Computation of eddy currents in highly conductive particles dispersed in a moderately conductive matrix

In this article, we report 3D numerical simulations of highly conductive non-magnetic particles dispersed in a moderately conductive matrix, subject to an AC magnetic field in a range of several hundred kHz. We address the issue of the scaling of current loops and heating power with respect to the volume fraction of the dispersed phase. Simulations are performed in two steps. First, a static electric potential gradient is imposed between two opposite faces of the simulation domain and an effective conductivity is computed in good agreement with percolation models. Second, the particles are constrained in a spherical sub-region and an AC magnetic field is imposed at the boundary of the domain. For small volume fractions, the induced Joule power is in good agreement with an analytical model of dilute dispersions. As the volume fraction increases, wider current loops form, until the percolation threshold is reached. Then the induced power in the spherical aggregate is well described by the power induced in an equivalent sphere with a volume-fraction-dependent conductivity

Comparison between numerical and experimental results on thermoconvective instabilities of a high-Prandtl-number liquid

International audienceThe flow structuration of silicon oil ͑Prandtl number of 10.3͒ in a open cylindrical pool heated from the center of the surface is investigated numerically. Our purpose is to perform the numerical simulation of experimental results obtained by Favre et al. ͓Phys. Fluids 9, 1473 ͑1997͔͒ who observed transitions between steady and axisymmetric flows at sufficiently low values of the Marangoni number ͑Ma͒ and various types of instability depending on the height of the fluid. The hydrothermal wave regime has been obtained at critical values of Ma which depend on the Bond number and on the aspect ratio. The numerical results are in good agreement with the experimental ones

Etude des instabilités thermocapillaires et thermogravitaires dans un bain d'huile

La structuration de l'écoulement d'une huile aux silicones (nombre de Prandtl Pr 10, 3 = ) dans une cellule cylindrique (rayon extérieure 2 50 R mm = ) à surface libre soumise à un gradient horizontal de température est étudiée numériquement. Le fluide de hauteur [ ] 1;15 h mm Î est chauffé par un plot horizontal de rayon 1 15 R mm = à la température 1 T posé en surface au centre de la cellule et refroidi par les parois inférieures et latérales de la cuve maintenues à une température 2 T telle que 1 2 0 T T T D = - ³ . Les forces motrices de l'écoulement sont les forces thermocapillaires traduites par le nombre adimensionnel de Marangoni (Ma) et les forces thermogravitaires traduites par le nombre de Rayleigh (Ra). L'objectif est d'acquérir des connaissances fines sur les types d'instabilités thermoconvectives et leur modélisation numérique en se basant sur des résultats expérimentaux obtenus dans les travaux de doctorat de E. Favre (1997). Les simulations numériques 2D ou 3D sont réalisées avec le logiciel commercial Fluent. Un écoulement de base stationnaire et axisymétrique est obtenu dès que l'écart de température imposé T D est différent de zéro. Pour 14, 9 h mm = , une confrontation des résultats numériques et expérimentaux de l'écoulement de base est réalisée. Lorsque la contrainte thermique appliquée est augmentée, l'écoulement de base se déstabilise en différents types d'instabilités stationnaires en fonction de la hauteur de fluide considérée. L'instabilité en rouleaux co-rotatifs se développant près du plot chauffant pour des petites hauteurs 3 h mm £ et celle en rouleaux d'axes longitudinaux pour les grandes hauteurs 6 h mm > ont pu être retrouvées numériquement. Au-delà d'un second seuil, les instabilités deviennent instationnaires. Pour de faibles hauteurs de fluide, des ondes hydrothermales, instabilités ondulatoires propagatives, apparaissent en superposition des rouleaux co-rotatifs. Leurs structures ont pu être visualisées et leurs seuils d'apparition déterminés par des méthodes numériques spécifiques. La longueur d'onde et la valeur de Marangoni critique ( ) c Ma d'apparition des instabilités sont en bon accord avec les valeurs expérimentales de E. Favre (1997)

Cold crucible for HLW vitrification : advances in the feed and PGM behavior modelling

International audienceA cold crucible induction melter is in operation in La Hague plant, which is operated by ORANO since 2010 for the vitrification of HLW arising from D&D operation and old reprocessing of high Molybdenum content fuel. The well-known advantages of the cold crucible compared to the hot metallic inductive melter are (i) a higher elaboration temperature (ii) an extended lifetime and (iii) a better homogeneity obtained by mechanical stirring and gas bubbling. As a result, the global production capacity is expected to be higher as well as the PM concentration in the glass.A part of the development of this technology is made with the help of numerical simulation of the glass flow and heating by direct Joule effect thanks to the high frequency induction power unit. In this paper, the last effort of 3D modelisation of the Platinum-Group-Metals (PGM) particles behavior in the glass and chemical reaction kinetics of the feed are detailed.During the melting, the glass generally shows a homogeneous liquid phase seeded with non-soluble heavy platinum-group-metal particles mainly made of palladium and ruthenium dioxide. Previous studies [1] reported spatial discrepancies of the local volume fraction of particles in the melt because of particles settling with time. This migration of particles towards less agitated bottom parts of the crucible affects the dynamical, electrical and thermal state of the melt because of concentration-dependent electrical conductivity and viscosity of the suspension. A theoretical one-fluid transport model was developed with the help of small scale experiments carried out with a HLW glass simulant. The model is coupled with existing 3D thermo-hydraulic numerical codes, thus enhancing the precision of heat flux predictions between the melt and the crucible.The feed modelling is also under development. From vitreous precursors (Sodium alumino-borosilicate glass frit) and calcined waste (a mixture of about thirty oxide and nitrate compounds obtained after calcination step), a succession of processes such as impregnation, denitration, crystallization and dis-solution occur in order to solubilize the waste into the vitreous network. Mainly focus on thermal and chemical (0D models) approaches the objective aim at coupling the reaction kinetics laws of thermally activated processes (denitration, crystallization, dissolution) to the magneto-thermo-hydraulic model in order to model both physic and chemistry of glass synthesis. The experimental methodology based on the characterization of thermo-activated reaction kinetics by differential and gravimetric thermal analysis (TGA/DTA) and the kinetic parameters identification (such as activation energy, reaction order, pre-exponential factor) [2] [3] will be described. Moreover, an example of simulation integrating the chemical equation solved with CFD tools will be presented and discussed

Cold crucible for HLW vitrification : advances in the feed and PGM behavior modelling

International audienceA cold crucible induction melter is in operation in La Hague plant, which is operated by ORANO since 2010 for the vitrification of HLW arising from D&D operation and old reprocessing of high Molybdenum content fuel. The well-known advantages of the cold crucible compared to the hot metallic inductive melter are (i) a higher elaboration temperature (ii) an extended lifetime and (iii) a better homogeneity obtained by mechanical stirring and gas bubbling. As a result, the global production capacity is expected to be higher as well as the PM concentration in the glass.A part of the development of this technology is made with the help of numerical simulation of the glass flow and heating by direct Joule effect thanks to the high frequency induction power unit. In this paper, the last effort of 3D modelisation of the Platinum-Group-Metals (PGM) particles behavior in the glass and chemical reaction kinetics of the feed are detailed.During the melting, the glass generally shows a homogeneous liquid phase seeded with non-soluble heavy platinum-group-metal particles mainly made of palladium and ruthenium dioxide. Previous studies [1] reported spatial discrepancies of the local volume fraction of particles in the melt because of particles settling with time. This migration of particles towards less agitated bottom parts of the crucible affects the dynamical, electrical and thermal state of the melt because of concentration-dependent electrical conductivity and viscosity of the suspension. A theoretical one-fluid transport model was developed with the help of small scale experiments carried out with a HLW glass simulant. The model is coupled with existing 3D thermo-hydraulic numerical codes, thus enhancing the precision of heat flux predictions between the melt and the crucible.The feed modelling is also under development. From vitreous precursors (Sodium alumino-borosilicate glass frit) and calcined waste (a mixture of about thirty oxide and nitrate compounds obtained after calcination step), a succession of processes such as impregnation, denitration, crystallization and dis-solution occur in order to solubilize the waste into the vitreous network. Mainly focus on thermal and chemical (0D models) approaches the objective aim at coupling the reaction kinetics laws of thermally activated processes (denitration, crystallization, dissolution) to the magneto-thermo-hydraulic model in order to model both physic and chemistry of glass synthesis. The experimental methodology based on the characterization of thermo-activated reaction kinetics by differential and gravimetric thermal analysis (TGA/DTA) and the kinetic parameters identification (such as activation energy, reaction order, pre-exponential factor) [2] [3] will be described. Moreover, an example of simulation integrating the chemical equation solved with CFD tools will be presented and discussed

Characterization and modeling of chemical reactions taking place during the vitrification of high level waste in cold crucible

International audienceSeveral chemical reactions take place during the vitrification of high-level nuclear waste. The integration of these chemical aspects in CFD codes is today one of the key points in the process of improving the prediction capabilities of numerical tools. In this study, based on simultaneous differential thermal analysis-thermogravimetry (DTA–TGA) and run/rerun method, we propose a modeling of the kinetics of chemical reactions taking place when the nuclear glass precursors are subject to constant heating rates. For the mathematical modeling, a weighted sum of n-order reactions is used to describe the overall mechanism. A hybrid approach coupling Kissinger and least squares method is performed to identify the apparent kinetic parameters. Along with the thermal characterization, we also propose a qualitative analysis of the reactions phenomenology through microstructural evolution and evolved gaz analysis based respectively on scanning electron microscope (SEM) and thermogravimetry–mass spectrometry (TGA–MS)

Contribution of the Numerical Simulations in the Development of the French HLLW Vitrification Processes

International audienceFor many years, the CEA (Commissariat à l'Énergie Atomique et aux Énergies Alternatives) Marcoule and AREVA have developed various processes dedicated to radioactive waste confinement, especially vitrification processes for HLLW. For 10 years now, the numerical simulation has become an important tool for research and developement projects held in the CEA-AREVA Joint Vitrification Laboratory (LCV). Induction heating, fluid mechanics and thermal simulations take part of all new R&D projects. The apports of such simulations are, first, the enhancement of the working knowledge of existing process. Those data are very useful to define optimisation choices, for example upgrades made on the hot metallic melter used since the 1990s at LaHague facility. Second, the simulations are, of course, also used at the conception stage of new processes as a tool allowing wide ranges parametric tests. This has been extensively used in the design of the cold crucible inductive melter (CCIM) commissioned in 2010 at La Hague plant. Finally, it is a powerful tool for prospective studies for processes of the future. Whatever the purpose, the potential benefits are gains on the reliability, the output capacity and the life time

Optimisation d'un procédé de vitrification chauffé par induction directe par couplage des simulations électromagnétique et Thermo-hydrodynamique

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Prédiction par modélisation statistique de la température d'une fonte verrière élaborée en four inductif

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