Vaporization and combustion in three-dimensional droplet arrays

The quasi-steady vaporization and combustion of multiple-droplet arrays is studied numerically. Utilizing the Shvab–Zeldovich formulation, a transformation of the governing equations to a three-dimensional Laplace’s equation is performed, and the solution to Laplace’s equation is obtained numerically to find the effects of droplet interactions in symmetric, multiple-droplet arrays. Vaporization rates, flame surface shapes, and flame locations are found for different droplet array configurations and fuels. The number of droplets, the droplet arrangement within the arrays, and the droplet spacing within the arrays are varied to determine the effects of these parameters. Computations are performed for uniformly spaced three-dimensional arrays of up to 216 droplets, with center-to-center spacing ranging from 3 to 25 droplet radii. As a result of the droplet interactions, the number of droplets and relative droplet spacing significantly affect the vaporization rate of individual droplets within the array, and consequently the flame shape and location. For small droplet spacing, the individual droplet vaporization rate decreases below that obtained for an isolated droplet by several orders of magnitude. A similarity parameter which correlates vaporization rates with array size and spacing is identified. Individual droplet flames, internal group combustion, and external group combustion can be observed depending on the droplet geometry and boundary conditions.     

Monte Carlo模拟空间各向异性势向列相液晶微滴

基于分子两体势,用Monte Carlo方法计算向列相液晶微滴.两体势基于格点模型,是空间各向异性的且依赖于液晶的弹性常数.假定向列相液晶微滴具有自由表面,引入切向内禀强度定量描写表面引发的分子内禀易取向的强弱.通过各向异性势的两种方案,在低温下计算切向内禀强度和二阶序参数在微滴内不同区域的变化,与Lebwohl-Lasher模型作对比分析.结果表明:只有一种方案在微滴表面产生内禀易取向,且内禀强度值的大小与K₃₃/K₁₁值有关;空间不完整的向列相液晶使得微滴由内层到外层有序度越来越低.     

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.     

溶液液滴在热等离子体射流中的运动和蒸发

为考察溶液注入热等离子体喷涂过程中喷雾参数对涂层质量的影响,本文建立了溶液液滴在热等离子体中运动蒸发的数学模型。模拟了液滴在不同参数下的运动和蒸发的过程,考虑了液滴、热等离子气流及液滴表面气体混合物随温度及组分的物性变化以及斯蒂芬流的影响,得到液滴的运动轨迹,蒸发速率以及半径和表面温度的变化。结果表明在一定范围内增大液...     

Computer simulations of the growth of breath figures

We describe computer simulations of the growth of breath figures, the patterns formed when droplets condense on a cold surface. The focus is on the coalescence of droplets, which is an important growth mechanism, and the conditions for self-similar patterns, which are experimentally observed. It is assumed that individual droplets grow according to a power law; droplets that touch coalesce instantly and are replaced by a new droplet at the center of gravity of the coalescing pair. The average droplet radius, distribution of droplet sizes, surface coverage, and radial distribution function are determined as a function of the time for a variety of initial coverages and polydispersities. These quantities are compared to those determined by experiment, and our simple model is found to be in good accord with the observed behavior. It is observed that the process of coalescence induces spatial correlation between droplets.This paper is dedicated to Howard Reiss on his 66th birthday.     

均匀液滴尺寸及间距的实验研究

液滴发生器产生液滴的尺寸和间距影响液滴层的辐射和蒸发特性,液滴尺寸及间距的可控性值得重点关注。根据Weber的射流不稳定修正方程,确定了均匀液滴流产生的无量纲波数及扰动频率范围,结合射流质量守恒,分析了均匀液滴流中液滴的尺寸和间距与无量纲波数的关系。在不同喷孔直径和射流压力下,对理论和实验结果进行了对比,验证了液滴尺寸和液滴间距的理论计算结果,为液滴层辐射蒸发特性的研究提供了依据。     

多喷嘴喷雾场数值模拟分析

本文建立了多喷嘴喷雾场的三维物理数学模型,运用欧拉-拉格朗日法对多喷嘴下的喷雾场进行了详细数值模拟,得到了双喷嘴及三喷嘴情况下喷射入口压力和流量变化对喷雾场内雾滴索太尔直径和雾滴速度的影响规律;并通过与单喷嘴情况进行对比,给出了喷嘴数目对喷雾场内雾滴速度及粒径大小分布的影响。     

Forming concentric double-emulsion droplets using electric fields

Double-emulsion droplets may be assembled into highly concentric shells using a uniform AC electric field to induce dipole/dipole interactions. The resulting force centers the inner droplet with respect to the outer shell if the outer droplet has a higher dielectric constant than the ambient, suspending liquid. The dielectric constant of the inner droplet does not influence this condition. Applying an electric field >10⁴ V_rms/m achieves centering of approximately 3–6 mm diameter droplets suspended in ～10 centipoise liquids within ～60 s. If the outer shell is electrically conductive, the effect depends strongly on frequency. In the case of the monomer-containing liquids requisite to forming foam shells for laser target fabrication, the electrical field frequency must be ～10 MHz or higher. Because of very stringent requirements imposed on the concentricity and sphericity of laser targets, electric field induced droplet distortion must be minimized. Consequently, the liquid constituents must be matched in density to ～0.1%.     

Electrostatic field and charge distribution in small charged dielectric droplets

The charge distribution in small dielectric droplets is calculated on the basis of continuum medium approximation. There are considered charged liquid spherical droplets of methanol in the range of nanometer sizes. The problem is solved by the following way. We find the free energy of some ion in dielectric droplet, which is a function of distribution of other ions in the droplet. The probability of location of the ion in some element of volume in the droplet is a function of its free energy in this element of volume. The same approach can be applied to other ions in the droplet. The obtained charge distribution differs considerably from the surface distribution. The curve of the charge distribution in the droplet as a function of radius has maximum near the surface. Relative concentration of charges in the vicinity of the center of the droplet does not equal to zero, and it is the higher, the less is the total charge of the droplet. According to the estimates the model is applicable if the droplet radius is larger than 10 nm.     

Reactions of QCD plasma droplets in hot mixed-phase matter

We introduce a droplet model to survey the interactions and reactions of blobs of QCD plasma in a mixed phase with low-density hadronic gas. We extend earlier work on the statistical mechanism of pionic emission by the droplets to include the fission of a large droplet into two smaller ones, and give quantitative estimates for the fission rate. We also consider the collisions of plasma droplets. By extending the proximity model to the case when the droplets are composed of relativistic particles, we show that collisions almost always lead to fusion of the plasma droplets, and give a simple estimate of the conditions necessary for fusion.     

Ultrasonic surface acoustic wave-assisted separation of microscale droplets with varying acoustic impedance

In droplet-based microfluidic platforms, precise separation of microscale droplets of different chemical composition is increasingly necessary for high-throughput combinatorial chemistry in drug discovery and screening assays. A variety of droplet sorting methods have been proposed, in which droplets of the same kind are translocated. However, there has been relatively less effort in developing techniques to separate the uniform-sized droplets of different chemical composition. Most of the previous droplet sorting or separation techniques either rely on the droplet size for the separation marker or adopt on-demand application of a force field for the droplet sorting or separation. The existing droplet microfluidic separation techniques based on the in-droplet chemical composition are still in infancy because of the technical difficulties. In this study, we propose an acoustofluidic method to simultaneously separate microscale droplets of the same volume and dissimilar acoustic impedance using ultrasonic surface acoustic wave (SAW)-induced acoustic radiation force (ARF). For extensive investigation on the SAW-induced ARF acting on both cylindrical and spherical droplets, we first performed a set of the droplet sorting experiments under varying conditions of acoustic impedance of the dispersed phase fluid, droplet velocity, and wave amplitude. Moreover, for elucidation of the underlying physics, a new dimensionless number AR_D was introduced, which was defined as the ratio of the ARF to the drag force acting on the droplets. The experimental results were comparatively analyzed by using a ray acoustics approach and found to be in good agreement with the theoretical estimation. Based on the findings, we successfully demonstrated the simultaneous separation of uniform-sized droplets of the different acoustic impedance under continuous application of the acoustic field in a label-free and detection-free manner. Insomuch as on-chip, precise separation of multiple kinds of droplets is critical in many droplet microfluidic applications, the proposed acoustofluidic approach will provide new prospects for microscale droplet separation.     

Formation and mobility of droplets on composite layered substrates

A mesoscale fluid film placed on a solid support may break up and form droplets. In addition, droplets may exhibit spontaneous translation by modifying the wetting properties of the substrate, resulting in asymmetry in the contact angles. We examine mechanisms for droplet formation and motion on uniform and terraced landscapes, i.e., composite substrates. The fluid film stability, droplet formation and velocity are studied theoretically in the isothermal case using a lubrication approach in one spatial dimension. The droplet properties are found to involve contributions from both the terraced layer thickness and molecular interactions via the disjoining potential.     

基于机器视觉的细水雾液滴尺寸测量与分析

为了满足科研与工程中对细水雾液滴尺寸测量的高精度低成本要求,对雾滴尺寸的机器视觉测量方法进行了深入研究.在自行建立的高压喷雾系统与雾滴采集装置上对细水雾液滴进行了采样,用显微镜及其CCD相机对雾滴样本进行了图像采集,用图像处理软件对采集的雾滴图像进行了处理与分析,测量并统计了5966个雾滴,得到了雾滴尺寸的频谱分布和累积分布以及雾滴平均直径和特征直径,将测量结果与相位多普勒粒子分析仪(PDPA)的测量结果进行了比较.结果表明,机器视觉方法町测量的最小雾滴直径约4.39 μm;机器视觉测量结果与PDPA测最结果相当接近,两种方法测得的细水雾液滴平均直径和特征直径的相对误差均在5%以内,雾滴尺寸均匀度指数的相对误差为0.27%.     

Droplet States in the XXZ Heisenberg Chain

We consider the ground states of the ferromagnetic XXZ chain with spin up boundary conditions. The ground state of this model, restricted to a sector with a fixed number of down spins, describes a droplet of down spins in an environment of up spins. We find the exact energy and the states that describe these droplets in the limit of an infinite number of down spins. We prove that there is a gap in the spectrum above the droplet states. As the XXZ Hamiltonian has a gap above the fully magnetized ground states as well, this means that the droplet states (for sufficiently large droplets) form an isolated band. The width of this band tends to zero in the limit of infinitely large droplets. We also prove the analogous results for finite chains with periodic boundary conditions and for the infinite chain. Received: 5 September 2000 / Accepted: 8 December 2000     

二甲基醚液滴超临界蒸发的数值模拟

建立了二甲基醚(DME)在N2介质中超临界蒸发的数学模型.采用状态方程法计算了DME-N2体系的高压气液相平衡,在相关文献的基础上得出一套流体热物性和输运参数的计算方法.完成了DME超临界蒸发过程的数值模拟.从数值模拟的结果讨论了超临界条件下环境压力、环境温度和液滴运动等因素对液滴升温、滴径变化以及生存时间的影响.     

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.     

液滴撞击不同湿润性固壁面上静止液滴的格子Boltzmann研究

采用单组分多相的伪势格子Boltzmann方法,在大小液滴粒径比为1.5的情况下,对大液滴竖直撞击壁面上静止小液滴的过程进行模拟,研究亲水与超疏水壁面上大液滴竖直碰撞小液滴的过程,得到液滴铺展因子和相对高度随时间的变化.结果表明:增大We数会使液滴的铺展因子增大,铺展直径变大,相对高度减小;并且随着We数的增加,在超疏...     

A semi-analytical,multizone model of droplet combustion with varying properties

A multizone droplet burn model is developed to account for changes in the thermal and transport properties as a function of droplet radius. The formulation is semi-analytical – allowing for accurate and computationally efficient estimates of flame structure and burn rates. Zonal thermal and transport properties are computed using the Cantera software, pre-tabulated for rapid evaluation during run-time. Model predictions are compared to experimental measurements of burning n-heptane, ethanol and methanol droplets. An adaptive zone refinement algorithm is developed that minimizes the number of zones required to provide accurate estimates of burn time without excess zones. A sensitivity study of burn rate and flame stand-off with far-field oxygen concentration is conducted with comparisons to experimental data. Overall agreement to data is encouraging with errors typically less than 20% for predictions of burn rates, stand-off ratio and flame temperature for the fuels considered.     

超声波电喷推力器羽流中和特性研究

超声波电喷推力器主要应用在小卫星(10 kg)平台,为解决该类型推力器的羽流在中和过程中产生的推力偏角以及能效低的问题,对超声波电喷推力器的羽流中和过程进行数值研究.为实现电喷推力器羽流特有物理过程的仿真,建立了一种带电液滴中和模型(NECD模型),对电子-正电液滴的中和过程进行捕捉,包括带电粒子的输运过程、电子液滴碰撞过程以及液滴的破碎与重组等过程;为验证模型的可行性和精度,开展推力测量和羽流高速照相试验,以工况相同试验和仿真结果进行对比.结果显示,该模型的综合计算误差在20%左右,在不同工况下可以和试验值形成趋势上的符合.基于该计算模型,对放电功率为2 W、放电电流为2 m A的超声波电喷推力器进行羽流输运过程的数值模拟,获得表征羽流中和特性的几种参数分布,包括数密度、电荷密度、液滴体积大小等,并统计出各类能耗所占比例,解释了推力偏角和能效低问题的内在机理以及为相应优化提供参考.     

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.