Conjugate heat and mass transfer in membrane-formed channels in all entry regions

Membrane-based energy recovery ventilators (or total heat exchangers) are key equipments to fresh air ventilation, which is helpful for the control of respiratory diseases like Swine flu (H1N1) and SARS. Parallel-plates narrow channels are common structure for membrane-based energy recovery ventilators. In practice, the exchanger channel lengths are limited due to the confinement in pressure drops and noises. In these channels, the hydraulically, thermally and concentrationally entry regions account for a large fraction of the total duct length. However, previous investigations neglected the entry issues for simplicity. Either hydraulically fully developed, or thermally or/and concentrationally fully developed flow were assumed, which would underestimate equipments performances seriously. This study provides a more accurate methodology: fluid flow, heat and mass transport equations were solved directly as they enter into the channel. In other words, both the fluid flow and the heat and mass transport are in simultaneously developing regions. The membrane and the two neighboring flows are considered as a conjugate problem. The conjugate heat transfer problem is solved with a commercial CFD code. Then the conjugate mass transfer problem is solved by transferring it to another conjugate heat transfer problem by heat mass analogy. The Nusselt and Sherwood numbers in the entry regions are calculated. The effects of three typical flow arrangements: cocurrent, counter and cross flow, on the boundary conditions and the consequent Nusselt and Sherwood numbers in the channels are evaluated.     

Analytical mass transfer solution of longitudinal laminar flow of Happel’s free surface model

The mass transfer problem of longitudinal laminar flow of the Happel’s free surface model (HFSM) is studied under constant wall flux and constant wall concentration boundary conditions. For different shell void fractions, the analytical solution of Sherwood’s number in fully developed region, the strict Graetz analytical solution of Sherwood’s number in the developing region and approximate Leveque solution are obtained. Furthermore, the expression of local Sherwood’s number for solving the whole range of shell void fraction is obtained by combining Leveque solution with analytical solution in fully developed region. The obtained equation is as accurate as Graetz analytical solution. In the HFSM, the mass transfer coefficient decreases with the increase of shell void fraction. The shell void fraction also affects the entrance region length of mass transfer. If Reynolds’s number, Schmidt’s number and radius of fiber are constant, the entrance region length will increase with the increase of shell void fraction.     

Double porous media model for mass transfer of hemodialyzers

Double porous media model for mass transfer of hollow fiber hemodialyzers is presented. In the model, the hollow fiber bundle is treated as a porous region composed of two interpenetrating porous regions i.e. the blood and dialysate flow regions, and the interface of the two regions is the porous membrane through which mass transfer is performed. Navier-Stokes equations with Darcy source terms are used to describe the flows within the two regions. Modified Kedem-Katchalsky equations as other source terms are added into conservation equations to simulate the permeating flux through the porous membrane. The model is validated with respect to the experimental data in the literature.     

Liquid-to-particle mass transfer in a micro packed bed reactor

The liquid-to-particle mass transfer in micro packed beds of different hydraulic to particle diameter ratio N and different channel geometries, i.e. circular and rectangular, was studied with the copper dissolution method. In this paper, we demonstrate that the shape of the channel has no influence on the liquid-to-particle mass transfer as long as N is constant. Moreover, higher Sherwood numbers were measured with increasing N for 4 < N < 24.4, but the opposite influence was observed for N < 4. In the case of N > 4 we highlight that the channeling effect, i.e. higher velocities in the wall region compared to the center of the packed bed, has a main impact on the mass transfer. For N < 4 the packed bed cross section cannot be differentiated anymore in a wall region and a center region, hence there is no wall effect and a better flow distribution through the packed bed cross section is induced which increases mass transfer, as proposed in this paper.     

Turbulent convection mass transfer of water with internal mass sources

Convective turbulent mass transfer in heated tubes is modeled with internal mass sources resulting from crystallization. The analysis considers the influence of internal mass sources on the concentration distribution, average concentration of colloidal particles and dissolved impurities, and the mass flux at the wall. It was found that if the mass transfer coefficient in the case which considers internal mass sources is defined properly, the Sherwood number and the mass transfer coefficient with internal mass sources are equal to those without internal mass sources. The mass flux and the increase in the wall temperature beneath the iron oxide deposit layer were predicted using two crystallization models. The model predicting crystallization at the wall only is recommended based on predictions of the maximum increase in the wall temperature beneath the deposit layer. © 2000 Scripta Technica, Heat Trans Asian Res, 29(3): 166–180, 2000     

CFD modeling of mass transfer in annular reactors

A computational fluid dynamics (CFD) investigation of single-phase flow mass transfer prediction in annular reactors was conducted. Different hydrodynamic models including laminar, standard k–ε, realizable k–ε, Reynolds stress (RSM), and the Abe-Kondoh-Nagano (AKN) (a low Reynolds number turbulence model) were evaluated against experimental data in terms of their mass transfer predication capabilities. The laminar model predicted successfully the average mass transfer in the flows under laminar regime (Re < 1500). Among the four evaluated turbulence models, the AKN model provided a better prediction of the average mass transfer rates in the systems when operated both under transitional and turbulent conditions (3000 < Re < 11000). The RSM performed very similarly to the AKN model, except for the entrance region of the reactors where it predicted lower mass transfer rates. These results make the AKN and RSM models very attractive to be integrated in CFD-based simulations of turbulent annular reactors.     

Measurements of mass transfer coefficient and effectiveness in the recovery region of a film-cooled surface

Mass transfer coefficients and film cooling effectiveness are measured downstream of a single row of holes (recovery region) inclined 30° with the surface and inline with the main turbulent boundary layer flow. The mass transfer coefficients are measured using a naphthalene sublimation technique. The effectiveness is determined through the injection of a trace gas into the secondary flow and measuring its concentration at the impermeable wall. Experiments are carried out in a subsonic, zero pressure gradient turbulent boundary layer, under isothermal conditions with three blowing ratios (U_j/U_∞): 0.4, 0.8, and 1.2. The data is collected in a region 7–80 jet diameters downstream of the injection location. From the data on mass transfer coefficients and effectiveness obtained under the same flow conditions a general mass transfer equation is derived. This paper presents extensive data and discussions; and is believed to be one of the few studies in which both of these variables are measured on the same surface and in a large area in the recovery region.     

Coupled heat and mass transfer characteristic in packed bed dehumidifier/regenerator using liquid desiccant

The dehumidifier and regenerator are two key components in liquid desiccant air conditioning systems. The heat transfer driving force and the mass transfer driving force influence each other, the air and desiccant outlet temperatures or humidity ratio may exceed the air and desiccant inlet parameters in the dehumidifier/regenerator. The uncoupled heat and mass transfer driving forces, enthalpy difference and relative humidity difference between the air and desiccant are derived based on the available heat and mass transfer model and validated by the experimental and numerical results. The air outlet parameter reachable region is composed of the air inlet isenthalpic line, the desiccant inlet equivalent relative humidity line and the linkage of the air and desiccant inlet statuses. Except the mass flow rate ratio and the heat and mass transfer coefficients, the air and desiccant inlet statuses and flow pattern have great effects on the dehumidifier/regenerator performance. The counter flow configuration expresses the best mass transfer performance in the dehumidifier and the hot desiccant driven regenerator, while the parallel flow configuration performs best in the hot air driven regenerator.     

Calculation procedure for mass transfer in fuel cells

An analysis of mass transfer losses, or concentration over-potentials in fuel cells is provided. An elementary theory, based on an equivalent film thickness, as proposed in some texts, is derived. This is followed by a more rigorous theoretical treatment of mass transfer theory, for which the mass transfer factor is obtained as a function of the driving force. The solution for the driving force is derived, for the well-known one-dimensional convection–diffusion problem. It is shown that mass transfer in planar and square geometries approximates this idealised situation. A linearised theory, appropriate for low mass flow rates is also presented. The methodology is illustrated using the simple example of a solid oxide fuel cell (SOFC). It is shown that the simplified theory is only applicable for very dilute binary mixtures. A step-by-step procedure for computing mass transfer in fuel cells is detailed, together with a discussion of the scope and range of application of the results.     

Entropy generation in combined heat and mass transfer

Irreversible entropy generation for combined forced convection heat and mass transfer in a twodimensional channel is investigated. The heat and mass transfer rates are assumed to be constant on both channel walls. For the case of laminar flow, the entropy generation is obtained as a function of velocity, temperature, concentration gradients and the physical properties of the fluid. The analogy between heat and mass transfer is used to obtain the concentration profile for the diffusing species. The optimum plate spacing is determined, considering that either the mass flow rate or the channel length are fixed. For the turbulent flow regime, a control volume approach that uses heat and mass transfer correlations is developed to obtain the entropy generation and optimum plate spacing.     

Radiation-induced mass transfer through membranes

The influence of excitation of molecules on mass transfer in membranes is investigated theoretically. It is shown that excitation of the molecules in the gas phase at one side of the membrane can lead to the occurrence of the resulting mass flux in an initially equilibrium system.     

Wall shear rates and mass transfer in impinging jets: Comparison of circular convergent and cross-shaped orifice nozzles

This article presents a study on the wall shear rate and mass transfer of impinging jets on a flat plate. The performance of a cross-shaped orifice nozzle was compared with a reference convergent circular nozzle having similar equivalent diameter. An array of electrodiffusion micro probes inserted into the plate was used for wall shear rates measurements. Mass transfer in the impinging region was calculated from the measured wall shear rates for a Reynolds number around 5500 and over a range of streamwise distances between the nozzle and the impinging plane within 1 to 5 nozzle equivalent diameters. The obtained Sherwood number of the reference convergent nozzle is close to the one given by Chin and Tsang (1978) [5]. The most important observation in the present investigation is that the wall shear rates and the mass transfer in the impingement region of the cross-shaped orifice nozzle are up to 175% and 40%, respectively, higher than that of the convergent nozzle. The performance of the cross-shaped orifice jet is probably related to its particular vortex dynamics characteristic of the near exit region. All the results confirm that the jet passive control enhance the mass transfer.     

Heat and mass transfer of ammonia-water in falling film evaporator

To investigate the performance of heat and mass transfer of ammonia-water during the process of falling film evaporation in vertical tube evaporator, a mathematical model of evaporation process was presented, the solution of which that needed a coordinate transformation was based on stream function. The computational results from the mathematical model were validated with experimental data. Subsequently, a series of parameters, such as velocity, film thickness and concentration, etc., were obtained from the mathematical model. Calculated results show that the average velocity and the film thickness change dramatically at the entrance region when x<100 mm, while they vary slightly with the tube length in the fully developed region when x>100 mm. The average concentration of the solution reduces along the tube length because of evaporation, but the reducing tendency becomes slow. It can be concluded that there is an optimalβrelationship between the tube length and the electricity generated. The reason for the bigger concentration gradient in the y direction is that the smooth tube is chosen in the calculation. It is suggested that the roll-worked enhanced tube or other enhanced tube can reduce the concentration gradient in the film thickness direction and enhance the heat and mass transfer rate.     

Co- and counter-current mass transfer in bubble column

Design and scale-up has gained considerable attention in recent years because of complex hydrodynamics and its influence on bubble column reactor performance. The concentration difference is important variable while characterizing bubble column reactor efficiency and this is a function of hydrodynamics prevailing inside the column. The efficiency of bubble column reactor is a function of physical properties of phases, geometry of column and operating conditions. Literature lacks on the simulation work on variation of concentration of solute for mass transfer rate and mass transfer efficiency in the complex system of bubble column device. In the present work a mechanistic model is formulated to predict the mass transfer efficiency of column and its dependency on various physical parameters, operating condition and column geometry. In this work the mass transfer efficiency has been analyzed based on a mechanistic model in the case of both co-current and counter current operations in bubble column reactor. The concentration variation of the phases obtained by simulation of model may be useful for further understanding the mass transfer phenomena in bubble column reactor.     

Local heat/mass transfer measurements on effusion plates in impingement/effusion cooling with rotation

This paper describes an investigation of the local heat/mass transfer for rotating impingement/effusion cooling. A study was conducted of parameters such as jet orientation and surface geometry. An experiment using the naphthalene sublimation method provided the local heat/mass transfer coefficients on the effusion plate. The heat/mass transfer distributions for the axial orientation were similar to those for the stationary cases, while the trailing orientation produced different Sherwood number features, with divided high Sh regions and one low Sh region around the stagnation area. The concave surface provided better and more uniform heat/mass transfer than the flat surface.     

质子交换膜燃料电池的多尺度传热传质

介绍了目前质子交换膜燃料电池(PEMFC)在膜、电极、单电池、电堆或系统等四个结构尺度上的传热传质过程研究;主要讨论了PEMFC内的多组分传输、膜内水管理和多孔电极内的传热、传质过程;认为建立在孔尺度水平的研究方法是深入探讨电池内多孔材料微结构传热传质的有效途径;多维、多尺度模型的建立及其模拟计算能准确反映PEMFC内部的传递过程机理,为进一步优化电池结构和操作条件提供有价值的参考。     

Charge transfer and mass transfer reactions in the metal hydride electrode

Charge transfer reaction across the electrode/electrolyte interface and hydrogen diffusion in the negative MH alloy electrode dominate the high-rate discharge capability of the metal hydride electrode in a nickel metal hydride (Ni/MH) battery. The mass transfer process in the MH electrode mainly involves hydrogen diffusion in the bulk MH alloy. The charge transfer reaction in the negative electrode reflects the capability of hydrogen reduction and oxidation reactions at the surface of the MH alloy powder. In this study, an AB₅-type hydrogen-absorbing alloy was used as the negative electrode material. The rate-determining mass transfer process in the bulk MH alloy electrode was studied and analyzed using anodic polarization measurements. The exchange current density, which is related to the charge transfer reaction, was analyzed by using the hydrogen equilibrium pressure. The estimation of hydrogen diffusion coefficient in the MH alloy is strongly dependent on the value of the effective reaction area of charge transfer reaction at the surface of the alloy powder.     

The heat/mass transfer analogy for a simulated turbine endwall

Heat transfer measurements in gas turbine cascades are often difficult because of thin boundary layers, complex secondary flows, and large variation in local heat transfer rates. Thus mass transfer techniques have often been used as an alternative method, the heat transfer coefficients being then calculated from the heat/mass transfer analogy.To ensure confidence in the quantitative conversion to the heat transfer coefficients from the mass transfer results, evaluation of the analogy factors is crucial. The present paper examines the validity of the heat/mass transfer analogy, evaluating the analogy factors on a simulated turbine endwall, with separate heat and mass transfer experiments with equivalent flow and geometric conditions. The Nusselt numbers, determined from the heat transfer experiments with a constant wall temperature boundary condition are compared to Sherwood numbers from the mass transfer experiments employing a constant wall concentration boundary condition to evaluate the heat/mass transfer analogy.     

Wave motion and heat and mass transfer of the disperse phase under the conditions of low-frequency gas pulsations

The article presents a system of equations of nonstationary wave motion of the disperse phase in a binary vapour–gas medium with account for the heat and mass transfer processes. The effect of pulsations of moist air and superheated steam on motion of gas suspension and rate of heat and mass transfer under the conditions of wave and resonance modes has been investigated in the course of the computational experiment. In particular, it is shown that in the range of the parameters studied gas pulsations lead to enhancement of interphase heat and mass transfer. Use of superheated vapour as a carrying phase results in heat and mass transfer enhancement compared with moist air. An analytical expression has been obtained for determining the frequency of gas velocity oscillations in a low-frequency region, at which maximum intensity of heat transfer is attained. A schematic of the pneumatic setup equipped with the chamber of pulse combustion with an aerodynamic valve is presented.     

Mass transfer enhancement by pulsatile flows in grooved channels (the effects of groove length on the resonant mass transfer enhancement)

The effect of groove length on the resonant mass transfer enhancement by pulsatile flow in grooved channel at intermediate Reynolds number was studied for five channels using experimental methods. The mass transport processes from the bottom of the groove to the channel were visualized by an electrolytic precipitation method, and overall and local mass transfer rates were measured by electrochemical methods with a high Schmidt number fluid. The qualitative and quantitative observations show that the range of Strouhal numbers, which gives inertia‐dominated flow, is extended to the low‐value region with groove length. Moreover, it is revealed that a noticeable enhancement is obtained in a specific channel at a Reynolds number for a wide range of Strouhal numbers. The results indicate that there is a specific Reynolds number for transfer enhancement in a channel. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(4): 240–257, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20197