A generalized model for bubble coalescence and breakup has been developed, which is based on a comprehensive survey of existing theories and models. One important feature of the model is that all important mechanisms leading to bubble coalescence and breakup in a turbulent gas-liquid flow are considered. The new model is tested extensively in a 1D Test Solver and a 3D CFD code ANSYS CFX for the case of vertical gas-liquid pipe flow under adiabatic conditions, respectively. Two kinds of extensions of the standard multi-fluid model, i.e. the discrete population model and the inhomogeneous MUSIG (multiple-size group) model, are available in the two solvers, respectively. These extensions with suitable closure models such as those for coalescence and breakup are able to predict the evolution of bubble size distribution in dispersed flows and to overcome the mono-dispersed flow limitation of the standard multi-fluid model. For the validation of the model the high quality database of the TOPFLOW L12 experiments for air-water flow in a vertical pipe was employed. A wide range of test points, which cover the bubbly flow, turbulent-churn flow as well as the transition regime, is involved in the simulations. The comparison between the simulated results such as bubble size distribution, gas velocity and volume fraction and the measured ones indicates a generally good agreement for all selected test points. As the superficial gas velocity increases, bubble size distribution evolves via coalescence dominant regimes first, then breakup-dominant regimes and finally turns into a bimodal distribution. The tendency of the evolution is well reproduced by the model. However, the tendency is almost always overestimated, i.e. too much coalescence in the coalescence dominant case while too much breakup in breakup dominant ones. The reason of this problem is discussed by studying the contribution of each coalescence and breakup mechanism at different test points. The redistribution of the gaseous phase from the injection position at the pipe wall to the whole cross section is overpredicted by the Test Solver especially for the test points with high superficial gas velocity. Besides the models for bubble forces, the simplification of the Test Solver to a 1D model has an influence on the redistribution process. Simulations performed using CFX show that a considerable improvement is achieved with comparison to the results delivered by the standard closure models. For the breakup-dominant cases, the breakup rate is again overestimated and the contribution of wake entrainment of large bubbles is underestimated. Furthermore, inlet conditions for the liquid phase, bubble forces as well as turbulence modeling are shown to have a noticeable influence, especially on the redistribution of the gaseous phase.

De nombreux procédés industriels nécessitent la manipulation de matériaux sous forme pulvérulente. L’émission de poussières générée par leur manipulation peut s’avérer dangereuse pour la santé des travailleurs ou bien causer un risque d’explosion. Afin de mieux comprendre les mécanismes de dispersion des poussières, le cas de la décharge d’un silo est étudié par simulation numérique avec une approche Euler-Euler. Deux configurations ont été étudiées au cours de cette thèse. La première, sans silo, a permis d’étudier l’influence de perturbations de vitesses imposées à l’entrée de la chambre de dispersion en lieu et place du silo. Cette étude a révélé que ces perturbations peuvent influencer l’élargissement du panache de poudre. Seules les perturbations avec une corrélation temporelle ont généré une ouverture importante du jet tombant semblable à celle relevée expérimentalement. Dans la deuxième configuration, le silo et la chambre de dispersion sont représentés afin d’étudier le couplage entre la dispersion du jet et l’écoulement dans le silo. L’une des difficultés de ces simulations est de prédire les différents régimes d’écoulements granulaires, allant de l’état quasi-statique dans le silo au régime très dilué lors de la dispersion du jet tombant, en passant par le régime collisionnel à la sortie du silo. La théorie cinétique permet de modéliser le régime dilué et collisionnel. En revanche pour la partie quasi-statique un modèle semi-empirique a été utilisé, implémenté et validé sur différentes configurations. La seconde étude a montré l’importance du rapport entre le diamètre de l’orifice et le diamètre des particules sur la structure du jet. En effet, lorsque ce paramètre est faible, le coeur du jet se contracte immédiatement après la sortie du silo puis s’ouvre en aval. Pour des valeurs grandes, l’ouverture du jet est négligeable. Cependant, il semblerait que l’angle du silo modifie le comportement de l’écoulement, ce qui nécessitera des études supplémentaires. / A wide range of industrial processes requires the handling of granular material in a pulverulent form. The subsequent dust emissions due to these processes can be harmful to the health of workers or hazardous explosion risks. In order to understand dust dispersion mechanisms, a case of a free falling granular jet discharged from a silo is studied by numerical simulations using an Euler-Euler approach. Two types of numerical simulation are conducted. First, the influence of velocity fluctuations at the inlet chamber is studied on the plume behavior, instead of the silo. This study reveals that fluctuations are enable to reproduce the jet expansion. It is established that only fluctuations with temporal correlation generate a large jet opening similar to the experiment. The second type of setup shows the coupling between the silo and the chamber. One of the major challenges is the ability to predict the different flow regimes going from quasi-static regime inside the silo, to the very dilute regime in the dust spread and include the collisional regime occurs through the silo. Kinetic theory allows modeling of the dilute and collisional regime. By contrast, frictional models have been used, implemented and validated in different cases. The second study highlights the key role of the ratio defined by the orifice diameter on the particle diameter. Indeed, when this parameter is small, the jet powder core contracts immediately after the exit of the silo dump plane and expands downstream. For high values, the granular jet does not exhibit dispersion anymore. This study suggests that the silo half-angle has an impact on the flow field which justifies the need for further investigations.

A generalized model for bubble coalescence and breakup has been developed, which is based on a comprehensive survey of existing theories and models. One important feature of the model is that all important mechanisms leading to bubble coalescence and breakup in a turbulent gas-liquid flow are considered. The new model is tested extensively in a 1D Test Solver and a 3D CFD code ANSYS CFX for the case of vertical gas-liquid pipe flow under adiabatic conditions, respectively. Two kinds of extensions of the standard multi-fluid model, i.e. the discrete population model and the inhomogeneous MUSIG (multiple-size group) model, are available in the two solvers, respectively. These extensions with suitable closure models such as those for coalescence and breakup are able to predict the evolution of bubble size distribution in dispersed flows and to overcome the mono-dispersed flow limitation of the standard multi-fluid model. For the validation of the model the high quality database of the TOPFLOW L12 experiments for air-water flow in a vertical pipe was employed. A wide range of test points, which cover the bubbly flow, turbulent-churn flow as well as the transition regime, is involved in the simulations. The comparison between the simulated results such as bubble size distribution, gas velocity and volume fraction and the measured ones indicates a generally good agreement for all selected test points. As the superficial gas velocity increases, bubble size distribution evolves via coalescence dominant regimes first, then breakup-dominant regimes and finally turns into a bimodal distribution. The tendency of the evolution is well reproduced by the model. However, the tendency is almost always overestimated, i.e. too much coalescence in the coalescence dominant case while too much breakup in breakup dominant ones. The reason of this problem is discussed by studying the contribution of each coalescence and breakup mechanism at different test points. The redistribution of the gaseous phase from the injection position at the pipe wall to the whole cross section is overpredicted by the Test Solver especially for the test points with high superficial gas velocity. Besides the models for bubble forces, the simplification of the Test Solver to a 1D model has an influence on the redistribution process. Simulations performed using CFX show that a considerable improvement is achieved with comparison to the results delivered by the standard closure models. For the breakup-dominant cases, the breakup rate is again overestimated and the contribution of wake entrainment of large bubbles is underestimated. Furthermore, inlet conditions for the liquid phase, bubble forces as well as turbulence modeling are shown to have a noticeable influence, especially on the redistribution of the gaseous phase.