In order to design a power transformer it is important to understand its internal electromagnetic behaviour. That can be obtained by measurements on physical transformers, analytical expressions and computer simulations. One benefit with simulations is that the transformer can be studied before it is built physically and that the consequences of changing dimensions and parameters easily can be assessed. In this thesis a time-domain transformer model is presented. The model includes core phenomena as magnetic static hysteresis, eddy current and excess losses. Moreover, the model comprises winding phenomena as eddy currents, capacitive effects and leakage flux. The core and windings are first modelled separately and then connected together in a composite transformer model. This results in a detailed transformer model. One important result of the thesis is the feasibility to simulate dynamic magnetization including the inhomogeneous field distribution due to eddy currents in the magnetic core material. This is achieved by using a Cauer circuit combined with models for static and dynamic magnetization. Thereby, all magnetic loss components in the material can be simulated accurately. This composite dynamic magnetization model is verified through experiments showing very good correspondence with measurements. Furthermore, the composite transformer model is verified through measurements. The model is shown to yield good correspondence with measurements in normal operation and non-normal operations like no-load, inrush current and DC-magnetization. / QC20100708

<p>In order to design a power transformer it is important to understand its internal electromagnetic behaviour. That can be obtained by measurements on physical transformers, analytical expressions and computer simulations. One benefit with simulations is that the transformer can be studied before it is built physically and that the consequences of changing dimensions and parameters easily can be tested.</p><p>In this thesis a time-domain transformer model is presented. The model includes core losses as magnetic static hysteresis, eddy current and excess eddy current losses. Moreover, the model comprises winding losses including eddy currents, capacitive effects and leakage flux. The core and windings are first modelled separately and then connected together in a total transformer model. This results in a detailed transformer model.</p><p>One important result of the thesis is the possibility to simulate dynamic hysteresis including the eddy current shielding in the magnetic core material. This is achieved by using Cauer circuit combined with analytical expression for static and dynamic hysteresis. Thereby, all magnetic loss components in the material can be simulated accurately. This dynamic hysteresis model is verified through experiments showing very good agreement.</p>

Ces travaux de thèse s’intègrent au sein du projet Biocarb lancé par le Commissariat à l’Énergie Atomique dont l’objectif est de développer des procédés de production de carburants liquides ou gazeux à partir de gaz de synthèse riche en H2 et CO obtenu par gazéification de la biomasse lignocellulosique. L’objectif de cette étude est d’étudier le comportement de particules de biomasse millimétriques lors de la pyrolyse dans des conditions types de gazéifieurs industriels tels que les réacteurs à lit fluidisé ou à flux entraîné, qui fonctionnent pour des flux de chaleur élevés (105 – 106 W⋅m-2) et pour de hautes températures (>800°C). Tout d’abord, des expériences de pyrolyse sont menées à 800 et 950°C dans un four à chute de laboratoire sur des particules de bois entre 350 et 800 μm. Les résultats montrent que dans les conditions de l’étude, l’augmentation de la taille de la particule augmente seulement la durée de la pyrolyse mais ne modifie pas les rendements ou la composition du solide et du gaz au cours de la pyrolyse. Par ailleurs, des mesures basées sur la technique de PTV (Particle Tracking Velocimetry) sont réalisées à température ambiante pour caractériser la taille et la densité des particules de bois brut et de résidu, et valider une corrélation donnant le coefficient de traînée qui sert à calculer le temps de séjour des particules dans le réacteur. On constate à la fin de la pyrolyse une diminution de la densité comprise entre 70 et 80% ainsi qu’une diminution de la taille des particules entre 25 et 40%. Les résultats montrent également que la vitesse de glissement de la particule et l’évolution de ses propriétés doivent être prises en compte lors du calcul de sa vitesse. Enfin, à partir des résultats expérimentaux, un modèle unidimensionnel à coeur rétrécissant est développé pour décrire le comportement d’une particule de bois lors de sa pyrolyse. Le modèle est capable de prévoir l’évolution du rendement en solide, en gaz total et en goudrons au cours de la pyrolyse ainsi que la vitesse de glissement de la particule et son temps de séjour dans le réacteur.L’analyse de sensibilité du modèle montre que même pour des particules millimétriques, une connaissance précise de la chaleur de réaction associée à la pyrolyse, de la densité du bois et de la conductivité thermique du résidu solide est essentielle / The present work is part of a project of the French energy research centre Commissariat à l’Energie Atomique. The goal of the project is to develop processes of production of gaseous or liquid fuel from synthesis gas obtained by gasification of lignocellulosic biomass. The objective of the present work is to study the pyrolysis behaviour of millimetric biomass particles under the operating conditions encountered in fluidized bed or entrained flow gasifiers, namely high external heat flux (105 – 106 W⋅m-2) and high temperature (> 800°C). First, pyrolysis experiments are conducted at 800 and 950°C in a lab-scale drop tube reactor on wood particles between 350 and 800 μm. The results show that under the explored conditions, the increase of the particle size only increases the time required for pyrolysis but does not affect the product distribution during pyrolysis. Since in the pyrolysis experiments, the particle residence time cannot be directly measured, PTV (Particle Tracking Velocimetry) measurements are performed at room temperature to characterize the evolution of the particle size and density along pyrolysis and to validate a drag coefficient correlation for the particle residence time calculation. The optical measurements show that at the end of pyrolysis there is a decrease of particle density of 70 – 80% and of particle size of 25 – 40%. It is also proven that the particle slip velocity cannot be neglected and that the change of these particle properties must be taken into account for the calculation of the particle slip velocity and residence time. Finally, based on these experimental results, a 1D shrinking-core model is developed that is able to predict the solid/gas/tar yields and the residence time of a single particle along pyrolysis in the drop tube reactor. It is validated on both the pyrolysis and optical experiments. The model sensitivity analysis shows that even for millimetric particles, the accurate knowledge of the heat of pyrolysis, of the wood density and of the char thermal conductivity is essential

M. Tech. (Department of Chemical Engineering, Faculty of Engineering and Technology): Vaal University of Technology / Strict environmental regulation on flue gas emission has led to the implementation of FGD technologies in power stations. Wet FGD technology is commonly used because it has high SO2 removal efficiency, high sorbent utilization and due to availability of the sorbent (limestone) used. SO2 is removed by passing flue gas through the absorber where it reacts with the slurry containing calcium ions which is obtained by dissolution. This study presents the findings of the dissolution of a calcium-based material (limestone) for wet FGD process. This was done using a pH stat apparatus and adipic acid as acid titrant. Adipic acid was used because of its buffering effect in wet FGD process. The conditions used for this study are similar to what is encountered in a wet FGD process. The extent of dissolution was determined by analyzing the amount of calcium ions in solution at different dissolution periods. The dissolution kinetics were correlated to the shrinking core model and it was found out that chemical reaction at the surface of the particle is the rate controlling step. This study also investigated the dissolution of coal fly ash and bottom ash. Their dissolution kinetics showed that the diffusion through the product layer was the rate controlling step due to an ash layer formed around the particle. The formation of ash layer was attributed to pozzolanic reaction products which is calcium-alumino-silicate (anorthite) compounds were formed after dissolution. The effect of fly ash on the dissolution of rate of limestone was also studied using response surface methodology. Limestone reactivity was found to increase with increase in the amount of fly ash added and the pH was found to be strong function of the rate constant compared to other dissolution variables. The presence of silica and alumina in fly ash led to a significant increase in the specific surface area due to hydration products formed after dissolution. / Eskom

Les gaz d'atomes ultrafroids offrent des possibilités sans précédent pour la réalisation et la manipulation des systèmes quantiques. Le contrôle exercé sur les interactions entre particules permet d'atteindre le régime de fortes interactions, pour des espèces d'atomes à la fois fermioniques et bosoniques. Dans la limite unitaire, où la force d'interaction est à son maximum, des propriétés universelles émergent. Pour les atomes bosoniques, celles-ci comprennent l'effet Efimov, l'existance surprenante d'une séquence infinie d'états liés à trois corps. Dans cette thèse, nous avons étudiés un système de bosons unitaires. Partant des cas à deux et à trois corps, nous avons montrés que le modèle choisi capturait correctement les caractéristiques universelles de l'effet Efimov. Pour le modèle à N-corps, nous avons développé un algorithme de Monte Carlo quantique capable de réaliser les différentes phases thermodynamiques du système : gaz normal à haute-température, condensat de Bose-Einstein, et liquide d'Efimov. Un unique composant de notre modèle resterait pertinent à la limite de température infinie, à savoir la répulsion corps dur à trois corps, qui constitue une généralisation du potentiel classique entre sphères dures. Pour ce modèle, nous avons proposé une solution au problème d'empilement compact en deux et trois dimensions, fondée sur une Ansatz analytique et sur la technique de recuit simulé. En étendant ces résultats à une situation de pression finie, nous avons montré que le système présente une transition de fusion discontinue, que nous avons identifié à travers la méthode de Monte Carlo. / Ultracold atomic gases offer unprecedented possibilities to realize and manipulate quantum systems. The control on interparticle interactions allows to reach the strongly-interacting regime, with both fermionic and bosonic atomic species. In the unitary limit, where the interaction strength is at its maximum, universal properties emerge. For bosonic atoms, these include the Efimov effect, the surprising existence of an infinite sequence of three-body bound states. In this thesis, we have studied a system of unitary bosons. Starting from the two- and three-body cases, we have shown that the chosen model correctly captures the universal features of the Efimov effect. For the corresponding many-body problem, we have developed a quantum Monte Carlo algorithm capable of realizing the different thermodynamic phases in which the system may exist: The high-temperature normal gas, Bose-Einstein condensate, and Efimov liquid. A single ingredient of our model would remain relevant in the infinite-temperature limit, namely the three-body hard-core repulsion, which constitutes a generalization of the classical hard-sphere potential. For this model, we have proposed a solution to the two- and three-dimensional packing problem, based on an analytical ansatz and on the simulated-annealing technique. Extending these results to finite pressure showed that the system has a discontinuous melting transition, which we identified through the Monte Carlo method.

DYN3D is an best estimate advanced code for the three-dimensional simulation of steady-states and transients in light water reactor cores with quadratic and hexagonal fuel assemblies. Burnup and poison-dynamic calculations can be performed. For the investigation of wide range transients, DYN3D is coupled with system codes as ATHLET and RELAP5. The neutron kinetic model is based on the solution of the three-dimensional two-group neutron diffusion equation by nodal expansion methods. The thermal-hydraulics comprises a one- or two-phase coolant flow model on the basis of four differential balance equations for mass, energy and momentum of the two-phase mixture and the mass balance for the vapour phase. Various cross section libraries are linked with DYN3D. Systematic code validation is performed by FZR and independent organizations.

Dans cette thèse, des simulations numériques tridimensionnelles instationnaires d'une installation expérimentale de combustion en boucle chimique sont réalisées. Le pilote expérimental, d'une puissance de 120 kWth, utilise un matériau perovskite, à base de Ca-Mn, comme transporteur d'oxygène. Les simulations numériques sont réalisées par le code NEPTUNE_CFD, selon une approche Euler-Euler pour les deux phases (solide et gazeuse), avec des modèles de fermeture spécifiques pour modéliser les transferts de masse, de mouvement et d'énergie. Les réactions hétérogènes (i.e. réactions gaz-solide) de réduction et d'oxydation sont décrites au moyen d'un modèle à cœur rétrécissant dans le grain, qui prend en compte les mécanismes compétitifs dans le processus global de réaction gaz-solide: réaction chimique à la surface interne des particules,diffusion à travers la couche de produits et transfert externe autour des particules. Les résultats des simulations numériques sont validées avec des mesures expérimentales et analysées afin de mieux comprendre le comportement local/instationnaire de l'écoulement gaz-particules réactif dans ce système de combustion en boucle chimique. L'outil théorique/numérique développé dans ce travail sera utilisé pour le dimensionnement d'une unité pilote à l’échelle des installations industrielles. / In this work, reactive unsteady three-dimensional numerical simulations of a Chemical Looping Combustion (CLC) plant are performed. The plant is a 120 kWth pilot working with Ca-Mn-based material as selected oxygen carrier. Numerical simulations are performed by NEPTUNE_CFD code using an Euler-Euler approach which computes both the gas and the solid phases in an Eulerian fashion accounting for specific closures in order to model interphase mass, momentum and energy transfers. Reduction and oxidation heterogeneous (i.e. gas-solid) reactions are modeled by means of a grain model (shrinking core model in the grain) accounting for both the competing mechanisms of chemical reaction at the particle internal surface and gaseous diffusion through the product layer. Results from numerical simulations are validated against experimental measurements and analyzed in order to gain insight in the local behaviour of the reactive gas-particle flow in the CLC system. The theoretical/numerical tool developed in this work will be used for design upgrade recommendation in the stage of scaling-up from pilot to industrial facilities.

Compact Models are the physically based accurate mathematical description of the cir-cuit elements, which are computationally efficient enough to be incorporated in circuit simulators so that the outcome becomes useful for the circuit designers. As the multi-gate MOSFETs have appeared as replacements for bulk-MOSFETs in sub-32nm technology nodes, efficient compact models for these new transistors are required for their successful utilization in integrated circuits. Existing compact models for common double-gate (CDG) MOSFETs are based on the fundamental assumption of having symmetric gate oxide thickness. In this work we explore the possibility of developing models without this approximation, while preserving the computational efficiency at the same level. Such effort aims to generalize the compact model and also to capture the oxide thickness asymmetry effect, which might prevail in practical devices due to process uncertainties and thus affects the device performance significantly. However solution to this modeling problem is nontrivial due to the bias-dependent asym-metric nature of the electrostatic. Using the single implicit equation based Poisson so-lution and the unique quasi-linear relationship between the surface potentials, previous researchers of our laboratory have reported the core model for such asymmetric CDG MOSFET. In this work effort has been put to include Non-Quasistatic (NQS) effects, different small-geometry effects, and noise model to this core, so that the model becomes suitable for practical applications. It is demonstrated that the quasi-linear relationship between the surface potentials remains preserved under NQS condition, in the presence of all small geometry effects. This property of the device along with some other new techniques are used to develop the model while keeping the mathematical complexity at the same level of the models reported for the symmetric devices. Proposed model is verified against TCAD simulation for various device geometries and successfully imple-mented in professional circuit simulator. The model passes the source/drain symmetry test and good convergence is observed during standard circuit simulations.

DYN3D is an best estimate advanced code for the three-dimensional simulation of steady-states and transients in light water reactor cores with quadratic and hexagonal fuel assemblies. Burnup and poison-dynamic calculations can be performed. For the investigation of wide range transients, DYN3D is coupled with system codes as ATHLET and RELAP5. The neutron kinetic model is based on the solution of the three-dimensional two-group neutron diffusion equation by nodal expansion methods. The thermal-hydraulics comprises a one- or two-phase coolant flow model on the basis of four differential balance equations for mass, energy and momentum of the two-phase mixture and the mass balance for the vapour phase. Various cross section libraries are linked with DYN3D. Systematic code validation is performed by FZR and independent organizations.

In order to design a power transformer it is important to understand its internal electromagnetic behaviour. That can be obtained by measurements on physical transformers, analytical expressions and computer simulations. One benefit with simulations is that the transformer can be studied before it is built physically and that the consequences of changing dimensions and parameters easily can be tested. In this thesis a time-domain transformer model is presented. The model includes core losses as magnetic static hysteresis, eddy current and excess eddy current losses. Moreover, the model comprises winding losses including eddy currents, capacitive effects and leakage flux. The core and windings are first modelled separately and then connected together in a total transformer model. This results in a detailed transformer model. One important result of the thesis is the possibility to simulate dynamic hysteresis including the eddy current shielding in the magnetic core material. This is achieved by using Cauer circuit combined with analytical expression for static and dynamic hysteresis. Thereby, all magnetic loss components in the material can be simulated accurately. This dynamic hysteresis model is verified through experiments showing very good agreement. / QC 20101116