降液膜波动的影响因素分析

建立了溴化锂溶液沿平板降膜的流动模型。利用VOF方法捕捉降液膜的自由流动表面,利用CSF模型考虑表面张力对降液膜流动的作用,研究了入口扰动频率、降膜Re和平板倾斜角对降膜波动演化过程的影响。模拟结果表明,入口处的低频率扰动发展为孤波,随着频率的增加,波长减小,孤波与孤波通过他们之间的波后、衬底和毛细波相互影响,而高频率扰动演变成毛细波。随着Re的增加,波幅和波长增加,同时波峰处的循环流动强度增加。随着倾斜角的增加,重力沿流动方向的分力增加,波发展速度增加。同时分析了波内的速度场和壁面切应力等动力学特性。     

波纹壁面上流动液膜涡的生成特性

针对液膜在非平整壁面上流动过程中生成涡的现象,基于VOF方法,采用FLUENT软件模拟了三维波纹壁面上的液膜流动。研究了波纹结构内涡结构的演化过程,分析入口Reynolds数、波纹结构、壁面倾角、流体黏度和表面张力对波纹结构内涡结构的影响。结果表明：随着时间的演化,涡的大小和形状不断变化,最终达到稳定。且涡结构变化对自由液面的波动影响显著。较低Re和波形度时,波纹结构内不易形成涡,随着Re和波形度增大,产生涡且涡呈增大趋势,涡的形态也随之改变,自由液面位置升高,其相位滞后于波纹壁面。当壁面倾角改变时,波纹结构内的涡特性变化较大,液膜厚度略有增加,而自由液面相位不明显。表面张力对涡结构有显著影响,液膜流动过程中不容忽视。流体黏性改变时,波纹结构内涡的大小和形状无明显的变化。黏度变小和忽略表面张力时,液膜厚度均变薄。以上结果为工业设备生产、运行和设计提供了一定参考依据。     

物性对油滴剪切变形影响的模拟分析

以简单线性剪切流为背景,采用VOF模型及若干自定义子程序进行模拟分析,探讨物性对油滴变形和破碎的影响关系。模拟表明:粘度对变形起到抑制作用,粘度越大,变形越难,更能耐受剪切流动;表面张力越小,油滴越容易发生变形和破碎,稳定性越差,表面张力阻止了油滴的变形;油滴粒径越大,越易变形、破碎,越不稳定。     

规整填料表面液体分布的计算流体力学

采用计算流体力学(computational fluid dynamics,CFD)中的VOF方法对规整填料表面液相分布进行了三维建模和仿真,实现规整填料内液相分布的可视化并得到了液膜厚度和有效相界面积比等相关定量信息。通过分析比较不同物系的数值模拟结果,发现液体的表面张力和黏度都对填料表面上液体分布有影响。表面张力越小,液相在填料片上分布越均匀,有效相界面积比越大,液膜厚度越小;黏度增加,有效相界面积比和液膜厚度也随之增加。较之于黏度对液相分布的影响程度,表面张力的影响程度更大,为主要影响因素。本文还提出一个预测有效相界面积比的新公式,并将数值模拟结果与已有文献进行对比,吻合性较好。     

Correlation between microscopic structural parameters and macroscopic flow behaviour of some polymer-surfactant systems suitable for tertiary oil recovery

Some materials, suitable for application in polymer-surfactant flooding used in tertiary oil recovery (EOR), have been characterized by static and dynamic light scattering and studied in respect to their suitability in various model brines by measuring their rheological and dynamic-rheological behaviour. Xanthan and polyacrylamides of different degree of hydrolysis have been used as polymers, an ethylene glycol sulfonate as surfactant. Correlations have been established between microscopic structural parameters (Hydrodynamic radius, radius of gyration, molar mass, shape of the polymers) and the macroscopic behaviour, such as viscosity and elasticity, being important rheological parameters for EOR application. Additional investigations on surface tension and long term viscosity study lead then to a judgement for the efficiency and long term stability of polymer-surfactant slugs in various reservoirs of different salinity.     

环己烯水合过程汽-液两相流动的CFD研究

环己烯直接水合是制备环己醇的一种新工艺,本文利用体积率函数法（VOF法）建立了环己烯水合流动过程的数学模型,考虑了汽-液相间摩擦力动量源项、表面张力动量源项和多孔介质动量源项,定量描述了汽-液两相逆流流动过程。根据计算流体力学（CFD）模型模拟了黏度、表面张力和流动阻力对流动的影响,结果表明黏度和表面张力影响液膜厚度和液面波动程度,进而影响传质;流动阻力过大会导致液膜断裂,不利于传质。     

界面物性对液滴聚结的影响

推导了分散相液滴运动轨迹模型和分层二相流基本流模型,并优化了计算方法。直接数值模拟结果表明：当层膜厚度和板宽变化时,流场的密度、黏度以及界面张力对界面处的液滴的作用方式不相同;黏度始终对液滴聚结产生明显的影响,而密度和界面张力对聚结的影响却因流场结构的变化而有所不同;密度、黏度及界面张力的变化使得液滴的顺时针或逆时针旋转运动更加具有倾向性。因此,密度和黏度在一定程度上可用于改善流场的聚结条件。数值模拟的结果与已知的实验结果吻合较好,而且,基本流的介入使得该模型能较好地反映液滴在分层二相流流场中的运动特性。     

Wave interactions on a viscous film coating a vertical fibre: Formation of bound states

Fibre coating has attracted considerable attention over the past years due to its engineering applications as well as fundamental interest generated by the fascinating complexity of the resulting flow. A liquid film coating axisymmetrically a vertical fibre and flowing under the action of gravity spontaneously breaks up into a regular drop-like wave train. This instability results primarily from the capillary pressure induced by the azimuthal curvature (Rayleigh–Plateau instability) while the pressure induced by the axial curvature has a stabilising effect. Streamwise viscous diffusion plays a dispersive role that dramatically affects the waves selection, speeds and shapes. When both surface tension and viscosity effects are strong, complex wave interactions lead to the formation of bound states. In this study, we investigate experimentally the details of those interactions and show that regular patterns of bound states can be obtained by external forcing. A qualitative theoretical explanation of the experimental findings is provided with a simple model for the flow.     

Computational fluid dynamic simulations on liquid film behaviors at flooding in an inclined pipe☆

The complex liquid film behaviors at flooding in an inclined pipe were investigated with computational fluid dynamic (CFD) approaches. The liquid film behaviors included the dynamic wave characteristics before flooding and the transition of flow pattern when flooding happened. The influences of the surface tension and liquid viscosity were specially analyzed. Comparisons of the calculated velocity at the onset of flooding with the available experimental results showed a good agreement. The calculations verify that the fluctuation frequency and the liquid film thickness are almost unaffected by the superficial gas velocity until the flooding is triggered due to the Kelvin–Helmholtz instability. When flooding triggered at the superficial liquid velocity larger than 0.15 m·s?1, the interfacial wave developed to slug flow, while it developed to entrainment flow when it was smaller than 0.08 m·s?1. The interfacial waves were more easily torn into tiny droplets with smaller surface tension, eventual y evolving into the mist flow. When the liquid viscosity increases, the liquid film has a thicker holdup with more intensive fluctuations, and more likely developed to the slug flow.     

A STUDY OF TRICKLING-TO-PULSING FLOW TRANSITION IN TRICKLE-BED REACTORS (TBR)

In this work, visual observation as well as the probability density function and the standard deviation of the pressure drop fluctuation were used for determination of the boundary of trickling-to-pulsing flow transition. Extensive experimental work was carried out with air and three liquid media on four kinds of packings. Experimental results indicate the transition depends on the physical properties of liquid (viscosity and surface tension), gas and liquid flow rates as well as the size and shape of packing particles. The Baker coordinates (Baker, 1954) used in the flow map as proposed by Charpentier and Favier (1975) was examined. It is found that application of the flow pattern map in this form is of rather limited universality. Two new correlations were proposed with satisfactory prediction of the liquid flow rates for the transition of trickling-to-pulsing flow.     

气升式内环流反应器内局部气含率径向分布

The local gas holdup profiles in an internal-loop airlift reactor were studied experimentally by using dual electrical conductivity probe under different conditions,including superficial velocity,surface tension and liquid viscosity.The results showed that the radial gas holdup profile has a parabolic shape,which was consistent with the empirical model of Luo.Local gas holdup distribution parameters were obviously influenced by flow regime and almost remained unchanged in the same flow regime.In the gas distributor region,the profiles were steeper in the homogenous flow regime than in the heterogeneous flow regime.However,in the stable region,there was an inverse change trend in two flow regimes.The increase of surface tension,superficial velocity and liquid viscosity made the profile of local gas holdup steeper in two flow regimes.     

Numerical simulation of transient roll-waves in two-phase pipe flow

This paper presents a transient roll-wave simulator based on a one-dimensional incompressible two-fluid model. Using an efficient numerical method and a grid cell length of one pipe diameter, the simulator is able to accurately predict experimental data in the roll-wave regime. Essential to obtaining the stable roll-wave solutions is the use of a modified version of the Biberg friction model. Based on an algebraic eddy viscosity model, the Biberg model yields mechanistic expressions for the wall and interfacial shear stresses in stratified two-phase flow. Input to the model is closure relations for the turbulence levels at either side of the interface. In order to model the increased dissipation of energy in breaking wave fronts, we propose to modify the original closures by adding a term proportional to the negative gradient of the liquid height. As the waves grow and come close to breaking, this term redistributes the shear stresses such that the waves are stabilized. The numerical method used in the simulator is an extension of a previously presented pseudospectral Fourier method. In particular, spectral vanishing viscosity terms are introduced to stabilize short wavelengths inconsistent with the long wavelength approximation of the two-fluid model. The result is a simulator that provides reliable and convergent numerical solutions for all stratified flow conditions. In the last part of the paper, the simulator is tested against roll-wave experiments with horizontal and upward inclined flows of water and a dense gas in a 10 cm pipe. Wave heights and speeds as well as mean pressure drops and holdups are predicted within 20% accuracy when compared with 474 experiments. Furthermore, the transient simulations enable us to study the dynamics and interactions between waves. In agreement with experimental observations, two different wave growth mechanisms are identified, and their influence on the evolution of the roll-wave flow regime is analyzed.     

Simulation and experiments of droplet deformation and orientation in simple shear flow with surfactants

The deformation and orientation behavior of three-dimensional (3D) viscous droplets with and without surfactants is studied in simple shear flow using simulations and experiments. Two added amounts of surfactants are considered, along with a range of viscosity ratios and capillary numbers. The numerical method couples the boundary integral method for interfacial velocity, a second-order Runge-Kutta method for interface evolution, and a finite element method for surfactant concentration. The algorithm assumes a bulk-insoluble, nonionic surfactant, and uses a linear equation of state to model the relationship between the interfacial tension and the surfactant concentration on the drop surface. The algorithm was validated by comparison with other numerical results and good agreement was found. The experiments are performed in a parallel-band apparatus with full optical analysis of the droplet. The simulated and measured 3D steady-state shape of the ellipsoidal drops and their orientation are in reasonably good agreement. It was found that the surfactants have a greater effect on drop geometry for smaller viscosity ratios and that the deformation increases as the transport of surfactant becomes more convection dominated. It was also found that surfactants cause the drops to align more in the flow direction and that, for both clean and surfactant-covered drops, this alignment increases with viscosity ratio. Finally, simulations showed a wider distribution of surfactant on the interface for smaller viscosity ratios.     

Resonance penetration of gas bubbles through a thin liquid layer: a capillary resonator and its use for the generation of droplets

As it is well known, a bounded layer of liquid forms an oscillatory system (resonator) for surface waves. We consider a capillary wave resonator that is composed by a thin liquid layer placed on a substrate and surrounded by a solid ring. The dimensions of the system are chosen so that gravity forces are small compared with forces from surface tension. Standing capillary waves are excited by a gas flow supplied through a small orifice in the substrate. Stable oscillations in various resonator modes are described which are accompanied by ordered formation and destruction of gas bubbles producing the regular streams of identical droplets. The mechanism of self-oscillatory behaviour based on the correspondence between bubble growth time and oscillation period is proposed and analysed. Possible applications of the phenomenon, in particular, for the generation of monodisperse droplets without special periodic stimulation, are discussed.     

溴化锂溶液垂直降膜过程中流动与传热的数值模拟

采用流体体积(VOF)法捕捉自由界面,并应用连续表而力(CSF)模型,建立了垂直降膜波动模型,研究了溴化锂溶液垂直降膜过程中的波动过程,根据波动模型得到的界面信息,建立了以波速为动坐标系的固定界面传热模型,同时考察了波动对传热的影响,并建立了垂直降膜波动模型和固定界面传热模型.结果表明:一定入口频率的扰动发展为孤波和毛...     

CFD-PBM耦合模型模拟气液鼓泡床的通用性研究

通过对不同操作压力和不同液体性质气液鼓泡床的模拟值与实验数据进行对比，从而验证CFD-PBM耦合模型的通用性。结果表明，CFD-PBM耦合模型在加入了气泡破碎修正因子后，可以很好地预测压力对鼓泡床流体力学行为的影响趋势，当压力升高时，气含率显著升高。不同液体黏度和表面张力条件下CFD-PBM耦合模型的模拟结果与实验结果均吻合较好。随液体黏度增大，气泡破碎速率减小，气泡尺寸分布变宽，曳力显著下降，气含率随之降低。随表面张力减小，气泡破碎速率增大，气泡变小，气含率升高。CFD-PBM耦合模型具有很好的通用性，原因在于考虑了压力、液体黏度和表面张力对气泡聚并、破碎和气液相间作用力的影响。     

Breakup dynamics for high‐viscosity droplet formation in a flow‐focusing device: Symmetrical and asymmetrical ruptures

The breakup mechanism of high‐viscosity thread for droplet formation in a flow‐focusing device is investigated using a high‐speed digital camera. Aqueous solution of 89.5%‐glycerol is used as the dispersed phase, while silicone oil as the continuous phase. The breakup process of the dispersed thread presents two categories: symmetrical rupture and asymmetrical rupture. Furthermore, the rupture behavior could be divided into two stages: the squeezing stage controlled by the squeezing pressure and the pinch‐off stage controlled by viscous stresses of both phases and surface tension. Specifically, it suggests that the differences in the shape of the liquid–liquid interface and the dynamics in the two breakup processes are caused by the disparity of the strain field at the point of detachment. Moreover, the thinning rate and the dynamics of the dispersed thread change with the viscosity of the continuous phase, but are less dependent of the flow rate of the continuous phase. © 2015 American Institute of Chemical Engineers AIChE J, 62: 325–337, 2016     

高炉焦炭层区渣、铁滞留特性的冷态模拟

为阐明高炉下部熔融物的滞留特性,对填料床内液体的滞留量进行了冷态模拟实验研究,考察了液体的粘度、密度和表面张力、填料的粒度和形状及液体的流速等影响因素. 结果表明,液体的粘度越大、表面张力越大、密度越小,则静态滞留量hs越大. 它们的影响程度为密度>表面张力>粘度. 填料的粒度、形状系数和孔隙度越小,则hs越大. 液体流量增加时,hs大的固液组合总滞留量ht仍然较大,因此影响hs的各种因素也是影响动态滞留量hd的主要因素. 得到了无气体流动条件下的hs和hd及气液逆流条件下载点至泛点间ht的计算式,计算结果与实验数据吻合较好. 对于实际过程,不考虑煤气流影响时,高炉内熔融物滞留量的大小由hs决定,焦炭粒度对hs的影响最大.     

波纹板表面流体流动特性的数值模拟

利用计算流体动力学CFD商业软件FLUENT,以多相流模型中的VOF模拟为基础,建立了波纹板式的计算模型,考察了液体流速、板面结构、液体物性对液膜流动的影响。模拟结果表明,在一定范围内,随着流速的增加,液膜的厚度和流动时间都减小;波纹板结构直接影响液膜流动,凹槽较浅的波纹板有利于连续液膜的产生,波谷滞留液内发生液体回流形成漩涡;液体的表面张力直接影响连续液膜的产生和断裂,表面张力、黏度、流量共同决定了液膜的厚度和相位差。     

Hydrodynamics and mass transfer in bubble column: Influence of liquid phase surface tension

According to literature, few experiments are performed in organic solvents which are mostly used in commercial gas-liquid reactors. However, it is commonly accepted that data obtained in aqueous solution allow to predict the surface tension effects, and to model the behaviour of organic solvents. In this work, we examine the validity of this approximation.In this objective, the flows observed in two pure media having similar viscosity but different surface tension—respectively, water (reference) and cyclohexane (solvent)—are successively compared at two scales: in a bubble column and in bubble plumes.In bubble plumes, as expected, the mean bubble size is smaller in the medium having the smallest surface tension (cyclohexane), but for this medium the destabilisation of flow is observed to occur at smaller gas velocity, due to break-up and coalescence phenomena. In bubble column, these phenomena induce the bubbling transition regime at lower gas velocity, whatever the operating conditions for liquid phase: batch or continuous. Consequently, when the two media are used at similar gas superficial velocity, but in different hydrodynamic regimes, greater gas hold-up and smaller bubble diameter can be observed in water; the interfacial area is then not always higher in cyclohexane.This result differs from the behaviour observed in the literature for aqueous solutions. The analysis of bubble plumes in aqueous solutions of butanol shows that this difference is due to a fundamental difference in coalescent behaviour between pure solvents and aqueous mixtures: the surface tension effect is less important in pure liquid than in aqueous solutions, because of the specific behaviour of surfactants.It is then still difficult to predict a priori the bubbling regime or the flow characteristics for a given medium, and all the more to choose an appropriate liquid as a model for industrial solvents.