Some physics inside drying droplets

Droplets are of great scientific interest because studying their formation and dynamics is important in many technologies such as in microfluidics [1, 2]. Evaporation of droplets is another interesting aspect that has caught much scientific attention. An example of an evaporating droplet is a coffee droplet spilled on a table. After evaporation the coffee droplet leaves a ring-like stain called a coffee ring or coffee stain [3]. This phenomenon, known as the ‘coffee ring effect’, occurs not only with coffee droplets but also with all droplets containing non-volatile solutes. There are a number of interesting physical processes going on within the droplets during evaporation to form the ‘coffee stains’. In this article, we will see what they are.     

Undercooling and Solidification of Sn—Pb Alloy Droplets Prepared by Uniform Droplet Spray

The undercooling and solidification of 150μm and 185μm droplets of Sn-5%Pb alloy prepared by the uniform droplet spray(UDS) process have been investigated.The enthalpy of the droplet has been measured by non-adiabatic calorimetric method as a function of the flight distance.A droplet solidification simulation model has been used to compare with the experimental data.The results show that the enthalpy released by the droplets in the calorimeter 11.88J/g and 22.29J/g less than the simulated values up to a certain flight distance at 0.485m and 0.460m for 150μm and 185μm droplets respectively,but agrees with the expected values at larger distance.The nucleation of the droplets takes place at the distance where the experimental and simulated enthalpy values agree.The droplets quenched before nucleation solidify into metastable supersaturated solid solution and have large undercooling.The formation of the metastable structure in the droplets has been verified metallographically and by calculations based on a thermodynamic model.     

基于欧拉方法的机翼表面水滴撞击特性数值模拟

基于欧拉方法分析了不同飞行速度和不同气象条件下NACA0012机翼水滴撞击的特性,使用Fluent商用软件计算空气流场,并通过UDS模块求解水滴相控制方程。从计算结果分析得出:局部水滴收集系数及水滴撞击范围随飞行速度和平均水滴容积直径的增加而明显增加;液态水含量对水滴撞击特性影响不大。     

Efficiency of Plate Separators for Dispersed Liquid-Liquid Systems

Plate packages are effective and promising appa-ratus for the separation of droplets from non-homoge-neous mixed systems of liquid-liquid dispersion inchemical unit operation for their features of compactconfiguration, low equipment cost and high efficien-cy[1]. The theories of plate separators were proposedand a lot of experimental researches were carried outin 1960s. Walteret alproved that in contrast to tradi-tional gravity settlers, plate packages are successfullyused to improve the separa…     

微波对O／W型化妆品乳化的影响

化妆品乳化过程中引入微波作用可以有效地细化乳化液滴颗粒,改善产品的外观,提高乳化体的稳定性。微波作用可以改变及降低乳化液滴表面的电性,降低体系粘度使得液滴界面膜的机械强度变低从而加快液滴的破裂,与机械均质乳化结合可以将液滴分散形成更小的颗粒。     

Characteristic of drop coalescence resting on liquid-liquid interface

The trajectory model of dispersed phase drops and the model of basic flow for drop motion between two inclined parallel plates are derived with the optimized calculation. The analytical results of direct numerical simulation indicate that the basic flow plays an important role in the drop coalescence on liquid-liquid interface. In the stratified two-phase flow field, the smaller droplets are difficult to drain the thin continuous film between the approaching droplets and bulk interfaces and eventually immerse into the trickling film to yield coalescence. They almost attain the velocity of their local surroundings. Moreover, the basic flow exerts a dominant influence on the motion of smaller droplet. The smaller droplets are easily entrained by the basic flow. On the contrary, the larger drop presents advantageous characteristics of coalescence due to its high velocity. The range of 0.3 mm δR≤ 0.75 mm is the advantageous drop coalescence condition since the rapidly varied velocity and its first derivative theoretically cause the forces acting on a drop to become imbalanced. On the other hand, the thin layer of the continuous phase drained from the interval between the drops and trickling film should not be neglected in the calculation of shearing force since it is important for drop rotation. The drop rotation is an indispensable factor in coalescence.     

液滴红外光电自动检测计数器的研制

将红外光电液滴传感技术和药理学与生理学实验技术相结合,研制了"液滴红外光电自动检测计数器",该装置具有抗干扰能力强,能准确、灵敏地检测到液滴通过而导致的信号瞬间变化,可更直观、更准确、更客观地反映各种理化、药物及病理因素对心、肝、肾功能的影响。与传统的实验装置相比,减少了实验过程中肉眼计数的繁琐劳动和受滴棒计数的不准确性,使学生有更多的时间对实验结果进行观察、分析,增加了实验的精确性和科学性。     

不互溶液体内泡滴的直接接触传热模型(英文)

研究垂直管内两不互溶液体——正戊烷 -水系统的直接接触传热 ,管内易汽化的液体从管底用氮气压入形成分散泡滴 ,不易汽化的连续相液体在加热到一定温度后 ,以并流形式流过 .在考虑气泡被连续相流体破碎的情况下 ,借助于平均体积传热系数模型 ,在理论分析基础上通过实验研究建立了单个气泡的传热系数模型 ,模型参数有较好的关联性     

波纹板表面凝结传热的数学模型与计算

基于波纹板表面与液膜之间的表面张力作用能减薄液膜厚度而强化传热的机理，建立了竖直波纹板表面的凝结过程的数学模型．计算了波纹板对应于当量平板的凝结传热增强系数．分析了波纹的主要几何参数如波纹节距、波纹板高度、波纹角、波纹倾斜角、波纹圆角半径等对强化传热的影响效应以指导波纹结构的优化．提出了一种能兼顾强化传热效果和适当的流动截面积的双尺度波纹，其凝结传热系数可比当量平板增强1～2倍．     

双液滴垂直冲击薄液膜的数值模

The normal impingement process of two water droplets upon a thin film on the solid surface was numerically investigated. The numerical treatment was based on the finite volume solution of the Navier-Stokes （N-S） equations combined with the volume of fluid method （VOF）. Physically reasonable results for the process of two droplets impacting on the thin film were obtained. The effects of the droplet velocity, the fihn thickness and the spacing between the two droplets on the splash and spread process of the impact were examined.     

Cloud <Subscript>Micro</Subscript>atlas

We begin by outlining the life cycle of a tall cloud, and then briefly discuss cloud systems. We choose one aspect of this life cycle, namely, the rapid growth of water droplets in icefree clouds, to then discuss in greater detail. Taking a single vortex to be a building block of turbulence, we demonstrate one mechanism by which we believe droplets grow rapidly.     

人体呼出气溶胶在封闭环境中的分布特性

研究了1.0～10.0μm的呼吸道颗粒物在混合送风、置换送风和地板送风的办公室环境中的浓度空间分布和瞬态演变规律.仿真结果表明：小于10.0μm的飞沫在混合送风的房间内几乎是均匀分布的.然而,在置换送风和地板送风中,垂直温度梯度使得小于10.0μm的飞沫在人体呼吸区高度聚集,导致一个较高的浓度水平,因此室内人员被感染的概率比较高.由咳嗽或者打喷嚏产生的飞沫在置换和地板送风中的稀释（浓度衰减）比在混合送风中慢,这是因为置换和地板送风中人体呼吸区的风速比较小,飞沫在此聚集,不易被气流携带排出室内空间.     

两种医疗操作过程中飞沫的产生

对2种会产生飞沫的医疗操作过程进行飞沫特性的初步实验研究：鼻咽分泌物（NPA）的抽吸和雾化治疗.抽吸鼻咽分泌物（NPA）时用玻片和水敏感试纸来收集产生的大飞沫,对玻片上飞沫留下的痕迹在显微镜下进行计数,然后进行粒径大小分析.雾化治疗时用粉尘监测仪来检测雾化器周围以及房间中不同位置的小飞沫和飞沫核.研究结果表明,鼻咽分泌物（NPA）的抽吸过程会引发咳嗽,产生大飞沫.雾化器每分钟可产生数千万0.3～20μm的细小飞沫,其中大部分飞沫会从雾化器面罩上的2个小孔中逸出,扩散到空气中.飞沫在面罩内壁以及志愿者脸上聚集凝结说明雾化治疗过程中药液极有可能被病人的分泌物污染.     

瓦楞纸板横向振动表述形式的研究

根据瓦楞纸板的结构特征,采用Hoff夹层板理论,研究分析瓦楞纸板横向振动的表述形式。该研究工作为瓦楞纸板缓冲防振包装的优化设计提供参考依据,对深入研究瓦楞纸板的缓冲防振性能具有良好的理论指导意义。     

Quasi 1D modeling of two-phase flow and deposit formation for urea-selective catalytic reduction systems

A quasi 1D model of two-phase flow for a urea-selective catalytic reduction (SCR) system is developed which can calculate not only the generation of reducing agent but also the formation of deposits in the exhaust pipe. The gas phase flow is solved through Euler method, variables are stored on staggered grids, and the semi-implicit method for pressure-linked equation (SIMPLE) algorithm is applied to decouple the pressure and velocity. The liquid phase is treated in a Lagrangian way, which solves the equations of droplet motion, evaporation, thermolysis, and spray wall interaction. A combination of a direct decomposition model and a kinetic model is implemented to describe the different decomposition behaviors of urea in the droplet phase and wall film, respectively. A new 1D wall film model is proposed, and the equations of wall film motion, evaporation, thermolysis, and species transport are solved. The position, weight, and components of deposits can be simulated following implementation of the semi-detailed kinetic model. The simulation results show that a decrease in the exhaust temperature will increase the wall film region and the weight of deposits. Deposit components are highly dependent on temperature. The urea-water-solution (UWS) injection rate can affect the total mass of wall film and expand the film region, but it has little influence on deposit components. An increase in exhaust mass flow can decrease the total weight of deposits on the pipe wall because of the promotion of the mass and heat transfer process both in the droplets and wall film.     

Dynamic modeling of micro- and nano-sized particles impinging on the substrate during suspension plasma spraying

Suspension plasma spraying (SPS) can be utilized to manufacture finely structured coatings. In this process, liquid suspended with micro- or nano-sized solid particles is injected into a plasma jet. It involves droplet injection, solvent evaporation, and discharge, acceleration, heating, and melting of the solid particles. The high-speed and high-temperature particles finally impact on the substrate wall, to form a thin layer coating. In this study, a comprehensive numerical model was developed to simulate the dynamic behaviors of the suspension droplets and the solid particles, as well as the interactions between them and the plasma gas. The plasma gas was treated as compressible, multi-component, turbulent jet flow, using Navier-Stokes equations solved by the Eulerian method. The droplets and solid particles were treated as discrete Lagrangian entities, being tracked through the spray process. The drag force, Saffman lift force, and Brownian force were taken into account for the aerodynamic drag force, aerodynamic lift force, and random fluctuation force imposed on the particles. Spatial distributions of the micro- and nano-sized particles are given in this paper and their motion histories were observed. The key parameters of spray distribution, including particle size and axial spray distance, were also analyzed. The critical size of particle that follows well with the plasma jet was deduced for the specified operating conditions. Results show that in the downstream, the substrate influences the flow field structure and the particle characteristics. The appropriate spray distances were obtained for different micro- and nano-sized particles.     

Simulation of droplet topography on super-hydrophobic surface

We established a novel droplet model (with-gravity model) to show the gravity effect of the droplet in the contact angle experiment.By using with-gravity model, we obtained a three-dimensional topography of the droplet including the height of the droplet, the shape of the baseline and the circumference of the baseline.Comparing the with-gravity model with the ideal spherical model, our model considered the measurement error caused by gravity effect in the contact angle experiment which is a key point to influence the three-dimension topography of the droplet.From the calculation of our model, we found that there were two important points to enhance the measurement error: the size of the droplet and the contact angle.With the droplet and the contact angle became larger, measurement error was obviously increased.     

SPH modeling of droplet impact on solid boundary

A droplet undergoes spreading,rebounding or splashing when it impacts solid boundary,which is a typical phenomenon of free surface flow that exists widely in modern industry.Smoothed particle hydrodynamics（SPH）method is applied to numerically study the dynamical behaviors of the droplet impacting solid boundary,and both the spreading and rebounding phenomena of the droplet are reproduced in the simulation.The droplet deformation,flow fields and pressure fields inside the droplet at different moments are analyzed.Two important factors,the initial velocity and diameter,are discussed in determining the maximum spreading factor,revealing that the maximum spreading factor increases with the increase of the impact velocity and droplet diameter respectively.     

Numerical study on heat transfer enhancement for use of corrugated,nodal and horizontal grain tubes

With isopentane as working fluid, the heat transfer performances for corrugated, nodal and horizontal grain tubes are simulated. The structural parameters of the three kinds of tubes are compared with those of the plain tube. The numerical results using computational fluid dynamics are validated with theoretical values. For the corrugated, nodal and horizontal grain tubes, the heat transfer enhancements(HTEs) are 2.31—2.53, 1.18—1.86 and 1.02—1.31 times of those of the plain tube, respectively. However, the improved HTEs are at the expense of pressure losses. The drag coefficients are 6.10—7.09, 2.06—11.03 and 0.53—1.83 higher, respectively. From the viewpoint of comprehensive heat transfer factor, the corrugated tube is recommended for engineering applications, followed by the horizontal grain tube.     

压型钢板受力蒙皮作用的有限元简化分析

采用ANSYS结构分析程序的四节点壳单元对压型钢板及等效的正交各向异性板进行了线弹性分析,结果表明在压型钢板受力蒙皮作用的有限元分析中正交各向异性板可以取代真实压型板进行分析,并得到了等效的正交各向异性板的弹性常数计算公式。