多液滴同步冲击液膜时中间薄膜射流形成机理与动力学特征

采用耦合水平集-流体体积(coupled level set and volume of fluid)方法结合高斯随机分布扰动对多液滴同步冲击平面液膜飞溅过程进行了三维数值模拟,通过分析压力、速度等细微场量分布,揭示了中间薄膜射流的生成、破碎以及后期柱状射流的形成机理。此外,讨论了Weber数、液膜厚度、液滴间距对薄膜射流高度的作用规律。结果表明,在液相加入高斯分布扰动后可以充分反映液滴冲击飞溅特征;相邻液滴颈部区域射流接触后,接触区压力梯度骤然升高,与流体运动间断共同作用下形成向上运动的薄膜射流,随后在流体不稳定性与气相涡流作用下发生破碎;薄膜射流高度随Weber数和液膜厚度升高而增大,液滴间距减小时,射流高度增大。     

对置液柱撞击液膜破裂特征

采用高速摄像仪对两液柱撞击产生液膜的破裂过程进行了实验研究。分析了撞击液膜的破裂过程及表面波产生和传播过程,考察了射流直径、喷嘴间距和射流Weber数（We）对撞击液膜破裂的影响;定量分析了液膜表面波频率的变化及液膜破裂后的粒径分布情况。研究结果表明,液膜表面波传播频率随We的增大而增大,并沿液膜径向方向逐渐减小;随着射流We的增加,液膜边缘的液滴脱落频率增加;当We>1000时,液膜表面产生大量液滴团,且液滴团对液膜破裂具有促进作用;液柱撞击液膜发生破裂后90%以上的量纲1液滴粒径分布在0～1之间。     

转盘离心雾化高黏非牛顿流体薄液膜纤维化特性

高黏性流体经转盘离心雾化以液滴形式进入气相可显著改善气液相间接触,其纤维化特性直接决定了雾化液滴的尺寸,是影响物料品质的关键因素。针对转盘离心雾化器,对非牛顿黏性液膜失稳纤维化过程的物理机制进行了分析,分析了高黏度流体,仅考虑黏性作用时,黏性力对纤维间距的贡献,建立了黏性力与表面张力耦合作用下液膜破碎的纤维间距预测模型,探讨了液膜纤维化的一般规律。结果表明,黏性对液膜的失稳纤维化起抑制作用,黏度提高,纤维数量减小,纤维间距增大。纤维数量与转盘边缘液膜线速度相关。进入完全纤维状模式后,纤维数量趋于稳定,提高转速引起纤维数量稳定的流量范围缩小,不利于雾化,可采用低转速、大直径转盘改善雾化效果。非牛顿高黏流体液膜破碎后的纤维数量与Weber数、等效Reynolds数和流变指数直接相关。研究结果对高黏流体的转盘离心雾化系统设计与优化提供了可借鉴的理论与应用基础。     

转盘离心雾化高黏非牛顿流体薄液膜纤维化特性

高黏性流体经转盘离心雾化以液滴形式进入气相可显著改善气液相间接触,其纤维化特性直接决定了雾化液滴的尺寸,是影响物料品质的关键因素。针对转盘离心雾化器,对非牛顿黏性液膜失稳纤维化过程的物理机制进行了分析,分析了高黏度流体,仅考虑黏性作用时,黏性力对纤维间距的贡献,建立了黏性力与表面张力耦合作用下液膜破碎的纤维间距预测模型,探讨了液膜纤维化的一般规律。结果表明,黏性对液膜的失稳纤维化起抑制作用,黏度提高,纤维数量减小,纤维间距增大。纤维数量与转盘边缘液膜线速度相关。进入完全纤维状模式后,纤维数量趋于稳定,提高转速引起纤维数量稳定的流量范围缩小,不利于雾化,可采用低转速、大直径转盘改善雾化效果。非牛顿高黏流体液膜破碎后的纤维数量与Weber数、等效Reynolds数和流变指数直接相关。研究结果对高黏流体的转盘离心雾化系统设计与优化提供了可借鉴的理论与应用基础。     

液滴碰撞液膜复合level set-VOF法的数值分析

液滴碰撞液膜现象广泛存在于化工领域中。采用CLSVOF法建立了液滴碰撞液膜数值模型,并开展实验验证了模型的准确性。通过分析结果,研究了碰撞速度对液滴运动形态的影响,揭示了液滴内部流动传热和飞溅机理,并探索了液滴撞击液膜动力学和传热特性随碰撞速度的变化规律。研究表明:液滴碰撞液膜后随碰撞速度的增加依次呈现出波动、皇冠射流和射流飞溅等形态;碰撞速度越大,射流飞溅特征越明显。碰撞中心区域较大的压力梯度是液滴铺展的主要原因;铺展边缘较大气液压差是产生射流的主要原因;射流区域内速度间断是皇冠射流发展的关键因素;空气剪切及毛细波的作用是射流颈部收缩和产生飞溅的关键。碰撞速度越大,液滴的铺展系数、无量纲射流高度和壁面最大平均热流密度越大,无量纲液面中心相对高度越小;随着液滴雷诺数的增加,壁面最大平均热流密度的碰撞速度效应逐渐减小。     

液滴冲击加热壁面沸腾现象特征分析

采用高速摄像仪对液滴冲击加热壁面过程进行实验观测,分析了不同实验流体的沸腾现象特征,探讨了中间射流及宝塔状气泡的形成机理。观测发现,壁温高于液体对应的Leidenfrost温度时水滴撞击后会出现暴沸现象,由于气泡夹带伴随强烈的核化作用,氯化钠溶液液滴撞击后可以观察到中间射流的产生,醇类液滴则发生完全反弹;壁温低于Leidenfrost温度时液滴在加热壁面会出现泡状沸腾现象,与半球形气泡不同,宝塔状气泡出现在液膜厚度较大的区域。此外定量考察了液滴在加热壁面完全反弹时的最大铺展因子,发现铺展因子仅受Weber数影响,与文献结果比较表明本研究得出的铺展因子经验公式可较好地预测液滴在加热壁面的铺展尺度。     

液滴冲击加热壁面沸腾现象特征分析

采用高速摄像仪对液滴冲击加热壁面过程进行实验观测,分析了不同实验流体的沸腾现象特征,探讨了中间射流及宝塔状气泡的形成机理。观测发现,壁温高于液体对应的Leidenfrost温度时水滴撞击后会出现暴沸现象,由于气泡夹带伴随强烈的核化作用,氯化钠溶液液滴撞击后可以观察到中间射流的产生,醇类液滴则发生完全反弹;壁温低于Leidenfrost温度时液滴在加热壁面会出现泡状沸腾现象,与半球形气泡不同,宝塔状气泡出现在液膜厚度较大的区域。此外定量考察了液滴在加热壁面完全反弹时的最大铺展因子,发现铺展因子仅受Weber数影响,与文献结果比较表明本研究得出的铺展因子经验公式可较好地预测液滴在加热壁面的铺展尺度。     

中空液滴撞击液膜的数值研究

采用耦合水平集-体积分数法(CLSVOF)建立了中空液滴撞击液膜的数值模型,进行了模型的准确性验证,对中空液滴撞击液膜的动态特性及传热特性进行了研究。结果表明,中空液滴撞击液膜的动态过程包含液壳破碎、液壳液膜融合、液滴铺展等过程。中空液滴撞击液膜时会出现空气卷吸现象及颈部射流现象,颈部射流现象产生的原因是由于液滴颈部位置存在较大的局部压差。随着时间的推移,近壁面处流体的温度逐步上升,壁面的热流密度逐步降低,撞击过程对于传热特性所产生的影响趋于明显。分析了液壳厚度、液膜厚度及撞击速度对传热特性所产生的影响,壁面的平均热流密度随着液壳厚度及撞击速度的增大而增大,随着液膜厚度的增大而减小。     

基于LBM的液滴撞击液膜数值模拟

结合格子玻尔兹曼方法(LBM)与VOF法对液滴撞击液膜的过程进行数值模拟。对不同分子结构下液滴撞击液膜的铺展、水花的形成以及飞溅等现象进行了分析描述,定量地讨论了水花飞溅的高度以及液滴撞击后在液膜上的铺展直径,并在此基础上对不同形状液滴撞击液膜的动态特性和最大涡量进行分析。结果表明:液滴在空气中撞击液膜更容易产生水花并伴随二次小液滴的产生,且铺展直径最大;液滴在氦气中仅会有水花产生,说明液滴在氦气中水花的飞溅受到了抑制;液滴在氮气中溅起的水花比甲烷高,但其铺展面积相近。在不同形状的液滴下,AR值较大的液滴不会出现射流现象,扁椭圆液滴撞击液膜后产生的最大涡量值更大。     

液—液分散雾化形成液滴的模式

基于图像采集与处理方法实验研究了水喷入非相溶油中的液液分散雾化过程.结果表明,通过改变操作条件使水破碎形成3种水滴模式:滴流、层流射流和湍流射流,揭示了其演变规律及其形成机理,获得了分散液体破碎强度、射流长度脉动和液滴粒径分布参数随雷诺数Re、韦伯数We的变化规律.随Re和We增加,分散液体破碎强度持续增强,在Re=467,We=8.6时液滴形成模式由滴流向层流射流发展,在Re=3169,We=241时演变为湍流射流,且液滴的粒径均匀性和最小平均粒径均出现在湍流射流模式下;在滴流模式下,液滴的形成、长大与脱离喷嘴均由液滴受力的平衡机制控制;在层流射流模式下,射流柱破碎形成液滴主要由两相界面的表面波扰动引起,且射流长度的脉动具有随机和非周期的特点;在湍流射流模式下,射流柱表面出现拟序涡结构,其在射流起始段失稳,在射流扩散段大量形成较小粒径的液滴.     

A Comprehensive Computational Fluid Dynamics Study of Droplet‐Film Impact and Heat Transfer

The Eulerian multiphase model and continuum surface force (CSF) are employed to simulate the liquid droplet impinging onto a solid wall with a pre‐existing thin film of the same liquid. The numerical results are compared with the experimental data reported in the literature, indicating a reasonable matching. The flow field and splashing behavior of a droplet impinging onto a liquid film are analyzed. The reason for the edge of the crown to eject into secondary drops is found. The splashing behavior can be influenced by the impacting velocity and fluid properties. The effects of impact velocity, droplet diameter, depth of film, liquid property, and droplet and wall temperature on the heat removal are investigated. Numerical results demonstrate that an increase in impact velocity, droplet diameter, film depth, cooling droplet, and wall temperature enhances the dissipated heat. These results can provide a reference for designing spray‐cooling systems.     

喷雾冷却中液滴撞击带气泡液膜的数值模拟

在喷雾冷却过程的核态沸腾区,液滴与液膜及液膜内气泡的撞击对过程传热有重要影响。本文建立以水为冷却工质的单液滴撞击带气泡液膜的二维数值模型,模拟研究过程现象和传热规律。结果表明,We为6.94、量纲为1的液膜厚度为0.5（对应液滴速度0.5m/s、液膜厚度1mm）时,撞击过程中液膜扰动不显著、运动形态近似波纹;当We增大到111.11（对应液滴速度2m/s）时,撞击过程中液滴与液膜接合处的表压达到6000Pa,成为颈部射流现象的推动力,并逐步发展形成冠状水花;撞击过程中气泡的存在会阻碍液滴与加热表面的直接接触,但随着气泡的破裂,液滴与加热表面直接接触换热,使撞击点附近表面传热系数远大于其他区域,提高了传热能力,且液膜厚度越小、液滴速度越大,表面传热系数峰值越高。研究结果可为喷雾冷却系统的进一步研究提供理论依据。     

An atomization model for splash plate nozzles

A model for the atomization and spray formation by splash plate nozzles is presented. This model is based on the liquid sheet formation theory due to an oblique impingement of a liquid jet on a solid surface. The continuous liquid sheet formed by the jet impingement is replaced with a set of dispersed droplets. The initial droplet sizes and velocities are determined based on theoretically predicted liquid sheet thickness and velocity. A Lagrangian spray code is used to model the spray dynamics and droplet size distribution further downstream of the nozzle. Results of this model are confirmed by the experimental data on the droplet size distribution across the spray. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

FLUID SHEET THICKNESS AND VELOCITY AT THE TIP OF A BLACK LIQUOR SPLASHPLATE NOZZLE

The fluid sheet thickness and velocity at the tip of a spray nozzle have previously been identified as being important to the ultimate droplet size distribution. In this investigation both the fluid sheet thickness and the velocity of the top surface just beyond the rim of a splashplate nozzle have been measured as functions of nozzle exit velocity and fluid viscosity. Measurements were made on two Babcock & Wilcox Co. splashplate nozzles, a 12-49 and a 15-52, specifically designed for black liquor, a by-product of wood pulping. Comparisons of the ratio of the sheet velocity to the nozzle velocity are shown to be very similar to previous data for the ratio of droplet velocity to nozzle velocity. Likewise, the dependence of the sheet thickness on nozzle exit velocity closely matches the previously measured dependence of the ultimate median droplet size on nozzle exit velocity. Both of these results are consistent with current theories of droplet formation mechanisms. Analysis of continuity and viscous drag on the splashplate are used to develop correlation equations for the data     

Splashing Characteristics of Monodisperse Sprays with Significant Viscosity Differences Impacting a Flat Surface

Drop impingement and splashing as a function of viscosity and, to a lesser extent, temperature are examined here. The working fluid is a mixture of water and glycerin with relative mass percentages varying from 0 to 100%, which spans a viscosity range of three orders of magnitude. First, a criterion that separates “on” and “off” for splashing as a function of glycerin percentage is expressed in terms of both Weber and Reynolds numbers and its highly nonlinear behavior is a function of the change in fluid viscosity. Next, the complex splashing characteristics of a rather simple monodisperse spray injected at a pressure of 2 bars onto a flat, 2-mm-diameter aluminum cylindrical rod are examined. Spatial variations in the fraction of splashed liquid, Sauter mean diameter, splashed droplet size distribution, and splash volume fraction as a function of radial distance for these mixtures are reported.     

生物转盘表面液膜流动特性数值模拟

采用计算流体动力学(CFD)方法,运用流体体积(VOF)模型研究生物转盘表面液膜流动特性,分析了传统实心转盘和新型开窗转盘表面液膜的成膜特性、厚度和速率等,并考察了不同转盘厚度对成膜和液膜厚度的影响。结果表明:实心转盘的液膜自上而下平缓过渡,而开窗转盘中,开窗区域附近出现液膜位置的急剧变化,并在槽内液体下方发现很多不规则的气泡;转盘在上升区的液膜厚度随角度的增加快速减小,而下降区的液膜厚度基本不变,趋于稳定;转盘开窗形成的自由膜速率大,更新频率快;在研究范围内,不同厚度转盘形成的液膜随转盘厚度增加,开窗区域的自由膜厚度增加。     

Régimes d'écoulement et mesure de l'épaisseur d'un film liquide ruisselant autour d'un tube horizontal

The flow characteristics and film thickness have been studied experimentally for the case of a falling liquid film flows on a horizontal cylinder. The optical method based on laser‐induced fluorescence is presented in this paper. All tests have been conducted with the adiabatic water/fluorescence mixture at atmospheric pressure. Five falling‐film modes are observed for different liquid mass flow. The wavelengths have been measured for each film Reynolds number value for droplet and column modes. The transitions between different falling‐film modes are studied and the hysteresis phenomenon is observed. The measured dimensionless wavelengths are compared to the results of Hu and Jacobi (1998). The experimental results show the local evolution of film thickness around the cylinder. They are compared to the results of Rogers and Goindi (1989), Rogers (1986) and Chyu and Bergles (1987). The local measurements of film thickness are correlated by including the effects of film Reynolds number.     

波纹板填料表面液体降膜流动的特性分析

基于降膜流动实验台,结合计算流体力学方法(Computational Fluid Dynamics, CFD)研究了波纹板表面液体的平均液膜厚度和有效润湿面积等定量信息,并通过三维模拟进一步分析了喷淋密度和波纹倾斜角度β对降膜流动特性的影响。结果表明,液体在波纹板表面的流动并非均匀,分为沟流和溪流两种形式;当喷淋密度较小时,液体在波谷内形成沟流,当喷淋密度达到400 m3/(m2?h),液体跨越相邻波纹进行溪流流动;两种形式波纹板整体的润湿性能均较差,且液膜厚度分布不均;波纹倾斜角度对降膜流动特性影响较大,90°时更有利于提高有效润湿面积。     

Parameter study of microdroplet formation by impact of millimetre-size droplets onto a liquid film

The objective of this work is to carry out an experimental investigation on the influence of different parameters such as liquid properties (viscosity and liquid–gas surface tension), impacting droplet properties (velocity v_i and diameter d_i) and the thickness of liquid film on the emission of airborne particles produced by the impact of millimetre-size droplets onto a liquid film. Our results show that in the variation range studied, the increase of v_i and d_i or the decrease in liquid film thickness produces an increase in the mean number of microdroplets emitted by impact in the size range 2–50 μm. Furthermore, it was also observed that an increase in viscosity involves a steep decrease in the mean number of microdroplets emitted. These microdroplets may be produced by the fingers pinching of the crown formed during impact. In the case of low surface tension liquid, the formation of bubbles involves high production of droplets smaller than 15 μm. The results presented here constitute a database of the micro-droplets produced to validate models of droplet impact outcomes.     

非平整倾斜表面上流动液膜的影响因素分析

基于流体体积函数法(VOF),分别从液体物性和液相入口流速等因素对非平整倾斜表面上的液膜流动特性进行数值模拟,得出流体黏度的增大可增大液膜厚度并降低液膜整体流动速度,但不影响液膜与壁面之间的相位角;流体表面张力影响液膜与壁面之间的相位差及液膜的均匀性;液相入口雷诺数的增大可以增大液膜厚度,当雷诺数增大到一定程度时,液膜厚度不再有明显的变化,且液膜表面趋于光滑。研究结果表明:液膜的流动形式由液体各项物性和液相入口流速共同决定。