Deformation of an oil droplet on a solid substrate in simple shear flow

Displacement of immiscible fluids is important in sub-surface processes such as enhanced oil recovery, oil sand processing and detergency. In this study, simulation of an oil droplet deformation on a solid substrate in simple shear flow has been carried out using computational fluid dynamics tool (Fluent 6.3) and the shape of the oil droplet is compared with that of the experimental observation. The dynamic behavior of a two-dimensional oil droplet subject to shear flow in a closed channel is considered under the condition of negligible inertial and gravitational forces. The volume of fluid method is used in Fluent to determine the dynamics of free surface of the oil droplet during the fluid flow. The oil droplet deformation increases with the increase in capillary number, Reynolds number and size of the oil droplet. The deformation of an oil droplet attached to channel surface in simple shear flow is studied experimentally in laminar flow through visual observation using microscope (Ziess, SV11 APO) with high speed camera (PCO). Aniline and isoquinoline was used to form oil droplet and distilled water was used as shearing fluid. The deformation of aniline and isoquinoline droplets was recorded using a high speed camera connected to a PC. The recorded image was replayed and the deformation of aniline and isoquinoline droplets was analyzed using Axio Vision software and compared with the results obtained from CFD simulation. The deformation of different sizes of aniline and isoquinoline droplets at different flow rates of shearing fluid and with time are well predicted by the CFD simulation.     

Validated simulation of droplet sedimentation with finite-element and level-set methods

In the present paper freely sedimenting n-butanol droplets in water are simulated by means of computational fluid dynamics. The finite-element and the extended finite-element methods were implemented and evaluated. The level-set function is used for capturing the interface movement. The three-dimensional nonstationary simulations included the stages of droplet acceleration, deformation, and stability in terms of shape and velocity. The influence of the grid resolution, the computational domain walls, and the droplet initial velocity was investigated and quantified. The droplet diameters that were studied spanned the region of spherical, deformed, and oscillating droplets. The simulation results were compared to experiments and empirical models in terms of droplet shape, oscillation behavior and terminal velocity, showing good agreement. The extended finite-element method was found to provide simulation results in better accordance to the experiments and empirical models than the conventional finite-element method.     

激波与亚毫米液滴相互作用的二维和三维数值模拟研究

为了研究激波与亚毫米液滴相互作用过程,基于现有的实验结果,利用Fluent平台,采用VOF (Volume of Fluid)多相流模型和k-ε湍流模型,通过二维数值模拟分析了不同韦伯数(We)对亚毫米液滴变形演化过程的影响规律,通过三维数值模拟揭示了亚毫米液滴爆炸式破碎机理。结果表明,韦伯数对液滴变形有促进作用,韦伯数越大,液滴在压缩变形阶段所需要的时间越短;在气动力不变的条件下,相同液滴直径条件下,马赫数越大,液滴所受的气动力越大,液滴质心位移的无量纲加速度越大,低韦伯数下液滴的横向展开速率随韦伯数增大而减小,而高韦伯数下液滴的横向展开速率随韦伯数增大而增大。数值模拟结果与对比实验结果相近,有效阐明了韦伯数对亚毫米液滴变形的作用。     

真空制冰过程中水滴动态特性

为研究真空制冰水滴温度影响因素并进行分析,搭建了真空制冰动态特性研究实验台,进行相关实验,采集了相关图像和实验数据。对采集的图像进行了定性分析。采集的实验数据主要是在不同环境温度、环境压力、供水水温、水质、粒径及水滴下落初速度等情况下水滴温度随时间的变化情况,并与模拟计算值一并进行了对比分析。分析得出环境温度、供水水温、下落初速度对其影响较小,而环境压力、水滴粒径对其影响较为明显,供水水质对其影响比较特殊,主要表现在液滴的最大过冷度上。     

Analysis of supersaturation and nucleation in a moving solution droplet with flowing supercritical carbon dioxide

A supercritical antisolvent (SAS) process is employed for production of solid nanoparticles from atomized droplets of dilute solution in a flowing supercritical carbon dioxide (SC CO₂) stream by attaining extremely high, very rapid, and uniform supersaturation. This is facilitated by a two‐way mass transfer of CO₂ and solvent, to and from the droplet respectively, rendering rapid reduction in equilibrium solubility of the solid solute in the ternary solution. The present work analyses the degree of supersaturation and nucleation kinetics in a single droplet of cholesterol solution in acetone during its flight in a flowing SC CO₂ stream. Both temperature and composition are assumed to be uniform within the droplet, and their variations with time are calculated by balancing the heat and mass transfer fluxes to and from the droplet. The equilibrium solubility of cholesterol with CO₂ dissolution has been predicted as being directly proportional to the Partial Molar Volume Fraction (PMVF) of acetone in the binary (CO₂–acetone) system. The degree of supersaturation has been simulated up to the time required to attain almost zero cholesterol solubility in the droplet for evaluating the rate of nucleation and the size of the stable critical nuclei formed. The effects of process parameters have been analysed in the pressure range of 7.1–35.0 MPa, temperature range of 313–333 K, SC CO₂ flow rate of 0.1136–1.136 mol s^?1, the ratio of the volumetric flow rates of CO₂‐to‐solution in the range of 100–1000, and the initial mole fraction of cholesterol in acetone solution in the range of 0.0025–0.010. The results confirm an extremely high and rapid increase in degree of supersaturation, very high nucleation rates and stable critical nucleus diameter of the order of a nanometre. Copyright © 2005 Society of Chemical Industry     

喷雾冷冻法单个液滴冻结过程模拟

喷雾冷冻液滴的冻结过程决定着干燥产品的微结构。本文以单个雾化液滴为研究对象,利用数值模拟的方法研究了液滴大小、气体流速和环境温度3个参数对其冻结过程的影响。结果发现,液滴越大冻结时所需的形核时间和完全固化时间越长,而且冻结过程随着气体流速的增大和环境温度的降低而缩短。通过方差分析发现,液滴大小较气体流速和环境温度对液滴完全固化时间的影响有较显著差异。液滴冷冻过程中,其质量损失率随着液滴大小的增大而略有减小,随着气体流速的增加及环境温度的降低而减小,其中环境温度对液滴质量损失率的影响最大。     

水下四氯化碳单液滴在凹壁面上的动态特性

采用实验观测与图像处理相结合,对CCl4液滴在水下撞击凹壁面后的动态特性进行了系统研究。结果表明,液滴撞击凹壁面的过程经历了下降、扩散、松弛、滚动和润湿五个阶段。液滴与凹壁面间的撞击角θ对液滴拉伸特性的影响大于液滴初始直径和壁面曲率半径。当θ=90°时液滴垂直撞击壁面最低点,液滴迅速弹跳并强烈回缩,铺展时间短且变形率最小。在θ=100°~150°时,随着撞击角增加液滴变形幅度增大,相邻时刻滑动变形率小于滚动变形率。110°<θ<130°时液滴以滑动和铺展为主。θ>130°时液滴沿壁面滚动现象更容易发生。θ=154.2°时液滴接近纯滚动状态。增大撞击角,液滴沿凹壁面滚动下滑有效降低壁面黏附和液滴破碎。     

Nucleation curves of model natural gas hydrates on a quasi‐free water droplet

Heterogeneous nucleation probability distributions of gas hydrates on a water droplet that was supported by inert and immiscible perfluorocarbon oil, perfluorodecalin is studied. The guest gas used was a mixture of 90 mol % methane and 10 mol % propane. The probability distribution was measured using a high pressure automated lag time apparatus under the guest gas pressure range of 6.7–12.5 MPa and the cooling rate range of 0.002–0.02 K/s. Nucleation curves were derived for unit area of water surface. The nucleation rate per unit area of water surface that was contained in a glass sample cell, which differed significantly from that on a quasi‐free water droplet, is also derived. It is concluded that the nucleation curves in the presence of a solid wall should be normalized to the unit length of the three‐phase line at which water, guest gas, and the solid wall meet. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2611–2617, 2015     

Simulation of interfacial mass transfer by droplet dynamics using the level set method

A unique approach to simulate mass transfer across the moving droplet where mass transport equations and governing equations of the levels set method are solved separately is proposed in this work. Mass transfer coefficients of the chemical species can be computed by equating the diffusive flux and the mass transfer flux at the interface, which are found to be of the same order of magnitude as of those obtained using an empirical correlation. Simulations underestimate mass transfer coefficients by roughly 25% across the range of low Reynolds number studied systematically. The level set method is used to track the motion of the interface to study droplet dynamics and mass transfer across a moving droplet because of the ease in defining the local curvature of the interface and in capturing any topological changes. We perform various numerical simulations by varying the physical properties of the system, in order to analyze the influence of dimensionless numbers such as the Reynolds number (Re), the Eotvos number (Eo) and the Morton number (M) on the shape of a buoyancy-driven droplet and compare them with the various shape regimes of drops and bubbles reported by Clift et al. [1978. Bubbles, Drops and Particles. Academic Press, New York]. It is shown that larger deformation occurs for buoyancy-driven droplets when interfacial forces are considerably greater than viscous forces (M?1 and Eo>10) and the droplets are almost undeformed when viscous forces dominate interfacial forces (M>10³ and Eo>10).     

Effect of crystallization kinetics on the properties of spray dried microparticles

A droplet chain technique was used to study the influence of the crystallization process on the morphology of spray dried microparticles. A piezoceramic dispenser produced a chain of monodisperse solution droplets with an initial diameter in the range of 60–80 µm. Aqueous solutions of sodium nitrate were prepared in concentrations ranging from 5 mg/ml to 5?10^?5 mg/ml. The solution droplets were injected into a laminar flow with gas temperatures varying from 25 to 150°C, affecting the droplet temperature and the evaporation rate, accordingly. Dried particles with diameters between 0.3 and 18 µm were collected. The properties of the collected microparticles were studied and correlated with a particle formation model which predicted the onset of saturation and crystallization. The model accounted for the dependence of the diffusion coefficient of sodium nitrate in water on droplet viscosity. The viscosity trend for sodium nitrate solutions was determined by studying the relaxation time observed during coalescence of two aqueous sodium nitrate droplets levitated in optical tweezers. The combination of theoretical derivations and experimental results showed that longer time available for crystallization correlates with larger crystal size and higher degrees of crystallinity in the final microparticles.

© 2016 American Association for Aerosol Research     

Monodisperse monocomponent fuel droplet heating and evaporation

The results of numerical and experimental studies of heating and evaporation of monodisperse acetone, ethanol, 3-pentanone, n-heptane, n-decane and n-dodecane droplets in an ambient air of fixed temperature and atmospheric pressure are reported. The numerical model took into account the finite thermal conductivity of droplets and recirculation inside them based on the effective thermal conductivity model and the analytical solution to the heat conduction equation inside droplets. The effects of interaction between droplets are taken into account based on the experimentally determined corrections to Nusselt and Sherwood numbers. It is pointed out that the interactions between droplets lead to noticeable reduction of their heating in the case of ethanol, 3-pentanone, n-heptane, n-decane and n-dodecane droplets, and reduction of their cooling in the case of acetone. Although the trends of experimentally observed droplet temperatures and radii are the same as predicted by the model taking into account the interaction between droplets, the actual values of the predicted droplet temperatures can differ from the observed ones by up to about 8 K, and the actual values of the predicted droplet radii can differ from the observed ones by up to about 2%. It is concluded that the effective thermal conductivity model, based on the analytical solution to the heat conduction equation inside droplets, can predict the observed average temperature of droplets with possible errors not exceeding several K, and observed droplet radii with possible errors not exceeding 2% in most cases. These results allow us to recommend the implementation of this model into CFD codes and to use it for multidimensional modelling of spray heating and evaporation based on these codes.     

Experimental study on fusion and break-up motion after droplet collision

The interactions between droplets have an important influence on the atomization of liquid fuel, the combustion efficiency, and the reduction of particulate matter emissions for an engine. For this reason, this paper presents results from an experimental study on the coalescence and break-up of droplets after collision. According to the shape and parameters of the droplets at different times after the collision of the droplets was captured by a high speed camera, analysis was done for the following effects of droplet collisions: the collision-coalescence motion for the collision between the droplets, the change history of the dimensionless length-to-width ratio of the oscillation motion, the critical size ratio of the breakup motion, and the liquid physical properties of the particles. The results show that the droplets collide and exhibit two forms of coalescence oscillation and break-up: for oscillating motion, at higher droplet collision velocities and dimensionless size ratios, there will be a larger dimensionless length-to-width ratio for the droplet oscillation; for the break-up motion, at higher collision velocities, there will be lower dimensionless size ratios, and lower liquid surface tension, shorter times over which the droplet breaks, and facilitated droplet break-up. The research results presented here can be used for atomization in engine cylinder, increasing the gas/liquid contact area and enhancing the combustion efficiency of gas/liquid heat transfer to improve the combustion efficiency of the engine.     

无机盐浓度及种类对电脱水过程水滴极化的影响

为了深入探究无机盐对电脱水过程水滴极化的影响机理,分别改变无机盐浓度及种类,对高频高压脉冲电场作用下水滴的极化变形进行显微实验研究。结果表明,不同NaCl浓度条件下水滴的变形度随电场频率和占空比的增加呈先增大后降低,由于电导率增大、离子冲击效应等的影响,随着NaCl浓度的增大,水滴的变形度随之增大。高价无机盐离子所受的电场力是一价离子的数倍,有利于其冲击速度和冲击动量的提升,水滴变形度随离子价位升高而增大。另外,由于离子活泼程度以及水解等原因,在同一高压脉冲电场中,不同钠盐溶液水滴变形度顺序为：磷酸钠 >碳酸钠 >硫酸钠 >氯化钠 >硝酸钠。不同氯盐溶液水滴变形度顺序为：氯化镁 >氯化钙 >氯化钾 >氯化钠 >氯化铵。研究成果为高压高频脉冲静电破乳机理的深入探讨奠定了基础。     

尿素水溶液液滴的蒸发特性

在石英管式炉上通过挂滴法来观察单个尿素水溶液（urea-aqueous-solution,UAS）液滴的具体蒸发过程,比较了不同环境温度以及不同初始直径大小下液滴的蒸发特性。结果表明,尿素溶液液滴在100～1300 ℃的温度范围内呈现出了不同的蒸发行为。在较高的温度下,液滴的蒸发行为较为复杂,如气泡的产生、液滴的变形以及发生微爆的现象;但是,随着环境温度的降低,这些现象就变得非常微弱甚至消失。同时,还定量分析了稳态蒸发常数与温度、液滴初始直径之间的变化关系,发现在初始直径为2.5 mm、温度在100～600 ℃之间变化的情况下,稳态蒸发常数从0.02075 mm2/s增加到了0.23953 mm2/s,增大了10倍左右。此外,还对气流流速为0.25～1.25 m/s范围内的液滴蒸发特性作了实验研究。当液滴周围有强迫气流存在时,液滴与气体间的换热方式由导热转变为对流换热,从而增强了液滴表面的传热传质能力,促进了液滴的蒸发。     

Bubble/droplet formation and mass transfer during gas–liquid–liquid segmented flow with soluble gas in a microchannel

Microchannels have great potential in intensification of gas–liquid–liquid reactions involving reacting gases, such as hydrogenation. This work uses CO₂–octane–water system to model the hydrodynamics and mass transfer of such systems in a microchannel with double T‐junctions. Segmented flows are generated with three inlet sequences and the size laws of dispersed phases are obtained. Three generation mechanisms of dispersed gas bubbles/water droplets are identified: squeezing by the oil phase, cutting by the droplet/bubble, cutting by the water–oil/gas–oil interface. Based on the gas dissolution rate, the mass transfer coefficients are calculated. It is found that water droplet can significantly enhance the transfer of CO₂ into the oil phase initially. When bubble‐droplet cluster are formed downstream the microchannel, droplet will retard the mass transfer. Other characteristics such as phase hold‐up, bubble velocity and bubble dissolution rate are also discussed. The information is beneficial for microreactor design when applying three‐phase reactions. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1727–1739, 2017     

Droplet evaporation and solute precipitation during spray pyrolysis

The process of spray pyrolysis was investigated theoretically using a model that describes the evolution of the droplet size, solvent vapor concentration in the carrier gas, and both droplet and gas temperatures along the reactor axis. The model also accounts for solute concentration profiles and solute precipitation in the solution droplets. The model was used to describe the evaporation of sodium chloride aqueous solution droplets in diffusion dryers and hot-wall reactors as a function of reactor residence time, droplet size (a few microns), solution molality (up to 2 M), droplet concentration (10⁶–10⁷ cm⁻³), relative humidity of the carrier gas (0–50%) and reactor wall conditions. Decreasing initial droplet size and solution molality accelerated droplet evaporation and resulted in smaller droplets at the onset of solute nucleation. Decreasing droplet concentration and carrier gas inlet relative humidity as well as increasing wall temperature (up to 350°C) or axial wall temperature gradient (up to 100°C cm⁻¹) increased the droplet evaporation rate, but did not change appreciably the droplet size at the point of precipitation for a given droplet size and solute concentration. Thus, control of droplet size at the onset of solute nucleation by varying process parameters other than the solution concentration and initial droplet size is limited.     

Models for fuel droplet heating and evaporation: Comparative analysis

The results of comparative analysis of liquid and gas phase models for fuel droplets heating and evaporation, suitable for implementation into computational fluid dynamics (CFD) codes, are presented. Among liquid phase models, the analysis is focused on the model based on the assumption that the liquid thermal conductivity is infinitely large, and the so-called effective thermal conductivity model. Seven gas phase models are compared. These are six semi-theoretical models based on various assumptions and a model based merely on the approximation of experimental data. It is pointed out that the gas phase model, taking into account the finite thickness of the thermal boundary layer around the droplet, predicts the evaporation time closest to the one based on the approximation of experimental data. In most cases, the droplet evaporation time depends strongly on the choice of the gas phase model. The droplet surface temperature at the initial stage of heating and evaporation does not practically depend on the choice of the gas phase model, while the dependence of this temperature on the choice of the liquid phase model is strong. The direct comparison of the predictions of various gas models, with available experimental data referring to droplet heating and evaporation without break-up, leads to inconclusive results. The comparison of predictions of various liquid and gas phase models with the experimentally observed total ignition delay of n-heptane droplets without break-up, has shown that this delay depends strongly on the choice of the liquid phase model, but practically does not depend on the choice of the gas phase model. In the presence of droplet break-up processes, the evaporation time and the total ignition delay depend strongly on the choice of both gas and liquid phase models.     

盐水液滴降压环境下蒸发过程

盐水溶液在降压环境下的蒸发过程的研究,主要集中在海水淡化领域的降膜蒸发过程应用方面和工业制盐方面的平坦表面的蒸发过程的研究。而本文主要集中研究各种实验因素对于液滴温度变化的影响。对于多组分液滴降压环境下相变过程的研究,实验采用浓度为15%和6%的盐水溶液作为一组比较工质,在初始环境压力为94.5～97 kPa,最终环境压力范围为50～3000 Pa,液滴的初始温度范围为7～30℃,初始直径范围为1～3 mm的条件下进行实验。通过实验数据分析可知:盐水液滴在降压蒸发过程中的中心温度变化有析盐和不析盐两种现象,随着水分不断地蒸发,当液滴浓度达到22.4%而且温度足够低时就会析出盐分,否则,不会出现析盐现象;同时分析不同浓度、不同最终环境压力、不同初始温度和不同初始直径对液滴相变过程和温度变化的影响,并且观察环境压力降低和液滴温度变化之间的关系。     

Convective Drying Kinetics of Single Droplets of Aqueous Glucose

The convective drying kinetics of single droplets of aqueous glucose was measured using a single droplet drying rig. The effects of air temperature and velocity were evaluated. It was found that the droplet of aqueous glucose shrank uniformly, retaining a nearly spherical shape during drying. The normalized volume (d/d₀)³ of the droplet decreased linearly with its moisture content. A constant-drying-rate-like period occurred when the moisture content of the droplet was higher than an amount of about 1.0 kg kg^?1 dry solid. The diameter of the droplet decreased sharply due to the evaporation of water, while its temperature remained at a wet-bulb-like temperature in this period. When the moisture content of the droplet was lower than the above-mentioned value, the drying transferred to a falling-drying-rate-period, during which the temperature of the droplet rose quickly and approached the air temperature as drying continued. The effect of air temperature on the drying of single droplets of aqueous glucose was more pronounced when compared with that of air velocity.     

单液滴撞击超疏水冷表面的反弹及破碎行为

对直径2.8 mm的液滴撞击冷表面的动态行为进行快速可视化观测，对比研究单液滴撞击普通冷表面以及超疏水冷表面的动力学特性，同时对初始撞击速度以及冷表面温度对液滴动态演化行为的影响进行了对比分析。实验结果表明：与液滴撞击普通冷表面（温度-25~-5℃）发生瞬时冻结沉积相比，液滴撞击超疏水冷表面时均未发生冻结，而且伴随铺展、回缩、反弹以及破碎行为；撞击速度越大，普通冷表面上液滴铺展因子越大，而且液滴越易冻结。液滴低速（We≤76）撞击超疏水冷表面会发生反弹现象，但速度对液滴最大铺展时间无影响；液滴高速（We≥115）撞击超疏水冷表面后会产生明显液指，而且破碎为多组卫星液滴。此外，冷表面温度仅影响液滴反弹高度，对液滴最大铺展因子以及液滴铺展时间影响较小。结果表明超疏水表面可显著抑制液滴撞击冷表面的瞬时冻结沉积。