Pour des raisons de coûts, les tests de catalyseur mis en forme (billes, extrudés, …) en réacteur est lit fixe sont mis en œuvre dans des réacteurs de plus en plus petits alors que la taille des objets catalytiques ne change pas. L'objet de cette thèse est d'explorer les domaines où la réduction d'échelle conduit à des questionnements nouveaux en termes de physique, répétabilité, représentativité et modélisation. Le document s'articule autour de 5 chapitres : 1) une introduction détaillée du contexte avec un état de l'art sur les réacteurs à lit fixe de petite taille et l'identification des questions en suspens, 2) une réflexion autour de la nature des écoulements gaz-liquide descendant dans des lits fixes quand les forces capillaires ne sont plus négligeables, 3) la présentation d'une méthodologie et d'un critère sur un nombre minimal de grain nécessaire pour limiter les conséquences d'une distribution granulométrique en présence de limitations au transfert interne, 4) l'étude des effets d'empilements aléatoires sur la performance apparente du réacteur, d'abord par un modèle réseau de pores qui décrit assez bien les tendances observées expérimentalement mais manque de capacité de prédiction, puis par simulation numérique directe d'écoulement réactif dans des empilements de 8 cylindres qui montre que les effets d'empilements apparaissent, avec les limitations au transfert externe, quand la diffusion moléculaire transverse n'est pas assez rapide pour compenser les gradients dus à la réaction et à la convection, 5) une conclusion qui résume les critères de conception de réacteurs de tests à petite échelle, et propose des pistes de continuation du travail / In order to lower costs, testing of catalytic pellets (spheres or extrudates) is performed in ever smaller fixed bed reactors whereas catalytic pellet size is unchanged. The object of this thesis is to explore domains where downsizing leads to new questions in terms of physics, repeatability and modeling. The thesis is built in 5 chapters, 1) a detailed introduction of the context and a review of the literature on small fixed bed reactors, 2) a discussion about flow patterns in fixed beds filled with fine powder where capilary effects are not negligible, 3) a presentation of a methodology to assess the impact of sampling small number of pellets out of a non uniform set and results for internally mass transfer limited reactions, 4) a study of the effect of randomness in fixed beds first through a pore network model with a good ability to predict trends but lacking accuracy, then using direct numerical simulation of a reactive flow in fixed beds made of 8 cylinders arranged in several configuration showing that packing effects occur, concurrently with external mass transfer limitations, when cross flow diffusion is not fast enough to level out convection and reaction induced gradients, 5) a conclusion that summarizes design criteria and offers a few perspectives for R&D in downsizing

L’hydrogène comme vecteur énergétique est une solution prometteuse pour réduire les émissions de gaz à effet de serre. En effet, l’hydrogène permet de stocker de grandes quantités d’énergie de façon totalement décarbonée. Pour favoriser l’utilisation à grande échelle de l’énergie hydrogène, il est essentiel de réduire le coût des piles à combustible et d‘augmenter leur durabilité et leurs performances. Les matériaux situés au coeur des piles à combustible ont un impact fort sur leurs performances et leur durabilité. Dans ce contexte, optimiser les matériaux est crucial. Nous développons dans cette thèse une démarche de modélisation des matériaux poreux des piles à combustible à membrane échangeuse de protons. Nous nous concentrons sur un matériau en particulier, celui intervenant dans les couches de diffusion des gaz (GDL). Les GDL ont de multiples fonctions, notamment de permettre en leur sein des transports simultanés de gaz, d’électrons, de chaleur et d’eau sous forme vapeur et liquide. Pour permettre ces transports, les GDL sont composées d’une phase fluide et d’une phase solide, elle-même constituée de plusieurs matériaux. La microstructure des GDL joue un rôle crucial sur les compromis entre les fonctions des GDL et l’efficacité des transports. Nous utilisons la tomographie aux rayons X pour imager la structure interne des GDL à l’échelle micrométrique, et développons des outils numériques pour simuler les transports sur les microstructures. Nous montrons que des simulations sur des images de grandes tailles sont réalisables en temps raisonnables. Nous validons les simulations de transports dans les GDL numériquement et expérimentalement. Le premier chapitre est consacré à la modélisation d’une expérience ex-situ d’injection d’eau dans les GDL. Nous développons un modèle réseau de pores extrait d’images tomographiques, pour simuler les écoulements d’eau dans les GDL en présence de forces capillaires. Nous validons les simulations réseaux de pores en utilisant des images tomographiques montrant l’eau liquide dans une GDL lors d’une expérience d’injection d’eau. Nous montrons que les courbes de pression capillaire peuvent être déterminées par simulations réseau de pores ou par simulations full morphology sur des images tomographiques. Le deuxième chapitre est consacré à la simulation des transports de gaz et d’électrons dans les GDL. Nous développons une méthode de simulation réseau de pores, consistant à décomposer l’image en régions de formes simples et à calibrer des modèles physiques sur ces régions. Cette approche à deux échelles est économe en temps de calcul. Nous comparons ces simulations à des simulations directes et à des formules analytiques. Une seconde partie concerne la comparaison des simulations directes à des mesures expérimentales. Nous montrons que les transports dans la phase fluide peuvent être déterminés avec fiabilité par simulation directe sur les images tomographiques, tandis que la simulation des transports dans la phase solide nécessite des informations non fournies par la tomographie aux rayons X. Le troisième chapitre est consacré à la modélisation de la condensation de l’eau dans les GDL. La vapeur d’eau produite par la réaction du dihydrogène avec le dioxygène traverse les GDL et condense dans les zones froides des GDL. Un modèle réseau de pores couplant diffusion de la vapeur d’eau, changement de phase et forces capillaires est développé. Nous étudions ce modèle sur des réseaux de pores générés virtuellement. Le dernier chapitre est consacré à l’étude de microstructures conçues virtuellement. Nous montrons qu’il est possible de produire virtuellement des microstructures proches de celles de matériaux réels, de chercher des microstructures optimales, et d’étudier des effets physiques par simulation sur matériaux virtuels. / Hydrogen as an energy carrier is a promising solution for reducing emissions of greenhouse gases. Indeed, hydrogen can be used to store large amounts of energy in a completely carbon-free way. To promote the widespread use of hydrogen energy, it is essential to reduce the cost of fuel cells and increase their durability and performance. The materials in the heart of fuel cells have a strong impact on their performance and durability. In this context, opti-mizing the materials is crucial. We develop in this thesis a modeling approach of porous materials in proton exchange membrane fuel cells. We focus on a specific material that takes part in the gas diffusion layers (GDL). The gas diffusion layers are crossed by gas, electron, heat and water fluxes. To allow such multiple transports, GDL are composed of a fluid phase and a solid phase, itself consisting of several materials. The microstructure of the GDL plays an essential role on the tradeoffs between transports. To model these tradeoffs, we use X-ray tomography to image the microstructure at micrometer scales, and develop digital tools to simulate the transport on tomographic images. We validate the simulations with experimental characterizations and tomographic images of GDL. Great care has been taken in the computer performance of the numerical tools, because tomographic images in three dimensions are a challenge because of the size of the data. The first chapter of this thesis is devoted to modeling of an ex-situ water injection experiment in a GDL. We develop a pore network model extracted from tomographic images, to simulate liquid water flows in GDL in the presence of ca-pillary forces. We validate pore networks simulations using tomographic images showing the liquid water in a GDL dur-ing a water injection experiment. We show that the capillary pressure curves can be determined reliably by pore net-work simulations or full morphology simulations on tomographic images. The second chapter is devoted to one-phase transport simulations in GDL. The first part of this chapter is devoted to the development of pore networks simulations for the diffusivity and the electrical conductivities of the GDL. We de-velop a two-scale simulation methodology, which consists of decomposing the image into elements having simple shapes, and to calibrate physical models on these elements. This method considers the effect of the microstructure on the physical transfers in an economical way, reducing the computing time. We compare the pore network simulations to direct simulation on microstructures and to analytical formulas. The second part is devoted to the comparison of transport simulations with experimental measurements. We show that the transports in the fluid phase can be deter-mined reliably by direct simulations on the tomographic images, while transports in the solid phase require additional information not provided by X-ray tomography. The third chapter is devoted to modeling of the condensation of water in the GDL. The steam produced by the reaction of the hydrogen with the oxygen passes through the GDL and condenses in the cold areas of the GDL. A pore network model coupling diffusion of steam, phase change and capillary forces is developed. We study this model on virtually generated pore networks. The last chapter is devoted to the study of virtually designed microstructures. Virtually exploring new materials designs has advantages over the experimental approach, in terms of speed, cost and control over the microstructures. We show that it is possible to virtually produce microstructures close to those of real materials, to seek optimal microstructures, and control the microstructure to better study some physical effects using simulation.

Cette thèse présente une contribution à l’étude des transferts d’eau au sein des piles à combustible de type PEMFC, un aspect clé de cette technologie. Une approche de simulation numérique est développée en couplant un modèle de type réseau de pores dans la couche de diffusion (DM), une approche mixte continue –réseau de pore dans la couche microporeuse (MPL) et une modélisation par compartiments dans la couche active. L’approche développée prend en compte les transferts couplés de chaleur et d’eau via notamment la modélisation des phénomènes de changement de phase dans la DM et la MPL (évaporation et condensation). Dans une première partie, nous étudions le cas où l’eau migre dans l’assemblage MPL-DM directement en phase liquide. L’impact de la variation de pression dans la phase gazeuse sur la distribution de la phase liquide est étudié. L’épaisseur optimale de la MPL est également étudiée. Dans une seconde partie, nous étudions des situations où l’eau se forme par condensation dans la couche de diffusion. Nous étudions tout d’abord l’impact des propriétés de la couche de diffusion et de la MPL sur le diagramme de condensation. Ensuite nous analysons l’impact de la formation de l’eau liquide sur la distribution de courant locale. Enfin, l’impact de la mouillabilité sur les figures de condensation est étudié. Cette dernière étude est vue comme un premier pas vers l’étude des mécanismes de dégradation dans le régime de condensation. / This thesis is a contribution to the study water transfers within PEM fuel cell, a key element of this technology. A numerical simulation tool is developed coupling a pore network model in the gas diffusion layer (DM), a mixed continuum – pore network approach in the microporous layer (MPL) and a model by compartments in the catalyst layer. The developed approach takes into account the coupled heat and water transfers through the modeling of phase change phenomena (evaporation – condensation) in the DM and in the MPL. In the first part, we study the case where water migrates into the MPL-DM assembly directly in liquid phase. The impact of gas pressure variation on liquid phase distribution is studied. The optimal thickness of MPL is studied too. In the second part we study situations where liquid water essentially formed by condensation in the diffusion layer. We first study the impact of DM and MPL properties on the condensation diagram. Then we analyze the impact of liquid water formation on the local current density distribution. Finally the impact of wettability modifications on the liquid water patterns is studied. This last study is considered as a first step toward the study of degradation mechanisms in the condensation regime.

[pt] A compreensão dos mecanismos e fenômenos de transporte relacionados ao fluxo multifásico em meios porosos é de grande relevância para diversas aplicações práticas como captura e sequestro de dióxido de carbono, transporte em células de combustível e recuperação avançada de reservatórios de hidrocarbonetos. A geometria do espaço poroso e as interações dos fluidos com sua parte sólida determinam propriedades macroscópicas como porosidade, permeabilidades relativas e pressão capilar. Porém, a visão em escala microscópica fornece uma melhor descrição e entendimento dos processos físicos e químicos do escoamento de fluidos no espaço poroso. Neste trabalho desenvolvemos um simulador de rede de poros para análise do escoamento bifásico de fluidos imiscíveis tanto para o processo de drenagem quanto para o de embebição. O modelo de rede 240×40 tem capilares com raios médios na ordem de 52.35 micrometro com constrição. Os padrões de escoamento e eficiências de deslocamento foram obtidos para diferentes razões de viscosidade e números de capilaridade. Os resultados encontrados, considerando deslocamento pistão, foram similares a de experimentos realizados previamente, injetando água no meio saturado de óleo. Na drenagem, a saturação residual de óleo cai com o aumento do número de capilaridade. O padrão de escoamento observado é de fingerings viscosos e, a frente de deslocamento torna-se mais estável com o aumento da razão de viscosidade. Na embebição, para números de capilaridade mais baixos, o escoamento foi dominado por fingers capilares. Para números de capilaridade altos, fingers viscosos foram predominantes e, com o aumento da razão de viscosidade, a frente apresentou maior estabilidade. / [en] Understanding the mechanisms and transport phenomena of multiphase flow in porous media has great relevance in several practical applications, such as capture and sequestration of carbon dioxide, transport in fuel cells and enhancement hydrocarbon recovery. The geometry of pore space and the fluid interactions with the solid determine macroscopic properties such as porosity, relative permeabilities and capillary pressure. However, microscopic analysis provides a better description and comprehension of physical and chemical processes of fluid flow in the pore space. In this work, we developed a pore-network simulator to analyze immiscible two-phase flow for both drainage and imbibition processes. The 240×40 pore-network model has constricted capillary channels with radius on the order of 52.35 micrometer. Flow patterns and displacement efficiencies evaluation were obtained at different viscosity ratios and capillary numbers. The results, considering piston-like displacement, were similar to experiments realized previously, injecting water in an oil saturated medium. In the drainage process, the oil saturation reduces with increasing capillary number. The observed flow pattern is viscous fingerings and the front is stable with the higher viscosity ratio. In imbibition, the flow was dominated by capillary fingers at low capillary numbers. At high capillary numbers, viscous fingers were predominant and, with increasing viscosity ratio, the front presented higher stability.

Accurate prediction of colloid deposition rates in porous media is essential in several applications. These include natural filtration of pathogenic microorganisms such as bacteria, viruses, and protozoa, transport and fate of colloid-associated transport of contaminants, deep bed and river bank filtration for water treatment, fate and transport of engineered nanoparticles released into the environment, and bioremediation of contaminated sites. Colloid transport in porous media is a multi-scale problem, with length scales spanning from the sub-pore scale, where the particle-soil interaction forces control the deposition, up to the Darcy scale, where the macroscopic equations governing particle transport are formulated. Colloid retention at the Darcy scale is due to the lumped effect of processes occurring at the pore scale. This requires the incorporation of the micro-scale physics into macroscopic models for a better understanding of colloid deposition in porous media. That can be achieved through pore-scale modeling and the subsequent upscaling to the Darcy scale. Colloid Filtration Theory (CFT), the most commonly used approach to describe colloid attachment onto the soil grains in the subsurface, is found to accurately predict the deposition rates of micron-sized particles under favorable conditions for deposition. But, CFT has been found to over predict particle deposition rates at low flow velocity conditions, typical of groundwater flow, and for nanoscale particles. Also, CFT is found to be inapplicable at typical environmental conditions, where conditions become unfavorable for deposition, due to factors not considered in CFT such as deposition in the secondary minimum of the interaction energy profile, grain surface roughness, surface charge heterogeneity of grains and colloids, and deposition at grain-to-grain contacts. To the best of our knowledge, mechanistic-based models for predicting colloid deposition rates under unfavorable conditions do not exist. Currently, fitting the colloid breakthrough curve (BTC), obtained from the laboratory column-or field-scale experiments, to the advection-dispersion-deposition model is used to estimate the values of deposition rate coefficients. Because of their small size (less than 100 nm), nanoparticles, a sub-class of colloids, may interact with the porous medium in a different way as compared to the larger colloids, resulting in different retention mechanisms for nanoparticles and micron-sized particles. This emphasizes the need to study nanoparticles separately from larger, micrometer-sized colloids to better understand nanoparticle retention mechanisms. The work reported in this thesis contributes towards developing mathematical models to predict nanoparticle movement in porous media. A comprehensive mechanistic approach is employed by integrating pore-scale processes into Darcy-scale models through pore-network modeling to upscale nanoparticle transport in saturated porous media to the Darcy scale, and to develop correlation equations for the Darcy-scale deposition parameters in terms of various measurable parameters at Darcy scale. Further, a one-dimensional mathematical model to simulate the co-transport of viruses and colloids in partially saturated porous media is developed to understand the relative importance of various interactions on virus transport in porous media. Pore-network modeling offers a valuable upscaling tool to express the macroscopic behavior by accounting for the relevant physics at the underlying pore scale. This is done by idealizing the pore space as an interconnected network of pore elements of different sizes and variably connected to each other, and simulating flow and transport through the network of pores, with the relevant physics implemented on a pore to pore basis (Raoof, 2011). By comparing the results of pore-network modeling with an appropriate mathematical model describing the macro-scale behavior, a relationship between the properties at the macro scale and those at the pore scale can be obtained. A three dimensional multi-directional pore-network model, PoreFlow, developed by Raoof et al. (2010, 2013) is employed in this thesis, which represents the porous medium as an interconnected network of cylindrical pore throats and spherical pore bodies, to upscale nanoparticle transport from pore scale to the Darcy scale. The first step in this procedure is to obtain relationships between adsorbed mass and aqueous mass for a single pore. A mathematical model is developed to simulate nanoparticle transport in a saturated cylindrical pore by solving the full transport equation, considering various processes such as advection, diffusion, hydrodynamic wall effects, and nanoparticle-collector surface interactions. The pore space is divided into three different regions: bulk, diffusion and potential regions, based on the dominant processes acting in each of these regions. In both bulk and diffusion regions, nanoparticle transport is governed by advection and diffusion. However, in the diffusion region, the diffusion is significantly reduced due to hydrodynamic wall effects. Nanoparticle-collector interaction forces dominate the transport in the potential region where deposition occurs. A sensitivity analysis of the model indicates that nanoparticle transport and deposition in a pore is significantly affected by various pore-scale parameters such as the nanoparticle and collector surface potentials, ionic strength of the solution, flow velocity, pore radius, and nanoparticle radius. The model is found to be more sensitive to all parameters under favorable conditions. It is found that the secondary minimum plays an important role in the deposition of small as well as large nanoparticles, and its contribution is found to increase as the favorability of the surface for adsorption decreases. Correlation equations for average deposition rate coefficients of nanoparticles in a saturated cylindrical pore under unfavorable conditions are developed as a function of nine pore-scale parameters: the pore radius, nanoparticle radius, mean flow velocity, solution ionic strength, viscosity, temperature, solution dielectric constant, and nanoparticle and collector surface potentials. Advection-diffusion equations for nanoparticle transport are prescribed for the bulk and diffusion regions, while the interaction between the diffusion and potential regions is included as a boundary condition. This interaction is modeled as a first-order reversible kinetic adsorption. The expressions for the mass transfer rate coefficients between the diffusion and the potential regions are derived in terms of the interaction energy profile between the nanoparticle and the collector. The resulting equations are solved numerically for a range of values of pore-scale parameters. The nanoparticle concentration profile obtained for the cylindrical pore is averaged over a moving averaging volume within the pore in order to get the 1-D concentration field. The latter is fitted to the 1-D advection-dispersion equation with an equilibrium or kinetic adsorption model to determine the values of the average deposition rate coefficients. Pore-scale simulations are performed for three values of Péclet number, Pe = 0.05, 5 and 50. It is found that under unfavorable conditions, the nanoparticle deposition at pore scale is best described by an equilibrium model at low Péclet numbers (Pe = 0.05), and by a kinetic model at high Péclet numbers (Pe = 50). But, at an intermediate Pe (e.g., near Pe = 5), both equilibrium and kinetic models fit the 1-D concentration field. Correlation equations for the pore-averaged nanoparticle deposition rate coefficients under unfavorable conditions are derived by performing a multiple-linear regression analysis between the estimated deposition rate coefficients for a single pore and various pore-scale parameters. The correlation equations, which follow a power law relationship with nine pore-scale parameters, are found to be consistent with the column-scale and pore-scale experimental results, and qualitatively agree with CFT. Nanoparticle transport is upscaled from pore to the Darcy scale in saturated porous media by incorporating the correlations equations for the pore-averaged deposition rate coefficients of nanoparticles in a cylindrical pore into a multi-directional pore-network model, PoreFlow (Raoof et al., 2013). Pore-network model simulations are performed for a range of parameter values, and nanoparticle BTCs are obtained from the pore-network model. Those curves are then modeled using 1-D advection-dispersion equation with a two-site first-order reversible deposition, with terms accounting for both equilibrium and kinetic sorption. Kinetic sorption is found to become important as the favorability of the surface for deposition decreases. Correlation equations for the Darcy¬scale deposition rate coefficients under unfavorable conditions are developed as a function of various measurable Darcy-scale parameters, including: porosity, mean pore throat radius, mean pore water velocity, nanoparticle radius, ionic strength, dielectric constant, viscosity, temperature, and surface potentials on the nanoparticle and grain surface. The correlation equations are found to be consistent with the observed trends from the column experiments available in the literature, and are in agreement with CFT for all parameters, except for the mean pore water velocity and nanoparticle radius. The Darcy-scale correlation equations contain multipliers whose values for a given set of experimental conditions need to be determined by comparing the values of the deposition rate coefficients predicted by the correlation equations against the estimated values of Darcy-scale deposition parameters obtained by fitting the BTCs from column or field experiments with 1-D advection-dispersion-deposition model. They account for the effect of factors not considered in this study, such as the physical and chemical heterogeneity of the grain surface and nanoparticles, flow stagnation points, grain-to-grain contacts, etc. Colloids are abundant in the subsurface and have been observed to interact with a variety of contaminants, including viruses, thereby significantly influencing their transport. A mathematical model is developed to simulate the co-transport of viruses and colloids in partially saturated porous media under steady state flow conditions. The virus attachment to the mobile and immobile colloids is described using a linear reversible kinetic model. It is assumed that colloid transport is not affected by the presence of attached viruses on its surface, and hence, colloid transport is decoupled from virus transport. The governing equations are solved numerically using an alternating three-step operator splitting approach. The model is verified by fitting three sets of experimental data published in the literature: (1) Syngouna and Chrysikopoulos (2013) and (2) Walshe et al. (2010), both on the co-transport of viruses and clay colloids under saturated conditions, and (3) Syngouna and Chrysikopoulos (2015) for the co-transport of viruses and clay colloids under unsaturated conditions. The model results are found to be in good agreement with the observed BTCs under both saturated and unsaturated conditions. Then, the developed model was used to simulate the co-transport of viruses and colloids in porous media under unsaturated conditions, with the aim of understanding the relative importance of various processes on the co-transport of viruses and colloids. The virus retention in porous media in the presence of colloids is greater under unsaturated conditions as compared to the saturated conditions due to: (1) virus attachment to the air-water interface (AWI), and (2) co-deposition of colloids with attached viruses on its surface to the AWI. A sensitivity analysis of the model to various parameters showed that virus attachment to AWI is the most sensitive parameter affecting the BTCs of both free viruses and total mobile viruses, and has a significant effect on all parts of the BTC. The free and the total mobile virus BTCs are mainly influenced by parameters describing virus attachment to the AWI, virus interactions with mobile and immobile colloids, virus attachment to solid-water interface (SWI), and colloid interactions with SWI and AWI. The virus BTC is relatively insensitive to parameters describing the maximum adsorption capacity of the AWI for colloids, inlet colloid concentration, virus detachment rate coefficient from the SWI, maximum adsorption capacity of the AWI for viruses, and inlet virus concentration.

Cette thèse est consacrée à l'étude des propriétés des transports diphasiques au sein des couches de diffusions (Gas Diffusion Layer = GDL) des piles à combustible PEMFC (Proton Exchange Membrane Fuel Cells). La GDL est faite d'une structure fibreuse (dont l'épaisseur est de quelques centaines de micromètres) traitée généralement avec une matière hydrophobe. Des images numériques de la GDL réelle obtenues par tomographie X sont d'abord analysées afin d'étudier des propriétés telles que la porosité, la perméabilité, ou le tenseur de diffusion. L'écrasement de la GDL est ensuite simulé en utilisant un algorithme comprimant les fibres dans un plan transversal. Les transports diphasiques (invasion quasi statique d'eau liquide) sont modélisés dans des réseaux de pores, milieux représentatifs de l'espace poreux de la GDL, en relation avec le problème de la gestion de l'eau dans les piles PEMFC. Deux algorithmes d'invasion, dénommés algorithmes séquentiel et cinétique, sont développés et comparés pour analyser les distributions de phases au sein des GDL. Un point clé est que l'eau rentre dans la couche poreuse par divers points d'injection indépendants, conduisant à la possibilité de multiples points de percée. Des expériences sur un système microfluidique sont conduites pour valider les algorithmes utilisés. Une étude statistique est menée pour caractériser le nombre de points de percée, les profils de saturation, l'accès au gaz, le transport diffusif, de même que l'influence du piégeage et de la mouillabilité mixte. / This thesis is devoted to the study of transport properties and two-phase flow in the Gas Diffusion Layer (GDL) of Proton Exchange Membrane Fuel Cells (PEMFC). A GDL is a thin fibrous structure (a few hundreds μm thick) treated generally with a hydrophobic agent. Numerical images obtained from X-ray computed tomography X are first exploited to study properties such as the porosity, permeability and diffusion tensors of a real GDL microstructure. The effect of GDL compression is also investigated using an algorithm mimicking the compression in GDL through plane direction. Then two phase flow (quasi-static water invasion) is studied in relation with the water management problem in PEMFC, using a structured pore network representation of the pore space. Two invasion algorithms, referred to as the sequential and the kinetic algorithm respectively, are developed and compared to study the fluid distributions within the GDL. A key point is that water enters the porous layer through multiple independent inlet injection points, leading to the possibility of many breakthrough points. Experiments are conducted on a microfluidic device to validate the algorithms. A numerical statistical study is performed to characterize the breakthrough point statistics, saturation profiles, gas access, diffusion transport as well as the influence of trapping and mixed wettability.

The catalycal oxidation of hydrogen sulphide in presence of small amounts of hexan has been investigated over a Titania catalyst. H2S is converted into sulphur. Nevertheless the catalyst undergoes a deactivation through coke formation. In order to explain the coke profile in the inner of the catalyst's pellet, the influence of the internal heat and mass transfers has been considered using a pore network. / Die katalytische Oxydation von Schwefelwasserstoff auf einem TiO2-Anatase-Katalysator wird in Anwesenheit von n-Hexan durchgeführt. Schwefel wird gebildet. Der Katalysator unterliegt jedoch eine schnelle Deaktivierung durch Russbildung, die anhand eines Porennetzwerksmodells simuliert werden wird. Sowohl Stoff- als auch Wärmetransport werden berücksichtigt.

[pt] As coquinas carbonáticas são rochas sedimentares constituídas principalmente por conchas e detritos. Elas apresentam diferentes características de textura que são afetadas pela sedimentação e diagênese, influenciando no arranjo de poros, e consequentemente, na permeabilidade. É necessário o entendimento do efeito da variação das tensões efetivas nessas formações rochosas e como problemas de dano mecânico irão afetar na produção e injeção de poços de sistemas de produção de petróleo e gás. Nesta tese, amostras de coquinas sintéticas foram confeccionadas e, com o auxílio de técnicas de microtomografia de raios-x (microCT), modelagem de rede de poros (PNM) e ensaios in situ, a influência da microestrutura dessas rochas na variação de suas propriedades devido a tensões de compressão confinada foi avaliada. O uso de microCT permitiu a análise da estrutura interna das amostras para a discriminação de poros. Uma célula para ensaios mecânicos, acoplável ao tomógrafo e capaz de aplicar tensões de compressão confinada, auxiliou no estudo da alteração da microestrutura das coquinas sob tensão. As amostras foram tomografadas em 3 condições: sem carregamento, com carregamento e após despressurização. PNM foi utilizada para simulação de transporte bifásico nas coquinas, além de relacionar dados como número de conectividade de poros, tortuosidade, permeabilidade, porosidade, diâmetro dos poros e capilares. Os resultados obtidos demonstram que a aplicação de tensões de compressão confinada resulta na redução da porosidade e permeabilidade das coquinas. A redução da porosidade total do estágio final em relação às condições iniciais das amostras foi de 87,5 por cento, 70,0 por cento e 85,9 por cento para CP1, CP2 e CP3, respectivamente. Já a permeabilidade absoluta em condições iniciais apresentou valores de 33,10, 11,60 e 22,90 mD para CP1, CP2 e CP3, respectivamente, reduzindo a zero com o aumento dos estágios de pressão, impossibilitando as coquinas de permear fluidos em seus interiores. Em relação aos estudos da literatura, a fabricação de coquinas sintéticas mostrou-se eficiente em termos de reprodutibilidade para coquinas carbonáticas autênticas, apresentando propriedades geomecânicas semelhantes. / [en] Carbonate coquinas are sedimentary rocks composed mainly of shells and fragments. They present different texture characteristics that are affected by sedimentation and diagenesis, influencing the pore arrangement and, consequently, the permeability. It is necessary to understand the effect of variations in effective stresses on these rock formations and how mechanical damage problems will affect the production and injection of wells in oil and gas production systems. In this thesis, samples of synthetic coquinas were made, and with the help of x-ray microtomography, pore network modeling techniques, and in situ tests, the influence of the microstructure of these rocks on the variation of their properties due to confined compression stresses was evaluated. The use of x-ray microtomography allowed the analysis of the samples internal structure for the discrimination of pores. Using a cell for mechanical tests, coupled with the tomograph and able to apply confined compressive stresses, helped study changes in the coquinas’ microstructure under tension. The samples were scanned under three conditions: without loading, with stress loading, and after depressurization. Pore network modeling was used to simulate single-phase transport in the coquinas, as well as for relating data such as pore connectivity number, tortuosity, permeability, porosity, pore, and capillary diameter. The results obtained demonstrate that the application of confined compressive stresses results in the reduction of porosity and permeability of the coquinas. The reduction of total porosity in the final stage relative to the initial conditions of the samples was 87.5 percent, 70.0 percent, and 85.9 percent for CP1, CP2, and CP3, respectively. Meanwhile, the absolute permeability under initial conditions presented values of 33.10, 11.60, and 22.90 mD for CP1, CP2, and CP3, respectively, which reduced to zero with the increase of pressure stages, making it impossible for the coquinas to permeate fluids within their interiors. Compared to literature studies, the production of synthetic coquinas proved to be efficient in terms of reproducibility for authentic carbonate coquinas, presenting similar geomechanical properties.

Afin de pouvoir utiliser les piles à combustible du type PEMFC dans une application automobile, leur coût doit être diminué et leur durée de vie doit être augmentée. De nombreux résultats montrent que la gestion de l'eau dans les piles PEMFC est essentielle sur ces aspects et qu’une meilleure maitrise contribuera a développer des piles plus performantes. La couche active cathodique (CCL, Cathode Catalyst Layer) est le lieu de production de l'eau ce qui en rend l'optimisation importante pour assurer une bonne gestion de l'eau. Dans ce travail, la méthode réseau de pores a été adaptée pour modéliser le transport diphasique dans la structure poreuse de la CCL. Dans l'état de l'art actuel, le modèle développé est le seul permettant d’analyser l'effet des proprietes locales de la CCL (structure, mouillabilite…) sur les mecanismes de transport diphasique. Cet outil de compréhension constitue également une base pour proposer des améliorations de la CCL afin d'améliorer les performances des piles. Les algorithmes d'invasion développés ont été analysés d'une façon détaillée. Le transport fluidique (gaz et liquide) est couplé avec le transport des charges (électrons et protons) par un modèle de réaction électrochimique. Les mécanismes de capillarité, de diffusion gazeuse et d’evaporation sont integres au modele afin d'avoir une représentation la plus complète possible du fonctionnement de la CCL. La description de la structure poreuse par un réseau de pore régulier, l'algorithme d'invasion de l'eau liquide et le modèle de la diffusion des gaz ont été validés par des comparaisons avec des résultats expérimentaux de la littérature ou spécifiques de ce travail. Le modèle est ensuite exploité pour analyser l'effet des paramètres de la CCL tels que la mouillabilité et la taille des pores sur les performances de la couche active. Les résultats permettent d’analyser de premieres idees de modifications de la CCL pour ameliorer la gestion de l’eau et les performances des PEMFC. / In order to use PEM fuel cells in an automotive application, their cost must be reduced and their lifetime must be increased. Many results show that water management is a critical issue in PEMFC optimization. The water is produced in the cathode active layer (CCL) which makes the optimization of this component very important to ensure a better water management in the PEMFC. In this work, the pore network method has been adapted to model the two-phase transport in the porous structure of the CCL. Considering the state of the art, this is the only model developed to analyze the effect of local properties of the CCL (structure, wetting ...) on the two-phase transport mechanisms. This model is proposed as a scientific tool to help understanding the fundamentals behind the transport phenomena inside the CCL and also to help in the conception of the future CCL. The liquid invasion algorithms developed in this work were analyzed in details. The fluids transport (gas and liquid) is coupled with the charges transport (electrons and protons) using an electrochemical reaction model. The capillary driven liquid transport, the gas phase diffusion and the evaporation process are all integrated into the model in order to have the most possible complete description of the CCL. The description of the porous structure by a regular network, the liquid invasion algorithm and the gas diffusion model all have been validated by comparisons with experimental results from literature or specific work . The model is then exploited to analyze the effect of parameters such as the CCL wettability and pore size distribution on the performance. The results allow analysis of initial ideas that can help in the conception of the CCL in order to improve the water management and the performances of the PEMFC.

The primary objective was to predict the relative storage capacity of carbonate rocks relevant for carbon dioxide sequestration. To achieve this, a detailed pore scale characterization of model carbonate rocks, Indiana Limestone and Pink Dolomite, was conducted utilizing micro-computed tomography (microCT) data using pore network modeling and invasion percolation simulations. For the first time in literature, Pink Dolomite’s pore space characteristics were analyzed. A secondary objective was to compare thresholding techniques as applied to carbonates which exhibit dual porosity (porosity at multiple length scales). The analysis showed the sensitivity of existing methods to the thresholding technique, imaging method and material. Overall, the contributions of this work provide an assessment of two carbonates relevant for carbon capture and storage at the pore scale; and a preliminary assessment into thresholding dual porosity carbonates.