1,721,030 research outputs found
Dynamic transition of dendrite orientation in the spinodal decomposition of viscous binary mixtures under a thermal gradient.
No full textIn this study, spinodal decomposition of a very viscous regular binary mixture bounded within two walls cooled at different temperatures is simulated by using the diffuse interface model. Under a temperature gradient, phase separation starts from the cooler wall forming dendritic structures growing anisotropically with time. Two remarkably different dynamics are identified depending on whether heat propagates slower or faster than mass. For small thermal conductivity (i.e., small Lewis number), dendrites grow parallelly to the temperature gradient, keeping such an alignment until the steady-state. On the other hand, for large Lewis number, during the early stages phase separation proceeds within stripes oriented along iso-temperature lines, i.e., with dendrites aligned perpendicularly to the temperature gradient, which, however, gradually shift their orientation parallel to the temperature gradient as the steady-state is approached. Such a dynamic transition of dendrite orientation upon a temperature gradient when heat propagates faster than mass is found to hold also for non-equimolar mixtures and for different species thermal conductivities. These results shed light on the dynamics of phase separation in constrained systems and anisotropic conditions
Modellazione multiscala di celle a combustibile ad ossidi solidi: dalla microstruttura alle prestazioni
No full textIn questo studio si presenta un approccio modellistico integrato per la simulazione di celle a combustibile ad ossidi solidi (SOFC). La modellazione copre in primis gli aspetti microstrutturali degli elettrodi porosi compositi costituenti la cella, le cui proprietà effettive di trasporto e reazione sono predette con la teoria di percolazione e/o con la ricostruzione numerica della microstruttura in funzione di composizione, distribuzione granulometrica delle polveri e condizioni di sinterizzazione. Le proprietà effettive costituiscono i parametri dei modelli di trasporto e reazione all'interno della cella, basati su bilanci di massa e carica, che permettono di ottenere la distribuzione delle variabili di campo e l'efficienza energetica del sistema. L'accoppiamento della modellazione microstrutturale con quella di trasporto e reazione permette di correlare i due livelli di scala cosicché i modelli tra loro integrati sono utilizzati come strumento interpretativo dei dati sperimentali e come strumento per l'ottimizzazione di prestazione e design.In this study an integrated modelling framework for the simulation of solid oxide fuel cells (SOFC) is presented. The modelling firstly covers the microstructural aspects of porous composite electrodes which constitute the cell, whose effective transport and reaction properties are predicted by means of percolation theory and/or numerical reconstruction of the microstructure as a function of composition, particle size distribution and sintering conditions. The effective properties are parameters of the models of transport and reaction within the cell, based on mass and charge conservation balances, which allow to obtain the field variables distribution and the energetic efficiency of the system. Coupling the microstructural modelling with the transport and reaction modelling enables the correlation of the two different scale levels, so that the integrated models are used as interpretative tool of experimental data as well as design tool for optimize the performance
Physically-based impedance simulation to decouple convoluted transport and reaction phenomena in SOFC cathodes
No full textA mechanistic model, based on mass and charge conservation equations [1], is presented for the physically-based simulation of impedance spectra in composite solid oxide fuel cell cathodes, taking into account the complex interaction between transport and reaction phenomena. The impedance simulation, which reproduces the same procedure used in laboratory frequency response analyzers, allows the de-convolution of distinct elementary processes and the identification of a specific double layer chemical capacitance, describing the possible accumulation of adsorbed species and reaction intermediates at the interface between electron-conducting and ion-conducting particles. The satisfactory agreement of simulated spectra with experimental data for different operating conditions and electrode thicknesses reveals that the model is capable to reproduce the transient behavior of composite electrodes by relying on only one fitted parameter. Model simulations show that mass-transfer processes within the electrode produce a resistive contribution in the impedance spectra related to the effect of the local oxygen partial pressure on the reaction kinetics. In addition, the pores act as a buffer for molecular oxygen, leading to a capacitive contribution in the frequency range 10^2-10^4Hz, more pronounced at high current densities
Constitutive relations of thermal and mass diffusion
No full textNon-equilibrium thermodynamics provides a general framework for the description of mass and thermal diffusion, thereby including also cross-thermal and material diffusion effects, which are generally modeled through the Onsager coupling terms within the constitutive equations relating heat and mass flux to the gradients of temperature and chemical potential. These so-called Soret and Dufour coefficients are not uniquely defined, though, as they can be derived by adopting one of the several constitutive relations satisfying the principles of non-equilibrium thermodynamics. Therefore, mass diffusion induced by a temperature gradient and heat conduction induced by a composition gradient can be implicitly, and unexpectedly, predicted even in the absence of coupling terms. This study presents a critical analysis of different formulations of the constitutive relations, with special focus on regular binary mixtures. It is shown that, among the different formulations presented, the one which adopts the chemical potential gradient at constant temperature as the driving force for mass diffusion allows for the implicit thermo-diffusion effect to be strictly absent while the resulting Dufour effect is negligibly small. Such a formulation must be preferred to the other ones since cross-coupling effects are predicted only if explicitly introduced via Onsager coupling coefficients
Microstructural modelling for prediction of effective properties in porous SOFC electrodes
No full textA modelling framework for the prediction of effective properties in porous SOFC electrodes is presented. The model consists of: i) a packing algorithm to numerically reconstruct the microstructure, and ii) a Monte Carlo method to calculate the effective transport properties. This modelling technique improves the accuracy of the prediction of effective properties beyond percolation theory estimates. In addition, the numerical reconstruction does not rely on existing samples and complex instrumentations (for example, X-ray tomography, FIB-SEM analyses) required by other reconstruction methods.
The packing algorithm enables to numerically generate a representative sample of the electrode microstructure with the desired particle size distribution, composition and porosity. Sintering phenomena are simulated by increasing the overlap among the particles, pore-former particles are accounted for during the packing generation [1]. The model is capable to simulate packings of spherical particles as well as of agglomerates of spheres.
The reconstructed samples are then analyzed with a Monte Carlo method [2]. Random walk simulations are used to determine the transport properties in gas and solid phase, such as the effective diffusivity and the effective electric conductivity. Other geometric quantities can be evaluated, such as the pore size distribution, the surface area exposed to the gas phase, the three-phase boundary length.
In this study, effective properties as a function of porosity and particle size for random packings of spherical particles are calculated. The results are compared with independent experimental data, revealing a good agreement for both gas and solid phase properties. Effective properties of agglomerates of particles are also presented and compared with the results obtained for spherical particles. The comparison highlights that particle agglomeration significantly increases the mean pore size while reducing the effective gas diffusivity and the specific surface area exposed to gas phase (Figure 1).
The presented modelling technique can be used to improve SOFC electrode design and to support the interpretation of experimental data
An integrated microstructural and electrochemical approach for cell-level modeling: the LSM-based Juelich cell
No full textThe typical weak point of existing SOFC cell-level models is the evaluation of the electrode effective properties, typically performed by using simple percolation models or by fitting the microstructural parameters on the polarization curves. In this study we present an integrated approach which incorporates a detailed microstructural modelling into the cell-level model. The three-dimensional microstructure of each porous layer is numerically reconstructed with packing algorithms for an accurate prediction of the effective properties [1]. The predicted effective properties are used in a two-dimensional electrochemical model, based on conservation equations written in continuum approach, describing transport and reaction phenomena within the cell.
The integrated approach allows the prediction of the polarization behaviour from the knowledge of operating conditions and powder characteristics, eliminating the need for empirical correlations and adjusted parameters. The simulation of a short stack (F-design) of planar LSM-based anode-supported cells, developed and tested by Forschungszentrum Jülich, shows that a quantitative agreement with experimental data is obtained without fitting any parameter [2]. Simulations show that at 800°C the activation resistance in the cathode functional layer is the main contribution to cell overpotential. In addition, gas concentration effects at the anode produce the parabolic shape of the polarization curve near OCV and lead to reduce the polarization resistance as the water molar fraction in the fuel stream increases
Erratum: A particle-based model for effective properties in infiltrated solid oxide fuel cell electrodes (Journal of the Electrochemical Society (2014) 161 (F1243))
No full textThis article was published online on September 11, 2014 before all of the corrections/changes the author requested had been made. ECS apologizes for these errors. The article was corrected online on September 15, 2014
A multiscale model for infiltrated SOFC anodes
No full textSOFC electrodes where the electrocatalyst is infiltrated into a porous electrolyte layer offer key advantages such as much higher electrochemical activity, a greater tolerance for thermal shock, higher redox tolerance (for anodes), etc. when compared to conventional composite electrodes. Another important development in recent times is the development of mathematical models that are able to relate the properties of fuel cell electrodes to their microstructure.
In this presentation, we will discuss the model development and results from two SOFC models: 1) a model that predicts the effective conductivities and triple-phase boundary density (a measure of reaction site density) using knowledge of the microstructure of Ni infiltrated anodes [1,2], and 2) a multiphysics model that takes the above computed electrode properties and uses them to simulate SOFC performance. The first model, a nano-micrometer scale model is based on percolation theory and uses experimentally controllable and measurable parameters as input. The second model is a micro-centimeter scale reaction-transport model that solves all the relevant coupled physics in a working SOFC to compute the current produced as a function of cell voltage. By coupling the two models together serially, we are able to evaluate the effect of microstructural parameters on fuel cell performance. We will present results that demonstrate how this approach can be used to evaluate and improve the design of infiltrated SOFC electrodes
Simulated impedance of diffusion processes in tomographically derived microstructures
No full textImpedance spectroscopy is a powerful and widely used technique for characterising processes in electrochemical devices, such as batteries and fuel cells. The performance of these devices is closely related to their 3D microstructures; however, the elements used for representing them are typically either zero dimensional (resistors, capacitors etc.) or occasionally 1D. The most commonly used 1D elements are Warburg diffusion elements, which are particularly useful as they have analytical solutions and so can be easily incorporated into standard EIS fitting algorithms. However, the transport processes that these elements are used to represent are inherently 3D, and so Warburg diffusion elements must capture transport phenomena with bulk parameters such as tortuosity factors and porosities.
Details of the geometry of porous electrodes have recently become routinely available using microtomography. This technique typically represents the geometry as an array of cuboid volume elements (voxels) that must be segmented from greyscale to a phase labelled volume. In 2016, the authors published an open-source software package, TauFactor, which allows for the rapid calculation of tortuosity factors from segmented tomography data. An updated version of the software is here presented to efficiently calculate diffusive impedance spectra in the frequency domain, directly from segmented tomography data.
Numerical results show that the diffusion impedance may significantly deviate from the Warburg analytical solution for structures showing an inhomogeneous distribution of pores. In particular, multiple peaks may appear in the high-frequency region in the complex plane, which may be misinterpreted as separate electrochemical processes in real impedance data (see fig.1 for a simple 2D example). Two classes of structures, namely with increasing or decreasing porosity distribution, can be identified from the analysis of diffusion impedance. Thus, the software can be used to overcome the limits of conventional equivalent circuit analysis in modelling transport phenomena in porous electrodes
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