伴有Rayleigh对流的气液传质理论

浓度梯度导致的Rayleigh对流会对界面传质产生重要影响。采用格子Boltzmann方法(LBM)对吸收过程伴有Rayleigh对流的气液传质理论进行了分析。针对表面更新理论,提出了将近界面处羽状对流结构的宽度作为特征尺度来求得表面更新时间。结果表明,Rayleigh对流的发生能够有效减小双膜理论中的液膜厚度;液相传质系数的LBM模拟值与表面更新理论预测值吻合较好。     

Two-dimensional numerical simulations of Marangoni-Bénard instabilities during liquid-liquid mass transfer in a vertical gap

Two-dimensional (2D) simulations of isothermal liquid-liquid mass transfer subject to surface tension- and buoyancy-driven hydrodynamic instabilities have been carried out. Simulation is based on the numerical solution of 2D equations of momentum and mass transport, using a combination of finite difference and finite volume methods. Two different cases have been considered: (1) buoyancy stable mass transfer, only surface tension-driven convection occurs; (2) surface tension-driven instability superseeded by buoyant convection. The faster attenuation of mass transfer coefficients in buoyancy stable situations is attributed to the merging of convection cells leading to a reduction in the number of renewal zones along the interface. Concentration profiles next to the interface reveal the diffusional nature of the mass transfer in the immediate interfacial neighbourhood.     

界面传质中Rayleigh对流的格子Boltzmann模拟和实验验证(英文)

Concentration gradient induced Rayleigh convection can influence effectively interfacial mass transfer processes, but the convection phenomena are known as mesoscopic and complex. In order to investigate this phe-nomenon, a two-equation Lattice Boltzmann Method (LBM) is proposed to simulate the velocity and the concentra-tion distributions of Rayleigh convection generated in the CO2 absorption into ethanol liquid. The simulated results on velocity distributions are experimentally verified by PIV (particle image velocimetry technique) measurements. In order to simplify the analysis, the convection in the simulation as well as in the experiment, the Rayleigh convec-tion was manipulated into a single down flow pattern. The simulated results show that the concentration contours agree qualitatively with the schlieren images in the literature. The experimental and simulated results show that the Rayleigh convection under investigation is dominated by the flow in the downward direction and impels exchange of the liquid between the interfacial vicinity and the liquid bulk promoting the renewal of interfacial liquid, and hence enhances mass transfer. The comparison between the simulated and experimental results demonstrated that the proposed LBM is a promising alternative for simulating mass transfer induced Rayleigh convection.     

采用格子Boltzmann方法模拟解吸过程中的Rayleigh对流

应用二维非稳态格子Boltzmann方法研究了异丙醇-水溶液和丙酮-乙酸乙酯溶液解吸过程中Rayleigh对流的临界开始时间、流动特征及其对界面传质的影响,并与相关文献对比. 结果表明,临界开始时间随界面浓度增加呈先缓慢增大再迅速增大最后趋于稳定的变化趋势. Rayleigh对流结构经历了从有序到无序的发展过程,是不断更新的耗散结构. Rayleigh对流主要作用于液相主体,使液相主体具有较大的湍动速度(10-4~10-3 m/s). 液相主体中存在许多循环流动,促进了界面更新及界面与液相主体之间液体的交换与混合. 传质增强因子(介于2~6之间)表明Rayleigh对流能有效提高解吸过程传质速率,强化界面传质过程.     

PIV Measurement for Rayleigh Convection and Its Effect on Mass Transfer☆

The velocity distribution in Rayleigh convection caused by acetone volatilization in acetone-ethyl acetate binary system was observed in a vertical cross section of an initially quiescent liquid layer ...     

吸收过程自然对流非稳态数值模拟

传质过程中液相密度变化导致的自然对流,改变了液相流动状态,对传质有着较大的影响。利用纹影仪光学方法,获得了乙醇吸收CO2过程中自然对流的纹影图像;并通过吸收传质流体CFD模型,应用有限元方法对气液吸收过程中出现的自然对流发生过程进行了数值计算,得到了液相自然对流发生时的非稳态流场分布和浓度分布信息。分析结果表明传质过程中当自然对流发生后,促进了液相表面更新,加强了传质效果,在考察相际传质过程时,应充分考虑自然对流对传质速率的影响。     

Transient rise velocity and mass transfer of a single drop with interfacial instabilities - experimental investigations

An experimental study of transient drop rise velocities and mass transfer rates was carried out in the system toluene/acetone/water which is known to show interfacial instabilities. The rise velocity of toluene drops was studied without added solute (acetone) in the diameter range 1-3 mm and with added solute for 2 mm drops. The initial concentration of the transferred solute was varied from 0 to 30 g/L. The transient drop rise velocities were used to quantify the Marangoni effect since the drag coefficient depends on the strength of the Marangoni convection patterns caused by interfacial tension gradients. In addition, mass transfer measurements were carried out in order to determine the modification of the mass transfer rate due to Marangoni convection. Velocity and mass transfer measurements were then correlated via the contact time. Results reveal the existence of a range in which a critical value for the solute concentration can be defined for Marangoni convection.     

Effect of interfacial instabilities and hydrodynamic interaction on liquid-liquid mass transfer

Mass transfer rate between two liquid phases with a plane interface has been studied in a modified Lewis stirred cell. The time interval between the successive concentration determinations was reduced to thirty seconds by the use of a continuous flow refractometer. The mass transfer coefficients were found to correlate satisfactorily with the surface renewal model unless interfacial motion is opposed by the change in physical properties of the system or interfacial shear stresses. Under these conditions Levich-Davies model could successfully explain the experimental mass transfer coefficients found in this work.Due to the effect of the shear stresses and the changes in the physical properties, bulk Reynolds numbers were not sufficient alone to explain the hydrodynamics at the interface which control the mass transfer rate. Instead, another dimensionless quantity such as the ratio of interfacial stresses to kinetic energies of the eddies should be used as a criterion of the hydrodynamics at the interface.     

气液传质过程中Marangoni对流的观测与定量分析

建立了一套气液传质装置和一套纹影系统对乙醇解吸CO₂过程中产生的Marangoni对流结构进行垂直界面方向上的观察,发现解吸过程中近界面处出现均匀分布的旋涡状的对流结构,并随时间聚合发展变大。此外还利用定量纹影法考察了传质过程中的浓度分布和界面张力梯度等信息。定量的结果表明较大的界面张力梯度位于对流胞型的边缘和中心处,驱动着界面流体的运动,并耦合浮力效应,形成了近界面环流的运动形式。这种环流运动造成了对流胞型边缘浓度较大、内部浓度小的分布,促进了相界面流体微元的更新,加强了传质过程。     

气液界面对流传质的观测与定量分析

采用光学纹影系统对乙醇和水双组分解吸传质过程的对流结构的界面湍动进行了定性观察和定量分析。建立了一套水平非稳态气液传质设备,试验观测了乙醇和水体系中液相组分向气相传质过程的Marangoni界面对流结构。还通过对传统纹影方法的改进,对乙醇解吸传质过程的浓度(本文用质量分数表示)梯度场进行了定量测量。定量分析表明乙醇和水系统解吸过程中,引发界面湍动的原因是局部较大的表面张力梯度。定量分析的结果很好的解释了伴随Marangoni效应的传质过程的混乱的对流结构,为进一步对界面湍动现象的分析提供了帮助。     

气液降膜流动中液相速度波动及其传质研究

为了研究降膜流动的动力学性质及其对气液传质过程的影响,在气液逆流的不同气液流动条件下采用激光多普勒(1aser Doppler anemometer,简称LDA)测量了降膜流动的液相速度分布和瞬时速度波动.和以往假定液膜外侧为自由表面,液膜表面处剪切力为零的Nusselt模型进行了比较,LDA测量结果表明气液逆流时降膜流动的最大液相速度出现在液膜表面之内,并且是以近界面区域的速度波动为特征的流动.在相同的降膜装置中进行了乙醇稀溶液的解吸实验,液相传质系数的实验测量值是渗透理论预测值的1～2倍.实验结果表明液相界面区域的速度波动加快了气液界面的表面更新速率,减小了传质阻力,强化了气液界面的传质过程.考虑液膜波动特征对气液接触情况的影响,从气液两相接触时间的角度出发,修正了渗透理论对液相平均传质系数的预测,预测结果和实验结果相吻合.     

Spatial Scale Effects on Rayleigh Convection and Interfacial Mass Transfer Characteristics in CO2 Absorption

Concentration gradient‐induced Rayleigh convections in the CO₂ absorption process were investigated by the hybrid Lattice‐Boltzmann/finite‐difference method (LBM‐FDM). The spatial scale effects on Rayleigh convection were studied by simulating Rayleigh convections with different liquid layers. The scale of convection increased with the liquid layer height but the average mass transfer coefficient showed the adverse tendency. The Rayleigh convection had a pronounced effect on the surface renewal. For better assessment of the renewal intensity, two statistical quantities were proposed. The transient variations of these quantities provided a good following feature with the mass transfer coefficient which confirms their accuracy and precision in characterizing the mass transfer process.     

Stability Modeling of Water-Based Surfactant Covered Micro-bubble Fluids

Currently, due to the decrease in easy access to crude oil reservoirs, oil and gas industries have focused on production from heavy oil and depleted reservoirs. In recent years, micro‐bubble fluids with surfactant and polymer layers around the bubbles are investigated as a part of drilling fluids and their positive effect on the formation damage is proven. Stability of the bubbles in the fluid is very important, and the optimum surfactant and polymer types should be chosen at optimum concentrations. In this work, an attempt is made to analyze the stability of bubbles of the drilling fluid, based on the diffusive mass transfer concept. Mass transfer and interfacial mass transfer coefficients become more important when surfactant concentration gradient exists in micro‐bubble layers. Interfacial mass transfer coefficients have an important effect on mass transfer phenomena in Aphron fluids system; so, the precise selection of these coefficients results in the conformity of modeling and experiments. We can say that reducing the mass transfer rate from bubble layers will result in stable bubbles in the fluid and, thus, the efficiency of the fluid during drilling will not decrease. It is shown that the interfacial mass transfer coefficient decreases with an increase in surfactant concentration.     

VIBRATION TECHNOLOGY IN FOOD INDUSTRY

ABSTRACT

This work present. a detailed study of heat and mass transport phenomena between a large sublimable object and a gas-solid fluidized bed of either inert or adsorbent fine panicles. Heat (h) and mass (k)transfer coefficients are related to the size and adsorption capacity of the fine particles, as well as to the diameter of the spherical immersed object and to the bed temperature. Convective and conductive components are also identified, and correlations are proposed to predict all of them. The significant differences which appear for the various working conditions may be accounted for considering the particular mechanisms that prevail in every case. A simple "surface renewal" model clearly shows the leading role of the object surface temperature. For adsorbent panicles, a local overheating phenomenon at the exchange surface is demonstrated.     

Studies of drop break-up in liquid-liquid systems in a rotating disc contactor. Part II: Effects of mass transfer and scale-up

The number fraction of drops of a given size which break up at rotor level in a rotating disc contactor has been observed during mass transfer in either direction to or from solvent or aqueous drops. Critical rotor speeds for a given drop size undergoing mass transfer can be used to find an effective interfacial tension. Using this interfacial tension value, the break-up fractions are correlated within experimental uncertainties in the same manner as for no mass transfer. Drop break-up fractions depend on column size and relevant empirical correlations of the data are presented. The results may be used to estimate the effect of mass transfer on drop size distributions in an RDC.     

Modelling mass transfer coefficient for liquid-liquid extraction with the interface adsorption of hydroxyl ions

A combined model was used for prediction of overall mass transfer coefficient of drops in the liquid-liquid extraction process, prone to the deleterious effect of adsorbed hydroxyl ions onto the interface. The importance is due to the use of different pH waters as aqueous phase. The work is based on single drop experiments with a chemical system of toluene-acetone-water where the pH of the continuous aqueous phase was within the range 5.5–8, appropriate to most industrial waters, and can lead to rigid behavior of circulating drops. The combined model in conjunction with the correlation developed here for the ratio of interfacial velocity to drop terminal velocity that links the film mass transfer coefficients of both sides can be used satisfactorily for design purposes. This model gives a maximum relative deviation of less than ±10% for the mass transfer directions of dispersed to continuous phase and vice versa.     

INFLUENCES OF INTERFACIAL CONVECTION AND PHYSICAL PROPERTIES OF DESICCANT SOLUTIONS ON THE MASS TRANSFER PERFORMANCE OF ABSORPTION SYSTEMS

In order to investigate the mass transfer phenomenon of the absorption of water vapor in desiccant solutions, a system of absorption cell was designed. The gradient in surface tension can result from the contact process between the desiccant solution and water vapor. This induced interfacial disturbance will affect the mass transfer performance. The experimental results demonstrate that the theoretical mass transfer coefficients predicted by boundary layer theory are less than those of the experimental values. In addition, the mass transfer performance of the absorption system can be enhanced by adding ethanol to the absorbent solutions. In order to describe the level of mass transfer performance affected by the interfacial convection resulting from the addition of ethanol, the enhancement factor was defined in this study. The enhancement factor is from 1.26 to 1.56 for adding ethanol to DEG (diethylene glycol) solution, and from 1.36 to 1.61 for adding ethanol to TEG (triethylene glycol) solution.     

单滴内的传质Ⅰ

本文应用Colburn-Welsh方法研究了正丁醇和异丁醇向下降的环流水滴的传质速率。实验结果经校正端效应后,曾与Kronig-Brink理论值以及Johnson和Heertjes等的数据作了比较。 实验结果表明,Kronig-Brink模型仅可适用于Re<50或60的情况,而在Re>80完全不能适用。实验结果和Johnson的单滴数据甚为符合,但较Heertjes的喷洒塔数据为高。异丁醇-水系的数据一般较正丁醇-水系的略高,但从光纹技术观察,两个系统都无可察见的介面骚动现象。     

Drop formation mass transfer coefficients in extraction columns

Mass transfer coefficients (MTC) of single liquid drops during drop formation period, in the presence and absence of down flow of the continuous phase, were measured in an extraction column. The effects of formation time, needle size, and flow rates of the continuous and dispersed phase were evaluated experimentally. It was found that the drop size increases with increasing formation time and decreasing down flow of the continuous phase. The mass transfer coefficients are the largest in the initial stages of drop formation when convection is the most significant. Both flow rates have a significant effect on the rate of the mass transfer, and the convection caused by the dispersed phase flow is more important than the continuous phase. The mass transfer coefficient and the degree of extraction increase with increasing down flow rate of the continuous phase.     

界面Rayleigh对流现象模拟的格子Boltzmann方法

采用格子Boltzmann方法中带有双分布函数的LBGK模型,在介观尺度上实现了对气液传质过程中界面对流现象的模拟,比较了不同Sc数、Ra数以及有局部浓度扰动的界面溶质扩散,对发生界面Rayleigh对流的临界条件进行了研究,发现临界Ra数在1.0×104—2.0×104之间。模拟结果和已有实验现象一致,证明双分布的LBGK模型适用于界面对流及扩散现象的模拟。