The origin of this PhD thesis lies in the study of Ebullated Bed Reactors (EBRs). These chemical reactors are very active research topics in chemical processes, notably thanks to their usage in heavy oil processing. Many complex phenomena take place within EBRs, and make their design and optimization difficult. In fluid mechanics, a lot of physical mechanisms present in EBRs are active fields of study (three-phase flow, fluid-body interaction...). Hence, in the present work, a study of the mechanisms participating in the hydrodynamics of an EBR with cylindrical catalysts is performed. In a first part, the impact of the catalyst anisotropy on its fall is investigated. In order to gain insight on the effect of the body anisotropy on its fall dynamics, we investigate experimentally the free fall of a solid cylinder in a fluid at rest. The sensitivity to two dimensionless parameters, the Archimedes number (Ar) and the aspect ratio of the cylinder (L/d) is examined. Experiments are conducted with two orthogonal cameras, and advanced image processing techniques are developed in order to measure the position and orientation of the cylinder in 3D. Within the range of parameters studied (200 < Ar < 1100, 2 < L/d < 20), the cylinders adopt different types of falling motion. Two main types of paths are observed, the first one is a rectilinear fall of the cylinder that keeps its axis horizontal, and the second one is a fluttering oscillatory motion. Other more complex types of motion are observed and discussed. The fluttering motion of the cylinder is analyzed in details. On top of the study of the body motion, the cylinder wake is also visualized and characterized. A large number of particles are present at the same time inside an EBRs (about 40% of the mass). Interactions between multiple objects have a strong impact on the motion of each individual particle, but are very complex. In a first approximation, we take into account the presence of numerous particles by introducing a confined medium. We study experimentally the fall of a single cylinder in a confined vertical thin-gap cell, where the cylinders are free to move in only two directions. The cylinder elongation ratio (3<L/d<40) and density ratio ( c / f = 1.16, 2.70, 4.50) are the two parameters of interest. The Archimedes number of the cylinder lies within the same range as in the unconfined medium, and the two main modes of motion of the cylinder are a rectilinear motion, and a fluttering one. However, for the same parameters (Ar,L/d), the motion of the cylinder in the confined cell is strongly different in form to that in the unconfined medium. We also studied the interaction between a freely falling cylinder and a rising swarm of bubbles. This investigation was performed experimentally, in the confined cell used in the second part. Cylinders of various density ratio ( c / f = 1.16, 2.70, 4.50) and elongation ratio (3<L/d<20) are released in a bubble swarm of gas volume fraction between 2% and 5%. The cylinder motion is greatly modified by the bubble swarm. Several mechanisms of interaction between the cylinder and the bubbles are identified (direct contact, interactions with fluid perturbations...), and their effect is characterized. We perform a statistical analysis of the cylinder motion in the swarm, and compare it to results in the confined fluid at rest. The cylinder density ratio and elongation ratio both play an important role in its motion in the bubble swarm. Conditional statistics allow us to further investigate the effect of the contact between the cylinder and a bubble, and of the cylinder orientation in the swarm. Finally, the dispersion of the cylinder motion in the swarm is characterized. A major effect of the bubble swarm is to increase, through bubble-cylinder contacts, the probability of the cylinder to be in nearly vertical orientations. This drastically changes the kinematics of the cylinder as compared to its motion in the fluid at rest

L’étude des Réacteurs à Lit Bouillonnant (RLB) est à l’origine de ce projet de thèse. Ce type de réacteur chimique est très étudié en génie des procédés, en raison notamment de son utilisation pour l’hydrocraquage des charges lourdes. Des phénomènes complexes ont lieu dans un RLB, ce qui rend leur design et leur optimisation difficiles. Certains des mécanismes physiques prenant place dans les RLBs sont également des champs de recherche actifs en mécanique des fluides. Par conséquent, cette étude se concentre sur des mécanismes locaux participant à l’hydrodynamique des RLBs avec des catalyseurs cylindriques. Dans un premier temps, l’impact de l’anisotropie du catalyseur sur sa chute est étudié. Nous réalisons une étude expérimentale de la chute libre d’un cylindre en fluide au repos, afin de déterminer l’effet de l’anisotropie du corps sur sa dynamique. Les paramètres d’intérêt du problème sont le nombre d’Archimède du cylindre (Ar) et son rapport d’élongation (L/d). Les expériences sont menées avec deux caméras orthogonales, et des techniques de traitement d’images avancées sont développées pour parvenir à une mesure précise de la position et de l’orientation du corps en 3D. Pour (200 < Ar < 1100, 2 < L/d < 20), les cylindres adoptent différents types de trajectoire. Les deux principaux sont la chute rectiligne, durant laquelle l’axe du cylindre reste horizontal, et un mouvement de fluttering, qui est analysé en détail. D’autres types de mouvement plus complexes sont observés et discutés. De surcroît, le sillage du cylindre est analysé et caractérisé. De nombreuses particules sont présentes dans un RLB (40% de fraction massique environ). Les interactions entre ces corps multiples ont un impact fort sur le mouvement de chacun d’entre eux, mais sont très complexes. En première approximation, nous rendons compte de la présence de multiples particules en introduisant un milieu confiné. Nous étudions expérimentalement la chute d’un seul cylindre dans une cellule confinée verticale, dans laquelle le cylindre n’est libre de se mouvoir que dans deux directions. Le rapport d’élongation du cylindre (3<L/d<40) et son rapport de densité ( c / f = 1,16, 2,70, 4,50) sont les deux paramètres d’intérêt. Le nombre d’Archimède du cylindre se trouve entre les mêmes bornes qu’en milieu non confiné, et les deux modes principaux de mouvement sont aussi la chute rectiligne et le fluttering. Cependant, pour des paramètres (Ar,L/d) comparables, il existe des différences importantes dans le déplacement du cylindre comparé au cas non confiné. Nous avons également étudié l’interaction entre un cylindre en chute libre et un nuage de bullesascendantes. Cette étude a été menée expérimentalement dans la cellule confinée utilisée pour la seconde partie de la thèse. Des cylindres de plusieurs rapports de densité ( c / f = 1,16, 2,70, 4,50) and rapports d’élongation (3<L/d<20) ont été lâchés dans un nuage de bulles de fraction volumique de gaz comprise entre 2% et 5%. Plusieurs mécanismes d’interaction entre le cylindre et les bulles ont été identifiés (contact direct, interaction avec des perturbations du fluide. . .), et leur effet a été caractérisé. Nous avons effectué une étude statistique du mouvement du cylindre dans le nuage de bulles, et nous l’avons comparée aux résultats obtenus en milieu confiné et en fluide au repos. Les rapports de densité et d’élongation du cylindre jouent tous deux un rôle important dans son mouvement au sein du nuage de bulles. Des statistiques conditionnelles nous permettent d’approfondir notre analyse du contact entre le cylindre et les bulles, ainsi que du rôle de l’orientation du cylindre. Enfin, la dispersion du mouvement du cylindre dans le nuage est caractérisée. Un des principaux effets du nuage de bulles est d’accroître, via les contacts bulle cylindre, l’orientation du cylindre jusqu’à-ce qu’il soit presque vertical, ce qui a un effet très fort sur sa cinématique en comparaison avec le fluide au repos / The origin of this PhD thesis lies in the study of Ebullated Bed Reactors (EBRs). These chemical reactors are very active research topics in chemical processes, notably thanks to their usage in heavy oil processing. Many complex phenomena take place within EBRs, and make their design and optimization difficult. In fluid mechanics, a lot of physical mechanisms present in EBRs are active fields of study (three-phase flow, fluid-body interaction...). Hence, in the present work, a study of the mechanisms participating in the hydrodynamics of an EBR with cylindrical catalysts is performed. In a first part, the impact of the catalyst anisotropy on its fall is investigated. In order to gain insight on the effect of the body anisotropy on its fall dynamics, we investigate experimentally the free fall of a solid cylinder in a fluid at rest. The sensitivity to two dimensionless parameters, the Archimedes number (Ar) and the aspect ratio of the cylinder (L/d) is examined. Experiments are conducted with two orthogonal cameras, and advanced image processing techniques are developed in order to measure the position and orientation of the cylinder in 3D. Within the range of parameters studied (200 < Ar < 1100, 2 < L/d < 20), the cylinders adopt different types of falling motion. Two main types of paths are observed, the first one is a rectilinear fall of the cylinder that keeps its axis horizontal, and the second one is a fluttering oscillatory motion. Other more complex types of motion are observed and discussed. The fluttering motion of the cylinder is analyzed in details. On top of the study of the body motion, the cylinder wake is also visualized and characterized. A large number of particles are present at the same time inside an EBRs (about 40% of the mass). Interactions between multiple objects have a strong impact on the motion of each individual particle, but are very complex. In a first approximation, we take into account the presence of numerous particles by introducing a confined medium. We study experimentally the fall of a single cylinder in a confined vertical thin-gap cell, where the cylinders are free to move in only two directions. The cylinder elongation ratio (3<L/d<40) and density ratio ( c / f = 1.16, 2.70, 4.50) are the two parameters of interest. The Archimedes number of the cylinder lies within the same range as in the unconfined medium, and the two main modes of motion of the cylinder are a rectilinear motion, and a fluttering one. However, for the same parameters (Ar,L/d), the motion of the cylinder in the confined cell is strongly different in form to that in the unconfined medium. We also studied the interaction between a freely falling cylinder and a rising swarm of bubbles. This investigation was performed experimentally, in the confined cell used in the second part. Cylinders of various density ratio ( c / f = 1.16, 2.70, 4.50) and elongation ratio (3<L/d<20) are released in a bubble swarm of gas volume fraction between 2% and 5%. The cylinder motion is greatly modified by the bubble swarm. Several mechanisms of interaction between the cylinder and the bubbles are identified (direct contact, interactions with fluid perturbations...), and their effect is characterized. We perform a statistical analysis of the cylinder motion in the swarm, and compare it to results in the confined fluid at rest. The cylinder density ratio and elongation ratio both play an important role in its motion in the bubble swarm. Conditional statistics allow us to further investigate the effect of the contact between the cylinder and a bubble, and of the cylinder orientation in the swarm. Finally, the dispersion of the cylinder motion in the swarm is characterized. A major effect of the bubble swarm is to increase, through bubble-cylinder contacts, the probability of the cylinder to be in nearly vertical orientations. This drastically changes the kinematics of the cylinder as compared to its motion in the fluid at rest