Les simulations numériques des équations d’Euler deux-fluides réalisé sur des maillages grossiers éliminent les structures fins d’écoulement gaz-solide dans les lits fluidisés. Pour précisément estimer l’hydrodynamique globale de lit, il faut proposer une modélisation qui prend en compte les effets de structure non-résolue. Dans ce but, les maillages sont raffinés pour obtenir le résultat de simulation pleinement résolue ce que les grandeurs statistiques ne modifient plus avec un autre raffinement pour le lit fluidisé périodique dilué gaz-particules sur une géométrie 3D cartésienne et ce résultat est utilisé pour tests "a priori". Les résultats de tests "a priori" montrent que l’équation filtrée de la quantité de mouvement est effectuée mais il faut prendre en compte le flux de la fraction volumique de solide de sous-maille en raison de l’interaction locale de la vitesse du gaz et la fraction volumique de solide pour la force traniée. Nous proposons les modèles fonctionnels et structurels pour le flux de la fraction volumique de solide de sous-maille. En plus, les modèles fermetures du tenseur de sous-maille de la phase dispersée sont similaires aux modèles classiquement utilisés en écoulement turbulent monophasique. Tous les modèles sont validés par test "a priori" et "a posteriori" / Eulerian two fluid approach is generally used to simulate gas-solid flows in industrial circulating fluidized beds. Because of limitation of computational resources, simulations of large vessels are usually performed by using too coarse grid. Coarse grid simulations can not resolve fine flow scales which can play an important role in the dynamic behaviour of the beds. In particular, cancelling out the particle segregation effect of small scales leads to an inadequate modelling of the mean interfacial momentum transfer between phases and particulate shear stresses by secondary effect. Then, an appropriate modelling ac counting for influences of unresolved structures has to be proposed for coarse-grid simu-lations. For this purpose, computational grids are refined to get mesh-independent result where statistical quantities do not change with further mesh refinement for a 3-D peri-odic circulating fluidized bed. The 3-D periodic circulating fluidized is a simple academic configuration where gas-solid flow conducted with A-type particles is periodically driven along the opposite direction of the gravity. The particulate momentum and agitation equations are filtered by the volume averaging and the importance of additional terms due to the averaging procedure are investigated by budget analyses using the mesh independent result. Results show that the filtered momentum equation of phases can be computed on coarse grid simulations but sub-grid drift velocity due to the sub-grid correlation between the local fluid veloc- ity and the local particle volume fraction and particulate sub-grid shear stresses must be taken into account. In this study, we propose functional and structural models for sub- grid drift velocity, written in terms of the difference between the gas velocity-solid volume fraction correlation and the multiplication of the filtered gas velocity with the filtered solid volume fraction. Particulate sub-grid shear stresses are closed by models proposed for single turbulent flows. Models’ predictabilities are investigated by a priori tests and they are validated by coarse-grid simulations of 3-D periodic circulating, dense fluidized beds and experimental data of industrial scale circulating fluidized bed in manner of a posteriori tests

No presente trabalho desenvolve-se um estudo de modelagem matemática e simulação numérica do escoamento bifásico gás-sólido na coluna ascendente de um leito fluidizado circulante. Utiliza-se o modelo euleriano de duas fases separadas considerando dois procedimentos diferentes para a modelagem do tensor das tensões da fase sólida: modelo tradicional e a teoria cinética dos escoamentos granulares (TCEG). As simulações numéricas são conduzidas com a utilização do código MFIX que é um software livre e disponível na rede (Internet). Os resultados da simulação numérica são avaliados por meio da análise da influência dos seguintes parâmetros: malha computacional, correlações para o computo do tensor das tensões da fase sólida e esquemas de discretização dos termos advectivos. Também se desenvolve estudo de caracterização de estruturas coerentes - \"clusters\". De forma complementar foram realizadas duas análises teóricas compreendendo: uma análise da influência das diversas correlações utilizadas na TCEG para o computo da viscosidade dinâmica do sólido; e uma análise enfocando o emprego de diversos esquemas de discretização para os termos advectivos presentes nas equações de conservação (Foup, Muscl, Van Leer, Minmod e Superbee). De todos os estudos e resultados apresentados no trabalho conclui-se que os escoamentos gás-sólido em leitos fluidizados circulantes são muito complexos, sendo necessário a realização de futuras pesquisas para uma melhor compreensão dos fenômenos físicos inerentes a esses escoamentos. / In the present work is described a mathematical model and numerical study simulation of the gas-solid flow in the riser of a circulating fluidized bed. It is used the two fluids eulerian model considering two different procedures for the solid phase stress tensor modeling: the traditional model and the kinetic theory of granular flows (KTGF). The numerical simulation results are evaluated through the influence analysis of the following parameters: computational mesh, correlations for computing the solid phase stress tensor and the discretization of the advective terms. It is also presented a study concerning the characterization coherent structures - \"clusters\". Complementing the above studies were accomplished two theoretical analyses comprehending: an influence analysis of several correlations used in the KTGF for computing the dynamic viscosity of the solid phase; and an analysis concerning several discretization schemes for the advective terms present in the conservative equations. Considering the developed studies and the obtained results it is concluded that the gas-solid flows in circulating fluidized beds are very complex, being necessary future research works for a better comprehension of the inherent physical phenomena to these flows.

No presente trabalho desenvolve-se um estudo de modelagem matemática e simulação numérica do escoamento bifásico gás-sólido na coluna ascendente de um leito fluidizado circulante. Utiliza-se o modelo euleriano de duas fases separadas considerando dois procedimentos diferentes para a modelagem do tensor das tensões da fase sólida: modelo tradicional e a teoria cinética dos escoamentos granulares (TCEG). As simulações numéricas são conduzidas com a utilização do código MFIX que é um software livre e disponível na rede (Internet). Os resultados da simulação numérica são avaliados por meio da análise da influência dos seguintes parâmetros: malha computacional, correlações para o computo do tensor das tensões da fase sólida e esquemas de discretização dos termos advectivos. Também se desenvolve estudo de caracterização de estruturas coerentes - \"clusters\". De forma complementar foram realizadas duas análises teóricas compreendendo: uma análise da influência das diversas correlações utilizadas na TCEG para o computo da viscosidade dinâmica do sólido; e uma análise enfocando o emprego de diversos esquemas de discretização para os termos advectivos presentes nas equações de conservação (Foup, Muscl, Van Leer, Minmod e Superbee). De todos os estudos e resultados apresentados no trabalho conclui-se que os escoamentos gás-sólido em leitos fluidizados circulantes são muito complexos, sendo necessário a realização de futuras pesquisas para uma melhor compreensão dos fenômenos físicos inerentes a esses escoamentos. / In the present work is described a mathematical model and numerical study simulation of the gas-solid flow in the riser of a circulating fluidized bed. It is used the two fluids eulerian model considering two different procedures for the solid phase stress tensor modeling: the traditional model and the kinetic theory of granular flows (KTGF). The numerical simulation results are evaluated through the influence analysis of the following parameters: computational mesh, correlations for computing the solid phase stress tensor and the discretization of the advective terms. It is also presented a study concerning the characterization coherent structures - \"clusters\". Complementing the above studies were accomplished two theoretical analyses comprehending: an influence analysis of several correlations used in the KTGF for computing the dynamic viscosity of the solid phase; and an analysis concerning several discretization schemes for the advective terms present in the conservative equations. Considering the developed studies and the obtained results it is concluded that the gas-solid flows in circulating fluidized beds are very complex, being necessary future research works for a better comprehension of the inherent physical phenomena to these flows.

Lors que nous examinons numériquement des phénomènes multiphasiques suite à un accidentgrave dans le réacteur nucléaire, la dimension caractéristique des zones multi-fluides(non-réactifs et réactifs) s’avère beaucoup plus petite que celle du bâtiment réacteur, cequi fait la Simulation Numérique Directe de la configuration à peine réalisable. Autrement,nous proposons de considérer la zone de mélange multiphasique comme une interface infinimentfine. Puis, le solveur de Riemann réactif est inséré dans la Méthode des ÉquationsDiscrètes Réactives (RDEM) pour calculer le front de combustion à grande vitesse représentépar une interface discontinue. Une approche anti-diffusive est ensuite couplée avec laRDEM afin de précisément simuler des interfaces réactives. La robustesse et l’efficacité decette approche en calculant tant des interfaces multiphasiques que des écoulements réactifssont à la fois améliorées grâce à la méthode ici proposée : upwind downwind-controlled splitting(UDCS). UDCS est capable de résoudre précisément des interfaces avec les maillagesnon-structurés multidimensionnels, y compris des fronts réactifs de détonation et de déflagration. / When numerically investigating multiphase phenomena during severe accidents in a reactorsystem, characteristic lengths of the multi-fluid zone (non-reactive and reactive) are foundto be much smaller than the volume of the reactor containment, which makes the directmodeling of the configuration hardly achievable. Alternatively, we propose to consider thephysical multiphase mixture zone as an infinitely thin interface. Then, the reactive Riemannsolver is inserted into the Reactive Discrete Equations Method (RDEM) to compute highspeed combustion waves represented by discontinuous interfaces. An anti-diffusive approachis also coupled with RDEM to accurately simulate reactive interfaces. Increased robustnessand efficiency when computing both multiphase interfaces and reacting flows are achievedthanks to an original upwind downwind-controlled splitting method (UDCS). UDCS is capableof accurately solving interfaces on multi-dimensional unstructured meshes, includingreacting fronts for both deflagration and detonation configurations.

This dissertation focused on development and utilization of numerical and experimental approaches to improve the CFD modeling of fluidization flow of cohesive micron size particles. The specific objectives of this research were: (1) Developing a cluster prediction mechanism applicable to Two-Fluid Modeling (TFM) of gas-solid systems (2) Developing more accurate drag models for Two-Fluid Modeling (TFM) of gas-solid fluidization flow with the presence of cohesive interparticle forces (3) using the developed model to explore the improvement of accuracy of TFM in simulation of fluidization flow of cohesive powders (4) Understanding the causes and influential factor which led to improvements and quantification of improvements (5) Gathering data from a fast fluidization flow and use these data for benchmark validations. Simulation results with two developed cluster-aware drag models showed that cluster prediction could effectively influence the results in both the first and second cluster-aware models. It was proven that improvement of accuracy of TFM modeling using three versions of the first hybrid model was significant and the best improvements were obtained by using the smallest values of the switch parameter which led to capturing the smallest chances of cluster prediction. In the case of the second hybrid model, dependence of critical model parameter on only Reynolds number led to the fact that improvement of accuracy was significant only in dense section of the fluidized bed. This finding may suggest that a more sophisticated particle resolved DNS model, which can span wide range of solid volume fraction, can be used in the formulation of the cluster-aware drag model. The results of experiment suing high speed imaging indicated the presence of particle clusters in the fluidization flow of FCC inside the riser of FIU-CFB facility. In addition, pressure data was successfully captured along the fluidization column of the facility and used as benchmark validation data for the second hybrid model developed in the present dissertation. It was shown the second hybrid model could predict the pressure data in the dense section of the fluidization column with better accuracy.