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。     

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.     

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。     

Leidenfrost boiling of methanol droplets on hot porous/ceramic surfaces

An experimental and analytical study of film boiling methanol droplets on a porous/ceramic surface is reported. Droplet evaporation times in the wetting and film boiling regimes were measured on a polished stainless-steel surface and three ceramic/alumina surfaces of 10%, 25% and 40% porosity. It was found that the Leidenfrost temperatures increased as surface porosity increased. The Leidenfrost point of the 10% and 25% porous surfaces were nearly 100 K higher and 200 K higher, respectively, than that of the polished stainless-steel surface; methanol droplets could not be levitated on the 40% porous surface at surface temperatures as high as 620 K, which was the maximum surface temperature which could be imposed on this particular material with our apparatus. The evaporation time of liquid deposited on this surface was thus almost two orders of magnitude lower than for levitated droplets on the three other surfaces tested at the same temperature. In the Leidenfrost regime droplets evaporated faster on the porous surfaces than on the stainless-steel surface, and the evaporation time decreased with increasing surface porosity at the same surface temperature. The reduced evaporation times were thought to have their origin in a decrease of the vapor film thickness separating the droplet from the ceramic surface due to vapor absorption and flow within the ceramic material. An analysis of flow in a horizontal channel bounded by an impermeable ẇall above and a permeable wall of finite thickness below was used to model the film boiling process. The results provided a basis for correlating our evaporation time measurements.     

Numerical investigation of the cooling effectiveness of a droplet impinging on a heated surface

Computational fluid dynamics numerical simulations for 2.0 mm water droplets impinging normal onto a flat heated surface under atmospheric conditions are presented and validated against experimental data. The coupled problem of liquid and air flow, heat transfer with the solid wall together with the liquid vaporization process from the droplet’s free surface is predicted using a VOF-based methodology accounting for phase-change. The cooling of the solid wall surface, initially at 120 °C, is predicted by solving simultaneously with the fluid flow and evaporation processes, the heat conduction equation within the solid wall. The range of impact velocities examined was between 1.3 and 3.0 m/s while focus is given to the process during the transitional period of the initial stages of impact prior to liquid deposition. The droplet’s evaporation rate is predicted using a model based on Fick’s law and considers variable physical properties which are a function of the local temperature and composition. Additionally, a kinetic theory model was used to evaluate the importance of thermal non-equilibrium conditions at the liquid–gas interface and which have been found to be negligible for the test cases investigated. The numerical results are compared against experimental data, showing satisfactory agreement. Model predictions for the droplet shape, temperature, flow distribution and vaporised liquid distribution reveal the detailed flow mechanisms that cannot be easily obtained from the experimental observations.     

A Level-Set Method for the Direct Numerical Simulation of Particle Motion in Droplet Evaporation

A sharp-interface level-set (LS) method is presented for direct numerical simulation (DNS) of particle motion in droplet evaporation. The LS formulation for liquid–gas flows is extended to liquid–gas–solid flows by treating the moving solid region as a high-viscosity fluid phase. The evaporation effect is accurately implemented by imposing the coupled temperature and vapor fraction conditions at the interface. The LS method is tested through computations of particle sedimentation in single-phase and two-phase fluids. The DNS of particle motion in droplet evaporation demonstrates the pinning phenomena of the liquid–gas–solid contact line.     

3-D modeling of the dynamics and heat transfer characteristics of subcooled droplet impact on a surface with film boiling

The impact of a subcooled water and n-heptane droplet on a superheated flat surface is examined in this study based on a three-dimensional model and numerical simulation. The fluid dynamic behavior of the droplet is accounted for by a fixed-grid, finite-volume solution of the incompressible governing equations coupled with the 3-D level-set method. The heat transfer inside each phase and at the solid–vapor/liquid–vapor interface is considered in this model. The vapor flow dynamics and the heat flux across the vapor layer are solved with consideration of the kinetic discontinuity at the liquid–vapor and solid–vapor boundaries in the slip flow regime. The simulated droplet dynamics and the cooling effects of the solid surface are compared with the experimental findings reported in the literatures. The comparisons show a good agreement. Compared to the water droplet, it is found that the impact of the n-heptane droplet yields much less surface temperature drop, and the surface temperature drop mainly occurs during the droplet-spreading stage. The effects of the droplet’s initial temperature are also analyzed using the present model. It shows that the droplet subcooling degree is related closely to the thickness of the vapor layer and the heat flux at the solid surface.     

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.     

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.     

Leidenfrost Droplets on Microstructured Surfaces

The lifetime of a droplet deposited on a hot plate decreases when the temperature of the plate increases, but above the critical Leidenfrost temperature, the lifetime suddenly increases. This is due to the formation of a thin layer of vapor between the droplet and the substrate, which plays a double role: First, it thermally insulates the droplet from the plate, and second, it allows the droplet to “levitate.” The Leidenfrost point is affected by the roughness or microstructure of the surface. In this work, a silicon surface with different microstructured regions of square pillars was prepared such that there is a sharp transition (boundary) between areas of different pillar spacing. The Leidenfrost point was identified in experiments using water droplets ranging in size from 8 to 24 μl and the behavior of the droplets was recorded using high-speed digital photography. The Leidenfrost point was found to vary by up to 120°C for pillar spacings from 10 to 100 μm. If the droplet is placed on the boundary between structured sections, the droplet becomes asymmetric and may move or spin. An axisymmetric computational fluid dynamics (CFD) model is also presented that shows qualitative agreement with experimental observations.     

The effect of gas and liquid properties and droplet size ratio on the central collision between two unequal-size droplets in the reflexive regime

A numerical investigation of various parameters affecting the collision process of two unequal sized droplets in a gaseous environment in the reflexive regime is presented. The investigation is based on the finite volume numerical solution of the Navier–Stokes equations, in their axial-symmetric formulation, expressing the flow field of the two phases, liquid and gas, coupled with the volume of fluid method (VOF) for tracking the liquid–gas interfaces; a recently developed adaptive local grid refinement technique is used in order to track more accurately the liquid–gas interface with reduced computational cost. A colour function is used in order to follow the mixing process of the two droplets after collision. The reliability of the procedure is first established by comparing predictions with available experimental data and important details as regards the time evolution of the deformed shapes, the ligament formation, the maximum deformation of the droplets and the penetration of one droplet into the other are presented. The collision process and its outcome are investigated having the liquid properties (Ohnesorge number based on the liquid properties), the gas phase properties (Reynolds number based on gas properties) and droplet size ratio as the controlling parameters.     

Leidenfrost drops on micro/nanostructured surfaces

In the Leidenfrost state, the liquid drop is levitated above a hot solid surface by a vapor layer generated via evaporation from the drop. The vapor layer thermally insulates the drop from the heating surface, causing deteriorated heat transfer in a myriad of important engineering applications. Thus, it is highly desirable to suppress the Leidenfrost effect and elevate the Leidenfrost temperature. This paper presents a comprehensive review of recent literature concerning the Leidenfrost drops on micro/nanostructured surfaces with an emphasis on the enhancement of the Leidenfrost temperature. The basic physical processes of the Leidenfrost effect and the key characteristics of the Leidenfrost drops were first introduced. Then, the major findings of the influence of various micro/nanoscale surface structures on the Leidenfrost temperature were presented in detail, and the underlying enhancement mechanism for each specific surface topology was also discussed. It was concluded that multiscale hierarchical surfaces hold the best promise to significantly boost the Leidenfrost temperature by combining the advantages of both micro- and nanoscale structures.     

单液滴撞击薄液膜产生二次雾化过程的数值模拟

应用数值模拟方法研究单液滴撞击薄液膜的动力学行为.在二维轴对称坐标系内,采用VOF方法与网格局部瞬时加密技术相结合,跟踪液滴和液膜与空气间的气液两相界面.结果表明,液滴撞击薄液膜的演化行为主要受液滴初始动能、表面张力以及液体黏性的影响.初始动能越大,则形成的空间液膜最大高度越大,达到稳定状态越晚,飞溅开始时刻越早,飞溅生成的二次液滴数量也越多;在扩展后期及回缩阶段,空间液膜的形成主要受液体黏度影响,增加液体黏度会阻碍空间液膜飞溅;表面张力增大,形成的空间液膜高度减小、厚度增加,同时阻碍二次液滴的生成.     

Three‐dimensional simulation of water droplet movement in PEM fuel cell flow channels with hydrophilic surfaces

Water management is one of the critical issues in proton exchange membrane fuel cells, and proper water management requires effective removal of liquid water generated in the cathode catalyst layer, typically in the form of droplets through cathode gas stream in the cathode flow channel. It has been reported that a hydrophilic channel sidewall with a hydrophobic membrane electrode assembly (MEA) surface would have less chance for water accumulation on the MEA surface. Therefore, a comprehensive study on the effect of surface wettability properties on water droplet movement in flow channels has been conducted numerically. In this study, the water droplet movements in a straight flow channel with a wide range of hydrophilic surface properties and effects of inlet air velocities are analyzed by using three‐dimensional computational fluid dynamics method coupled with the volume‐of‐fluid (VOF) method for liquid–gas interface tracking. The results show that the water droplet movement is greatly affected by the channel surface wettability and air flow conditions. With low contact angle, droplet motion is slow due to more liquid–wall contact area. With high air flow velocities, increasing the contact angle of the channel surface results in faster liquid water removal due to lesser liquid–wall contact area. Copyright © 2010 John Wiley & Sons, Ltd.     

Pattern analysis of a single droplet impinging onto a heated plate

The time scale for measuring the heat transfer phenomena, such as the Leidenfrost phenomenon, is much longer than the hydrodynamic transient period of a droplet impinging onto a heated surface. However, the Leidenfrost temperature has been long since taken as the criterion to classify characteristic regimes for not only heat transfer but also hydrodynamic impact patterns of liquid‐solid‐interface problems in the literature. The impingement phenomena of a single droplet onto a heated plate for We <200 were experimentally classified as five different characteristic patterns in this paper. They are [I] completely wet, [II] wet film boiling, [III] transition, [IV] dry rebounding, and [V] satellite dry rebounding impact patterns. The former two patterns belong to wet impact, while the latter two are dry impact. This work reveals that the hydrodynamic impact parameter does influence the classification of characteristic impact patterns, especially for the dry impact patterns. For high impact Weber numbers, the dry impact happens to the plate surface temperature much lower than the commonly believed Leidenfrost temperature due to the squeeze film effect. The evolutions that impact changes from one to another characteristic patterns are also discussed. This paper offers a systematic and reliable database for future numerical simulations. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(8): 579–594, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20089     

Numerical solution for film evaporation of a spherical liquid droplet on an isothermal and adiabatic surface

A numerical solution for the problem of film evaporation of a liquid droplet on a horizontal surface is presented. The droplets are small enough to be assumed spherical. Two principal cases are considered: (1) the horizontal surface is maintained at a constant temperature (case I), and (2) the surface is insulated while the ambience is hot (case II). The complete set of equations governing this problem were solved under the following assumptions : (1) evaporation is quasi-steady, (2) no internal liquid circulation, (3) constant properties, and (4) the droplet temperature is spatially uniform but temporally varying. The Lewis number is not assumed to be unity; gas phase viscous effects, Stefan type convection, and gas phase inertia are included in the analysis. The total droplet evaporation time was found to decrease with increasing plate (I) or ambient (II) temperature as expected, and the droplet progressively moves away from the plate as it evaporates. The numerical results agree with the analytical solution for film evaporation of a droplet above an adiabatic surface in a hot ambience in the limit of large effective Reynolds number (i.e. potential flow).     

Numerical simulation of droplet impact and evaporation on a porous surface

Numerical simulation is performed for the evaporation of a droplet impacted on a porous surface. A level-set formulation for tracking the droplet deformation is extended to include the effects of evaporation coupled to heat and mass transfer, porosity and porous drag and capillary forces. The local volume averaged conservation equations of mass, momentum, energy and vapor fraction for the porous region are simultaneously solved with the conservation equations for the external fluid region. The computations demonstrate not only the evolution of the liquid-gas interface during the whole period of droplet penetration and evaporation in a porous medium, but also the associated flow, temperature and vapor fraction fields. The effects of impact velocity, porosity and particle size on the droplet deformation and evaporation are quantified.     

Thermal Fouling of Heat Exchanger Tubes due to Heavy Hydrocarbon Droplets Impingement

ABSTRACT

This work discusses fouling in the vapor–steam mixture overheater in the convection section of an industrial steam cracker due to the thermal degradation of heavy hydrocarbon droplets deposited on the tube wall. A spray of heavy hydrocarbon multicomponent droplets is injected in a tube of the vapor–steam mixture overheater and the path of the droplets through the tube is followed by an Eulerian–Lagrangian computational fluid dynamics simulation. To study tube fouling, the droplet impingement behavior on the wall, the evaporation of the deposited liquid, and a coking model describing thermal coke formation due to degradation of heavy hydrocarbons are required. To describe the droplet impingement behavior, a regime map for single component millimeter-sized droplets is taken from the literature. Two simulations are performed to study fouling problems in a vapor-mixture overheater tube. Simulation results are found to be grid sensitive. By analyzing and comparing simulation results it is concluded that reliable fouling data require a regime map for the impingement of multicomponent heavy hydrocarbon micron-sized droplets.     

波纹管降膜蒸发器传热性能数值模拟

为了理解波纹管降膜蒸发过程中涉及的液膜传热过程,本文采用VOF法建立了二维气-液两相分层流动CFD模型,考虑了液相流量,传热温差,蒸发温度,液相粘度等参数对传热效果的影响,根据模拟结果给出了波纹管降膜蒸发器的流量可操作范围。模拟结果和实验数据比较吻合。     

Transient modeling of heat,mass and momentum transfer of an evaporating cerium nitrate solution droplet with a surrounding shell in a rf thermal argon–oxygen plasma under reduced pressure

A model was developed to study the evaporation of a solution droplet surrounded by a porous crust in a stagnant rf Ar–O₂ thermal plasma under reduced pressure. This model considered a three phase system: a liquid core of dissolved Ce(NO₃)₃ · 6H₂O in water, a porous crust of homogeneously precipitated spherical crystals of equal size containing water vapor, and an Ar–O₂ plasma under reduced pressure. The model was solved considering a receding solution/crust interface in an ALE frame using temperature and composition dependant thermophysical properties. Darcy flow with a Knudsen correction to account for the gaseous flow through a porous media composed of nano-sized crystals was employed. The strength of the solid/liquid interface was calculated by computing the strength of liquid bridges formed at this interface. This value was compared to the pressure build-up caused by solvent evaporation and the point of crust breakage was determined at different operating conditions. The effects of plasma gas temperature, pressure and composition, droplet size, size of precipitated crystals and crust porosity on crust bursting were studied. The results showed that crust bursting occurred for all the conditions analyzed and that plasma temperature, droplet size and the size of the precipitated crystals had a significant effect on pressure build-up.