激波诱导的液滴变形和破碎

 建立液滴变形与破碎的模型,提出初始雾化时间概念,由此分析激波后液滴变形和破碎雾化的特征。液滴变形率、完全破碎时间的数值分析结果与实验结果基本一致。     

Y型微通道内双重乳液流动破裂机理

基于体积分数法建立了Y型微通道中双重乳液流动非稳态理论模型,数值模拟研究了Y型微通道内双重乳液破裂情况,详细分析了双重乳液流经Y型微通道时的流场信息以及双重乳液形变参数演化特性,定量地给出了双重乳液流动破裂的驱动以及阻碍作用,揭示了双重乳液破裂流型的内在机理.研究结果表明:流经Y型微通道时,双重乳液受上游压力驱动产生形变,形变过程中乳液两端界面张力差阻碍双重乳液形变破裂,两者正相关;隧道的出现将减缓双重乳液外液滴颈部收缩速率以及沿流向拉伸的速率,并减缓了内液滴沿流向拉伸的速率,其对于内液滴颈部收缩速率影响不大;隧道破裂和不破裂工况临界线可以采用幂律关系式l~*=βCa~b进行预测,隧道破裂和阻塞破裂工况临界线可以采用线性关系l~*=α描述;与单乳液运动相图相比,双重乳液运动相图各工况的分界线关系式系数α和β均相应增大.     

T型微通道中液滴半阻塞不对称分裂行为研究

液滴不对称分裂是获得不同尺寸微液滴的优选方法,研究液滴不对称分裂行为对于生物医学、能源化工及食品工程等领域具有重要意义.本文研制T型微通道芯片并设计搭建T型微通道液滴半阻塞不对称分裂行为可视化实验平台,研究流量调控对微液滴分裂比的影响规律,并建立理论模型对分裂比进行预测,得到以下结论:液滴不对称挤压分裂过程分为挤压前期、挤压后期和快速夹断阶段,在挤压前期,液滴颈部宽度随时间呈线性变化,在挤压后期,颈部宽度随时间呈指数关系,而在快速夹断阶段,液滴颈部向心收缩的界面附加压力占主导,液滴颈部宽度剧烈收缩,呈断崖式减小;调控分支通道流量可对液滴不对称分裂比进行调控,且调控作用受毛细数影响较大;基于液液流动压降模型的液滴分裂比预测模型能够有效预测液滴分裂比.     

超声速气流中液体横向射流雾化过程数值模拟

为了更加深入了解超燃冲压发动机燃烧室中的燃料雾化机理,对来流Mach数为1.94的超声速气流中液体横向射流的雾化过程进行了数值模拟研究.计算采用Euler-Lagrange方法,液滴二次破碎模型采用K-H/R-T模型.计算结果表明:考虑液滴二次破碎时,采用雾化锥模型获得的射流穿透深度以及液滴速度分布与实验结果符合得很好;初始液滴直径对射流穿透深度和液滴分布的影响很小;随着初始雾化锥角的增加,相同横截面上的射流穿透深度逐渐减小.当不考虑液滴二次破碎时,液滴穿透深度及分布与所选的初始液滴直径有很大关系.      

A Predictive Model for the Initial Droplet Size and Velocity Distributions in Sprays and Comparison with Experiments

The distribution of sizes and velocities of droplets initially formed in sprays is an important piece of information needed in the spray modelling, because it defines the initial condition of the spray droplets in the predictive calculations of the downstream two‐phase flow fields. A predictive model for the initial droplet size and velocity distributions in sprays is formulated in this study. The present model incorporates both the deterministic and the stochastic aspect of spray formation process. The deterministic aspect takes into account of the unstable wave motion before the liquid bulk breakup through the linear and nonlinear instability analysis, which provides information for the liquid bulk breakup length, the mass‐mean diameter and a prior distribution for the droplet sizes corresponding to the unstable wave growth of various wavelengths. The stochastic aspect deals with the final stage of droplet formation after the liquid bulk breakup by statistical means through the maximum entropy principle based on Bayesian entropy. The two sub‐models are coupled together by the various source terms signifying the liquid‐gas interaction, the mass mean diameter and the prior distribution based on the instability analysis. The initial droplet size and velocity distributions are measured experimentally by phase‐Doppler interferometry for sprays generated by a planar research nozzle and a practical gas turbine airblast nozzle. For the two nozzles, the liquid bulk sheet is formed before its breakup in a coflowing air stream. It is found that the model predictions are in satisfactory agreement with the experimental data for all the cases measured. Hence the present model may be applied to a variety of practical sprays to specify the initial conditions for the spray droplets formed in practical spray systems.     

激波诱导液滴变形和破碎现象实验研究

本文介绍了用于激波诱导液滴变形和破碎现象研究的实验系统及方法,详细分析了激波与液滴相互作用以及液滴加速、变形和破碎过程,为进一步研究激波诱导的液滴内流场性质及气液相间相互作用对液滴变形和破碎的影响机制提供了基础实验数据。     

剪切流动条件下液滴变形和断裂的数值模拟

本文采用扩散界面法研究了剪切流动条件下悬浮液滴变形和断裂的动力学机制。控制方程采用考虑表面张力影响的Navier-Stokes-Cahn-Hilliard方程描述。计算网格采用均匀矩形交错网格。采用基于压力增量的近似投影法计算 Navier-Stokes方程,采用完全近似多重网格法计算Cahn-Hilliard方程。稳态液滴变形规律及液滴拉伸断裂过程的计算结果与试验结果符合较好,表明本文模型能够很好的研究液滴变形及断裂机理。     

Breakup of an uncharged droplet in an electrostatic field

The breakup of an uncharged electrically conducting liquid dropet placed in a uniform electrostatic field into two daughter droplets caused by strong nonspheroidal deformation is qualitatively analyzed by applying the principle of minimum potential energy under spontaneous virtual variations of the droplet’s state. It is shown that the breakup mechanism involves asymmetric mass distribution among the daughter droplets, which are found to be stable with respect to their respective intrinsic charges.     

T型微通道内溶胶液滴形成过程

以制备空心玻璃微球的前体溶胶和硅油为原料,采用实验观测和数值模拟的方法,对T型微通道内溶胶乳液形成过程进行研究。基于液滴的受力分析,建立了液滴形成过程的数学模型,探讨了液滴大小的变化规律。研究结果表明:对于给定的物料体系和T型微通道,通过改变两相流量可以有效地控制液滴尺寸;在相同的分散相流量条件下,增大连续相流量可以减小液滴尺寸,但连续相流量大到一定程度后,这种效果逐渐减弱;在给定的连续相流量条件下,分散相流量越大,液滴直径越大;利用数学模型计算出的液滴直径与实验值偏差在10%左右。根据模拟结果和摄像分析,液滴产生过程经历了静态长大和缩颈剥离两个主要阶段。     

Experimental analysis of shape deformation of evaporating droplet using Legendre polynomials

Experiments involving heating of liquid droplets which are acoustically levitated, reveal specific modes of oscillations. For a given radiation flux, certain fluid droplets undergo distortion leading to catastrophic bag type breakup. The voltage of the acoustic levitator has been kept constant to operate at a nominal acoustic pressure intensity, throughout the experiments. Thus the droplet shape instabilities are primarily a consequence of droplet heating through vapor pressure, surface tension and viscosity. A novel approach is used by employing Legendre polynomials for the mode shape approximation to describe the thermally induced instabilities. The two dominant Legendre modes essentially reflect (a) the droplet size reduction due to evaporation, and (b) the deformation around the equilibrium shape. Dissipation and inter-coupling of modal energy lead to stable droplet shape while accumulation of the same ultimately results in droplet breakup.     

Stochastic modeling of atomizing spray in a complex swirl injector using large eddy simulation

Large-eddy simulation of an atomizing spray issuing from a gas-turbine injector is performed. The filtered Navier–Stokes equations with dynamic subgrid scale model are solved on unstructured grids to compute the swirling turbulent flow through complex passages of the injector. The collocated grid, incompressible flow algorithm on arbitrary shaped unstructured grids developed by Mahesh et al. (J. Comp. Phys. 197 (2004) 215–240) is used in this work. A Lagrangian point-particle formulation with a stochastic model for droplet breakup is used for the liquid phase. Following Kolmogorov’s concept of viewing solid particle-breakup as a discrete random process, the droplet breakup is considered in the framework of uncorrelated breakup events, independent of the initial droplet size. The size and number density of the newly produced droplets is governed by the Fokker–Planck equation for the evolution of the pdf of droplet radii. The parameters of the model are obtained dynamically by relating them to the local Weber number and resolved scale turbulence properties. A hybrid particle-parcel is used to represent the large number of spray droplets. The predictive capability of the LES together with Lagrangian droplet dynamics models to capture the droplet dispersion characteristics, size distributions, and the spray evolution is examined in detail by comparing it with the spray patternation study for the gas-turbine injector. The present approach is computationally efficient and captures the global features of the fragmentary process of liquid atomization in complex configurations.     

电场对协流式微流控装置中乳液液滴生成行为的调控机理

建立了直流电场作用下协流式微流控装置中单乳液液滴乳化生成过程的非稳态理论模型,并开展了数值模拟研究,揭示了电场对液滴乳化生成动力学行为的调控机理,阐明了流场/电场参数对液滴乳化生成特性的影响规律.研究结果表明:沿流体流动方向施加静电场可在电物性参数不同的两相流体界面法线方向上产生指向内相流体的电场力,进而强化了内相流体界面的颈缩和断裂,提升了液滴生成速率和形变程度,减小了液滴生成尺寸;在同一毛细数下,随着电毛细数的增大,乳液乳化流型由每周期仅有单一液滴生成的滴式流型转变为每周期有一个主液滴并伴随有卫星液滴生成的滴式流型;随着毛细数和电毛细数的增大,黏性拖曳力以及电场力作用增强,使内相流体颈缩过程后期更容易形成细长型液线,从而有助于诱发液线上产生Rayleigh-Plateau不稳定现象,继而促进卫星液滴的形成.     

Phase-field modeling droplet dynamics with soluble surfactants

Using lattice Boltzmann approach, a phase-field model is proposed for simulating droplet motion with soluble surfactants. The model can recover the Langmuir and Frumkin adsorption isotherms in equilibrium. From the equilibrium equation of state, we can determine the interfacial tension lowering scale according to the interface surfactant concentration. The model is able to capture short-time and long-time adsorption dynamics of surfactants. We apply the model to examine the effect of soluble surfactants on droplet deformation, breakup and coalescence. The increase of surfactant concentration and attractive lateral interaction can enhance droplet deformation, promote droplet breakup, and inhibit droplet coalescence. We also demonstrate that the Marangoni stresses can reduce the interface mobility and slow down the film drainage process, thus acting as an additional repulsive force to prevent the droplet coalescence.     

Detailed simulation of primary atomization mechanisms in Diesel jet sprays (isolated identification of liquid jet tip effects)

In this study, the atomization characteristics of Diesel jet front tip have been investigated to elucidate the physical mechanisms by detailed numerical simulation. The computations are carried out with the finest grid resolutions ever that can resolve the final droplet generation by surface tension. The numerical methods are based on level-set interface tracking. The methods were validated by test cases and the grid resolution survey shows that the resolutions for the present study are sufficient. The present flow setup excludes nozzle disturbances to investigate how the disturbances from the liquid jet front would lead to atomization where the liquid jet impacts against the quiescent gas. The liquid jet front becomes an umbrella-like shape. From the front umbrella tip edge, ligament breakup first occurs. Ligament breakup is strongly correlated with the gas motion in the vicinity. The gas region behind the front is highly disturbed by atomization. By the gas recirculation motion here, air and some droplets are entrained and mixed. Also, the disturbances are fed back to the front umbrella by this motion and become synchronized with the breakup. Droplet pinch-off is mainly in the short-wave mode, but some ligaments are elongated by local gas stretch to finally have a long-wave mode shape, namely a mode shift occurs. The above findings of liquid jet front umbrella formation, atomization at the umbrella edge, mixing and atomization loop in the recirculation flow region and droplet generation mode give an insight to the modeling of droplet generation in actual sprays.     

带凹槽的微通道中液滴运动数值模拟

运用改进的耗散粒子动力学方法模拟了液滴在由凹槽所构成的粗糙表面微通道内的运动行为.改进的耗散粒子动力学方法采用新近提出的一种短程排斥、长程吸引相互作用势能函数,从而可以模拟带有自由面的流体,如液滴等.模拟了新势能函数下液滴与固体壁面的静态接触角,并用2次多项式拟合了"接触角-a_wf/a_f"变化曲线.研究了液滴在带凹槽的微通道中运动时,微通道壁面浸润性、外场力、液滴温度对液滴流动特性的影响.研究表明壁面浸润性和外场力对液滴流动特性的影响较大,液滴温度对液滴流动特性的影响较小.研究结果对运用耗散粒子动力学方法模拟并分析微流体在复杂微通道的流动有一定的参考价值.     

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.     

Drop deformation in microconfined shear flow

The deformation and breakup of a drop in an immiscible equiviscous liquid undergoing unbounded shear flow has been extensively investigated in the literature, starting from the pioneering work of Taylor. In this Letter, we address the case of microconfined shear flow, a problem which is relevant for microfluidics and emulsion processing applications. The main effects of confinement include complex oscillating transients and drop stabilization against breakup. In particular, very elongated drop shapes are observed, which would be unstable in the unbounded case and can be explained in terms of wall-induced distortion of the shear flow field. We show that wall effects can be exploited to obtain nearly monodisperse emulsions in microconfined shear flow.     

Numerical analysis on behavior of dilute water droplets in detonation

Two-dimensional numerical simulations are conducted based on the Eulerian-Lagrangian method to model a gaseous detonation laden with monodispersed water droplets. The premixed mixture is a slightly diluted stoichiometric hydrogen oxygen mixture at low pressure. The outcome of the interactions of the droplet breakup with the cellular instabilities and the non-uniform flow behind the leading front is analyzed. The simulation results are also analyzed using instantaneous flow fields and Favre average profiles for water droplets. Breakup occurs mainly near the detonation front. The mean final diameter of the water droplets at the end of the breakup process is the same regardless of the initial strength of the leading shock or whether it is lower or greater than the Chapman-Jouguet value. The polydispersity comes from local phenomena behind the leading shock, such as forward jets coming from triple point collisions, transverse waves and a combination of both. The total breakup time is longer than that estimated from post-shock conditions and the present finding is in line with the previous experimental results on spray detonation.     

Study of the breakup of liquid droplets in the vortex wake behind pylon at high airspeeds

The study is devoted to the establishment of regularities in the process of liquid-droplet breakup in the vortex wake behind pylon at high subsonic airspeeds. The article describes the laboratory setup, the diagnostic tools, and the experimental procedure. Structure of the unsteady gas flow behind pylon was examined, and the main characteristics of the generated vortex wake were evaluated. Experimental data concerning the variation of droplet diameters in the gas-dynamic fractionation process versus the flow conditions and liquid injection regimes were obtained. Typical distribu-tions of droplet diameters and velocities in the vortex wake behind pylon are reported. A comparison of experimental data on the rate of the gas-dynamic fractionation process with calculations made using previously developed evaluation procedures was performed. The results of the study may prove useful when choosing the configuration of systems for implementation of liquid injection into a high-speed flow and, also, for validation of mathematical models intended for calculation of parameters of two-phase flows.     

气体中心式离心喷嘴喷雾实验与三维仿真

在实验的基础上, 基于RNG k-ε模型对常压下气体中心式同轴离心(gas-centered swirl coaxial,GCSC)喷嘴喷雾形态和破碎模式进行了三维仿真研究。采用网格自适应加密(adaptive mesh refinement,AMR)技术、耦合水平集和流体体积(coupled level-set and volume of fluid, CLSVOF)方法对气液界面进行捕捉。结果表明, 液体质量流率($\dot{m}_{\mathrm{l}}$)不变, 随着气体质量流率($\dot{m}_{\mathrm{g}}$)的增加, 中心气流的引射作用增强, 液膜内外压差增大, 雾化锥角减小, 并对其流动特性进行了分析; 而$\dot{m}_{\mathrm{g}}$不变时, 液膜在喷嘴出口的径向速度与切向速度随$\dot{m}_{\mathrm{l}}$的增大而增大, 导致雾化锥角增大。同时根据气液质量流率比(gas-liquid mass flow rate,GLR), 将喷雾的破碎模式分为穿孔破碎、气泡破碎和气动破碎。