Defect and Diffusion Forum Vols. 273-276

Abstract: In order to model the diffusion in multicomponent and multiphase systems we define and use the volume fixed frame of reference. In this unique frame all diffusion fluxes are expressed by the combined Darken-Nernst-Planck formulae. The components mobility is estimated using the NIST data and Calphad approach. The resulting CADiff method is used for the modeling (inter)diffusion process in the multicomponent diffusion multiple. We shows results for the IN718-Rene88 terminal compositions. Comparison of the simulation results with experimental data shows good agreement. The presented CADiff method allows to model interdiffusion in multicomponent non-ideal systems. When the thermodynamical data are available, the method allows to include reactions, variable partial molar volumes and effect of stress.

Abstract: Numerical simulations of evolution of the potentials and impedance spectra of ionselective membranes (ISEs) with ionic sites are presented. The Nernst–Planck–Poisson and continuity equations (NPP) are solved numerically by means of the finite difference method, the Rosenbrock solver and with the use of Matlab platform. Transient solutions for ion-selective electrodes under open- and closed-circuit conditions are computed. The potential-time response to small-current perturbation is used for determination of complex impedances. We present simulations of ISEs as a function of varying diffusivities and ionic concentrations in the “bathing” solutions at interfaces. It is shown that the non-Nernstian behavior of passive membrane electrodes is a result of kinetic constraints at the interfaces, which is manifested in the appearance of an additional arc between the high-frequency bulk and the low-frequency (Warburg) arcs. The presented approach directly relates the diffusivities in the membrane and the interface properties (heterogeneous rate constants determining the transport across interfaces) to the characteristic features of impedance spectra (dimensions and characteristic radial frequencies). NPP problem solved on the Matlab platform allows simulating of the non-linear effects in electro-diffusion.

Abstract: Increase of the emission of CO2, which is mostly the result of the combusted fossil fuels into the atmosphere, exponentially increases. Through increased energy efficiency there is lower CO2 emission. There is a tendency to reduce exhaust gases temperatures down from their original value referred to as “acid dew point”, 115-160°C. A result is vapor condensation of wet flue in chimney. Condensation occurs when the surface temperature is below the dew point of the vapor-gas mixture. Therefore, Vapor-Liquid Equilibrium models are required in order to determine the dew point of the mixture. Wet flue gas is simulated with vapor-air mixture. A numerical model was presented to calculate the velocity and thermal field of turbulent vapor-air mixture flow trough a chimney. The momentum and temperature field were calculated via a finite-volume CFD code, using the k – e turbulence model. The validation of this calculation was conducted employing an experimental set for heat and mass transfer in vertical upward vapor-air mixture. Measurements were done using a stainless steel tube of 13.2 mm I.D. (internal diameter) and 70 I.D. lengths. Flow rates of steam and air were varied as the experimental parameters. The experiment involves two-phase, two-component, heat and mass transfer. Comparisons of wall temperature and condensate rate were made and the model was shown to give an acceptable results.

Abstract: A series of biodegradable superabsorbent polymers (SAP’s) based on starch grafted with acrylic acid (AA) and crosslinked with N,N’-methylenebisacrylamide (MBIS) were obtained. The parameters that define SAP’s properties, such as starch type, amount of initiator, acrylic acid and crosslinker concentrations, and degree of neutralization were varied. The dynamic swelling was interpreted in terms of both a simple power of time equation and a more detailed model based on a coupled diffusion–relaxation mechanism. The diffusivity coefficients calculated from diffusion–relaxation model were compared with the microimages obtained by optical microscopic.

Abstract: Mass transfer around a slightly soluble cylinder, aligned with the direction of liquid flow in a packed bed of sand particles, is analysed for transport by advection and molecular diffusion. The theoretical analysis applies to cylinders that are large in comparison with the inert particles, so that the bed may be treated as a continuum. Experimental measurements of mass transfer in water were carried out at temperatures that differ significantly from ambient value. The soluble cylinders used in the experiments were made of either salicylic acid or cinnamic acid and the range of temperatures covered were 278 to 343 K, for the dissolution of salicylic acid, and 278 to 328 K, for cinnamic acid in water. Using the mass transfer theory presented in this work the molecular diffusion coefficient of the two solutes was determined, and good agreement with literature values was found.

Abstract: Osmotic dehydration of pumpkin (Cucurbita pepo, L.) fruits was carried out with binary solutions of sucrose and NaCl at different temperatures and solute concentrations. Water loss and solids gain kinetics were experimentally determined and fitted using a diffusional model. Pumpkins samples were considered as finite cylinders and the analytical solution of the unsteady diffusion equation was used considering the external resistance to the mass transfer negligible. The influence of shrinkage and temperature on the effective diffusion coefficients was also assessed in this work.

Abstract: This study considers numerical simulations of the combustion of propane with air, in a burner element due to high temperature and velocity gradients in the combustion chamber. The effects of equivalence ratio (φ) and oxygen percentage (γ) in the combustion air are investigated for different values of φ between 0.5 and 1.0 and γ between 10 and 30%. In each case, combustion is simulated for the fuel mass flow rate resulting in the same heat transfer rate (Q) to the combustion chamber. Numerical calculations are performed individually for all cases with the use of the Fluent CFD code. The results shown that the increase of equivalence ratio corresponds to a significantly decrease in the maximum reaction rates and the maximum temperature increase with the increases of oxygen percentage. Mixing hydrogen with propane causes considerable reduction in temperature levels and a consequent reduction of CO emissions.

Abstract: Moisture is an important factor when analyzing the behaviour of building elements and materials. A deeper knowledge on moisture transport under transient conditions of environmental air temperature and humidity is required. In this experimental work, the measurements of moisture diffusion coefficients, w D , of three different types of building materials commonly used in Portugal (gypsum plasterboard, gypsum plaster and gypsum+lime plaster) were obtained for different values of temperature and relative humidity. The results obtained were in good agreement with the values found in literature and show that the increase in temperature resulted in an increase in the diffusion coefficient and an increase in relative humidity (54% to 69%) resulted in a decrease in w D .

Abstract: The sorption and transport of water in two porous building materials, clay brick and autoclaved aerated concrete, was studied in detail. The evolution of the distribution of liquid in the porous medium was analysed in terms of the Boltzmann transform method and anomalous diffusion equation proposed by Küntz and Lavallée [1]. The apparent moisture diffusion coefficients of water were determined from the total water profiles using a modified Boltzmann-Matano analysis, and a good agreement with literature values was found. The application of anomalous diffusion model to building materials indicates that the previous 1/ 2 t relation is not entirely accurate to estimate the volume of absorbed water. This result has particular relevance for evaluating the durability of building structures.

Abstract: This work describes an experimental and numerical study to determine the corresponding detailed thermal and fluid dynamic fields developed in a burner element for domestic water heaters. A three-dimensional burner element was designed in real scale and numerical studies using Fluent code were carried out in steady-state conditions to allow continuous observations and measurements of the influence of burner element geometry and fuel-air velocity on flame characteristics, namely colour, length and stability. Air has been used as a fluid. The computational results, specifically the velocity and flame characteristics were verified with experimental data obtained in order to ensure a stable flame. In order to evaluate the power generated and the emissions of CO2 and CO associated to different burner element geometries, some experiments were performed using single burner elements with a stainless steel prismatic tank covered with thermal insulation to minimize heat losses.