单液滴撞击液膜的颈部射流模拟及机理分析

本文采用DNS耦合ACLS方法对喷雾中常见的单液滴撞击水平液膜现象进行二维的数值模拟,通过撞击后形态演变及内部场信息对比,重点分析颈部射流的形成机理,随后将研究对象拓展到倾斜液膜。本文验证了ACLS方法的准确性,成功捕捉到液膜倾角对空气卷吸现象的促进作用。研究表明,局部压差是液滴撞击水平液膜后产生颈部射流的直接原因,而对于撞击倾斜液膜的案例,前后两端颈部射流成因不同:前端射流由局部压差产生而后端射流随着液膜倾斜角的增大,旋涡机理的影响比重逐渐增强,两者处于共存竞争关系。     

液滴冲击液膜过程实验研究

实验显示了液滴撞击物体表面液膜后产生水花、发生飞溅,特别是产生“钟形”水花等的流动现象.根据实验结果,探讨了液滴冲击速度、液体黏性、表面张力、液滴直径和液膜厚度等对液滴冲击后产生的流动现象,以及液膜形状演化的影响,分析了观测到的鲜有文献报道的液滴撞击液膜后产生“钟形”水花的现象.     

液滴冲击液膜过程实验研究

实验显示了液滴撞击物体表面液膜后产生水花、发生飞溅,特别是产生“钟形”水花等的流动现象.根据实验结果,探讨了液滴冲击速度、液体黏性、表面张力、液滴直径和液膜厚度等对液滴冲击后产生的流动现象,以及液膜形状演化的影响,分析了观测到的鲜有文献报道的液滴撞击液膜后产生“钟形”水花的现象. 关键词： 液滴 液膜 冲击 流动显示     

单液滴撞击倾斜液膜飞溅过程的耦合Level Set-VOF模拟

采用耦合水平集--体积分数法(CLSVOF)对液滴撞击倾斜表面液膜后液膜的形态演化及飞溅过程进行数值模拟, 并对液滴撞击液膜过程中形成的空气卷吸现象进行研究并探讨了撞击角对此的影响, 分析了液滴撞击后液体内部的压力和速度分布, 对液滴撞击倾斜表面液膜的飞溅过程进行讨论, 并与实验结果进行了对比, 验证了CLSVOF方法研究液滴撞击倾斜液膜的可行性. 结果表明, 液滴撞击倾斜液膜时前后两部分飞溅现象产生的机理不同, 前半部分飞溅是由于压差引起的颈部射流, 而后半部分则是由液膜径向流动产生的飞溅现象. 随着撞击角的增大, 空气卷吸气泡数量减少.     

基于MRT-LB伪势模型的液滴撞击液膜数值研究

运用改进的格子玻尔兹曼（LB）伪势多松弛多相模型，研究单/双液滴撞击液膜的流动特性.考察单液滴在不同气液相密度比时撞击液膜的发展.随着密度比的减小，冠状水花顶端开始向内弯曲，且底部半径显著减小.在大密度比情况下研究双液滴撞击液膜.结果表明：双液滴撞击液膜有中心射流的形成；液滴水平间距的增大，延缓中心射流的出现，并降低初始中心射流的高度；随Re数的增加，中心射流的高度明显增大.     

液滴撞击液膜过程的格子Boltzmann方法模拟

本文采用格子Boltzmann方法对液滴撞击液膜过程进行了研究, 主要考察了雷诺数(Re)、韦伯数(We)、相对液膜厚度 (h) 以及表面张力 (σ) 等物理参数对界面运动过程的影响. 首先, 随着Re数和We数的增加, 可以明显观察到液滴撞击液膜过程中形成的皇冠状水花以及卷吸现象; 当Re数较大时, 液体会发生飞溅, 由液体飞溅形成的小液滴则会继续下落, 并与液膜再次发生碰撞. 其次, 当相对液膜厚度较小时, 液滴撞击液膜并最终导致液膜断裂; 然而随着相对液膜厚度的增大, 尽管撞击过程溅起的液体会越来越多, 但是液膜并不会发生断裂. 再次, 随着表面张力的增大, 界面变形阻力增大, 撞击过程中产生的界面形变也逐渐减弱. 最后还发现皇冠(由液滴溅起形成)半径r 随时间满足r/(2R) ≈ α√Ut/(2R), 这一结果与已有结论是一致的.     

液滴撞击液膜的流热耦合界面追踪方法数值模拟

采用界面追踪方法研究蒸馏过程中液滴撞击高温薄液膜的流动和传热特性，将数值结果与解析解和实验进行比较验证模型的正确性，研究气液界面和热流分布的演变过程.同时，分析液滴We数和无量纲液膜厚度对传热的影响.液滴撞击后的热流密度分布显示：液膜可分为撞击区、过渡区和静态区.由于液滴的撞击作用，强制对流是撞击区内主要的传热机制.增大液滴的韦伯数或减小无量纲液膜厚度会加强热量传递.随着液滴韦伯数的增加，冲击引起的扰动增强，在动量和能量共同作用下，平均热流密度明显增大，撞击区冠状水花越明显.无量纲液膜厚度越小，平均热流密度越大，且有更长的时间保持高热流密度换热.     

液滴撞击固体表面铺展特性的实验研究

利用高速摄影仪记录了不同液滴撞击固体表面的形态变化过程, 并探讨了液滴撞击参数对撞击过程液滴形态的影响. 结果表明: 液滴黏度对液滴铺展过程起着决定性作用, 液滴表面张力对液滴铺展后的回缩起到主导作用, 两者的共同作用决定着液滴的震荡特性; 撞击速度的提高会增大液滴的最大铺展因子, 但达到最大铺展的时间因表面张力的不同呈现不同变化规律.     

双液滴撞击平面液膜的流动与传热特性

采用耦合的水平集-体积分数法(CLSVOF)对双液滴连续撞击恒定壁温壁面上的热液膜的流动和换热特性进行了数值模拟及分析,得到了双液滴撞击热液膜后形态演变的过程.分析了液滴垂直间距、撞击速度、液膜厚度以及液滴直径对双液滴撞击液膜后的流动与传热特性的影响,结果显示,壁面平均热流密度随液滴撞击速度的增大而增大,液滴垂直间距、液膜厚度和液滴直径对平均热流密度的影响较小,但会对热流密度在撞击区域和交界区的分布产生重要影响.     

自由空间内气泡破碎过程的实验研究

利用高速摄像对自由空间内悬挂气泡的自然破碎过程进行了研究,分析了气泡破碎过程中环形断面的发展规律、膜液滴的形成过程及末期射流的产生原因。气泡成孔后,在液膜表面张力和气泡内部气流喷射的综合作用下,会形成不稳定的环形断面,其铺展过程中边缘会出现诸多不稳定液线,液线末端断裂形成膜液滴,膜液滴飞行方向与断裂点切线方向一致;随着液膜的破碎,半封闭球状液膜的曲率半径逐渐减小,环状断面以一定的加速度推进;在破碎末期,环形断面将在初始破碎点的对称位置附近聚焦并形成射流。表面张力系数和液膜厚度均匀性对膜液滴分布和射流强度具有一定的影响;相对于液膜厚度,作用于液膜单位面积上的表面张力对气泡破碎时间的影响更为显著。     

Numerical simulation of two droplets impacting upon a dynamic liquid film

The impact of droplets on the liquid film is widely involved in industrial and agricultural fields. In recent years, plenty of works are limited to dry walls or stationary liquid films, and the research of multi-droplet impact dynamic films is not sufficient. Based on this, this paper employs a coupled level set and volume of fluid (CLSVOF) method to numerically simulate two-droplet impingement on a dynamic liquid film. In our work, the dynamic film thickness, horizontal central distance between the droplets, droplets' initial impact speed, and simultaneously the flow velocity of the moving film are analyzed. The evolution phenomenon and mechanism caused by the collision are analyzed in detail. We find that within a certain period of time, the droplet spacing does not affect the peripheral crown height; when the droplet spacing decreases or the initial impact velocity increases, the height of the peripheral crown increases at the beginning, and then, because the crown splashed under Rayleigh-Plateau instability, this results in the reduction of the crown height. At the same time, it is found that when the initial impact velocity increases, the angle between the upstream peripheral jet and the dynamic film becomes larger. The more obvious the horizontal movement characteristics, the more restrained the crown height; the spread length increases with the increase of the dynamic film speed, droplet spacing and the initial impact velocity. When the liquid film is thicker, more fluid enters the crown, due to the crown being unstable, the surface tension is not enough to overcome the weight of the rim at the end of the crown, resulting in droplets falling off.     

液体横向射流在气膜作用下的破碎过程

为了研究液体横向射流在气膜作用下的破碎过程,采用背景光成像技术及VOF TO DPM方法进行了实验研究和仿真研究,模拟介质为水和空气.研究结果表明,液体射流在气膜作用下主要存在两种破碎过程:柱状破碎和表面破碎.Rayleigh-Taylor(R-T)不稳定性产生的表面波是液体射流发生柱状破碎的主要原因,气流穿透表面波的波谷导致射流柱破碎,破碎后的液丝沿流向逐渐发展呈带状分布.Kelvin-Helmholtz(K-H)不稳定性产生的表面波是液体射流发生表面破碎的主要原因,液丝和液滴从射流表面剥离.局部动量比对液体横向射流的破碎过程具有重要影响,当局部动量比较低时,液体射流的破碎由K-H不稳定性主导;随着局部动量比的增大液体射流的破碎逐渐由R-T不稳定性主导.液体射流的破碎长度及穿透深度均随局部动量比的增大而增大.     

Comparative visualized investigation of impact-driven high-speed liquid jets injected in submerged water and in ambient air

This paper is a comparative study on the characteristics of high-speed liquid jets injected in surrounding water and air using shadowgraph technique. One of the main objectives is to investigate the effects of liquid’s physical properties, used to generate the high-speed liquid jets, on jet generation’s characteristics. Moreover, comparative investigations on effects of those liquid jets after injected in water and air are reported. The high-speed liquid jets were generated by the impact of a projectile launched by a horizontal single-stage power gun. The impact-driven high-speed liquid jets were visualized by shadowgraph technique and images were recorded by a high-speed digital video camera. The process of impact-driven high-speed liquid jet injection in air and water, oblique shock waves, jet-induced shock waves, shock waves propagation, the bubble behavior, bubble collapse-induced rebound shock waves and bubble cloud regeneration were clearly observed. It was found that different properties of liquid (surface tension and kinematic viscosity) affect the jet maximum velocity and shape of the jet. Bubble behaviors were only found for the jet injected in water. From the shadowgraph images, it is found that the maximum average jet velocity, expansion and contraction velocities of bubble in axial direction increase when the value of the multiplied result of surface tension by kinematic viscosity increases. Therefore, surface tension and kinematic viscosity are the significant physical properties that affect characteristics of high-speed liquid jets.     

Spreading of a viscous liquid film over the corrugated surface and the heat and mass transfer calculations

There are a lot of industrial applications of structured packing. Distillation columns are one of the examples where the liquid flows over the corrugated surface as a thin film to provide a good mass-transfer surface between the liquid and vapor phase. The purpose of the present paper is to study the hydrodynamics and the heat-mass transfer of the liquid film spreading down the corrugated surfaces when the corrugation amplitude is comparable with Nusselt’s film thickness (the amplitude corresponds to a small texture of the structured packing). As a result, a nonlinear type diffusion equation is obtained to describe the evolution of the film thickness profile. The nonlinear diffusion coefficient is obtained for three cases: a smooth inclined plate, a corrugated plate with large ribs, and an inclined corrugated plate with small ribs. The equations are solved numerically. As a result, it has been obtained that the small texture significantly increases the rate of the film thickness evolution in comparison with a smooth plate. To obtain the nonlinear diffusion coefficient in the case of a small texture, the hydrodynamics of the film flow over an inclined corrugated surface are studied. The viscosity, inertia, and surface tension forces are taken into account. The calculations were carried out on the basis of the Navier-Stokes equations. The influence of the microcorrugations on both the heat transfer from the wall and the mass transfer through the free surface was investigated.     

Study on the temperature distribution of heated falling liquid films

A 2-D theoretical model was derived to present the temperature distribution of falling liquid films flowing over a vertical heated/cooled plate with constant temperature. And the temperature gradients for different flow rates and fluids were also discussed for different liquid films. The temperature distributions for liquid films of water, ethanol aqueous solutions and glycerol aqueous solutions were experimentally investigated with a sensitive thermal imaging system. It is found that the surface temperature of a film flowing over a vertical heated solid plate has a characteristic relationship with the film flow distance. A lower flow rate of the film or a higher temperature of the wall generally leads to a higher surface temperature in the film inception. For films of glycerol aqueous solutions under the same heating conditions, a lower glycerol concentration causes a higher surface temperature of the film, due to the decrease of the liquid viscosity, whereas the ethanol concentration is found to have little influence on the temperature distribution of the film surface. Comparisons of the experimental data and the theoretical model show that the model can adequately describe the surface temperature distribution of a heated falling liquid film.     

非平整基底上受热液膜流动稳定性研究

本文研究了二维黏性流体薄膜沿非平整不均匀加热基底流动时非线性表面波的演化及其流动稳定性.利用长波摄动法推导出非平整线性加热基底上非线性表面波的零阶和一阶演化方程,基于所得演化方程,绘制出正弦波纹基底上液膜的表面波形图,并研究液膜流动的线性稳定性,分析了各无量纲参数对液膜线性稳定性的影响.分析结果表明:在正弦波纹基底上,液膜自由表面随同壁面作相同频率的正弦型波动,且液膜厚度沿流动方向逐渐变小;Marangoni数为稳定影响因素,随Marangoni数的增大,液膜稳定区域增大;Peclet数和倾角θ均为不稳定影响因素,随Peclet数和倾角θ的增大,液膜稳定区域减小;在非平整基底的波峰和波谷处,Peclet数、Marangoni数和倾角θ对稳定性的影响趋势一致,但基底波谷处的液膜稳定区域小于波峰处区域,流动更易失稳.     

Instabilities and decay rates of charged viscous liquid jets

The eigenmodes and characteristic frequencies of surface deformations of real fluids in the geometry of a cylindrical jet, which is homogeneously charged, are calculated using the linearized Navier-Stokes equation with free surface boundary conditions. A stability phase diagram in the wavenumber-fissility plane is given. Two types of instabilities are described, surface tension and Coulomb force induced. The known modes for ideal potential flow are extended to arbitrary viscosity. In addition new types of modes are described (eigenfunctions and decay rates) which represent rotational flow and have no correspondence to Rayleigh's jet instability.