Defect and Diffusion Forum Vols. 273-276

Abstract: This work deals with a stationary axisymmetrical heat transfer problem in a combined domain. This domain consists of half-space joined with a bounded cylinder. An important feature of the problem is the possible flux singularity along the edge points of the transmission surface. Domain decomposition is used to separate the subdomains. The solution for an auxiliary mixed boundary value problem in the half space is found analytically by means of Hankel integral transform. This allows us to reduce the main problem in the infinite domain to another problem defined in the bounded subdomain. In turn, the new problem contains a nonlocal boundary conditions along the transmission surface. These conditions incorporate all basic information about the infinite sub-domain (material properties, internal sources etc.). The problem is solved then by means of the Finite Element Method. In fact it might be considered as a coupled FEM-BEM approach. We use standard MATLAB PDE toolbox for the FEM analysis. As it is not possible for this package to introduce directly a non-classical boundary condition, we construct an appropriate iterative procedure and show the fast convergence of the main problem solution. The possible solution singularity is taken into account and the corresponding intensity coefficient of the heat flux is computed with a high accuracy. Numerical examples dealing with heat transfer between closed reservoir (filled with some substance) and the infinite foundation are discussed.

Abstract: Interdiffusion coefficients of the refractory elements in Fe-W-Re and Fe-Cr-X (X=Mo, W) ternary alloys have been measured on the basis of the modified Boltzmann-Matano method for ternary system. Both the cross interdiffusion coefficients, Fe ReW ~D and Fe WRe ~D were negative in Fe-W-Re ternary alloys. This result indicates that attractive interaction exists between W and Re atoms in iron alloys [1]. This is consistent with our previous experimental results that Re suppresses W diffusion in Fe-15Cr alloy [1]. In addition, the value of cross interdiffusion coefficient Fe CrW ~D was positive in Fe-Cr-W diffusion system, whereas Fe MoCr ~D was negative in Fe-Cr-Mo diffusion system.

Abstract: A mathematical model of sulfur transfer in a moving metal and slag melts has been developed. Sulfur is transported simultaneously both by diffusion and by laminar natural convection of the melting metal and oxide fluxes. The diffusion of sulfur is described by the 2nd Fick's law. We have computed the distribution of sulfur in the metal and slag phases and the concentration changes of sulfur in the volume of the metal and slag phases, both functions of space and time. Numerical results are provided to show the validity of this approach.

Abstract: Many food materials must be dried in order to decrease its water activity and to increase the shelf-life. Also, rehydration operation must be carried out as previous step before consumption. Both operations are commonly employed in some industrial chestnut processing. These processes can be carried out at different temperatures and in all cases the quality of the final product can be affected. In this work, convective drying with hot air is the employed method for water removal and rehydration is carried out by immersion of chestnut in water. In both cases, mass transfer processes are governed by water diffusion in the bulk of the solid. The aim of this work is to determine experimentally the drying and rehydration kinetics of chestnut samples at different conditions of temperature and moisture content. Obtained data are modeled with a diffusional model taking into account volume variations and the corresponding values of the coefficients of diffusion of water are obtained. Analysis of the results indicates that drying/rehydration rates increase with temperature and rehydration kinetics are also depending on the initial moisture content of chestnut. Finally, leaching flow during rehydration is only important at high temperature due to starch gelatinization processes.

Abstract: The main aim of this work was to assess the influence of power ultrasound on mass transfer process during convective drying of a low porosity product submitted to different acoustic energy levels. Drying kinetics of carrot cubes (side 8.5 mm) were carried out at 40 °C and 1 m/s applying different electric power levels to the ultrasonic transducer: 0, 10, 20, 30, 40, 50, 60, 70, 80 and 90 W. Drying kinetics were modelled considering the diffusion theory. From the results, a significant (p<0.05) influence of power ultrasound application on drying kinetics of carrot cubes was found. Drying rate increased as the electric power applied got higher. The influence was only observed above an acoustic energy threshold, which corresponded to an electric power applied to the transducer of 20-30 W. From this threshold, a linear relationship was found between the average effective moisture diffusivity or the mass transfer coefficient and the electric power applied to the transducer.

Abstract: In this work the use of Membrane Bioreactors to treat leachate effluents is discussed. The problem of membrane fouling is addressed, and some of the efforts being done to overcome this problem presented. The process optimization requires the reduction of other parameters, some directly related to mass transfer, such as the total amount of solids, the total dissolved solids and the total suspended solids. The preliminary results obtained show the high potential of this technique for the treatment of leachates, mainly for the reduction of solids in leachate streams.

Abstract: Two models comprising external and intraparticle mass transfer resistances developed to describe ion exchange in microporous materials are compared. Maxwell-Stefan and Nernst-Planck equations account for both concentration and electric potential gradients. However, under certain conditions, Maxwell-Stefan approach can be more advantageous particularly due to taking into account ion-ion and ion-solid interactions separately. The models were tested and compared with data available in the literature, namely batch experiments on cadmium (II) removal from aqueous solution using ETS-4 microporous titanosilicate. Calculated results reveal both models provide good and similar representations as well as fine predictive capability. Therefore, under the conditions investigated, both models can be successfully applied to describe intraparticle ionic transport.

Abstract: In this paper the numerical modelling of the behaviour of a channel of a hygroscopic compact matrix is presented. The heat and mass transfer phenomena occurring in the porous medium and within the airflow are strongly coupled, and some properties of the airflow and of the desiccant medium exhibit important changes during the sorption/desorption processes. The adopted physical modelling takes into account the gas side and solid side resistances to heat and mass transfer, as well as the simultaneous heat and mass transfer together with the water adsorption/desorption process in the wall domain. Two phases co-exist in equilibrium inside the desiccant porous medium, the equilibrium being characterized by sorption isotherms. The airflow is treated as a bulk flow, the interaction with the wall being evaluated by using appropriated convective coefficients. The model is used to perform simulations considering two distinct values of the channel wall thickness and different lengths of the channel. The results of the modelling lead to a good understanding of the relationship between the characteristics of the sorption processes and the behaviour of hygroscopic matrices, and provide guidelines for the wheel optimization, namely of the duration of the adsorption and desorption periods occurring in each hygroscopic channel.

Abstract: The present work describes the mass transfer process between a moving fluid and a slightly soluble plane surface buried in a packed bed, in alignment with the direction of flow. The bed of inert particles is taken to have uniform voidage. The elliptic equation resulting from a differential mass balance was solved numerically over a wide range of the relevant parameters and the resulting values of Sherwood number are seemed to depend only of the Peclet number. Experiments measurements of mass transfer in water were performed on the dissolution of plane surfaces of 2-naphthol and benzoic acid at temperatures that differ significantly from ambient value. The soluble plane surfaces used in the experiments were made of either 2-naphthol or benzoic acid and the range of temperatures covered were 278 to 368 K, for the dissolution of 2-naphthol, and 278 to 338 K, for benzoic acid in water. The results illustrate a simple and accurate method for the measurement of the coefficient of molecular diffusion of slightly soluble solutes.

Abstract: A set of experiments is done to study the phenomenon of free convection heat transfer from an isothermal vertical flat plate embedded in a saturated porous medium in steady state condition. The porous medium consisting of 0.8 cm spheres. The aspect ratio of the isothermal flat plate, H/W, is equal to 2. Where H is the height and W is the width of the vertical plate. The investigations were cared out for Darcy modified Rayleigh number between 100 and 500. The results indicate that heat transfer increases linearly with increasing the Darcy modified Rayleigh number. In addition, the present results are in good agreement with the higher-order boundary layer theory obtained by Cheng and Hsu [1].