Chemically reactive solute transfer in a boundary layer slip flow along a stretching cylinder

This paper presents the distribution of a solute undergoing a first order chemical reaction in an axisymmetric laminar boundary layer flow along a stretching cylinder. Velocity slip condition at the boundary is used instead of no-slip condition. Similarity transformations are used to convert the partial differential equations corresponding to momentum and concentration into highly nonlinear ordinary differential equations. Numerical solutions of these equations are obtained by the shooting method. The velocity decreases with increasing slip parameter. The skin friction as well as the mass transfer rate at the surface is larger for a cylinder than for a flat plate.     

Multiphase mass transport with partitioning and inter-phase transport in porous media

We derive the effective mass-transfer coefficient between two fluid phases in a porous medium, one of which is flowing and the other is immobile. A passive tracer is advected by the flowing phase, becomes partitioned at the fluid-fluid interface and diffuses in the immobile phase. We use traditional volume-averaging methods to obtain a unit-cell boundary-value problem for the calculation of the effective mass-transfer coefficient. The problem is controlled by the Peclet number of the flowing phase, by a second dimensionless parameter that captures diffusion and partition in the two phases and by the geometrical properties of the porous medium.We derive asymptotic results for the scaling of the mass-transfer coefficient under various limiting conditions. Then, we use numerical methods that solve for the flow velocity field under Stokes flow conditions, and for the transport problem. The numerical results verify the asymptotic scaling expressions and provide estimates of the coefficient for a number of special cases. In particular, we find that when the immobile phase is wetting the solid (in the form of films), the mass transfer coefficient is larger than in the non-wetting case (where the phase is distributed in the form of blobs). Shape factors for practical applications are also obtained.     

On MHD heat and mass transfer over a moving vertical plate with a convective surface boundary condition

In this paper, the magnetohydrodynamics (MHD) boundary layer flow with heat and mass transfer over a moving vertical plate in the presence of magnetic field and a convective heat exchange at the surface with the surrounding has been studied. The similarity solution is used to transform the system of partial differential equations, describing the problem under consideration, into a boundary value problem of coupled ordinary differential equations, and an efficient numerical technique is implemented to solve the reduced system. The results are presented graphically and the conclusion is drawn that the flow field and other quantities of physical interest are significantly influenced by these parameters.     

Effects of higher order chemical reaction on micropolar fluid flow on a power law permeable stretched sheet with variable concentration in a porous medium

An analysis has been carried out to obtain the effects of higher order chemical reaction on flow and mass transfer characteristics of micropolar fluids past a nonlinear permeable stretching sheet immersed in a porous medium with variable concentration of the reactant. The local similarity solutions for the flow, microrotation and mass transfer are obtained numerically and are illustrated graphically for various material parameters. Comparisons carried out with results from previously published work present excellent agreement. The results show that rate of mass transfer from the sheet to the surrounding fluid decreases significantly with the increase of nonlinear stretching parameter and increases with the increase of concentration parameter. The mass transfer of the reactive species strongly depends on the reaction rate parameter as well as order of the chemical reaction. It is stronger for the first‐order reaction than that for the higher order reaction. The rate of surface mass transfer decreases with the increase of the Darcy parameter. The results also show that the effect of these parameters on the micropolar fluids are less compared to the Newtonian fluids.     

Electro-kinetically enhanced mass transport of charged macro-solutes through a microchannel with porous walls

Electrokinetics of the solute transport across the porous walls of micro channel is important from its practical application but less explored. Transport of the charged macro-solutes across perm-selective walls in a microchannel is investigated. The extended Nernst–Planck equation describes the charged macro-solutes distribution in the mass transfer boundary layer over the porous wall. The transverse electromigration of the charged macro-solute either augments or suppresses the concentration polarization and the permeation rate depending on the wall and solute surface potential (attractive or repelling). The wall potential is screened due to the electrical double layer interaction of the wall and charged solute. It is observed that the charged solute concentration over the channel wall enhances by two times in case of oppositely charged interactions (unlike solute and channel wall) compared to like charges. The findings of this study can facilitate understanding of electrokinetic based drug delivery and separation systems involving charged solutes.     

Hydrodynamics,mass transfer,and photocatalytic phenol selective oxidation reaction kinetics in a fixed TiO2 microreactor

Photocatalytic phenol dissociation was studied in a microreactor, with a TiO₂ layer immobilized on the reactor inner walls. Experiments were conducted for various residence times, initial concentrations, pH values, and UV light irradiation intensities. The intermediates and products (catechol, hydroquinone, and resorcinol) were quantitatively investigated to determine the predominant reaction pathways for the investigated anatase catalyst. A three‐dimensional mathematical model was used to simulate the heterogeneous photocatalysis reaction conditions with Langmuir–Hinshelwood mechanism, considering the adsorption/desorption thermodynamic equilibria, and for kinetic parameter estimation via regression analysis. The effectiveness factor, Thiele modulus, and the correction function were calculated to determine the pore diffusion effects. The value of pH had the dramatic effect of lowering the reaction rate due to the competitive adsorption of hydroxide ions and protons on the catalyst surface. A phenol conversion of 79.5% was achieved at the residence time of 7.22 min, but without total mineralization. © 2014 American Institute of Chemical Engineers AIChE J, 61: 572–581, 2015     

具有预制孔隙多孔介质冷冻干燥的多相传递模型

基于局部质量非平衡假设,建立了多相多孔介质热、质耦合传递数学模型,理论验证具有预制孔隙的初始非饱和多孔物料对冷冻干燥过程的强化作用。模型考虑了多孔介质的吸湿效应,构建了3种吸附-解吸平衡关系。模型使用基于有限元法的COMSOL Multiphysics软件平台数值求解,并与实验数据进行了比较。结果表明,初始非饱和冷冻物料能够有效地强化冷冻干燥过程。采用不同函数形式的吸附-解吸平衡关系模拟的干燥曲线均与实验数据非常吻合。通过分析物料内部的饱和度、温度和质量源分布,探讨了初始非饱和物料冷冻干燥过程的传热传质机理。初始非饱和物料的干燥速率控制因素主要是传热。模拟考察环境辐射温度对冷冻干燥过程影响的结果表明,所建模型具有良好的预测能力。     

Elucidating two-phase transport in a polymer electrolyte fuel cell, Part 1: Characterizing flow regimes with a dimensionless group

This paper explores the through-/in-plane characteristics of water transport in the cathode gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC). Theoretical analysis is performed on the non-isothermal two-phase flow under flow channels. A dimensionless group Da (Damkohler number for PEFC operation), defined as the ratio of water generation rate to water vapor-phase removal rate, is formulated to characterize the flow regimes in a PEFC. This group, lumping geometrical parameters and physical properties, compares the water vapor-phase removal capability (via water diffusion and holding capacity) with the rate of water production by the oxygen reduction reaction. We find that this dimensionless group can be used to characterize the non-isothermal, two-phase phenomena: when Da→0, the fuel cell is subjected to single-phase operation; while as Da→∞ we have full two-phase operation. A more precise expression is explored for the dimensionless group at the channel central line, i.e. Da₀: when Da₀>1 the entire cathode GDL–CL (catalyst layer) interface is in two-phase region, whereas part of the interface is free of liquid water for Da₀<1. The latter scenario is the concept that this paper proposes for improving fuel cell water management: the consequent co-occurrence of single- and two-phase flows in the in-plane direction at Da₀<1 is beneficial to avoid severe dryout and flooding. A two-phase transport model, describing the water and heat transport on the PEFC cathode side, is employed to perform a two-dimensional numerical study. Detailed liquid and temperature distributions are displayed. Simulation predictions are in reasonably good agreement with the dimensionless-group analysis.     

几何结构对滤压式电解槽内部传质的影响

Effect of configuration （structure of electrode, interelectrode gap, positions of inlet and outlet, volume of the cell and additional nets） on mass transfer characteristic of a filter-press type electrochemical cell has been studied. The mass transfer coefficients on the electrodes were obtained by using the well-known technique based on the determination of limiting diffusion current. It is found that mass transfer coefficients with mesh electrode are greater than that of with plate electrode. Mass transfer coefficient is decreased with interelectrode gap. While interelectrode gap achieved a certain value （7 ram）, mass transfer coefficient is steady, no more declining. Mass transfer characteristic for different positions of inlet and outlet are different and dimensionless number groups correlated equations are obtained by experiment. Mass transfer characteristic is the best when inlet located on the top and outlet on the bottom of the cell respectively. While magnified the volume of the cell to eight times, mass transfer characteristic changes little. Mass transfer characteristic without nets is lower than that of with additional nets in the exit region, but higher than that of with additional nets in the entry region.     

Numerical prediction of wall mass transfer rates in turbulent flow through a 90° two-dimensional bend

The present paper reports a numerical study of mass transfer in a curved channel flow using a low Reynolds number k–ε model modified to include the influence of curvature on the turbulent transport. Predictions were obtained of flow through a 90° 2-D bend for Reynolds numbers of Re = 2 × 10⁴, 10⁵ and 10⁶, curvature ratio's of R_c/d = 1, 1.5 and 3, and Schmidt numbers of Sc = 500, 1 500 and 5 000. Based on the numerial simulations, the value of the mass transfer coefficient, k, was observed to generally decrease and increase along the inner and outer walls, respectively, compared to the value in a straight duct. The behaviour appears to be sensitive to both the curvature ratio and Reynolds number, being most pronounced for the case of tight bends and low Reynolds numbers.     

Numerical investigation of mass transfer enhancement through a porous body affected by electric field

The use of electrohydrodynamic (EHD) actuator has emerged recently as an effective mean to enhance mass transfer. However, the effect of nonthermal EHD-induced mass transfer in such porous body has remained unclear. Hence, in this paper, the mass transfer enhancement with EHD technique in a porous body is numerically investigated using the finite volume method. The flow patterns and the moisture content have been studied for different Reynolds numbers and the applied voltages. The numerical results show that the EHD flow field amplifies the mass transfer by a factor of 20.5 for Re?=?200 and 2.12 for the Re?=?4,000 and V?=?28?kV in comparison to the case without the electric field.     

Modeling of CO2 mass transport across a hollow fiber membrane reactor filled with immobilized enzyme

The enzyme‐based contained liquid membrane reactor to capture CO₂ from the closed spaces is a very complicated process with large numbers of interdependent variables. A theoretical and experimental analysis of facilitated transport of CO₂ across a hollow fiber membrane reactor filled with immobilized carbonic anhydrase (CA) by nanocomposite hydrogel was presented. CO₂ concentration profiles in the feed gas phase and the membrane wall were achieved by numeric simulation. The effects of CO₂ concentration, CA concentration, and flow rate of feed gas on CO₂ removal performance were studied in detail, and the model solution agrees with the experimental data with a maximum deviation of up to 18.7%. Moreover, the effect of CO₂ concentration on the required membrane areas for the same CO₂ removal target (1 kg/day) was also investigated. This could provide real‐world data and scientific basis for future development toward a final efficient CO₂ removal device. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

非热平衡多孔介质内反应与传热传质耦合过程

采用局部热不平衡假设,对发生强吸热化学反应的多孔介质体系建立了反应与传热、传质耦合问题的数学模型,采用Ergun-Forchheimer-Brinkman方程描述多孔介质中的流体流动.运用交替方向隐式(ADI)方法对模型离散求解,并采用文献中的实验数据对模型进行验证.计算了不同条件下颗粒物料层内气体和固体骨架的温度场、产物气体浓度场以及固体转化率分布,以得到多孔介质体系内固有化学反应时的传热、传质规律.结果表明,不能忽略固体骨架与流体间的温度差.入口渗流速度、入口气体温度以及固体颗粒尺寸是影响系统反应特性的重要参数.研究结果对具有强吸热反应的固定床反应器的设计和运行具有一定的参考作用.     

Modelling of turbulent flow and mass transfer with wall function and low-reynolds number closures

A recent laser doppler anemometer study, by other researchers, of turbulent flow through a sudden expansion has been simulated using a kinetic energy of turbulence-dissipation rate of turbulence (k - ?) model. The near wall region was modelled in two different ways; using wall functions (WF), and a low-Reynolds number (LRN) formulation. The reattachment length under particular flow conditions was obtained. Radial profiles for the mean axial and radial velocity, turbulence kinetic energy and Reynolds shear stress were determined for six locations downstream of the sudden expansion. Both models performed reasonably well in predicting the flow with the LRN approach performing slightly better than the WF approach near to the wall. The application of the LRN approach to the calculation of local mass transfer rates in wall bounded complex turbulent flows is demonstrated.     

Effect of ring promoter on ionic mass transfer in a fluidized bed

Mass transfer data were obtained at the wall of a batch fluidized bed in the presence of a ring promoter used for the reduction of ferricyanide ion. The ring promoter was a copper rod on which rings of uniform size were fixed at equal distances. In these studies, the cross‐sectional diameter of the ring, the spacing between the rings, the particle size and the void fraction were varied and the limiting current data were measured. It was observed that the mass transfer coefficients were augmented with increased particle diameter, decreased spacing between the rings, and increased cross‐sectional diameter of the ring. Copyright © 2003 Society of Chemical Industry     

微孔扩散器在气液传质过程中的研究进展

阐述了微孔扩散器的定义及气泡传质理论,介绍了微孔扩散器在曝气、臭氧氧化、气浮、塔板、纳米材料制备等方面的研究现状和发展趋势。与其他技术相比较,以微孔介质为核心的微孔扩散技术具有混合、传质性能好,能耗低,操作简单,利于工程放大等优点。     

Bridging the gap between Graetz's and lévêque's analyses for mass/heat transfer in a channel with uniform concentration or flux at the wall

A new approximate solution which bridges the gap between the classical theories of Graetz and Lévêque for heat/mass transfer in channel flow is presented. The results include expressions, uniformly valid in the axial direction, for the mixing‐cup concentration (or temperature) profile 〈c〉 when transport towards the wall is slow (Dirichlet limit), and for the Sherwood number Sh when the wall flux can be considered uniform (Neumann limit). The technique employed provides insight into the mathematical structure of both quantities 〈c〉 (or conversion X_R) and Sh identifying explicitly the contributions from fully developed and developing behaviors, while maintaining accuracy in the transition region. Criteria to bound the different convection‐diffusion regimes are suggested, which critically systematize previous results. These results are important for model selection in the design and simulation, among others, of heat exchangers and wall‐coated microreactors where fast heterogeneous reactions occur. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1880–1892, 2012     

气液液三相体系中的气液传质特征

选取CO2-K2CO3/KHCO3为吸收体系,次氯酸钠为催化剂,甲苯、异戊醇为第2液相,应用Danckwerts图来同时确定液侧传质系数KL和界面面积a,通过实验研究了分散第2液相的加入对气液传质的影响。实验结果表明,随着分散相体积分数φ(1%—10%)的增大,或分散相形成的液滴直径的减小,以及传质组分在分散相和连续相中溶解度的比值m(即传质组分在实验体系的分配系数)增加,或在二相间的相对扩散系数增加时,可显著增强气液传质,为气液液三相体系的系统化研究提供了实验依据。     

Effect of circulation rate on mass transfer in a forming drop

The prediction of mass transfer rate in an internally circulating drop forming in a continuous gaseous medium depends upon the residence time of a liquid element on the drop surface. A theoretical analysis has been made for the estimation of this contact time. The experimental results on comparison with the model show that the mass transfer rate is inversely proportional to the square-root of the contact time as conceived by a modified penetration theory.     

Effect of swelling on mass transfer performance in membrane extraction

The membrane extraction experiments were performed with tributyl phosphate/acetic acid (HAc)/water, n-butanol/HAc/water and 20% Alamine (in kerosene); HAc; and water as working systems. HAc was transferred from the aqueous phase to the organic phase. The effect of flat membranes swelling on mass transfer was studied. The membranes were microporous polysulfone and microporous polytetrafluoroethylene. The overall mass transfer coefficients based on the water phase were calculated and compared between nonswollen and swollen membranes. The experimental results show that the physical structure of the flat membranes used in our experiments was changed if soaked by organic solvents; however, change in thickness was not found. The overall mass transfer coefficients clearly were decreased after the flat membranes were swollen. The most likely reason is that the mass transfer resistance was increased because of the change of the membrane structure. The results also show that it is better to choose a hydrophilic membrane to reach high mass transfer performance when the equilibrium constant is very low. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1555–1561, 1999