柴油、正丁醇及其混合燃料单液滴蒸发特性的试验

为了探明添加正丁醇对柴油蒸发特性的影响,采用石英丝挂滴技术研究了不同温度下正丁醇、柴油及其混合燃料的蒸发特性,并利用高速摄像技术记录了液滴蒸发过程中直径和形态的变化.研究表明:与柴油两阶段蒸发特性相比,正丁醇瞬态加热阶段较短,正丁醇比柴油蒸发快,且提高环境温度可以降低正丁醇与柴油蒸发特性的差异性.正丁醇/柴油混合燃料比柴油蒸发快,正丁醇添加主要影响柴油蒸发过程的前阶段.高温下,与柴油相比,正丁醇/柴油混合燃料的蒸发特性发生根本变化,其蒸发过程呈现三阶段蒸发特性,液滴出现气泡生成、膨胀和喷气现象,液滴直径波动剧烈,这是由于正丁醇/柴油混合燃料沸点差异性导致的.     

燃料液滴超临界蒸发综述

从常压液滴蒸发的传热传质过程出发,建立了常压液滴蒸发的数学模型。通过比较液滴超临界蒸发和常压蒸发的差异,研究液滴超临界蒸发所涉及的关键问题。以二甲基醚(DME)在氮气介质中超临界蒸发为例,介绍了状态方程法计算DME-N2体系的高压气液相平衡,在相关文献的基础上,总结了流体热物性和输运参数的变化特性。     

纳米添加剂对麻疯树甲酯和乙醇混合物蒸发特性的影响

采用悬浮液滴技术研究在873 K和973 K的环境温度下,麻疯树甲酯-乙醇混合物(J70E30)添加不同质量浓度的Fe3O4纳米粒子(0.25%、1%、2%)燃料液滴的蒸发特性。结果表明,在873 K和973 K两个温度下,含有不同浓度的纳米流体燃料液滴蒸发过程均可以分为瞬态加热阶段、波动蒸发阶段和平衡蒸发阶段,三种Fe₃O₄纳米粒子浓度液滴的归一化平方直径在平衡蒸发阶段符合d2定律。在873K温度下,由于纳米粒子较强的布朗运动导致传热效率提高,促进了燃料液滴蒸发速率,其蒸发速率随着纳米粒子浓度增加不断提高;在973 K温度下,纳米流体燃料液滴的蒸发速率则是先减小后增大,但在973 K温度下纳米流体燃料液滴蒸发速率要大于其在873 K时的蒸发速率。     

固着液滴蒸发研究进展及展望

固着液滴是指附着于壁面上的液滴,其蒸发行为及传热传质特性是喷雾冷却、喷墨打印等相变传热传质领域的基础问题之一。文中重点针对固着液滴蒸发过程所涉及的自身形态演变规律、气液固三相耦合传热/传质/流动特性进行了综述。结合毫微尺度固着液滴基本蒸发模式、热质传递形式、气液两相流动特征和界面输运行为,分析了液滴性质、壁面条件、气相环境条件等关键因素对固着液滴蒸发过程的内在作用机制和影响规律,提出了微纳尺度固着液滴（群）热质传递过程与机理的相关研究展望。     

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。     

石墨烯-铜纳米流体液滴蒸发特性的实验研究

文中针对纯石墨烯、纯铜纳米流体液滴以及石墨烯-铜混合纳米流体液滴在铜基底表面的蒸发特性开展了实验研究,分析了纳米粒子质量分数、石墨烯与铜配比对液滴蒸发过程中接触角和接触直径动态演化以及蒸干后粒子沉积形貌的影响。结果表明:纯石墨烯纳米流体液滴的初始平衡接触角大于纯铜纳米流体液滴、蒸发过程平均接触角小于纯铜纳米流体液滴;纯石墨烯纳米流体液滴蒸干后粒子密集地堆积在中心和边缘位置、边缘形成沉积环、中心形成粒子均布图案,纯铜纳米流体液滴蒸干后形成明显的咖啡环。在混合纳米流体液滴中,随着石墨烯含量的增加,初始平衡接触角增大、蒸发过程平均接触角减小、液滴蒸发速率增大、液滴蒸干后铜基底表面中心区域粒子密度增大。     

加热板上液滴沸腾实验研究

采用高速摄像技术研究了不同加热表面上液滴蒸发和沸腾的相变特性和壁面温度变化特性,讨论了局部相变行为对壁面温度变化的影响.同时定量的研究了三种不同表面特性的加热板对沸腾和传热的影响,以及液滴初始体积对相变的影响.结果表明,表面特性和液滴尺寸对沸腾传热有较大影响.     

亚临界和超临界压力下燃料液滴的蒸发特性

以实际气体状态方程为基础,建立了适用于高压下的导热系数、扩散系数等物性参数的计算方法,并将高压汽液相平衡、混合物临界点以及蒸发焓的概念引入到液滴表面的传热传质过程中,以此为基础建立了单个燃料液滴的高压蒸发模型.研究了亚临界和超临界压力下壬烷液滴在氮气中的蒸发过程及其物理控制因素,重点探讨了超临界压力下液滴蒸发过程中液滴表面自亚临界状态向超临界状态的迁移过程及迁移条件.结果表明,在亚临界压力下,液滴蒸发始终受相变控制.在超临界压力下,当液滴表面由燃料和环境气体组成的混合物达到其临界点时,液滴表面将发生自亚临界状态向超临界状态的迁移.在液滴表面迁移之后,液滴表面消失,燃料自高浓度的燃料核心向远方场的扩散过程不受相变控制.另外,随着环境温度的升高液滴表面发生迁移所需的最低环境压力逐渐降低.     

600 MW电厂脱硫废水旋转雾化蒸发特性的数值研究

对广东某600 MW燃煤电厂脱硫废水干燥塔进行了数值建模,利用离散相模型(DPM)计算气液两相的传热传质,得到流场分布和液滴蒸发情况,并研究了盐质量分数、废水体积流量、进口烟温和液滴初始粒径对液滴蒸发的影响.研究发现:设计工况下塔内轴心处速度较高,形成回流区,液滴在塔内上方完全蒸发;随着进口烟温提高,蒸发时间逐渐减小,...     

纳米流体动态湿润的格子-Boltzmann模拟

纳米流体动态湿润特性与纳米颗粒的微观运动密切相关。由于缺乏纳米尺度的实验观测技术及相关理论描述,纳米流体动态湿润的研究极具挑战,相关机理仍未明晰。采用格子-Boltzmann方法研究纳米颗粒在纳米尺度下(10-9 m)的微观运动及颗粒沉积所导致的基液流体表面张力、流变性改变及结构分离压力对宏观动态湿润(10-3 m)的影响机制。结果表明,纳米颗粒对基液的表面张力的改性影响纳米流体平衡湿润特性,决定纳米流体是完全浸润还是部分浸润。而纳米颗粒对基液流体流变性的改变影响纳米流体动态湿润过程的铺展指数。纳米颗粒在液滴底部的沉积对动态湿润过程影响较小,而在接触线区域的沉积显著改变纳米流体的动态湿润行为。研究尝试从跨尺度的角度阐释纳米颗粒微观运动对宏观动态湿润行为的影响,探索从微观层面调控纳米流体动态湿润的新方法。     

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.     

不同润湿性表面液滴铺展及蒸发过程的LBM数值模拟

基于格子玻尔兹曼方法（Lattice Boltzmann Method, LBM）对固着在加热基板上的液滴铺展及蒸发过程进行模拟,主要研究重力场、基板润湿性以及初始环境温度对液滴铺展及蒸发过程的影响。通过预测蒸发过程中液滴与基板的接触直径变化和液滴剩余质量变化,分析液滴形状及体积变化。研究结果发现,液滴形貌及蒸发过程受重力影响较大,重力作用下液滴铺展现象明显且蒸发加快。基板的接触角越小,液滴铺展现象越明显,其接触直径越大,蒸发越快。当环境温度与基板温度相差较大时,液滴内部出现涡流,强化换热使蒸发过程加快。     

Interaction of the transfer processes in semitransparent liquid droplets

The study presents the mathematical model of unsteady heat transfer in evaporating semitransparent droplets of non-isothermal initial state and the numerical research method, evaluating selective radiation absorption and its influence on the interaction of transfer processes. The relation of the transfer processes inside droplets and in their surroundings and the necessity of thorough research of these processes are substantiated. When modeling the combined energy transfer in water droplets, the evaluation of thermoconvective stability in evaporating semitransparent liquid droplets is presented; the influence of the droplet initial state on its heating and evaporation process is investigated. The influence of heat transfer peculiarities on the change of the evaporating droplet state is indicated. Main parameters, which decide the peculiarities of the interaction of unsteady transfer processes in droplets and their surroundings, are discussed. The results of the numerical research are compared to the known results of the experimental studies of water droplet temperature and evaporation rate.     

重力热管内部相变及传热传质过程的数值模拟

重力热管内部包含复杂的两相流动以及相变传热过程,传统理论分析及实验手段不能直观给出其内部流动、相变、热质传递的详细信息。采用VOF(volume of fluid)多相流模型对重力热管内气液两相流动及传热进行模拟,捕捉到蒸发段气泡产生、合并、长大、上升,以及冷凝段壁面附近液滴形成、合并、下滑、汇集到液池的全过程,得到的壁温分布与实验测量值对比体现良好一致性,表明数值模拟的正确性。同时,以热阻、传热量和热效率为评价标准,研究不同充液率和倾斜角度下对重力热管运行性能的影响。结果表明:在所研究的参数范围内,随着充液率的增加,热阻逐渐减小,冷凝段传热量逐渐增大。且工质初始充注量充满蒸发段时热管性能较好;倾角对热阻的影响不明显,冷凝段传热量和热效率均随倾角增加而增长。     

Effect of surface wettability on boiling and evaporation

Titanium dioxide, TiO₂, is one of the photocatalysts that has a very unique characteristic. The surface coated with TiO₂ exhibits extremely high affinity for water by exposing the surface to UV light and the contact angle decreases nearly to zero. On the contrary, the contact angle increases when the surface is shielded from UV light. We applied this superhydrophilic nature to enhancement of boiling and evaporation heat transfer. Experiments of pool boiling and evaporation of single water droplet have been performed to manifest the effect of high wettability on heat transfer characteristics. Both of TiO₂-coated and non-coated surfaces were used for comparison in each experiment. It is found that (1) the critical heat flux (CHF) of TiO₂-coated surface is about two times larger than that of non-coated one, and (2) Leidenfrost temperature increases as the contact angle decreases. The superhydrophilic surface can be an ideal heat transfer surface.     

Marangoni convection flow and heat transfer characteristics of water–CNT nanofluid droplets

ABSTRACT

The heat transfer characteristics of liquid droplets are influenced by the hydrophobicity of the surfaces. Fluid properties and surface energy play important roles in heat transfer assessment. In the present study, the influence of the contact angle on the flow field developed inside a nanofluid droplet consisting of a mixture of water and carbon nanotubes (CNT) is investigated. Flow field and heat transfer characteristics are simulated numerically in line with the experimental conditions. It is found that the flow velocity predicted numerically is in good agreement with the experimental data. Nusselt and Bond numbers increase at large contact angles and Marangoni force dominates over buoyancy force.     

New approaches to numerical modelling of droplet transient heating and evaporation

New approaches to numerical modelling of droplet heating and evaporation by convection and radiation from the surrounding hot gas are suggested. The finite thermal conductivity of droplets and recirculation in them are taken into account. These approaches are based on the incorporation of new analytical solutions of the heat conduction equation inside the droplets (constant or almost constant h) or replacement of the numerical solution of this equation by the numerical solution of the integral equation (arbitrary h). It is shown that the solution based on the assumption of constant convective heat transfer coefficient is the most computer efficient for implementation into numerical codes. This solution is applied to the first time step, using the initial distribution of temperature inside the droplet. The results of the analytical solution over this time step are used as the initial condition for the second time step etc. This approach is applied to the numerical modelling of fuel droplet heating and evaporation in conditions relevant to diesel engines, but without taking into account the effects of droplet break-up. It is shown to be more effective than the approach based on the numerical solution of the discretised heat conduction equation inside the droplet, and more accurate than the solution based on the parabolic temperature profile model. The relatively small contribution of thermal radiation to droplet heating and evaporation allows us to take it into account using a simplified model, which does not consider the variation of radiation absorption inside droplets.     

高频感应等离子场中液滴运动蒸发过程模拟

建立在高频感应热等离子体环境下单个溶液液滴的运动蒸发模型,采用数值计算的方法模拟了液滴在等离子体射流中的运动和传热过程,分析了不同操作参数对液滴运动蒸发过程的影响.结果表明:液滴初始入射尺寸越小,表面溶质质量分数达到饱和状态所用时间越短;初始入射速度越快,表面溶剂蒸发速度越快,溶质结晶析出时间越短;入射角较大时,液滴会被反向涡流卷吸,表面浓度达到饱和状态的时间较长.     

Modeling gas oil spray coalescence and vaporization in gas solid riser reactor

The sprayed feed droplet behavior, including coalescence and vaporization into gas–solid flow, is complex especially near the atomizer region in fluid catalytic cracking (FCC) riser reactor. A three dimensional CFD model of the riser reactor has been developed, which takes into the account three phase hydrodynamics, heat transfer and evaporation of the liquid droplets into a gas–solid flow as well as phase interactions. A hybrid Eulerian–Lagrangian approach was applied to numerically simulate the collision and vaporization of gas oil droplets in the gas–solid fluidized bed. This numerical simulation accounts the possibility of coalescence of feed spray droplets in computing the trajectories and its impact on droplet penetration in the reactor. The modeling result shows that droplet coalescence mainly occurs at the initial part of the atomizing region and where three phase flow hits the reactor wall and bounces back. The model has the ability of inspecting the effects of feed injector geometry on the overall reactor hydrodynamic and heat transfer. The CFD simulation results showed that the evaporated droplet gas caused higher local velocities of the gas and solid particles and gas–solid flow temperature reduction.