用格子Boltzmann方法模拟液滴合并过程

用格子Boltzmann方法中的伪势模型对两个液滴的合并过程进行了数值模拟。详细研究了两液滴能否合并临界尺寸和液滴合并过程中液桥的形成与演化的关系，并研究了表面张力对合并速度的影响。研究结果发现当两个液滴之间距离小于2倍界面厚度时，两个液滴在不受外力的作用下能够自动合并；液桥的宽度与演化时间有一定的指数关系；表面张力越大，合并速度越快，这个结果与前人的理论预测和实验结果一致。     

悬链线形渠道正常水深与临界水深的计算方法

基于悬链线形渠道断面几何特点,根据均匀流基本方程推导出了悬链线形渠道正常水深的直接计算公式,并根据临界流方程,通过适当的数学变换,得到了计算悬链线形渠道临界水深的无量纲关系式,从而获得了两种水深的计算方法。采用Matlab语言编程对某悬链线形渠道断面临界水深的计算结果表明,该计算方法简单、结果精确,便于在实际工程中推广应用。     

亚/超临界压力下碳氢燃料液滴的燃烧特性

建立了一个适用于超临界压力下包括能够正确描述跨临界迁移现象的液滴燃烧模型,提出了跨临界迁移时刻液滴表面处燃料质量流束有限的新观点.利用开发的计算模型,以碳氢燃料液滴自燃着火为研究对象,研究了亚临界和超临界压力下单液滴的燃烧特性,并从传热传质过程出发,阐明了跨临界迁移前、后液滴燃烧过程的热质输运机理及物理控制因素.结果表明:在亚临界压力下,液滴燃烧不会发生跨临界迁移现象,燃烧过程始终受燃料在液滴表面处相变的控制,液滴的燃烧速度取决于传热过程,并且液滴温度受该压力下燃料沸点的限制而增长缓慢.而在超临界压力下,液滴着火之后很快发生跨临界迁移现象,此后燃料向燃烧反应区域的扩散不存在相变,液滴的燃烧速度取决于传质过程,并且液滴温度不再受燃料沸点的限制而持续升高.     

基于分子动力学的燃油液滴超临界蒸发特性

基于分子动力学模拟的方法,对氮气环境中单个烷烃液滴的蒸发过程进行了模拟研究,揭示了液滴在亚临界和超临界条件下液滴蒸发特性的显著差异．对正十二烷液滴在氮气环境内的蒸发过程进行分子动力学模拟,结果表明：在超临界温度和压力条件下,液滴的温度持续上升,能够超过燃油组分的临界温度;此时,液滴与周围气相区的密度差异近乎消失,气-液相交界变得难以辨别,明显不同于亚临界条件下典型的气-液两相蒸发特征;蒸发速率随环境温度的升高而增大．在较低的压力范围内,升高环境压力能够提升液滴蒸发速率,但当压力达到一个特定值后,随着环境压力的升高蒸发速率反而会降低,同时液滴转变为超临界蒸发状态所需的最小压力随环境温度的升高而降低．对于双组分混合液滴,在亚临界环境条件下,液滴内的轻质组分优先蒸发;而在超临界环境条件下,液滴内各个组分近乎保持同步蒸发,两个燃油组分共同主导液滴的完整蒸发过程．     

液滴在倾斜表面上的固化特征研究

采用平滑铝表面为基底，以去离子水为介质，对低温条件下倾斜表面上运动液滴的固化特征进行了研究，分析了液滴的固化时间特征，并探讨了基底倾斜角、液滴体积以及基底表面浸润性对液滴固化时间的影响。结果表明：当基底倾斜角小于液滴在铝表面上的临界滑动角时，液滴固化时间保持不变；当基底倾斜角大于临界滑动角时，随着倾斜角增大，液滴固化时间变短。随着液滴体积增大，虽然湿接触面积增大，但是液滴高度也增大，液滴固化时间随之延长。基底表面的疏水性越好，液滴与固体表面的湿接触面积越少，液滴高度越高，因此液滴固化时间越长。基于传热学理论建立了双圆法模型，利用其预测运动液滴的固化时间，并将计算值与实验值进行了比较，发现两者吻合良好。     

分子动力学方法研究纳米尺度下液滴蒸发的物理规律

采用分子动力学方法对纳米尺度下氩液滴在氩蒸气中蒸发过程进行了模拟,其中液相分子采用球形截断的Lennard-Jones势能函数描述。模拟过程首先在三维模拟空间产生准稳态平衡的液滴和周围气相环境,随后控制液滴的外界物理条件形成蒸发现象,同步记录气液两相分子坐标和动量变化,从微观信息中统计计算出相应的宏观物理信息。研究了蒸发初始液滴半径的不同研究其对液滴蒸发过程的影响,结果表明纳米尺度下液滴蒸发现象与微米以上尺度液滴蒸发现象存在差异;引入等效辐射能的概念在分子动力学方法中实现了对辐射能传递过程的模拟,证实了辐射传递能量会对纳米尺度液滴蒸发过程产生很大的影响。     

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.     

丁烷-空气超临界燃烧中温度对迁移过程的影响

建立了单一燃料液滴燃烧过程的计算模型和数值方法,并利用之计算了丁烷-空气系统的蒸发和燃烧过程,研究了环境温度对迁移过程的影响.计算结果表明,在环境压力为2倍丁烷临界压力下,当环境温度为丁烷临界温度3倍时,液滴表面状态几乎在着火的同时实现了亚临界向超临界状态的迁移.环境温度低于3倍丁烷临界温度时,液滴会在迁移前着火,并借助火焰产生的热量完成迁移过程;当环境温度高于3倍丁烷临界温度时迁移时间短于着火延迟,液滴不利用燃烧产生的热量而依靠自周围高温介质传来的热量提高自身温度,进而完成迁移过程.此外,随着环境温度的升高,着火延迟时间和迁移时刻均逐渐变短.     

闭式循环柴油机排出气体喷淋冷却传热模型及实验关联研究

闭式揭环柴油机(CCD)排出气体处理技术是各种不依赖空气动力装置(AIP)的共性技术，优选喷淋冷却方式并对其内部的气液传热模型化是该技术的核心。为此，分析对比了横流式、并流式、逆流式等喷淋冷却方式，对实际选用的CCDAIP横流式喷淋冷却器进行了气液传热分析。从单个液滴传热模型出发，采用分层计算方法建立了喷淋冷却传热计算模型，并应用Matlab语言编程计算各种工况下冷却效果。喷淋冷却实验结果与模型计算结果对比表明，该模型能够较精确地预测CCD排出气体喷淋冷却效果。     

质量力作用下液滴变形及破裂现象的LBM模拟

基于伪势模型的两相格子Boltzmann方法(lattice Boltzmann method-LBM)模拟研究了液滴在质量力作用下的运动、变形和破裂现象.根据Young-Laplace定律确定模拟中所需的表面张力系数值,并对模型中表面张力所应具有的各向同性特性进行验证;计算了不同Bond数和Ohnesorge数时液滴的运动和变形过程.结果表明:随着Bond数的增大,液滴变形不断加剧并最终破裂;随着Ohnesorge数的增大,液滴趋向于维持其原始形状,运动过程中破裂现象的发生将受到抑制.     

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.     

Numerical simulations of the equilibrium shape of liquid droplets on gradient surfaces

The equilibrium shape of liquid droplets on horizontal and inclined plates that have a surface energy gradient is simulated numerically by applying a finite element method based on the principle of energy minimum in the present study. The numerical results show that the liquid droplet shape changes with locations under the influence of the unbalanced surface tension created by the gradient surface. It is shown that the contact angle reaches the maximum value at the one end of the droplet (2D), but it becomes minimum at the other end; the triple-phase contact line deforms toward the region with a smaller contact angle. It is further shown that the length of the liquid droplet increases with an increase in the surface energy gradient on the surface. More interestingly, an inflexion point appears when the droplet length varies with the center contact angle of the droplet, where the liquid droplet just locates at the transition region from the hydrophilic side to the hydrophobic side. It shifts to the hydrophilic side with the increase in the surface energy gradient. On the inclined gradient surface, the gravity induces a significant deformation of the equilibrium droplet shape towards the bottom of the surface. And the surface energy gradient further enhances the deformation when the unbalanced surface tension is directed to the bottom of the surface. However, the droplet shrinks back when the unbalanced surface tension is opposite to the component of gravity.     

Dynamic behavior of droplet transport on realistic gas diffusion layer with inertial effect via a unified lattice Boltzmann method

The dynamic behavior of liquid droplets on a reconstructed real gas diffusion layer (GDL) surface with the inertial effect produced by the three dimensional (3D) flow channel is investigated using an improved pseudopotential multiphase model within the unified lattice Boltzmann model (ULBM) framework, which can realize thermodynamic consistency and tunable surface tension. The microstructure of the GDL (Toray-090) including carbon fibers and polytetrafluoroethylene (PTFE) is reconstructed by a stochastic and mixed-wettability model. The critical force formulation for the Cassie-Wenzel transition of a droplet on GDL surface is derived. The effects of inertia and contact angles on the liquid droplet transport process on a reconstructed real GDL surface with a 3D flow channel are investigated. The results show the normalized center-of-mass coordinate X may enter the channel wall area or fluctuate around the initial position. With increased inertia applied on the droplet, the normalized center-of-mass coordinate Y grows faster and the normalized center-of-mass coordinate Z decreases. It is found by the ULBM for the first time that the liquid droplet is pushed back into the GDL by inertial effect. With the increase of inertia and the decrease of contact angle of GDL, both the droplet penetration depth in GDL and the droplet invasion fraction increase. The droplet invasion fraction in GDL is up to 30%.     

Transient burning of a convective fuel droplet

The transient burning of an n-octane fuel droplet in a hot gas stream at 20 atmosphere pressure is numerically studied, with considerations of droplet regression, deceleration due to the drag of the droplet, internal circulation inside the droplet, variable properties, non-uniform surface temperature, and the effect of surface tension. An initial envelope flame is found to remain envelope in time, and an initial wake flame is always transitioned into an envelope flame at a later time, with the normalized transition delay controlled by the initial Reynolds number and the initial Damkohler number. The initial flame shape is primarily determined by the initial Damkohler number, which has a critical value of Da₀=1.02. The burning rates are modified by the transition, and are influenced by the intensity of forced convection which is determined by initial Reynolds number. The influence of surface tension is also studied as the surface temperature is non-uniform. Surface tension affects the liquid motion at the droplet surface significantly and affects the change of surface temperature and burning rate modestly. The influence of surface tension generally increases with increasing initial Reynolds number within the range without droplet breakup. We also studied cases with constant relative velocity between the air stream and the droplet. The results show that in these cases the initial envelope flame still remains envelope, but the evolution from an initial wake flame to an envelope flame is inhibited. Validation of our analysis is made by comparing with a published porous-sphere experiment Raghavan et al. (2005) [6] which used methanol fuel.     

Solidification characteristics of a droplet on a horizontal cooled wall

An experimental and numerical study of solidification characteristics of a droplet on a horizontal cooled wall is reported. Pure water and molten salt were utilized as the testing liquids. The droplet was cooled under a variety of conditions such as wall temperature and initial liquid temperature in a static atmosphere. Extensive observations of both solidification characteristics and morphologies of the droplet were made. Numerical calculations based on the Landau method considering the effects of both surface tension at the droplet surface and the density inversion at 4 °C within the droplet were carried out. It was found that the numerical results are qualitatively in good agreement with the experimental results except for the terminating period of solidification. © 1998 Scripta Technica, Heat Trans Jpn Res, 26(7): 469–483, 1997     

CaCO3结垢过程控制机理分析

为了掌握结垢过程的控制机理,通过分析CaCO3在换热面上的结垢过程,得到了控制结垢过程的阻力关系式。计算结果表明流速越大,过饱和度越小,结垢过程越易为表面反应所控制;流速越小,过饱和度越大,结垢过程越易为对流传质所控制,且往往发生在液壁温差较大,壁温较高的情况下。垢层生长过程中,在恒壁温条件下,对流传质与表面反应共同控制的结垢过程可能转变为只由表面反应控制结垢过程;在恒热流条件下,控制机理没有变化。     

Numerical Simulation of Droplet Deposition and Self-Alignment on a Microstructured Surface

Numerical simulation is performed for droplet impact and deposition on a microstructured surface. The droplet deformation is calculated by a sharp-interface level-set method which is extended to treat the immersed solid structure and the contact angle at the liquid–gas–solid interline. The computations are further carried out to investigate the droplet self-alignment behavior derived by the interfacial characteristics between the liquid-gas-solid phases, which can be used to overcome a droplet placement error and to improve the accuracy in film formation. The effects of contact angle, surface tension, and microstructure configuration on the droplet deposition are quantified.     

Liquid Ejection Characteristics of Piezoelectric Inkjet Printhead for Varied Work Liquids

This research applied computational fluid dynamics in the study of liquid ejection characteristics of a piezoelectric inkjet printhead for varied work liquids. The results of the fixed work liquid viscosity coefficient and the change of surface tension showed that larger surface tension increases the flying speed of the ejected droplet, but irregularity may occur if the viscosity coefficient is too small. Also, given fixed surface tension of the work liquid and changed viscosity coefficient, larger viscosity coefficient increases the tendency of the liquid surface in the nozzle to move toward the pressure chamber, and decreases the flying speed of the ejected droplet. When the nozzle diameter is changed, smaller nozzle diameter produces faster ejected droplet flying speed.     

Water film coated composite liquid metal marble and its fluidic impact dynamics phenomenon

A composite liquid metal marble made of metal droplet coated with water film was proposed and its impact dynamics phenomenon was disclosed. After encapsulating the liquid metal into water droplets, the fabricated liquid marble successfully avoided being oxygenized by the metal fluid and thus significantly improved its many physical capabilities such as surface tension modification and shape control. The striking behaviors of the composite liquid metal marbles on a substrate at room temperature were experimentally investigated in a high speed imaging way. It was disclosed that such marbles could disintegrate, merge, and even rebound when impacting the substrate, unlike the existing dynamic fluidic behaviors of liquid marble or metal droplet. The mechanisms lying behind these features were preliminarily interpreted. This fundamental finding raised profound multiphase fluid mechanics for understanding the complex liquid composite which was also critical for a variety of practical applications such as liquid metal jet cooling, inkjet printed electronics, 3D printing or metal particle fabrication etc.     

SIMULATION OF METAL DROPLET DEPOSITION WITH SOLIDIFICATION INCLUDING UNDERCOOLING AND CONTACT RESISTANCE EFFECTS

A numerical procedure for the deformation and solidification of a metal droplet impinging on a flat surface is developed and a sample calculation is presented. A previously derived second-order ordinary differential equation (ODE) that approximates the splat as a cylinder and describes the droplet size evolution based on the mechanical energy equation in conjunction with kinematic and geometrical compatibility is used. The thermal energy equations for the liquid and solid regions of the splat and the substrate are separately solved coupled by boundary conditions such as contact resistance and undercooling in a regularized calculation domain produced by algebraic grid generation. The solidified layer thickness is calculated by solving a hyperbolic partial differential equation (PDE) resulting from the interface energy equation at the phase change boundary. Physical processes such as convective heat loss, substrate heat loss, viscous dissipation, and surface tension are modeled through appropriate nondimensional parameters.