液滴撞击不同固体表面的数值模拟研究

采用CLSVOF(coupled level set and volume of fluid)方法,对低We情况下液滴撞击不同固体表面的过程进行了数值模拟研究。分析了液滴撞击平板表面的动态过程,构建了液滴衰减振荡的数学模型,探究了不同的表面润湿性、撞击速度及表面微尺度结构对液滴动态特性的影响。结果表明:液滴撞击固体表面的过程包含铺展、回缩、振荡等多个现象,其最大铺展因子及振荡周期随着表面接触角的增大而减小,随着撞击速度的增大而增大;撞击表面的微尺度结构会对液滴的动态特性产生影响,微尺度结构会对液滴的铺展及回缩运动产生阻碍作用,导致液滴的振荡特性减弱;液滴在矩形沟槽表面达到最大铺展因子的用时最短,在三角形沟槽表面的最大铺展因子最小。     

液滴撞击细纤维的研究进展

液滴撞击细纤维现象在空气过滤、淡水收集及纤维涂层等工程领域很常见,研究该过程的动力学行为及捕集机理,对提高纤维过滤器效率、优化捕获液滴数量具有重要意义。简要分析了液滴碰撞动力学;综述了国内外学者对液滴撞击细纤维动力学过程进行的可视化及数值研究,讨论了撞击速度与丝径、偏心距和纤维表面温度、纤维弹性等参数对碰撞后流动状态及液膜厚度的影响,以及液滴在细纤维上的形状和临界体积等问题。结合电场作用下液滴的动力学特性,耦合电场和重力场是未来研究液滴碰撞动力学的重要方向。     

液滴撞击低温超疏水表面运动状态的数值模拟研究

本文针对液滴撞击固体表面后出现的反弹和黏附两种运动状态,通过引入基于连续温度函数的能量方程,建立了用于计算液滴运动、传热和相变过程的数值模型,并与实验结果对比,验证了数值模型的准确性。进而模拟了具有不同韦伯数We和奥内佐格数Oh的液滴撞击低温超疏水表面的运动、传热和相变过程。结果表明：液滴撞击后的运动状态主要取决于奥内佐格数Oh,临界范围是0.022-0.026,而基本不受韦伯数We的影响。当液滴撞击后的运动状态为反弹时,韦伯数We越大的液滴,最大铺展直径越大。而当韦伯数相同时,随着奥内佐格数Oh的增大,液滴与表面的接触时间越大。     

连续液滴撞击热壁面传热特性的研究

为了研究不同碰撞速度以及不同换热方式对连续液滴撞击热壁面传热特性的影响,以8滴去离子水和铜板为试验材料,设置了速度分别为0.63 m/s、0.77 m/s、0.89 m/s、0.99 m/s的8滴去离子水液滴在膜态蒸发(铜板温度60℃)和核态沸腾(铜板温度110℃)两种换热方式下的液滴撞击热铜板试验,探究铜板热流密度值的变化。结果表明:膜态蒸发换热方式下,液滴撞击热壁面速度越大,铜板的最大热流密度值越大,液滴与壁面之间的换热效果越好。核态沸腾换热方式下,以0.89 m/s的速度值为转折点,当连续液滴撞击热壁面速度小于0.89 m/s时,液滴撞击速度越大,铜板热流密度值越大;当连续液滴撞击热壁面速度大于0.89 m/s时,随着液滴撞击速度的增大,铜板的热流密度值减小。     

单液滴撞击球面液膜水花形成特征分析

应用VOF(volume of fluid)方法对液滴撞击球面液膜的飞溅过程进行数值模拟,由计算结果分析了不同液滴撞击速度和液膜厚度对撞击产生的水花形态、高度和直径的影响。结果表明:随着液滴撞击速度和液膜厚度的增加,水花高度增大;随着液滴撞击速度增加,水花直径增大,但当液膜厚度增加时,水花直径减小。     

液滴撞击超疏水管状壁面的动态特性研究

利用相界面追踪的CLSVOF(coupled level set and volume of fluid)方法,对液滴在不同速度下撞击超疏水管状壁面的演化过程展开研究。研究结果表明,不同初速度会对液滴在管壁上的动态特性产生较大影响。当撞击速度较小时,液膜会以整体的形态反弹;继续增大撞击速度后,由于受到外缘液膜的牵扯,液膜内部开始出现断裂;撞击速度足够大时,液膜发生解体破碎。同时给出了对于初速度在2.00m/s以内、直径为2.58mm的水滴撞击管径为8.00mm的超疏水壁面时运动状态发生转变的临界条件。     

固体表面振荡液滴接触角演化

固体表面液滴振荡存在于很多实际生产当中,了解和掌握其中的界面特性对于实际的生产具有指导意义。利用可视化实验台对吹风条件下固体表面液滴的振荡现象进行了观察。实验在不同的条件下进行,着重风速、液滴尺寸和表面粗糙度等因素对实验现象的影响。实验观察到液滴与固体壁面接触角在振荡过程中有一定的变化规律,而接触线在振荡过程中始终保持不动。通过对实验结果的分类整理和对比,发现固体表面振荡液滴接触角变化和风速、液滴尺寸及表面粗糙度存在一定的关系。最后,通过接触线力平衡条件初步分析解释了接触角的演化规律。     

单液滴撞击平面液膜飞溅过程的CLSVOF模拟

采用CLSVOF(coupled level set and volume of fluid)方法对单液滴撞击平面液膜初期的飞溅过程进行了数值模拟,探讨了撞击速度和液膜厚度对撞击后形成冠状水花形态及扩展直径的影响.结果显示,扩展直径随撞击速度的增大而增大,随液膜厚度的增大而减小,CLSVOF方法可较好地用于液滴撞击过程的数值计算.     

液滴撞击液膜动力学特性的FTM模拟

采用界面追踪法(front-tracking method)对液滴撞击液膜动力学特性进行数值模拟,通过撞击后的形态演变及内部物理场信息分析,研究了韦伯数和无量纲液膜厚度对界面运动过程的影响,并阐述了撞击过程中形成卷吸现象的机理。研究表明,液滴撞击之后,会在颈部区域产生小射流,该射流是后期水花形成的基础;水平方向上,在撞击影响不到的区域压力不变化,而在射流形成处的颈部附近存在局部压差;液滴撞击液膜时其间的气体层被压缩,在流体黏性和剪切力的作用下,压力高的气层中的气体逃逸速度减慢,从而形成卷吸现象。     

粘性力和表面张力对液滴撞击超疏水管壁动力学特性的影响研究

利用相界面追踪的耦合水平集-流体体积(CLSVOF)方法,对液滴撞击超疏水管状壁面的动态特性展开研究,主要讨论粘性系数和表面张力系数对撞击过程的影响。研究结果表明:液滴粘性系数越小,液膜的铺展速度越快,回缩速度也越快,同时越早发生反弹,反弹高度也越大;随着表面张力系数的增大,液膜达到最大铺展所需时间减少,会阻碍液膜的铺展,但会促进其回缩,当表面张力系数为0.01 N/m时,液膜外边缘出现飞溅且液膜主体不发生反弹;还对液滴撞击超疏水管壁后出现的反弹、飞溅现象的机理展开分析。     

Internal flow in evaporating droplet on heated solid surface

This work numerically studies the evaporation process of a liquid droplet on a heated solid surface using a comprehensive model. The internal flow within the evaporating liquid droplet is elucidated, while considering the effects of buoyancy force, thermocapillary force, and viscous resistance. The evaporation process is modeled by simultaneously solving the Navier–Stokes equations and energy equation for the liquid domain and the heat conduction equation for the solid domain, while assuming the liquid–vapor interface is a free surface. Three dimensionless parameters are utilized to describe the contribution of individual driving forces to internal flow. Evolutions of the thermal and internal flows during evaporation are discussed. The volume evolution and experimental data are in good agreement.     

加热板液滴蒸发的理论分析及实验研究

液滴蒸发是由气-液浓度差驱动的一种常见而复杂的扩散现象.通过实验与理论相结合对去离子水在玻璃表面和有机硅油表面的蒸发特性进行研究,测量了液滴接触角和接触直径随时间的动态演变过程.结果 发现:玻璃表面的液滴蒸发为典型的定底半径模式和混合模式;而液滴在有机硅油表面较为特殊,除了定底半径模式和混合模式还有周期性的黏滑模式.出...     

A numerical study of air entrapment during the droplet impact on a solid substrate

A numerical investigation is conducted to study the air entrapment phenomenon when two different liquids such as water and diesel droplet are impacted on the solid surface. The beginning of the air entrapment process was observed during droplet impact on a solid substrate forming a dimple underneath the droplet. The air film thus trapped underneath the droplet started evolving into the air bubble. This journey of evolution mainly comprises phases like an inertial retraction of air film, contraction, and pinch-off of the secondary droplet inside the air bubble for a water droplet impact case. The volume of fluid approach has been utilized to track the progress of air film evolution. The influence of surface wettability has been observed on the evolution of air film into the air bubble by taking four different values of contact angle pertaining to the hydrophilic surface (θ = 10° and 35°) and hydrophobic surface (θ = 90° and θ = 120°). The air bubble was found to get detached from the substrate for the hydrophilic surface (θ = 35°) and observed to remain attached to the substrate for the hydrophobic surface. The variation of pressure underneath the droplet was also investigated as the droplet reaches the substrate. The effect of surface tension has been studied on the evolution of air film by impacting the diesel droplet on the same substrate keeping the same wettability condition (θ = 35°). The lower surface tension of the diesel droplet as compared to the water droplet delayed the process of air film evolution and consequently decreases the retraction speed of air film. Also, the air bubble remains attached to the surface. Furthermore, the air bubble detaches from the surface for an even higher wettability condition (θ = 10°). Thus surface wettability and surface tension become two important factors governing the development of entrapped air film and bubble elimination in many practical applications.     

Solidification characteristics of a droplet on a horizontal cooled wall

An experimental and numerical study of solidification characteristics of a droplet on a horizontal cooled wall is reported. Pure water and molten salt were utilized as the testing liquids. The droplet was cooled under a variety of conditions such as wall temperature and initial liquid temperature in a static atmosphere. Extensive observations of both solidification characteristics and morphologies of the droplet were made. Numerical calculations based on the Landau method considering the effects of both surface tension at the droplet surface and the density inversion at 4 °C within the droplet were carried out. It was found that the numerical results are qualitatively in good agreement with the experimental results except for the terminating period of solidification. © 1998 Scripta Technica, Heat Trans Jpn Res, 26(7): 469–483, 1997     

水平矩形管中气固两相流的PDA实验研究

应用三维颗粒动态分析仪(3D-PDA)对方形水平管道内的气固两相流进行了测试。实验采用的颗粒为玻璃微珠，对不同工况下的水平方向(主流方向)的平均速度和湍流强度进行了讨论。发现在垂直截面上的速度分布呈上部高而下部低的分布特点，且随平均风速、颗粒体积分数和粒径的增大这种不均匀分布有加剧的趋势。湍流强度中心位置较低，而靠近壁面的位置较高，尤其是底部湍流强度更大一些。在大部分位置颗粒相的速度滞后于气相，在边壁附近特别是底部壁面附近颗粒速度较气相速度稍大。颗粒体积分数沿垂直方向上分布较均匀，越靠近壁面颗粒体积分数越高，在管道的底部和垂直壁面的交角附近颗粒体积分数最高。     

Theoretical and experimental study of natural convection on a horizontal plate

The authors report a numerical and experimental study of the natural convection flow above an upward facing horizontal heated plate placed in a semi-infinite medium. The equations, which govern the phenomenon, are discretized by using the control volume approach. The linkage between the momentum and continuity equations is realized with the SIMPLE algorithm. The results show the existence of two regions situated near the sides of the plate, which break away in the middle of the heated surface and form a thermal plume. The local heat transfer coefficient is higher near the sides of the plate and decreases at the central heated region. The validity of the above numerical results is experimentally checked with measurements of the temperature in the air surrounding the plate, which leads to experimental Nusselt numbers.     

Experimental study on the effect of surface conditions on evaporation of sprayed liquid droplet

The present study investigates experimentally the effects of thermal properties of the hot surface and droplet characteristics on the droplet evaporation. Cylindrical blocks made of Stainless Steel, Aluminum and Brass with different degrees of surface roughness were used. The droplet diameter and velocity were controlled independently. The behavior of droplet during the collision with hot surface was observed with a high-speed camera. The results presented the effect of the thermal properties of the hot surface, droplet Weber number, droplet velocity, droplet size, hot surface conditions; surface superheat and degree of surface roughness on the solid–liquid contact time and the maximum spread of droplet over the surface. Empirical correlations have been deduced describing the relationship between the hydrodynamic characteristics of an individual droplet impinging on a heated surface and concealing the affecting parameters in such process. Also, the comparison between the current results and the results due to others investigators shows good agreement in which the difference between them ranged from 5% to 25%.     

城市生活垃圾燃烧特性的实验研究

分析研究生活垃圾的燃烧特性,通过实验分析垃圾的燃烧热值、着火温度、燃尽温度、失重速率等燃烧特性参数,并计算各种生活垃圾活化能。     

Interfacial temperature measurements,high-speed visualization and finite-element simulations of droplet impact and evaporation on a solid surface

The objective of this work is to investigate the coupling of fluid dynamics, heat transfer and mass transfer during the impact and evaporation of droplets on a heated solid substrate. A laser-based thermoreflectance method is used to measure the temperature at the solid–liquid interface, with a time and space resolution of 100 μs and 20 μm, respectively. Isopropanol droplets with micro- and nanoliter volumes are considered. A finite-element model is used to simulate the transient fluid dynamics and heat transfer during the droplet deposition process, considering the dynamics of wetting as well as Laplace and Marangoni stresses on the liquid–gas boundary. For cases involving evaporation, the diffusion of vapor in the atmosphere is solved numerically, providing an exact boundary condition for the evaporative flux at the droplet–air interface. High-speed visualizations are performed to provide matching parameters for the wetting model used in the simulations. Numerical and experimental results are compared for the transient heat transfer and the fluid dynamics involved during the droplet deposition. Our results describe and explain temperature oscillations at the drop–substrate interface during the early stages of impact. For the first time, a full simulation of the impact and subsequent evaporation of a drop on a heated surface is performed, and excellent agreement is found with the experimental results. Our results also shed light on the influence of wetting on the heat transfer during evaporation.