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.     

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。     

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。     

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。     

液液雾化特性与粒径分布规律

液液雾化过程是液液循环流化床的关键技术之一,在流化床常态实验装置上,采用快速摄像与图像处理相结合的方法,获得了实验流量范围内液滴形成过程的图像以及形成液滴的粒径信息,运用数学分布函数对液滴的粒径分布进行了研究。研究结果表明,各流量工况下,液滴的粒径分布与Rosin-Rammler分布符合得较好;在实验流量范围内,形成液滴的区域有单液滴形成区域、过渡区域和多液滴形成区域,且液滴中位径的总体趋势是减小的,当水的流量为50 ml·min-1时,形成液滴的粒径主要集中在0.7～1.0 mm之间;在单液滴形成区域和多液滴形成区域,液滴粒径的均匀性先减小后增大,中位径呈减小趋势,但在多液滴形成区域两者的变化幅度较小,在过渡区域,液滴粒径的均匀性与中位径基本不变。研究结果可为液液循环流化床基于设计粒径的要求合理选取运行流量提供可靠依据。     

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。     

循环流化床液液雾化过程转型的实验研究

建立了循环流化床液液雾化的实验装置,对水在非相溶介质中喷射雾化进行了研究。采用高分辨率数码摄像仪实时采集不同工况条件下的雾化形态,对比分析了雾化过程转型现象;通过图像处理与数值计算相结合的方法,对实验结果进行了分析。结果表明,雾化过程与相间相对雷诺数密切相关,在相对雷诺数为1 632时,由鼓泡型转变为连续射流型,而当相对雷诺数增加到1.14×105时,雾化则由连续射流型转变为紊态射流型;在雷诺数大于1.62×105后,液滴粒径急剧减小。在紊态射流形态中,雾化液滴数多且粒径微小,符合液液循环流化床制取流体冰技术对液滴尺度的要求。     

液滴行为与液-液混合设备

总结了液-液非均相混合过程中的液滴运动速率计算模型,分析了液滴破碎及合并的机理。在结合各种工业应用的基础上,总结了搅拌釜、射流混合器、静态混合器、撞击流混合器及纤维液膜混合器等五种混合设备的混合原理及研究现状。展望了液-液混合设备的研究前景。     

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。     

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。     

The formation and breakup of molten oxide jets under periodic excitation

The experiments on the capillary breakup of slag jets at high temperatures are presented in this article. The impact of external excitations on the disintegration process was investigated in a furnace with optical access filmed at frame rates up to 10,000 fps. A synthetic calcia‐alumina slag was used to form jets at different temperatures (1570–1660°C) and jet velocities (0.6–1.4 ms^?1). The impact of external vibration on the breakup was evident: for low jet velocities, the jet length decreased, the droplet size increased, satellite droplet formation was hindered, and a distinct “pumping mechanism” was observed. For jets with higher velocity, the jet length decreased by 30%, the droplet generation frequency increased from 20 to 250 droplets per second, the drop sizes were uniform, and satellite formation was also suppressed. In this case, the ideal case in which the volume of one wave instability forms one droplet was achieved. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3350–3361, 2014     

转盘边缘黏性薄液膜不同破碎模式临界转变特性

液膜在离心粒化器边缘的破碎模式直接决定了雾化后的液滴形态和尺寸分布,是影响物料品质的关键因素。针对转盘粒化器,提出临界转变系数表征液膜由膜状向纤维状破碎的转变条件,并拓展至其他破碎模式,建立了滴状向纤维状、完全纤维状及纤维状向膜状破碎转变的临界关系。结果表明,转盘表面润湿性对于液膜呈滴状以及滴状向纤维状模式转变影响显著,未完全润湿导致临界流量存在一定的随机性,转盘直径与临界流量间无明确规律;而完全纤维状以及膜状时,大直径转盘临界流量明显升高。转速、流量、密度及黏度的提高,破碎模式趋向于膜状;而增大表面张力,即使对于较大流量和转速,液膜也能维持纤维状或滴状模式。调整转盘直径将引起表面张力与离心力同时变化,若未打破平衡,其破碎模式不会改变。研究结果为转盘粒化器的设计与优化提供了可借鉴的理论与应用基础。     

Mean Droplet Size and Local Velocity in Horizontal Isothermal Free Jets of Air and Water,respectively, Viscous Liquid in Quiescent Ambient Air

Measurements using two‐dimensional Phase Doppler Anemometry as well as high speed cinematography in free jets at several nozzle exit pressures and mass flow rates, show that the Sauter mean droplet diameter decreases with increasing air and liquid‐phase mass flow ratio due to the increase of the air stream impact on the liquid phase. This leads to substantial liquid fragmentation, respectively primary droplet breakup, and hence, satellite droplet formation with small sizes. This trend is also significant in the case of a liquid viscosity higher than that of water. The increased liquid viscosity stabilizes the droplet formation and breakup by reducing the rate of surface perturbations and consequently droplet distortions, ultimately also leading, in total, to the formation of smaller droplets. The droplet velocity decreases with the nozzle downstream distance, basically due to the continual air entrainment and due to the collisions between the droplets. The droplet collisions may induce further liquid fragmentation and, hence, formation of a number of relatively smaller droplets respectively secondary breakup, or may induce agglomeration to comparatively larger liquid fragments that may rain out of the free jet.     

水煤浆气流雾化的初次破裂特性

以水煤浆和空气作为实验介质,利用高速摄像仪对同轴双通道喷嘴水煤浆气流雾化的初次破裂过程进行了实验研究,重点考察了在喷嘴出口附近水煤浆的射流核心长度和振荡频率等特征。研究结果显示,实验中水煤浆的破裂模式为非轴对称雷利破裂模式,根据实验结果,得到了浆体的量纲1射流核心长度与气速和液速之间的关系式。雾化过程中浆滴的产生与水煤浆射流振荡有关,研究了其Strouhal数与Weber数和液体Reynolds数之间的关系。     

The Influence of the External Imprinted Flow Field on Capillary Instability Driven Jet Breakup

The breakup of a flowing liquid jet surrounded by a second flowing liquid is governed by capillary instabilities. These instabilities are caused by minor perturbations in the flow field that build up sinusoidal waves at the jet interface and eventually break it up. The phenomenon of such fatal wavelengths was described first by Lord Rayleigh and later extended by several authors to quiescent and flowing liquid‐liquid systems. The jet formation in a co‐flowing environment was only recently investigated. Originating from the forced breakup of liquid jets in air, this paper reports on the breakup mechanism of liquid jets surrounded by a fluid. By using different flow cells and consequently different flow profiles along the jet trajectory, the breakup mechanism given by the material and process parameters could be excited or suppressed. Two questions concerning droplet generation and the droplet size distribution are addressed: the suppression of the fatal wavelength in transient flow fields and the use of external flow fields to support the fatal wavelength.     

Spray and mixing characteristics of liquid jet in a tubular gas–liquid atomization mixer

For the design and optimization of a tubular gas–liquid atomization mixer,the atomization and mixing characteristics of liquid jet breakup in the limited tube space is a key problem.In this study,the primary breakup process of liquid jet column was analyzed by high-speed camera,then the droplet size and velocity distribution of atomized droplets were measured by Phase-Doppler anemometry (PDA).The hydrodynamic characteristics of gas flow in tubular gas–liquid atomization mixer were analyzed by computational fluid dynamics (CFD) numerical simulation.The results indicate that the liquid flow rate has little effect on the atomization droplet size and atomization pressure drop,and the gas flow rate is the main influence parameter.Under all experimental gas flow conditions,the liquid jet column undergoes a primary breakup process,forming larger liquid blocks and droplets.When the gas flow rate (Q_g) is less than 127 m~3·h~(-1),the secondary breakup of large liquid blocks and droplets does not occur in venturi throat region.The Sauter mean diameter (SMD) of droplets measured at the outlet is more than 140μm,and the distribution is uneven.When Q_g127 m~3·h~(-1),the large liquid blocks and droplets have secondary breakup process at the throat region.The SMD of droplets measured at the outlet is less than 140μm,and the distribution is uniform.When 127Q_g162 m~3·h~(-1),the secondary breakup mode of droplets is bag breakup or pouch breakup.When 181Q_g216 m~3·h~(-1),the secondary breakup mode of droplets is shear breakup or catastrophic breakup.In order to ensure efficient atomization and mixing,the throat gas velocity of the tubular atomization mixer should be designed to be about 51 m·s~(-1)under the lowest operating flow rate.The pressure drop of the tubular atomization mixer increases linearly with the square of gas velocity,and the resistance coefficient is about 2.55 in single-phase flow condition and 2.73 in gas–liquid atomization condition.     

表面张力变化对含气泡液体射流破裂的影响

使用高速相机研究了表面张力变化对含气泡液体射流破裂过程的影响。通过改变表面活性剂浓度获得了不同表面张力的液体射流。实验发现当液体射流速度保持不变时,减小液体表面张力会增加射流破裂长度。表面活性剂一方面降低了液体动态表面张力,减小了射流表面不稳定波的增长率,增大了射流破裂长度;另一方面表面活性剂在射流表面的非均匀分布会产生Marangoni应力,促使液体向射流变形区运动,从而推迟了射流破裂的发生,增大了射流破裂长度。通过理论分析得到了液体射流破裂长度表达式。发现射流内部气泡会显著缩短含表面活性剂射流的破裂长度。通过气泡扰动射流速度和吸附表面活性剂的分析,揭示了内部气泡对含表面活性剂射流破裂的影响规律。     

液滴碰撞球形凹曲面复合level set-VOF法的数值分析

采用复合水平集法和流体体积法建立液滴冲击球形凹曲面的数值模型,通过分析计算结果揭示了液滴撞壁流动及破碎机制。研究表明:液滴的撞壁特性与液滴碰撞速度密切相关;液滴凹曲面撞壁与平面撞壁相比,铺展系数较小,回弹射流出现时间超前,回弹射流速度较大。量纲1分析得出:液滴的最大铺展系数和相对最大铺展速度与Reynolds数近似幂递增,液滴的相对最大射流长度与Reynolds数近似对数递增,液滴的相对最大射流速度与Reynolds数近似幂递减。对比分析现有液滴撞壁最大铺展系数理论解析模型,提出了液滴凹曲面撞壁最大铺展系数解析模型的发展方向。     

Prediction of Droplet Velocities and Rain Out in Horizontal Isothermal Free Jet Flows of Air and Viscous Liquid in Stagnant Ambient Air

Two‐dimensional phase Doppler anemometer measurements of droplet size and velocity conducted under several nozzle conditions and a systematic variation of the air mass flow quality and liquid phase viscosity show that the air entrainment process is enhanced when keeping all test conditions constant except for increasing the Newtonian liquid viscosity above of that of water. A two‐zone entrainment model based on a variable two‐phase entrainment coefficient is proposed with the normalized axial distance allowing for a change in the jet angle. Thus, the jet perimeter is lower and the breakup length is longer in the case of air/relatively higher viscosity liquid phase. It provides the most accurate reproduction of the experimental droplet velocity in comparison with that of other models in the literature and, hence, is recommended for the prediction of the droplet velocity in the case of two‐phase air/liquid phase free jet flow in stagnant ambient air. A model for predicting the droplet rain out, considering the droplet trajectories in the free jet flow, allows also for an adequate reproduction of the experimental data.     

喷淋塔液滴粒径分布及比表面积的实验研究

以水和空气为实验介质,通过拍照法获得喷淋塔内液滴粒径分布,考察了不同喷淋量及空塔气速对塔内不同高度处液滴Sauter平均直径(SMD)的影响,并对液滴粒径分布进行了理论分析.结果表明,喷淋塔顶部液滴分布密集,底部稀疏,液滴群在下落过程中,平均粒径减小且趋于均匀化;塔顶处液滴SMD随喷淋量的增加而增大,处在塔中下部的液滴SMD则随喷淋量增大而减小,提高空塔气速,可减小平均粒径;理论分析认为,液滴粒径减小主要是由于发生了碰撞破碎的缘故,而塔内液滴大小不一是碰撞的主要原因;通过量纲1化拟合得到喷淋塔内液滴SMD经验关联式,其计算结果与实验值吻合较好;考虑液滴破碎的喷淋塔比表面积比不考虑破碎的比表面积大70%左右.