Marangoni effect.     

Mixed convection heat transfer enhancement through film evaporation in inclined square ducts

The investigation of mixed convection heat transfer enhancement through film evaporation in inclined square ducts has been numerically examined in detail. The main parameters discussed in this work include the inclined angle, the wetted wall temperature and the relative humidity of the moist air mixture. The numerical results of the local friction factor, Nusselt number and Sherwood number are presented for moist air mixture system. Attention was particular paid to the effects of latent heat transport on the heat transfer enhancement. Results show that the latent heat transport with film evaporation augments tremendously the heat transfer rate. The heat transfer rate can be enhanced to be 10 times of that without mass transfer, especially for a system with a lower temperature. Besides, better heat and mass transfer rates related with film evaporation are found for case with a higher wetted wall temperature. The increase in the relative humidity of moist air in the ambient causes the decrease in heat transfer enhancement.     

Thermal and water management in irrigating lands in the arid and semi-arid regions

Excess heat and scarcity of water are the two major problems, which are usually encountered in irrigating lands especially in the arid and semi-arid regions. This paper introduces a technical approach of managing agricultural lands in the arid and semi arid regions through determination of daily water requirement and amount of heat the land is being exposed at various meteorological conditions. Through setting up a mathematical model consisting of basic heat and mass transfer equations and fluid properties, daily rate of water evaporation, different modes of heat transfer such as radiation, convection and heat transfer by evaporation at a wide range relative humidities are determined. Furthermore, the analyses are performed at two different scenarios at average air velocities of 1 and 5 m/s. Our findings showed that the volume of water evaporation at relative humidity and air temperature of φ=50% and T_∞=20 °C is 22% higher than at φ=100% and T_∞=20 °C. Moreover, at a specified φ and T_∞, the total rate of heat transfer at air velocity of 5 m/s is at least 25% higher than the total rate of heat transfer at air velocity of 1 m/s.     

Simulation of transient Rayleigh-Bénard-Marangoni convection induced by evaporation

Numerical simulation of thermal convection induced by solvent evaporation in an initially isothermal fluid is considered. Both thermocapillarity and buoyancy driving forces are taken into account, and a criterium based on the Peclet number is used to analyze the stability of this transient problem. Critical Marangoni and Rayleigh numbers are obtained for a large range of Biot and Prandtl numbers. Results of the non-linear simulations are compared with a previous linear transient stability analysis based on a non-normal approach and with visualizations performed during polyisobutylene (PIB)/toluene solutions drying experiments. A scaling analysis is developed for the Marangoni problem and correlations are derived to predict the order of magnitude of temperature and velocity as a function of Bi, Ma and Pr numbers.     

Onset of oscillatory thermocapillary convection in acetone liquid bridges: The effect of evaporation

Experiments have been carried out for half-zones of acetone (Pr = 4.3) to investigate the effects of evaporative cooling on the flow structures and temperature fields during transition from steady to oscillatory convection. The unstable flow phenomena have been measured using a variety of diagnostic techniques to determine the effects of evaporative cooling on Marangoni convection in liquid bridges of intermediate Prandtl number. The results show that Marangoni convection in acetone liquid bridges with and without strong evaporation becomes unstable due to the same mechanism but the evaporation has a strong stabilizing effect on the onset of oscillatory Marangoni convection.     

正丁醇水溶液液滴的蒸发特性实验研究

自湿润流体是一种具有特殊的表面张力特性的二元流体,了解其蒸发传热特性对于揭示其强化传热机理十分重要.为了探究添加自湿润流体液滴的蒸发特性,采用液滴形状分析仪(DSA100)研究了不同温度(30、40、50、60℃)下铜底板上去离子水、正丁醇水溶液(质量分数为0.5％)液滴的蒸发特性.结果 表明:加入少量正丁醇溶液并不影...     

蒸发液滴内部非稳态流动的数值计算

在气液两相流VOF(volume of fluid,VOF)模型的基础上耦合CSF(continuum surface force,CSF)表面张力模型,建立了高温平板上的铺展液滴与高温空气中悬浮液滴蒸发过程中内部非稳态流动模型,对液滴蒸发过程中内部非稳态流动进行了研究。基于相变理论,采用用户自定义函数将流体相变模型加入非稳态流动模型中进行耦合计算,获得了高温平板上的铺展液滴与高温空气中悬浮液滴蒸发过程中的内部流动及变化过程。液滴蒸发过程中非稳态内部流动由液滴表面的温度梯度引发,Marangoni流动在液滴内部形成的时间非常短,流体从液滴表面高温区域流向低温区域。计算结果表明:高温平板上随着液滴蒸发的进行,液滴内部一直保持两个对称的涡流,Marangoni流动比较稳定;高温空气环境中随着液滴蒸发的进行,液滴内部四个涡流逐渐转变成两个对称的涡流;液滴内部温度分布因Marangoni流动加强传热而变得均匀,同时由于温度分布变得均匀,Marangoni流动被削弱。     

Transient heat and mass transfer at the ignition of vapor and gas mixture by a moving hot particle

The processes of heat and mass transfer at the ignition of air and typical combustible liquid vapors mixture by a moving single metal particle heated till high temperatures are numerically investigated. The researches are carried out on the base of gas phase ignition model considering processes of heat conductivity, liquid evaporation, diffusion and convection of fuel vapors in oxidizing medium, ignition source crystallization, kinetic of evaporation and ignition processes, dependences of thermo physical properties of interacting substances from temperature, air humidity, heating source movement in vapor and gas mixture. The dependences of main characteristic of investigated process – ignition time delay from rate of particle motion, air humidity and temperature, distance between heating source and liquid surface, initial temperature and sizes of particle are determined.     

The effect of soluble anionic surfactants on rise velocity and mass transfer at single droplets in systems with Marangoni instabilities

The toluene/acetone/water system without surfactants shows strong Marangoni instabilities with distinct fluid dynamics and enhanced mass transfer. The present study investigates the impact of an anionic surfactant (SDS) on rise velocity and mass transfer both in the spherical and the oscillating droplet regime in time resolved measurements. Results are compared with the respective pure system and with correlations from literature. Velocity measurements reveal that the smaller the droplets are the lower is the surfactant concentration where the terminal velocity of the corresponding rigid sphere is attained. Mass transfer measurements show that mass transfer is still significantly enhanced by Marangoni convection although the surfactant concentration was believed to be high enough to hinder interfacial movement.     

Pr对具有表面蒸发环形池内热毛细对流的影响

为了了解普朗特数对具有表面蒸发的环形液池内热毛细对流的影响,对流体在其纯蒸汽环境中蒸发时的热毛细对流进行了数值模拟,流体Pr变化范围为0.01至50.00,环形液池半径比和深宽比分别为0.5和1.0。结果表明,随着Pr的增加,表面温度逐渐升高,表面蒸发质量通量增大,热毛细对流流胞逐渐向外壁和自由表面移动,这种影响随着蒸发Biot数增加而逐渐减小;当蒸发Bi较小时,总蒸发质量随Pr增加而增大,当Bi较大时,Pr对总蒸发质量影响很小。     

大气对流对土壤内热湿迁移影响的实验研究

对土壤内热湿迁移过程进行了研究,通过对土壤内热湿迁移机理分析,根据质量守恒和能量守恒原理,建立了土壤非饱和区热湿迁移的理论模型,对大气对流环境条件下砂土内热湿迁移过程进行了实验研究,实验测量和数值计算,获得了不同大对流速度作用下土壤中温度,含水率分布以及水分蒸发强度的变化。     

基于CFD技术的太阳能污泥干燥室内热湿环境研究

基于传热传质理论,建立一种太阳能污泥干燥室内热湿耦合传递的数学模型。综合考虑空气流动以及对流和辐射传热,利用CFD软件Fluent的k~ε湍流模型、组分输运模型及辐射模型,初步分析了在太阳辐射条件下不同干燥室结构、排风形式以及通风量对太阳能污泥干燥室内干燥区域的温度、相对湿度以及速度分布的影响。模拟结果表明:干燥室内温、湿度模拟值与实测值吻合较好,平均相对误差分别为3.55%和5.39%。对比分析不同结构下干燥室内的流场分布,两出口排风形式的太阳能污泥干燥室可以形成良好的干燥微环境。当两出口排风风速≥5 m/s时,室内干燥区域温度高于室外环境温度,同时相对湿度低于室外环境相对湿度,且增大出流风速,在干燥区域内空气扰流强度增强,有利于干燥室内污泥水分的蒸发。     

Interfacial temperature measurements,high-speed visualization and finite-element simulations of droplet impact and evaporation on a solid surface

The objective of this work is to investigate the coupling of fluid dynamics, heat transfer and mass transfer during the impact and evaporation of droplets on a heated solid substrate. A laser-based thermoreflectance method is used to measure the temperature at the solid–liquid interface, with a time and space resolution of 100 μs and 20 μm, respectively. Isopropanol droplets with micro- and nanoliter volumes are considered. A finite-element model is used to simulate the transient fluid dynamics and heat transfer during the droplet deposition process, considering the dynamics of wetting as well as Laplace and Marangoni stresses on the liquid–gas boundary. For cases involving evaporation, the diffusion of vapor in the atmosphere is solved numerically, providing an exact boundary condition for the evaporative flux at the droplet–air interface. High-speed visualizations are performed to provide matching parameters for the wetting model used in the simulations. Numerical and experimental results are compared for the transient heat transfer and the fluid dynamics involved during the droplet deposition. Our results describe and explain temperature oscillations at the drop–substrate interface during the early stages of impact. For the first time, a full simulation of the impact and subsequent evaporation of a drop on a heated surface is performed, and excellent agreement is found with the experimental results. Our results also shed light on the influence of wetting on the heat transfer during evaporation.     

Numerical investigation of heat and mass transfer from an evaporating meniscus in a heated open groove

The process of evaporation from a meniscus into air is more complicated than in enclosed chambers filled with pure vapor. The vapor pressure at the liquid–gas interface depends on both of the evaporation and the vapor transport in the gas environment. Heat and mass transport from an evaporating meniscus in an open heated V-groove is numerically investigated and the results are compared to experiments. The evaporation is coupled to the vapor transport in the gas domain. Conjugate heat transfer is considered in the solid walls, and the liquid and gas domains. The flow induced in the liquid due to Marangoni effects, as well as natural convection in the gas due to thermal expansivity and vapor concentration gradients are simulated. The calculated evaporation rates are found to agree reasonably well with experimentally measured values. The convection in the gas domain has a significant influence on the overall heat transfer and the wall temperature distribution. The evaporation rate near the contact lines on either end of the meniscus is high. Heat transfer through the thin liquid film near the heated wall is found to be very efficient. A small temperature valley is obtained at the contact line which is consistent with the experimental observation.     

Effects of Non-Darcian and Inlet Conditions on the Forced Convection Along a Vertical Plate With Film Evaporation

The present study analyzes theoretically the non-Darcian effects and inlet conditions of forced convection flow with liquid film evaporation in a porous medium. The physical scheme includes a liquid–air streams combined system; the liquid film falls down along the plate and is exposed to a cocurrent forced moist air stream. The axial momentum, energy, and concentration equations for the air and water flows are developed based on the steady two-dimensional (2-D) laminar boundary layer model. The non-Darcian convective, boundary, and inertia effects are considered to describe the momentum characteristics of a porous medium. The paper clearly describes the temperature and mass concentration variations at the liquid–air interface and provides the heat and mass transfer distributions along the heated plate. Then, the paper further evaluates the non-Darcian effects and inlet conditions on the heat transfer and evaporating rate of liquid film evaporation. The numerical results show that latent heat transfer plays the dominant heat transfer role. Carrying out a parametric analysis indicates that higher air Reynolds number, higher wetted wall temperature, and lower moist air relative humidity will produce a better evaporating rate and heat transfer rate. In addition, a non-Darcy model should be adopted in the present study. The maximum error for predictions of heat and mass transfer performance will be 21% when the Darcy model is used.     

快速降压环境下盐水液滴蒸发过程的传热传质研究

采用VOF（Volume of Fluid）自由表面捕捉方法对盐水液滴蒸发过程中气液界面进行追踪,建立了降压环境下单个盐水液滴的蒸发模型,并通过盐水液滴蒸发的实验数据验证了此模型。通过对盐水液滴在相变过程中的形态变化以及传热传质特性的分析,研究了液滴内部温度、速度、蒸汽分布以及液滴形态等随时间的变化情况,分析了影响盐水液滴降压蒸发过程的主要因素。结果表明：在降压蒸发过程中液滴形态变化和环境中蒸汽的分布会随速度场的变化而变化;蒸发过程中初始盐组分质量浓度越大的液滴蒸发速率越缓慢,最终能达到的液滴最低中心温度越高,且液滴中心温度回升速度越慢、回升时间也越晚;液滴初始温度对蒸发速率影响较大,初始温度越高,表面蒸发速率越快,液滴中心温度回升速度越快。     

Three-dimensional modeling of heat transfer and fluid flow in laminar-plasma material re-melting processing

Modeling study is performed concerning the heat transfer and fluid flow for a laminar argon plasma jet impinging normally upon a flat workpiece exposed to the ambient air. The diffusion of the air into the plasma jet is handled by using the combined-diffusion-coefficient approach. The heat flux density and jet shear stress distributions at the workpiece surface obtained from the plasma jet modeling are then used to study the re-melting process of a carbon steel workpiece. Besides the heat conduction within the workpiece, the effects of the plasma-jet inlet parameters (temperature and velocity), workpiece moving speed, Marangoni convection, natural convection etc. on the re-melting process are considered. The modeling results demonstrate that the shapes and sizes of the molten pool in the workpiece are influenced appreciably by the plasma-jet inlet parameters, workpiece moving speed and Marangoni convection. The jet shear stress manifests its effect at higher plasma-jet inlet velocities, while the natural convection effect can be ignored. The modeling results of the molten pool sizes agree reasonably with available experimental data.     

Thermal and solutal effects on convection inside a polymer solution droplet on a substrate

The convective flow inside polymer solution droplets drying on a lyophobic substrate is numerically studied. The evaporating droplet is presumed as a hemisphere shrinking with time at the constant contact angle. The thermal and solutal effects are simultaneously considered in the computation. The thermal Marangoni convection is induced due to the quick thermal diffusion, and this convection transports the solute resulting in the solutal Marangoni flow. The solutal dependence corresponds to our previous experimental work, but the flow pattern does not. Consideration of the pseudo evaporation rate distribution depending on the contact angle yields to the flow pattern correspondence.     

The effect of thermal boundary conditions on the heat transport in vertical channels heated from below

A numerical analysis is carried out of the effect of perfectly conducting and adiabatic vertical walls on the heat transport by convection in a fluid heated from below. The equations of motion for a high Prandtl number fluid and the heat equation have been solved by the Galerkin method. Because of the side walls, the convection velocity field has three velocity components and depends on all three spatial coordinates. A two velocity component approximation, as well as the full three component representation of the velocity field, is employed in the numerical analysis. The results indicate the surprising accuracy of the two component approximation. Converged solutions are determined for a range of aspect ratios A between A = 0 (Bénard convection) and A = 20 (Hele-Shaw convection).     

Numerical investigation on the evaporation of droplets depositing on heated surfaces at low Weber numbers

The evaporation of water droplets, impinging with low Weber number and gently depositing on heated surfaces of stainless steel is studied numerically using a combination of fluid flow and heat transfer models. The coupled problem of heat transfer between the surrounding air, the droplet and the wall together with the liquid vaporisation from the droplet’s free surface is predicted using a modified VOF methodology accounting for phase-change and variable liquid properties. The surface cooling during droplet’s evaporation is predicted by solving simultaneously with the fluid flow and heat transfer equations, the heat conduction equation within the solid wall. The droplet’s evaporation rate is predicted using a model from the kinetic theory of gases coupled with the Spalding mass transfer model, for different initial contact angles and substrate’s temperatures, which have been varied between 20–90° and 60–100 °C, respectively. Additionally, results from a simplified and computationally less demanding simulation methodology, accounting only for the heat transfer and vaporisation processes using a time-dependent but pre-described droplet shape while neglecting fluid flow are compared with those from the full solution. The numerical results are compared against experiments for the droplet volume regression, life time and droplet shape change, showing a good agreement.