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.     

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.     

实验观测液滴撞击倾斜表面液膜的特殊现象

采用高速摄像仪以10000帧/s 的拍摄速度对液滴撞击倾斜表面液膜的过程进行了实验观测, 分析了液滴撞击倾斜表面液膜后的铺展、水花形成以及飞溅等现象, 考察了撞击角对液滴震荡变形过程的影响; 在此基础上, 定量讨论了液滴铺展速度随时间的变化规律, 揭示了液滴撞击速度和撞击角对前、后铺展因子及初始铺展速度的影响.观测发现, 在撞击角为28.0°–74.7°范围内, 随着撞击角的减小, 液滴在液膜表面的震荡变形程度增大; 前铺展因子随撞击速度的增大而增大, 随撞击角的减小而增大; 后铺展因子随撞击速度的增大几乎不发生变化, 但是随撞击角的增大而增大; 液滴初始铺展速度随撞击速度和撞击角的升高而增大. 关键词： 液滴撞击 倾斜液膜 铺展因子 铺展速度     

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.     

单液滴正碰球面动态行为特性实验研究

在考虑空气阻力影响,确定液滴撞击球面速度的基础上,对较高韦伯数液滴撞击干燥球面动态行为过程进行了实验研究,分析了球面曲率与韦伯数对液滴撞击行为和铺展因子的影响,并与前人撞击平面结果进行了对比.实验表明,靠近撞击球面时,液滴降落速度出现明显波动;球面曲率对液滴撞击后行为影响明显,曲率较大时,液滴撞击后铺展液膜会超出球面直径并滑落,曲率较小时,液滴撞击后在球面上呈现明显的铺展、回缩、震荡、着附动态变化行为,此时最大铺展因子受曲率影响小,随曲率减小,逐渐趋向于撞击平面时的最大铺展因子;韦伯数对液膜铺展速率影响较小,但对液膜回缩时间影响明显,最大铺展因子随韦伯数增加逐渐增大,获得的关联式呈指数变化.     

液滴撞击加热壁面传热实验研究

本文采用高速摄像仪对水滴和乙醇液滴撞击加热壁面后的蒸发过程进行了实验观测, 分析了液滴撞击加热壁面后的蒸发特性参数. 实验中, 两种液体初始温度均为20 ℃, 不锈钢壁面初始温度范围为68-126℃. 水滴初始直径为2.07 mm, 撞击壁面时Weber 数为2-44; 乙醇液滴初始直径为1.64 mm, Weber数为3-88. 结果表明, 液滴受到重力、表面张力及流动性的影响, 在蒸发过程的大部分时间内, 水滴高度持续降低而接触直径几乎不变; 蒸发后期, 液滴发生回缩, 水滴的接触直径、高度和接触角出现振荡现象. 乙醇液滴的接触角随时间的增加呈现先减小随后保持不变的趋势, 而接触直径和高度则持续减小, 直到液滴完全蒸发. 液滴蒸发总时长与液体物性和壁面温度有关, 随壁面温度的升高而减小, 与液滴撞击壁面时的Weber 数无关. 同时, 随着壁面温度的升高, 液滴显热部分占总换热量的比重增大, 显热部分能量不可忽略, 本文实验条件下得到水滴的平均热流密度为0.014-0.110 W·mm^-2.     

Simulation of impact of a hollow droplet on a flat surface

Despite many theoretical and experimental works dealing with the impact of dense continuous liquid droplets on a flat surface, the dynamics of the impact of hollow liquid droplets is not well addressed. In an effort to understand dynamics of the hollow droplet impingement, a numerical study for the impact of a hollow droplet on a flat surface is presented. The impingement model considers the transient flow dynamics during impact and spreading of the droplet using the volume of fluid surface tracking method (VOF) coupled with the momentum transport model within a one-domain continuum formulation. The model is used to simulate the hydrodynamic behaviour of the impact of glycerin hollow droplet. It is found that the impact and spreading of the hollow droplet on a flat surface is distinctly different from the conventional dense droplet and has some new hydrodynamic features. A phenomenon of formation of a central counter jet of the liquid is predicted. With the help of simulations the cause of this phenomenon is discussed. Comparison of the predicted length of the central counter jet and the velocity of the counter jet front shows good agreements with the experimental data. The influence of the droplet initial impact velocity and the hollow droplet shell thickness on the impact behaviour is highlighted.     

基于范德瓦尔斯表面张力模式液滴撞击疏水壁面过程的研究

液滴撞击疏水壁面过程的研究在介观流体力学和微流体作用材料科学的研究中具有重要的理论意义和工程价值. 论文在SPH方法中引入范德瓦尔斯状态方程处理液滴表面张力, 考虑流体粒子之间远程吸引, 近程排斥的内部作用力, 提出了流体粒子与疏水壁面粒子间势能函数与表面张力相结合的作用模式. 通过模拟真空条件下两个静止的等体积液滴相互融合的过程, 验证了计算模式在模拟液滴的表面张力中的有效性. 采用该模式模拟的液滴撞击疏水壁面过程, 不仅能够有效地模拟液滴撞击壁面后的变形过程, 而且清晰地模拟出液滴的回弹、腾空以及二次撞壁现象的完整过程. 模拟结果与液滴撞击疏水壁面的实验结果以及VOF模拟结果符合较好, 表明本文所提出的表面张力和疏水壁面作用力处理模式对模拟液滴撞壁过程具有实际应用价值.     

Transient convective burning of interactive fuel droplets in single-layer arrays

The transient convective burning of n-octane droplets interacting within single-layer arrays in a hot gas flow perpendicular to the layer is studied numerically, with considerations of droplet surface regression, deceleration due to the drag of the droplets, internal liquid motion, variable properties, non-uniform liquid temperature and surface tension. Infinite periodic arrays, semi-infinite periodic arrays with one row of droplets (linear array) or two rows of droplets, and finite arrays with nine droplets with centers in a plane are investigated. All arrays are aligned orthogonal to the free stream direction. This paper compares the behavior of semi-infinite periodic arrays and finite arrays with the behavior of previously studied infinite periodic arrays. Furthermore, it identifies the critical values of the initial Damköhler number for bifurcations in flame behavior at various initial droplet spacing for all these arrays. The initial flame shape is either an envelope flame or a wake flame as determined by the initial Damköhler number, the array configuration and the initial droplet spacing. The critical initial Damköhler number separating initial wake flames from initial envelope flames decreases with increasing interaction amongst droplets at intermediate droplet spacing (when the number of rows in the array increases or the initial droplet spacing decreases for a specific number of rows in the array). In the transient process, an initial wake flame has a tendency to develop from a wake flame to an envelope flame, with the moment of wake-to-envelope transition advanced for the increasing interaction amongst droplets at intermediate droplet spacing. For the array with nine droplets with centers in a plane, the droplets at different types of positions have different critical initial Damköhler number and different wake-to-envelope transition time for initial wake flame.     

矩形通道内横流喷雾掺混流场的实验研究

应用PIV系统对矩形通道内横流-喷雾掺混过程中液滴沿横流方向的流场进行了实验测量.分别获得了单喷嘴和双喷嘴下横流掺混中的流场结构,以及液滴的水平速度分布.横流作用下,液滴的最大水平速度为横流速度的两倍左右;反旋涡对降低了液滴的水平速度,提高了液滴在横流中的停留时间.双喷嘴条件下,横流截面上的液滴密集区域呈狭长的三角形,...     

Crown evolution kinematics of a camellia oil droplet impacting on a liquid layer

The phenomenon of droplet impact on an immiscible liquid is encountered in a variety of scenarios in nature and industrial production. Despite exhaustive research, it is not fully clear how the immiscibility of the liquid on which a droplet impacts affects the crown evolution. The present work experimentally investigates the evolution kinematics of a crown formed by the normal impact of a camellia oil droplet on an immiscible water layer. Based on discussion of dynamic impact behaviors for three critical Weber numbers (We), the radius of the crown and its average spreading velocity are compared with those of previous theoretical models to discuss their applicability to the immiscible liquid. The evolution kinematics (morphology and velocity) are analyzed by considering the effects of the We and layer thickness. Furthermore, the ability of crown expansion in radial and vertical directions is characterized by a velocity ratio. The results show that our experimental crown radius still follows a square-root function of evolution time, which agrees with the theoretical predictions. The dimensionless average spreading velocity decreases with We and follows a multivariate power law, while the dimensionless average rising velocity remains constant. The velocity ratio is shown to linearly increase with We, demonstrating that the rising movement in crown evolution gradually enhances with We. These results are helpful for further investigation on the droplet impact on an immiscible liquid layer.     

非对称布置液滴在波纹状基底上的聚并过程

基于润滑理论建立在波纹基底上两个含表面活性剂液滴聚并的演化模型，模拟液滴位于波纹基底波峰和波谷处的聚并过程，分析液滴初始间距和基底高度对聚并的影响，并与对称布置液滴进行比较.结果表明：液滴高度随时间呈现五个阶段变化；活性剂浓度在短时间内完成三个阶段变化；初始间距的增大将延长液滴及活性剂聚并时间；增大基底高度将缩短液滴及活性剂聚并时间；非对称液滴相较于对称液滴聚并时间短，聚并速度快.     

Field dependent shape variation of magnetic fluid droplets on magnetic dots

The morphology of magnetic fluid droplets on magnetic thin film dots is studied experimentally, including the aspect ratio and the contact angle variation of the droplets. Under a uniform external magnetic field, the droplet's aspect ratio increases with the external field and with the diameter of the magnetic dot due to the concentrated magnetic flux inside the magnetic fluid droplet. Similar to the electrical wetting phenomenon, the induced magnetic dipoles in the magnetic film and in the magnetic fluid near the solid–liquid interface change the solid–liquid interfacial tension, and in consequence reduce the apparent contact angle of the magnetic fluid droplet.     

液滴撞击液膜的射流与水花形成机理分析

建立了单液滴撞击平面液膜的物理与数学模型,采用Coupled Level Set and Volume of Fluid方法对这种现象进行了数值模拟,探讨了黏度和表面张力对冠状水花形态的影响.通过分析撞击后液体内部的压力和速度分布,揭示了液滴颈部射流的产生机理,验证了Yarin和Weiss提出的运动间断理论.研究显示,表面张力对冠状水花形态的影响远大于黏度的影响.颈部射流的产生主要是由于撞击后颈部区域局部较大压差造成的,随着撞击过程的继续,压差作用减弱;液膜内流体的径向运动对射流发展成冠状水花具有推动作用.     

液滴碰撞液膜润湿壁面空气夹带数值分析

采用复合水平集-流体体积法并综合考虑传热及接触热阻的作用, 对液滴碰撞液膜润湿壁面空气夹带现象进行了数值分析. 揭示了夹带空气形成机理, 探索了夹带空气特性参数随碰撞速度和液膜厚度的变化规律, 获得了夹带空气作用下液滴碰撞润湿壁面的传热机理. 研究结果表明: 撞壁前气液两相压力差是引起气液相界面拓扑结构变化以及夹带空气形成的主要原因; 液滴碰撞速度与压缩空气层内压力以及相界面形变高度密切相关; 液滴接触液膜时, 碰撞轴上液滴底部和液膜表面速度相等, 大约是碰撞速度的1/2; 碰撞速度对夹带空气层底部到破碎点的无量纲弧长和最大无量纲夹带空气直径均存在较大的影响; 液滴和液膜的无量纲形变高度与斯托克斯数密切相关; 液膜初始厚度对液滴和液膜的无量纲形变高度和最大无量纲夹带空气直径影响较大; 撞壁初始阶段, 碰撞中心区域夹带空气对壁面热流密度分布存在较大的影响.     

Microgravity experiments of flame spreading along a fuel droplet array in fuel vapor-air mixture

Combustion experiments of fuel droplet array in fuel vapor-air mixture were performed at microgravities to investigate growth mechanism of group combustion of fuel droplets. A 10-droplet array was inserted into the test section filled with a saturated fuel vapor-air mixture as a simple model of prevaporized sprays. Gas equivalence ratio of the fuel vapor-air mixture was regulated by the test section temperature. n-Decane droplets of 0.8 mm in the initial diameter were suspended at the crossing points of 10 sets of X-shaped suspenders. The first droplet was ignited by a hot wire to initiate flame spread along a fuel droplet array. Flame spread speed was obtained from the history of the leading edge position of a spreading flame. Effects of droplet spacing and gas equivalence ratio on the flame spreading behavior and the flame spread speed were examined. The droplet spacing and the gas equivalence ratio were varied from 1.6 to 10.2 mm and from 0.2 to 0.7, respectively. The gas equivalence ratio has little effect on the relationship between the flame spreading behavior and the droplet spacing. The flame spread speed increases as the increase in the gas equivalence ratio at all droplet spacings. The influence of the gas equivalence ratio on the flame spread speed becomes strong as the increase in the droplet spacings. The flame spread speed increases as the increase in the droplet spacing, and then decreases. The maximum flame spread speed appears in the range from 2.4 to 3 mm at all gas equivalence ratios.     

液滴撞击圆柱内表面的数值研究

针对液滴撞击圆柱内表面的过程,利用基于相场的格子Boltzmann方法模拟液滴以不同初速度、从不同初始高度、撞击不同大小的圆柱内表面时液滴的形态变化,分析了液滴自身物性(如密度和黏性等)和圆柱内表面润湿性等因素对撞击现象的具体影响.研究发现:撞击韦伯数、密度比及动力黏性比、圆柱半径等对液滴撞击后沿圆柱内表面的铺展均有一定影响,较高的韦伯数下液滴可能会发生分裂;液滴初始高度对大密度比和动力黏性比的撞击影响较小;液滴反弹现象可能出现在接触角较大时;重力作用会抑制撞击后液滴的振荡.     

Persistent droplet motion in liquid-liquid dewetting

When a nonvolatile liquid film dewets from a partly compatible liquid substrate, the advancing dewetting front leaves behind droplets formed through a Rayleigh instability mechanism at its rim. We have found that these droplets continue to move in the direction of the dewetting front for extended periods (of order one day) with an initial droplet velocity varying linearly with the droplet size, and a displacement varying logarithmically with time. We attribute this persistent motion to a transient surface tension gradient on the substrate liquid surface trailing the dewetting front.     

Numerical simulation on partial coalescence of a droplet with different impact velocities

Partial coalescence is a complicated flow phenomenon. In the present study, the coalescence process is simulated with the volume of fluid(VOF) method. The numerical results reveal that a downward high-velocity region plays a significant role in partial coalescence. The high-velocity region pulls the droplet downward continuously which is an important factor for the droplet turning into a prolate shape and the final pinch-off. The shift from partial coalescence to full coalescence is explained based on the droplet shape before the pinch-off. With the droplet impact velocity increasing, the droplet shape will get close to a sphere before the pinch-off. When the shape gets close enough to a sphere, the partial coalescence shifts to full coalescence. The effect of film thickness on the coalescence process is also investigated. With large film thickness,partial coalescence happens, while with small film thickness, full coalescence happens. In addition, the results indicate that the critical droplet impact velocity increases with the increase of surface tension coefficient but decreases with the increase of viscosity and initial droplet diameter. And there is a maximum critical Weber number with the increase of surface tension coefficient and initial droplet diameter.     

Transient convective burning of a periodic fuel-droplet array

The transient convective burning of fuel-droplets interacting within 3-D infinite periodic arrays in a hot gas stream is numerically studied for the first time, with considerations of droplet regression, deceleration due to the drag of the droplets, internal liquid motion, variable properties, non-uniform liquid temperature, surface tension, and n-octane one-step oxidation kinetics. Depending upon the initial conditions and other constraints, a flame is established early as either a wake flame or an envelope flame. An initial envelope flame remains an envelope flame, and an initial wake flame has a tendency to develop from a wake flame to an envelope flame. The flame shows no strong tendency to modify significantly the standoff distance during the lifetime of the droplet. For an initial wake flame, the moment of wake-to-envelope transition is advanced as the initial droplet spacing (intermediate) is decreased, but tends to be postponed as the initial droplet spacing is further reduced. The burning rate at smaller initial droplet spacing or smaller initial Reynolds number might be greater for some period during the lifetime because of an earlier wake-to-envelope transition which elevates the average surface temperature. Lower ambient temperature yields a later wake-to-envelope transition time and smaller mass burning rate. At the lower ambient pressure with the same initial relative stream velocity, the average surface temperature is reduced, the wake-to-envelope transition is later, and the mass burning rate is smaller. Validation of our analysis is made by comparing with the results of an isolated droplet Wu and Sirignano [11].