Couplages multi-physique et multi-échelle pour la modélisation prédictive de la combustion de matériaux énergétiques intégrés dans leur environnement

Cette thèse présente le développement et l'exploitation d'un modèle qui simule à la fois l'initiation et la réaction complète de propagation de nanothermites à base de poudre avec la seule prise en compte de mécanismes en phase condensée. Trois objectifs ont été poursuivis. - Un modèle prédictif de l'initiation et de la propagation de la réaction nanothermite pour des systèmes à base de poudres, en mettant en œuvre le mécanisme découvert récemment de "fusion réactive" sous différentes rampes de chauffe. - Une étude de l'influence de nombreux paramètres clés tels que la taille du système expérimental, la taille des particules, le rapport stœchiométrique des matériaux ou le taux de compaction, sur les deux aspects de la réaction : initiation et propagation. Cela permet à la fois d'explorer les effets de ces facteurs pour la conception du système et d'accélérer les étapes de conception par des comparaisons systématiques théorie/expérience. - Une comparaison de ce modèle avec une approche purement en phase gazeuse pour expliquer les travaux expérimentaux récents explorant l'importance de la "fusion réactive" et pour contribuer à la discussion dans le domaine sur les mécanismes fondamentaux de la réaction de combustion des nanothermites. La réalisation de ces objectifs est détaillée dans ce manuscrit, organisé en deux chapitres, complétés par deux articles publiés. Dans un premier chapitre, un bref état de l'art des nanothermites est présenté pour donner la motivation et le contexte scientifique de ce projet. De nombreux travaux expérimentaux sont cités pour résumer les méthodes de fabrication et de synthèse des nanothermites, leurs principales caractéristiques et l'effet de la nanostructure sur les performances en termes de vitesse de combustion et de délais d'initiation. Cela inclut ensuite un aperçu des arguments actuels dans le débat sur les mécanismes fondamentaux qui dominent la combustion. Nous poursuivons par une présentation des approches de modélisation existantes, leurs objectifs, leurs formulations et leurs limites. Le chapitre 2 présente la base théorique de notre modèle qui s'appuie sur une formulation de mécanismes de combustion liés à la phase condensée, développée spécifiquement pour une application aux thermites à nanopoudres. Une première formulation propose l'initiation de deux nanoparticules (un combustible, un oxydant) en contact, couplées à une équation thermique. Ce modèle est ensuite étendu à la dimension de la propagation, qui comprend les différents éléments du transfert de chaleur macroscopique. Ainsi, la formulation finale du modèle combine les mécanismes hétérogènes à l'échelle nanométrique, les réactions chimiques aux interfaces et la propagation du front global de combustion à l'échelle de l'observation macroscopique. Nous ajoutons à ces deux chapitres un article publié dans Journal of Applied Physics, où nous nous concentrons sur le modèle de combustion élémentaire, c'est-à-dire à l'échelle d'un couple de nanoparticules en contact, afin d'étudier l'importance des mécanismes en phase condensée sur l'initiation des thermites. L'article porte particulièrement sur le processus et l'impact sur la combustion de la fusion réactive, pour différents couples de thermites et différentes vitesses de chauffe. Les résultats sont d'abord validés par comparaison avec des récents travaux expérimentaux puis sont comparés avec une simulation considérant seulement la phase gazeuse comme médiatrice de la combustion. Nous y ajoutons également un deuxième article, qui porte sur l'exploitation du modèle complet de propagation auto-entretenue, avec une discussion des facteurs probables qui influencent la vitesse de réaction. L'article se termine par une discussion sur la propagation du front de combustion fonction de la formulation du terme de conductivité thermique, en correspondance avec les bases théoriques discutées dans le chapitre 2.This thesis presents the development and exploitation of a model that simulates both the initiation and propagation reaction of powder-based nanothermites with purely condensed phase mechanisms. Three main goals have been targeted: - A predictive model of both the initiation and propagation of the nanothermite reaction for powder-based systems implementing the recently discovered "reactive sintering" mechanism under different external heating regimes at low computational cost with flexibility to adapt to newly interesting materials. - A study of the influence of numerous key parameters such as the size of the experimental apparatus, particle size, stoichiometric ratio of materials, or the amount of compaction on both aspects of the reaction. This permits an exploration of the effects of these factors for system design, as well as acts as a method of validation through comparison with experimental studies. - A comparison of this model with a purely gas phase approach to expound on the recent experimental works exploring the importance of reactive sintering and to contribute to the discussion within the domain on the fundamental driving mechanisms of the nanothermite combustion reaction. The fulfillment of these goals is detailed in this manuscript, organized into five chapters. In a first chapter, a state of the art of nanothermites is presented to outline the motivation and scientific context of this project. Numerous experimental works are cited to summarize the methods of manufacturing and synthesis of nanothermites, their principal characteristics, and the effect of the nanostructure on performance in terms of the burn rate and initiation delays. We include an overview of the current arguments in the debate on the fundamental driving mechanisms of reaction, followed by a presentation of the existing modelisation approaches, their aims, formulations, and limitations. Chapter 2 presents the theoretical basis for the proposed model exclusively based on a condensed state formulation of the combustion, developed specifically for application to nanopowder thermites. A first formulation considers a base model of the initiation of two nanoparticles (one fuel, one oxidizer) sintered into contact, coupled to a thermal equation. Then, the base model is expanded into a full propagation model, which includes theoretical constructions of different macroscopic heat transfer mechanisms that were compared to find the best reproduction of experimental results. Thus, the final iteration combines the heterogeneous nanoscale mechanisms and chemical reactions with the overall macroscale propagation reaction. In Chapter 3, the base model is utilized to investigate the importance of condensed-phase mechanisms on the initiation of these thermites, with particular insights into reactive sintering for different thermite couples and different heating rates, and comparison with recent experimental works in addition to a purely gas phase simulation. This is supplemented by a benchmark study of the initiation of nanothermites with different varying parameters including the particle size, stoichiometric ratio, native oxide thickness, and the fuel and oxide material species. Chapter 4 continues with the exploitation of the full self-sustaining propagation model with discussion of the probable factors that most influence the reaction rate. This begins with a presentation of the basic results for an Al:CuO system for each of the three model formulations presented in Chapter 2. Once the chosen system was validated, this version was then used to test the effect of varying important parameters such as the compaction rate, the material species', and the different heat transfer mechanisms also discussed previously. Finally, this is followed by a general conclusion to summarize this work and its implications, as well as explore the perspectives for future work. A Software Architecture Document is available in Appendix A

Compacting solid waste materials generated in Missouri to form new products: final technical report

Presented at the 32nd Annual Conference of Missouri Waste ControlThe unique high-pressure compaction technology developed at Capsule Pipeline Research Center (CPRC) of University of Missouri-Columbia was used to study the compaction of combustible components of municipal solid waste and flyash generated from coal-fired power plants. By compaction, the combustible wastes can be turned into uniform, densified solids for use as fuel; the flyash can be turned into high-valued building elements such as bricks and blocks.This research project was sponsored by the Solid Waste Management Program, Missouri Department of Natural Resources (MDNR) for the period from January 1, 2001 to December 29, 2001. (MDNR Award Project no. 00038-1

Equine Surfaces White Paper

This white paper has been drafted as a collection of published scientific papers and data. It is considered a work in progress and will be updated as new scientific studies and surface data become availabl

Key Topics in Deep Geological Disposal : Conference Report (KIT Scientific Reports ; 7696)

The current state of knowledge of central aspects of radioactive waste repository research was presented in the course of the DAEF conference "Key topics in deep geological disposal". For the first time socio-economic and socio-technical issues played an important role within a conference focusing on the disposal of radioactive waste. Scientists from about 16 different countries presented their scientific work in 8 sessions and during a poster session

Optimisation of ITER Nb3Sn CICCs for coupling loss, transverse electromagnetic load and axial thermal contraction

The ITER cable-in-conduit conductors (CICCs) are built up from sub-cable bundles, wound in different stages, which are twisted to counter coupling loss caused by time-changing external magnet fields. The selection of the twist pitch lengths has major implications for the performance of the cable in the case of strain sensitive superconductors, i.e. Nb3Sn, as the electromagnetic and thermal contraction loads are large but also for the heat load from the AC coupling loss. Reduction of the transverse load and warm-up cool-down degradation can be reached by applying longer twist pitches in a particular sequence for the sub-stages, offering a large cable transverse stiffness, adequate axial flexibility and maximum allowed lateral strand support. Analysis of short sample (TF conductor) data reveals that increasing the twist pitch can lead to a gain of the effective axial compressive strain of more than 0.3 % with practically no degradation from bending. For reduction of the coupling loss, specific choices of the cabling twist sequence are needed with the aim to minimize the area of linked strands and bundles that are coupled and form loops with the applied changing magnetic field, instead of simply avoiding longer pitches. In addition we recommend increasing the wrap coverage of the CS conductor from 50 % to at least 70 %. The models predict significant improvement against strain sensitivity and substantial decrease of the AC coupling loss in Nb3Sn CICCs, but also for NbTi CICCs minimization of the coupling loss can be achieved. Although the success of long pitches to transverse load degradation was already demonstrated, the prediction of the combination with low coupling loss needs to be validated by a short sample test.Comment: to be published in Supercond Sci Techno

Effects of cosmic rays on single event upsets

The efforts at establishing a research program in space radiation effects are discussed. The research program has served as the basis for training several graduate students in an area of research that is of importance to NASA. In addition, technical support was provided for the Single Event Facility Group at Brookhaven National Laboratory

CropWatch No. 95-23, Sept. 15, 1995

Inside Scout for corn stalk rot; early harvest an option .................. 175 Quick cooling key to storing wet grain .................. 175 Test drought-stressed forages to ensure nitrate-nitrogen not at toxic levels.................. 176 Plan to avoid compaction at harvest .................. 177 Improve CR-P stands before grazing .................. 178 Evaluate resources, goals before grazing post-CRP acres.................. 179 Frost probabilities .................. 180 How available are waste nutrients?.................. 181 Estimate maximum \u27shelf life\u27 for temporary storage of wet corn .................. 182 Nebraska weather date*.................... 18

CropWatch No. 95-23, Sept. 15, 1995

Inside Scout for corn stalk rot; early harvest an option .................. 175 Quick cooling key to storing wet grain .................. 175 Test drought-stressed forages to ensure nitrate-nitrogen not at toxic levels.................. 176 Plan to avoid compaction at harvest .................. 177 Improve CR-P stands before grazing .................. 178 Evaluate resources, goals before grazing post-CRP acres.................. 179 Frost probabilities .................. 180 How available are waste nutrients?.................. 181 Estimate maximum \u27shelf life\u27 for temporary storage of wet corn .................. 182 Nebraska weather date*.................... 18