Lagrangian stochastic modeling of turbulent gas-solid flows with two-way coupling in homogeneous isotropic turbulence

Dans ce travail de thèse, réalisé à l'IMFT, nous nous sommes intéressés aux écoulements turbulents diphasiques gaz-solides et plus particulièrement au phénomène de couplage inverse qui correspond à la modulation de la turbulence par la phase dispersée. Ce mécanisme est crucial pour les écoulements à forts chargements massiques. Dans cette thèse, nous avons considéré une turbulence homogène isotrope stationnaire sans gravité dans laquelle des particules sont suivies individuellement d'une façon Lagrangienne. La turbulence du fluide porteur est obtenue par des simulations directes (DNS). Les particules sont sphériques, rigides et d'une taille inférieure aux plus petites échelles de la turbulence. Leur densité est bien plus grande que la densité du fluide. Dans ce cadre, la force la plus importante agissant sur les particules est celle de traînée. Les interactions inter-particules ainsi que la gravité ne sont pas prises en compte. Pour modéliser ce type d'écoulement, une approche stochastique est utilisée pour laquelle l'accélération du fluide est modélisée par une équation de Langevin. L'originalité de ce travail est la prise en compte de l'effet de la modulation de la turbulence par un terme additionnel. Nous avons proposé deux modèles : une force de couplage moyenne qui est définie à partir des vitesses moyennes des phases, et une force instantanée qui est définie à l'aide du formalisme mésoscopique Eulérien. La fermeture des modèles s’appuie sur la fonction d’autocorrélation Lagrangienne et l’équation de transport de l’énergie cinétique. Les modèles sont testés en terme de prédiction de la vitesse de dérive et des corrélations fluide-particule. Les résultats montrent que le modèle moyen, plus simple, prend en compte les effets principaux du couplage inverse. Cependant, le problème de fermeture pratique est reporté sur la modélisation de l’échelle intégrale Lagrangienne et l’énergie cinétique de la turbulence du fluide vue par les particules. ABSTRACT : In this thesis, performed in IMFT, we are interested in the turbulent gas-solid flows and more specifically, in the phenomenon of turbulence modulation which is the modification of the structure of the turbulence due to the solid particles. This mechanism is crucial in flows with high particle mass-loadings. In this work, we considered a homogeneous isotropic turbulence without gravity kept stationary with stochastic type forcing. Discrete particles are tracked individually in Lagrangian manner. Turbulence of the carrier phase is obtained by using DNS. The particles are spherical, rigid and of a diameter smaller than the smallest scales of turbulence. Their density is very large in comparison to the density of the fluid. In this configuration the only force acting on the particles is the drag force. Volume fraction of particles is very small and inter-particle interactions are not considered. To model this type of flow, a stochastic approach is used where the fluid element accel- eration is modeled using stochastic Langevin equation. The originality in this work is an additional term in the stochastic equation which integrates the effect of the particles on the trajectory of fluid elements. To model this term, we proposed two types of modeling: a mean drag model which is defined using the mean velocities from the mean transport equations of the both phases and an instantaneous drag term which is written with the help of the Mesoscopic Eulerian Approach. The closure of the models is based on the Lagrangian auto- correlation function of the fluid velocity and on the transport equation of the fluid kinetic energies. The models are tested in terms of the fluid-particle correlations and fluid-particle turbulent drift velocity. The results show that the mean model, simple, takes into account the principal physical mechanism of turbulence modulation. However, practical closure problem is brought forward to the Lagrangian integral scale and the fluid kinetic energy of the fluid turbulence viewed by the particles

Numerical study of solid particle axial mixing in a fixed cylindrical drum with rotating paddles

Axial mixture characterization is a wide spread problem in granular particle blending processes such as in an horizontal drum mixer. The homogeneous mixture of particles is obtained by blending the particles via rotating paddles in a fixed cylindrical drum. This problem, common to many technological devices, is crucial in the manufacture of a broad variety of industrial products, such as polypropylene. The granular flow behavior in these systems is still poorly understood and the numerical study of such configurations receives increasing academic and industrial attention. In this paper, a study is conducted to investigate the effects of different aspects of the reactor design on the axial transport of monodisperse, uniform density and spherical polypropylene particles. Results show that principally the shape of the paddles is the important design consideration to enhance the axial transport of particles

Numerical Simulation of Liquid Injection into an Anisothermal Dense Fluidized Bed

Fluidized beds are widely used in reactive industrial processes such as: olefins production, oil cracking or uranium fluorination because of their high efficiency mixing properties. In such processes, the chemical reaction is strongly dependent on the temperature and liquid is injected into the reactor in order to cool it. The experimental data of the time evolution of the gas temperature in an anisothermal dense fluidized bed with a liquid injection provided by INEOS is first compared with the results predicted by a simple model with an assumption of a perfectly mixed fluidized bed (uniform solid and temperature distribution in the bed). The results of this simple model show that the time evolution of the gas temperature is accurately predicted. Additionally, we point out that the wall-to-bed heat transfer plays a crucial role of the gas temperature in the bed. Then, we performed 3-D numerical simulations that let us investigate local interactions between phases and heat transfer with wall. The simulations show that the liquid evaporates quickly and the temperature is in a satisfactory agreement with the experiment data

3D Numerical Simulation of Catalyst Injection into a Dense Fluidized Bed

In this study, the injection of catalyst particles into a dense gas-solid fluidized bed reactor is investigated using the Euler/Euler approach. Numerical simulations of horizontal catalyst+gas mixture injection through a circular-sectioned nozzle have been performed in order to study the effect of the operating conditions of an injector on the flow hydrodynamics. The study is in the context of a larger project to optimize the nozzle geometry. The nozzle is composed of two annular sections, one outer and one inner. Catalyst is injected from the central section of the nozzle with a transport gas called here “gas 1”. Another gas is injected in the outer section: “gas 2”. The results show that the good dispersion of the catalyst depends highly on the ratio of the velocities in the outer and the inner sections of the nozzle

3D Numerical Simulation of Catalyst Injection into a Dense Fluidized Bed

International audienceIn this study, the injection of catalyst particles into a dense gas-solid fluidized bed reactor is investigated using the Euler/Euler approach. Numerical simulations of horizontal catalyst+gas mixture injection through a circular-sectioned nozzle have been performed in order to study the effect of the operating conditions of an injector on the flow hydrodynamics. The study is in the context of a larger project to optimize the nozzle geometry. The nozzle is composed of two annular sections, one outer and one inner. Catalyst is injected from the central section of the nozzle with a transport gas called here "gas 1". Another gas is injected in the outer section: "gas 2". The results show that the good dispersion of the catalyst depends highly on the ratio of the velocities in the outer and the inner sections of the nozzle

Prevalence of sleep disorders in the Turkish adult population epidemiology of sleep study

Sleep disorders constitute an important public health problem. Prevalence of sleep disorders in Turkish adult population was investigated in a nationwide representative sample of 5021 Turkish adults (2598 women and 2423 men, response rate: 91%) by an interviewer‐administered questionnaire. Insomnia was defined by the DSM‐IV criteria, habitual snoring and risk for sleep‐related breathing disorders (SDB) by the Berlin questionnaire, excessive daytime sleepiness (EDS) by the Epworth sleepiness scale score, and restless legs syndrome (RLS) by the complaints according to the International Restless Legs Syndrome Study Group criteria. Mean age of the participants was 40.7 ± 15.1 (range 18 to 90) years. Prevalence rates (men/women) were insomnia 15.3% (10.5%/20.2%; P < 0.001), high probability of SDB 13.7% (11.1%/20.2%; P < 0.001), EDS 5.4% (5.0%/5.7%; P: 0.09), RLS 5.2% (3.0%/7.3%; P < 0.001). Aging and female gender were associated with higher prevalence of sleep disorders except for habitual snoring. Prevalence rates of the sleep disorders among Turkish adults based on the widely used questionnaires were close to the lower end of the previous estimates reported from different parts of the world. These findings would help for the assessment of the health burden of sleep disorders and addressing the risk groups for planning and implementation of health care

Modélisation lagrangienne stochastique des écoulements gaz-solides turbulents avec couplage inverse en turbulence homogène isotrope stationnaire

Dans ce travail de thèse, réalisé à l'IMFT, nous nous sommes intéressés aux écoulements turbulents diphasiques gaz-solides et plus particulièrement au phénomène de couplage inverse qui correspond à la modulation de la turbulence par la phase dispersée. Ce mécanisme est crucial pour les écoulements à forts chargements massiques. Dans cette thèse, nous avons considéré une turbulence homogène isotrope stationnaire sans gravité dans laquelle des particules sont suivies individuellement d'une façon Lagrangienne. La turbulence du fluide porteur est obtenue par des simulations directes (DNS). Les particules sont sphériques, rigides et d'une taille inférieure aux plus petites échelles de la turbulence. Leur densité est bien plus grande que la densité du fluide. Dans ce cadre, la force la plus importante agissant sur les particules est celle de traînée. Les interactions inter-particules ainsi que la gravité ne sont pas prises en compte. Pour modéliser ce type d'écoulement, une approche stochastique est utilisée pour laquelle l'accélération du fluide est modélisée par une équation de Langevin. L'originalité de ce travail est la prise en compte de l'effet de la modulation de la turbulence par un terme additionnel. Nous avons proposé deux modèles : une force de couplage moyenne qui est définie à partir des vitesses moyennes des phases, et une force instantanée qui est définie à l'aide du formalisme mésoscopique Eulérien. La fermeture des modèles s’appuie sur la fonction d’autocorrélation Lagrangienne et l’équation de transport de l’énergie cinétique. Les modèles sont testés en terme de prédiction de la vitesse de dérive et des corrélations fluide-particule. Les résultats montrent que le modèle moyen, plus simple, prend en compte les effets principaux du couplage inverse. Cependant, le problème de fermeture pratique est reporté sur la modélisation de l’échelle intégrale Lagrangienne et l’énergie cinétique de la turbulence du fluide vue par les particules.In this thesis, performed in IMFT, we are interested in the turbulent gas-solid flows and more specifically, in the phenomenon of turbulence modulation which is the modification of the structure of the turbulence due to the solid particles. This mechanism is crucial in flows with high particle mass-loadings. In this work, we considered a homogeneous isotropic turbulence without gravity kept stationary with stochastic type forcing. Discrete particles are tracked individually in Lagrangian manner. Turbulence of the carrier phase is obtained by using DNS. The particles are spherical, rigid and of a diameter smaller than the smallest scales of turbulence. Their density is very large in comparison to the density of the fluid. In this configuration the only force acting on the particles is the drag force. Volume fraction of particles is very small and inter-particle interactions are not considered. To model this type of flow, a stochastic approach is used where the fluid element accel- eration is modeled using stochastic Langevin equation. The originality in this work is an additional term in the stochastic equation which integrates the effect of the particles on the trajectory of fluid elements. To model this term, we proposed two types of modeling: a mean drag model which is defined using the mean velocities from the mean transport equations of the both phases and an instantaneous drag term which is written with the help of the Mesoscopic Eulerian Approach. The closure of the models is based on the Lagrangian auto- correlation function of the fluid velocity and on the transport equation of the fluid kinetic energies. The models are tested in terms of the fluid-particle correlations and fluid-particle turbulent drift velocity. The results show that the mean model, simple, takes into account the principal physical mechanism of turbulence modulation. However, practical closure problem is brought forward to the Lagrangian integral scale and the fluid kinetic energy of the fluid turbulence viewed by the particles

Simulation 3D instationnaire d’un jet tombant de particules avec un modèle hybride granulaire-cinétique

L'écoulement 3D d’un jet tombant de particules fines sortant librement d'un silo est simulé en utilisant l'approche à deux fluides, dite Euler-Euler, et les résultats sont comparés à des données expérimentales. Les simulations montrent que la prise en compte de la viscosité frictionnelle entre les particules et du frottement entre les particules et la paroi a une grande influence sur le débit des particules en sortie du silo et permet de retrouver les valeurs expérimentales. Cependant, la vitesse moyenne verticale des particules en air libre est mal prédite. Les particules tombent en masse, pratiquement sans interaction avec l’air environnant. En revanche, si les évents disposés sur la partie haute du silo sont fermés, les simulations montrent un ralentissement marqué de la vitesse du jet de particules proche des données expérimentales. Cet effet est lié à la remontée de bulles d'air dans le silo dues à la conservation de volume du mélange qui entraine une dispersion radiale des particules

Numerical Simulation of Liquid Injection into an Anisothermal Dense Fluidized Bed

International audienceFluidized beds are widely used in reactive industrial processes such as: olefins production, oil cracking or uranium fluorination because of their high efficiency mixing properties. In such processes, the chemical reaction is strongly dependent on the temperature and liquid is injected into the reactor in order to cool it. The experimental data of the time evolution of the gas temperature in an anisothermal dense fluidized bed with a liquid injection provided by INEOS is first compared with the results predicted by a simple model with an assumption of a perfectly mixed fluidized bed (uniform solid and temperature distribution in the bed). The results of this simple model show that the time evolution of the gas temperature is accurately predicted. Additionally, we point out that the wall-to-bed heat transfer plays a crucial role of the gas temperature in the bed. Then, we performed 3-D numerical simulations that let us investigate local interactions between phases and heat transfer with wall. The simulations show that the liquid evaporates quickly and the temperature is in a satisfactory agreement with the experiment data