L'optimisation des systèmes d'allumage est un paramètre critique pour la définition des foyers de combustion industriels. Des simulations aux grandes échelles (ou LES pour Large-Eddy Simulation) d'un brûleur de type bluff-body non pré-mélangé ont été menées afin de comprendre l'influence de la position de la bougie sur la probabilité d'allumage. La prise en compte de la combustion est basée sur une méthode de tabulation de la chimie détaillée (PCM-FPI pour Presumed Conditional Moments - Flame Prolongation of ILDM). Les résultats de ces simulations ont été confrontés des résultats expérimentaux disponibles dans la littérature. Dans un premier temps, les mesures de vitesse et du champ de richesse à froid sont comparées aux résultats de la simulation pour évaluer les capacités de prédiction en terme de structure de l'écoulement et de mélange turbulent. Un suivi temporel des vitesses et de la fraction de mélange est réalisé à différents points pour déterminer les fonctions de densité de probabilité (ou PDF)des variables caractéristiques de l'écoulement, à partir des champs résolus en LES. Les PDFs ainsi obtenues servent l'analyse des phénomènes d'allumages réussis ou déficients rencontrés expérimentalement. Des simulations d'allumage forcé ont été effectuées pour analyser les différents scénarios de développement de la flamme. Les corrélations entre les valeurs locales (fraction de mélange, vitesse) autour de la position d'allumage et les chances de succès de développement du noyau de gaz brûlés sont alors discutées. Enfin, une extension de la méthode PCM-FPI avec prise en compte des effets d'étirement est développée à l'aide d'une analyse asymptotique, puis confrontée aux résultats de mesures expérimentales. / The optimization of the ignition process is a crucial issue in the design of many combustion systems. Large eddy simulation (LES) of a conical shaped bluff-body turbulent non-premixed burner has been performed to study the impact of spark location on ignition success. The chemistry part of the simulation is done using tabulated detailed chemistry approach. This burner was experimentally investigated by Ahmed et al at Cambridge (UK). The present work focuses on the case without swirl for which detailed measurements are available. First, cold fkow measurements of velocities and mixture fraction are compared with their LES counterparts, to assess the prediction capabilities of simulations in terms of flow and turbulent mixing. Time history of velocities and mixture fraction are recorded at selected spots, to probe the resolved probability density function (pdf) of flow variables, in an attempt to reproduce, from the knowledge of LES resolved instantaneous flow conditions, the experimentally observed reasons of success or failure of spark ignition. A flammability map is also constructed from the resolved mixture fraction pdf and compared with its experimental counterpart. LES of forced ignition is then performed using flamelet fully detailed tabulated chemistry combined with presumed pdfs (PCM-FPI). Various scenarios of flame kernel development are analyzed and correlated with typical flow conditions observed in this burner. The correlations between velocities and mixture fraction values at the sparking time and the success or failure of ignition are then further discussed and analysed. The rate of flame development during successful or unsuccessful ignition events are analysed and compared against experimental observations. Finally, from asymptotic flame analysis, a novel approach has been proposed to include flame straining effects in the PCM-FPI method developped at CORIA-CNRS. The new model overcomes the problem associated with classical PCM-FPI closure to model kernel quenching due to intense local turbulence. Computations are done including the flame straining effects and the effect brought by the new model on kernel development is analysed in detail.

Les enjeux environnementaux et sociétaux de la combustion de combustibles fossiles pour la production d'énergie (électrique, chauffage ou transport), nécessitent le développement de nouveaux modes de combustion, de nouvelles technologies de brûleurs et de combustibles alternatifs (gazéification de la biomasse, biofuels, ...). La vitesse de combustion laminaire est un des paramètres fondamentaux utilisé pour caractériser la combustion pré-mélangée de ces nouveaux mélanges combustibles. Cette vitesse est une donnée de référence pour le processus de validation et d'amélioration des schémas cinétiques ainsi qu'un paramètre d'entrée pour estimer la vitesse de combustion turbulente de la plupart des codes de combustion turbulente. Mais bien qu'étudiée depuis plus de 100 ans, la détermination expérimentale précise de cette vitesse reste encore un défi de par les limitations inhérentes aux configurations expérimentales utilisées, en particulier pour les conditions de pression et de température élevées. Dans ce contexte, les objectifs de ces travaux de thèse concernent l'étude, l'analyse et la caractérisation des techniques de détermination de la vitesse de combustion laminaire à partir des flammes en expansion sphérique, en proposant une réflexion sur la minimisation de l'ensemble des sources d'incertitudes possibles sur la détermination de cette vitesse. Cette approche est réalisée pour la configuration de flamme en expansion sphérique, permettant des températures et pressions élevées et maitrisées.Dans une première partie, le formalisme des définitions des vitesses de flamme laminaire existantes dans cette configuration est rappelé afin de définir les facteurs d'incertitudes liés à la mesure expérimentale de ces vitesses (grandeurs cinématiques locales et cinétique globale). En particulier, les effets liés à l'estimation de l'état thermodynamique des gaz brûlés, du rayonnement et de la diffusion différentielle sont discutés. Dans une seconde partie, plusieurs dispositifs numériques et expérimentaux utilisés au cours de cette thèse et permettant l'étude de flammes sphériques en expansion sont présentés. Une étude utilisant quatre dispositifs expérimentaux différents est proposée afin d'analyser et caractériser les incertitudes inhérentes aux mesures et à leur traitement. Enfin dans une troisième partie, une définition rigoureuse de la vitesse de consommation est proposée et une nouvelle méthodologie pour la mesurer est développée. Une validation numérique complète est présentée. Puis les incertitudes liées aux rayonnement, à la diffusion différentielle et à l’extrapolation des données mesurées sont étudiées en détails. Cette dernière étape introduit un biais qui peut être conséquent, et une nouvelle méthodologie pour exploiter des mesures brutes est proposée par une comparaison directe avecdes simulations DNS reproduisant les expériences. / Environmental and social challenges concerning the combustion of fossil fuels for energy production (electricity, building and transport) require the development of new combustion processes, new burner technologies and alternative fuels (gasification of biomass, biofuels, ...). Laminar burning velocity is one of the fundamental parameters used to characterize premixed combustion for these new fuels. This speed is a reference for the validation and improvement of kinetic schemes and an input parameter to estimate the turbulent burning velocity of most turbulent combustion codes. But even if it has been studied over 100 years, the precise experimental measurement of this velocity is still complicated due to inherent limitations in experimental configurations used, especially for high pressure and temperature conditions. In this context, this thesis work focuses on the study, analysis and characterization of the different techniques used to determine the laminar burning velocity from spherically expanding flames and proposes a reflection on the minimization of all possible uncertainty sources. This approach is achieved with confined spherical flames which allow to obtain high temperature and pressure initial conditions. In the first part, the formalism of existing laminar flame speeds in spherical expanding configuration is reminded to define the factors of uncertainty related to the experimental measurement (local kinematic and global kinetic variables). In particular, the effects associated with the estimation of the burned gases thermodynamic state, radiation and differential diffusion are discussed. In the second part, several numerical and experimental devices used in this thesis are presented. A study on four different experimental setups is proposed to analyze and characterize the uncertainties in the measurements and processing. Finally, in the third part, a rigorous definition of the consumption speed is proposed and a new methodology to measure it is developed. A complete validation based on numerical results is presented. Then uncertainties related to radiation, differential diffusion and extrapolation to zero stretch rate of measured data are detailed. This last step introduces a non-negligible bias and a new methodology to exploit raw data by a direct comparison with DNS reproducing the experiments is proposed.

A pulsed Nd:YAG laser is used to study laser spark ignition of methane counter-flow diffusion flames with the use of helium and argon as diluents to achieve a wide range of variations in transport properties. The global strain rate and Damkohler number on successful ignition were investigated for the effects of Lewis number and transport properties, which are dependent on the diluent type and dilution level. A high-speed camera is used to record the ignition events and a software is used for pre-ignition flow field and mixing calculations. It is found that the role of effective Lewis number on the critical global strain rate, beyond which ignition is not possible, is qualitatively similar that on the extinction strain rate. With the same level of dilution, the inert diluent with smaller Lewis number yields larger critical global strain rate. The critical Damkohler number below which no ignition is possible is found to be within approximately 20% for all the fuel-inert gas mixtures studied. When successful ignition takes place, the ignition time increases as the level of dilution of argon is increased. The ignition time decreases with increasing level of helium dilution due to decreases in thermal diffusion time, which causes rapid cooling of the flammable layer during the ignition process. However, the critical strain for ignition with helium dilution rapidly decreases as the dilution level is increased. The experimental results show that with the increase of strain rate the time to steady flame decreases, and that with the increase of dilution level time for the flame to become steady increases. For the same level of dilution, the time for steady flame is observed to be longer for He-diluted flames than for Ar-diluted flames due to its thermal diffusivity being larger than that of Ar.

Vätgas är en energibärare som kan vara en av pusselbitarna i omställningen till en mer klimatneutral värld. Infrastrukturen byggs ut, industrin växer och vätgasfordon blir vanligare. Vätgas är ett ämne med de bra egenskaperna att det kan lagra kemisk energi och från förnybar el tillverkas med elektrolys, men det har också den riskabla egenskapen att det är mycket brandfarligt. Vid läckage av vätgas finns risken att en jetflamma med höga temperaturer uppstår vilket påverkar omgivningen där det sker. Standarder för jetbrandtester är idag baserade på tester med propan, men då vätgas har andra egenskaper behövs ökad kunskap för att minska riskerna vid olyckor. Studien syftar till att via litteraturstudier samla information om storskaliga jetbrandtester, vilken utrustning som används, hur material påverkas av jetflammor och vilka standarder som finns för jetflammor och jetbrandtester. Ett av syftena är också att med intervjustudie ta reda på hur räddningstjänsten i Luleå ser på utvecklingen av vätgasinfrastruktur och om de har några rutiner för olyckor med vätgas. Det finns en vision vid Luleå tekniska universitet att det i framtiden ska finnas en anläggning för att utföra jetbrandtester av vätgas och andra bränslen i Luleå. Studien syftar därför också till att via intervju ta reda på om räddningstjänsten i Luleå skulle ha någon användning för en sådan anläggning. Jetbrandtester kan enligt standarden SS-ISO 22899-1:2021 utföras i mindre skala med propan som bränsle och ändå ge liknande resultat som vid storskaliga jetbrandtester med naturgas. Jetbrandtestet utförs för att testa brandmotstånd genom integritet (E) och isolerande förmåga (I) för passiva brandskyddsmaterial som används till rör, paneler, konstruktionsstål och rör- och kabelgenomföringar. Testerna utförs enligt standarden med utrustning såsom munstycke, åter-cirkuleringskammare, skyddskammare, med mera. Enligt standarden SS-ISO 22899-1:2021 träffas testobjektet vid ett jetbrandtest med en jetflamma av propan på 1 meters avstånd. Det korta avståndet medför att propanet inte fullt hinner förbrännas vilket skapar temperaturskillnader på testobjektets yta när det träffas av flamman. Det bildas en ”kall” och en ”varm” zon på ytan där den ”kalla” zonen är den punkt som i direkt kontakt med jetflamman utsätts för mekanisk kraft i form av erosion. För vätgas hinner flamman stabilisera sig på en meters avstånd vilket gör att testobjektet träffas av en fullt utvecklad flamma och därför både utsätts för termiska laster i form av en enhetlig ”varm” zon och mekaniska laster i form av erosion. Denna skillnad kan göra att passiva brandskyddsmaterial vid jetflammor av vätgas inte klarar av att upprätthålla det krav på brandmotstånd som ställs. Detta är främst aktuellt att undersöka för reaktiva passiva brandskyddamaterial då de är mer känsliga för erosion än icke-reaktiva passiva brandskyddsmaterial. Om så är fallet att passiva brandskyddsmaterial inte klarar av att motstå jetflammor av vätgas lika bra som för propan kanske en standard för jetbrandtester med vätgas skulle behöva tas fram. Det kan vid intervjun med PärJohan Fredrickson som är sektionschef för myndighetsutövningen vid Luleå räddningstjänst konstateras att de verkar vara väl informerade om utvecklingen av vätgasinfrastrukturen och att de har varit delaktiga i vätgasfrågor sedan några år tillbaka. De har samarbeten med andra räddningstjänster och de försöker tidigt vara med i dialogen när nya processer och verksamheter utvecklas. De har i dagsläget inte några operativa övningar med jetflammor av vätgas men de arbetar förebyggande genom att ta fram insatsplaner tillsammans med de industriella verksamheter som hanterar vätgas. De kan från ett förebyggande perspektiv se hur räddningstjänsten kanske skulle kunna ha användning av en anläggning för att genomföra jetbrandtester med vätgas. Men om det finns något operativt behov behöver vidare utredas med personal på räddningstjänsten som arbetar inom de operativa resurserna. / Hydrogen is an energy-carrier that can be a piece in the change for a climate neutral world. The infrastructure and industry expand, and hydrogen vehicles becomes more common. Some good characteristics with hydrogen are that it from renewable energy can be produced through electrolysis and store chemical energy, but it also has the risky characteristic that its very flammable. If hydrogen gas would leak from a container there is the risk of a jet flame with high temperatures that can affect the surroundings. Today’s standards for jet fire testing are based on propane gas, but because hydrogen has so many different characteristics there might be a need for more knowledge to prevent risks and accidents. Through a literature study this report aims to gather information about large scale jet fire testing, what equipment that is used, how materials react to jet flames, and what standards that are current for jet fire testing and jet flames. A purpose is to through an interview-study figure out how the rescue service in Luleå sees on the development in hydrogen infrastructure and if they have any routines for accidents with hydrogen. Luleå University of Technology has a vision to in the future have a facility in Luleå where they can perform jet fire testing with hydrogen and other flammable fuels. One purpose of the study is therefore to interview the rescue service in Luleå to investigate if they would have any interest in such facility and what use they could have of it. According to the standard SS-ISO 22899-1:2021, jet fire tests with propane gas can be performed in a smaller scale and still give similar results as for large scale jet fire tests with natural gas. The jet fire test is performed to determine the fire resistance regarding integrity (E) and isolating capacity (I) for passive fire protection materials that are used for pipes, panels, structural steelwork, and pipe penetration seals. The gear that is used for the tests are a nozzle, flame re-circulation chamber, protective chamber etc. A propane flame hits the object of testing from 1 meter according to the standard. Because of the short distance, the propane flame does not reach full combustion which leads to a temperature difference on the object of testing that is encountered with the jet flame. A “cold” and “hot” zone is therefore created at the surface where the “cold” zone appears at the center core of the jet flame. At this core, the object of testing is exposed to a higher mechanical force in terms of erosion than other parts of the object. For hydrogen the jet flame stabilizes in 1 meter which changes how the flame affects the object of testing. The object is instead hit by a fully combusted jet flame that exposes it to high thermal load with a uniform “hot” zone and mechanical load of erosion. That difference could change how passive fire protection (PFP) materials are able to resist jet flames with hydrogen as the tests for PFP materials are done with propane. This would mainly be topical for reactive PFP materials as they are more sensitive to erosion than non-reactive PFP materials and a standard for jet fire tests with hydrogen may become relevant if PFP materials fail to maintain the requirements that are set for propane. The interview with section manager PärJohan Fredrickson that works at Luleå rescue service shows that Luleå rescue service seems to be well informed about the development of hydrogen infrastructure. Since a few years back they have been involved with questions relating to hydrogen, they have collaborations with other rescue services around the country and they try to get involved as early as possible when new processes and operations are formed. They do not seem to do any exercises where they train for jet fire accidents with hydrogen involved but they are working to prevent accidents with hydrogen. Action plans has been and are being developed together with the operations that are handling the hydrogen. The rescue service could from a preventive perspective have a use of a facility for jet fire testing with hydrogen. But it must be further investigated if there is an operative need to use such a facility.

Cette thèse de doctorat analyse la valeur symbolique du feu dans la Divine Comédie de Dante, dans le but de produire une étude générale sur ce sujet. L’approche méthodologique qui a été retenue pour cette recherche est le commentaire de texte, bien que des analyses iconographiques l’enrichissent pour certains passages, et ce en lien permanent avec la théologie chrétienne et la riche exégèse dantesque existante. La première partie de ce travail fournit le contexte de l’objet d’étude : elle vise à établir une cartographie des différentes apparitions du feu et des flammes physiques au long du poème. Dans la seconde partie, divisée en trois chapitres correspondant aux trois cantiche de la Comédie, l’étude s’étend et tente de lister à la fois les occurrences du feu et des flammes physiques, mais aussi métaphoriques. Les principaux passages étudiés sont les chants des hérétiques, des luxurieux et d’Ulysse en Enfer, la barrière de feu au Purgatoire, et ensuite, au Paradis, la sphère de feu, Mars, la croix de Cacciaguida, la figure de l’aigle, le ciel des étoiles fixes et l’Empyrée. Enfin, la dernière partie est consacrée aux métaphores transversales liées au symbole du feu : d’abord, les manifestations de l’ardeur et des sentiments enflammés, puis le plus brûlant d’entre eux, l’amour, et le plus important du chef d’œuvre de Dante, la charité. Les images qui ont été sélectionnées sont certains des dessins de Sandro Botticelli, des peintures de William Blake et des gravures de Gustave Doré. / This doctoral thesis analyses the symbolic meaning of fire in Dante’s Divine Comedy, in order to generate a general study about this topic. The chosen methodology of this research is a textual analysis although an iconographic analysis enriches the poem’s study as well, with permanent connections to Christian theology and the rich existing Dante exegesis. The context of the study is provided in the first part, which aims to establish a mapping of the various occurrences of physical fire and flames throughout the poem. In the second part, which is divided into three chapters corresponding to the three poem’s cantiche, the study widens in its scope and attempts to list both physical and metaphorical occurrences of fire and flames. The key passages that are analysed are the heretics’, the luxurious’ and Ulysse’s verses in Inferno, the fire barrier in Purgatorio and then, in the Paradiso, the sphere of fire, Mars, the Cacciaguida cross, the eagle figure, the sky of the Fixed Stars and the Empyrean. Finally, the third and final part aims to identify some crosscutting metaphors related to the fire symbol: first, the expression of ardour and ignited feelings, followed by love, which is the most inflamed of them, and charity, the most significant for Dante in his masterpiece. Images that have been chosen are drawings from Sandro Botticelli, William Blake’s paintings and Gustave Doré’s engravings.

Interactions between flames and thermal phenomena are the guiding thread of this work. Flamesproduce heat indeed, but can also be affected by it. Large Eddy Simulations (LES) are used hereto investigate these interactions, with a focus on two main topics: wall heat transfer andcombustion control. In a first part, wall heat transfer in a rocket engine sub-scale CH4/O2 burner isstudied. In the context of launchers re-usability and cost reduction, which are major challenges,new propellant combinations are considered and wall heat fluxes have to be precisely predicted.The aim of this work is to evaluate LES needs and performances to simulate this kind ofconfiguration and provide a computational methodology permitting to simulate variousconfigurations. Numerical results are compared to experimental data provided by the TechnischeUniversität München (Germany). In a second part, combustion control by means of NanosecondRepetitively Pulsed (NRP) plasma discharges is studied. Modern gas turbine systems use indeedlean combustion with the aim of reducing fuel consumption and pollutant emissions. Lean flamesare however known to be prone to instabilities and combustion control can play a major role in thisdomain. A phenomenological model which considers the plasma discharges as a heat source isdeveloped and applied to a swirl-stabilized CH4/Air premixed lean burner. LES are performed inorder to evaluate the effects of the NRP discharges on the flame. Numerical results are comparedwith experimental observations made at the King Abdulla University of Science and Technology(Saudi Arabia).

The Fire Dynamics Simulator (FDS) code recently developed by the National Institute of Standards and Technology (NIST) is particularly well-suited for use by fire protection engineers for studying fire behavior. It makes use of Large Eddy Simulation (LES) techniques to directly calculate the large-scale fluid motions characteristic of buoyant turbulent diffusion flames. However, the underlying model needs further development and validation against experiment in the areas of soot formation/oxidation and radiation before it can be used to calculate flame heat transfer and predict the burning of solid or liquid fuels. WPI, Factory Mutual Research, and NIST have undertaken a project to make FDS capable of calculating the flame heat transfer taking place in fires of hazardous scale. The temperatures predicted by the FDS code were generally too high on the fuel side and too low on the oxidant side when compared to experimental data from small-scale laminar diffusion flames. For this reason, FDS was reformulated to explicitly solve the conservation of energy equation in terms of total (chemical plus sensible) enthalpy. This allowed a temperature correction to be applied by removing enthalpy from the fuel side and adding it to the oxidant side. This reformulation also has advantages when using probability density function (PDF) techniques in larger turbulent flames because the radiatively-induced nonadiabaticity is tracked locally with each fluid parcel. The divergence of the velocity field, required to obtain the flow-induced perturbation pressure, is calculated from an expression derived from the continuity equation. A new approach to soot modeling in diffusion flames was developed and added to the FDS code. The soot model postulated as part of this work differs from others because it is intended for engineering calculations of soot formation and oxidation in an arbitrary hydrocarbon fuel. Previous models contain several fuel-specific constants that generally can only be determined by calibration experiments in laminar flames. The laminar smoke point height, an empirical measure of a fuel?s sooting propensity, is used in the present model to characterize fuel-specific soot chemistry. Two separate mechanisms of soot growth are considered. The first is attributed to surface growth reactions and is dependent on the available surface area of the soot aerosol. The second is attributed to homogeneous gas-phase reactions and is independent of the available soot surface area. Soot oxidation is treated empirically in a global (fuel-independent) manner. The local soot concentration calculated by the model drives the rate of radiant emission. Calibration against detailed soot volume fraction and temperature profiles in laminar axisymmetric flames was performed. This calibration showed that the general approach postulated here is viable, yet additional work is required to enhance and simplify the model. The essential mathematics for modeling larger turbulent flames have also been developed and incorporated into the FDS code. An assumed-beta PDF is used to approximate the effect of unresolved subgrid-scale fluctuations on the grid-scale soot formation/oxidation rate. The intensity of subgrid-scale fluctuations is quantified using the principle of scale similarity. The modified FDS code was used to calculate the evolution of soot in buoyant turbulent diffusion flames. This exercise indicated that the subgrid-scale fluctuations are quantitatively important in LES of turbulent buoyant diffusion flames, although no comparison of prediction and experiment was performed for the turbulent case.

Simulações acopladas do escoamento reativo e dos processos de transferência de calor para o estudo de chamas são problemas dispendiosos computacionalmente. A transferência de calor por radiação em processos de combustão, devido às elevadas temperaturas, é o processo de troca energética dominante. Ainda, o comportamento altamente irregular do coeficiente de absorção com o comprimento de onda se constitui em uma dificuldade adicional na modelagem da transferência radiativa em meios participantes. Para contornar essa dificuldade modelos espectrais foram desenvolvidos com o objetivo de simular o comportamento de um gás real. Dentre esses modelos destacam-se o gás cinza (GG: Gray Gas), o mais simples, que negligencia o comportamento espectral do coeficiente de absorção, e o modelo da soma-ponderada-de-gases-cinza (WSGG: Weighted-Sum-of-Gray-Gases) onde a integração sobre todo o espectro é substituída por um número finito de gases cinza. Com o avanço de ferramentas computacionais, principalmente códigos CFD (Computational Fluid Dynamics), abordagens computacionais se tornaram atrativas frente ou em complemento às abordagens experimentais. Desse modo, o presente trabalho tem por objetivo a aplicação dos modelos WSGG e GG com novas correlações na simulação detalhada de chamas difusivas laminares de metano diluído com dióxido de carbono e nitrogênio com o código CFD comercial ANSYS/Fluent. Foram desenvolvidas rotinas de usuário (UDF: User-Defined Functions) para o acoplamento dos modelos espectrais ao código CFD. A verificação das rotinas de usuário foi realizada comparando os resultados obtidos via simulação Fluent com dados obtidos pelo modelo WSGG com um código FORTRAN próprio desenvolvido pelo grupo de pesquisa do Laboratório de Radiação Térmica (LRT/UFRGS) para o problema unidimensional de superfícies negras e infinitas preenchidas por um meio não-isotérmico e não-homogêneo. Os erros encontrados para o fluxo de calor radiativo nas superfícies e para o termo fonte radiativo ao longo do meio foram da ordem de 1% indicando o funcionamento correto das rotinas UDF acopladas ao Fluent. Por fim, as rotinas foram aplicadas na simulação numérica para chamas de potência constante com diluição dos reagentes e os dados obtidos com a solução numérica foram comparados com dados experimentais para a fração radiante e fluxo de calor radiativo. Os desvios médios encontrados para o fluxo de calor radiativo ficaram em torno de 10% para todas as chamas, excetuando as chamas com diluição de CO2 de 30%, 40% e 50%, em volume, para as quais os desvios médios ficaram em torno de 15%. O termo fonte para as chamas apontou para a predominância da emissão do meio em relação à absorção. Todas as chamas estudadas se encontram no regime opticamente fino (optically thin) para o qual, segundo apontam estudos da literatura, a escolha do modelo espectral possui impacto pequeno em resultados globais da chama como a temperatura e a concentração das espécies na mistura. Nesse aspecto os resultados encontrados concordaram com a previsão da literatura, entretanto para a transferência radiativa, o modelo GG se mostrou sensivelmente menos preciso em comparação ao modelo WSGG, principalmente para a fração radiante e para o fluxo radiativo na região da pluma aquecida, indicando a dependência do modelo espectral adotado. / Coupled simulations of the reactive flow with the heat transfer processes for flame studying are computationally demanding problems. The radiative transfer in combustion processes is the main heat transfer mechanism due to the high temperatures involved. However, the highly irregular behavior o f the absorption coefficient with the wavenumber composes in an additional difficulty on modeling the radiative transfer in participating media. In order to overcome this issue, spectral models were developed with the objective of simulate the behavior of real gases. Some of the most known models are the gray gas (GG) for which the spectral behavior of the radiative properties of the medium is neglected and the weighted-sum-of-gray-gases (WSGG) for which the integration over the entire spectrum is replaced by a summation over a finite number of gray gases with constant absorption coefficients. With the development of computational tools, mainly Computational Fluid Dynamics (CFD) codes, numerical approaches became attractive instead or in complement of experimental set ups. In this way, the present work aims to couple the WSGG and the GG models with new correlations in a detailed simulation of diffusive laminar flames of methane diluted with carbon dioxide and nitrogen with the commercial CFD code ANSYS/Fluent. User-defined functions (UDF) were developed to the coupling of the spectral models. The verification was carried out through the WSGG model by comparing the Fluent solution with a solution obtained with a FORTRAN code developed by the Thermal Radiation Laboratory (LRT/UFRGS) research group for the one-dimensional system of black surfaces filled with a non-homogeneous and non-isothermal medium. The deviations for the radiative heat flux for the walls and the radiative heat source along the domain were of 1% or less, indicating the correct coupling between the UDF routines and the CFD code. Finally, the UDF were applied in the solution of constant power flames with fuel diluted with carbon dioxide and nitrogen. The obtained data was then compared with experimental measurements for the radiant fraction and the radiative heat flux along the flame axis. The average deviations found were in order of 10% for all flames, except for the flames with 30%, 40% and 50% of CO2 dilution, in volume, for which the deviatioms found were in order of 15%. The radiative heat source was plotted and indicated for the medium emission predominance in comparison with the medium absorption. All flames studied were optically thin flames for which, studies pointed, the spectral model have minor impact over global results as flame temperature and mixture concentration. For this aspect the results found showed agreement with the literature studies predictions, however the GG model showed itself less accurate in comparison with the WSGG model for the radiant fraction and the radiative heat flux computations. So the spectral models have influence on the radiative transfer even if its effect on flame structure can be negligible.

Modelos algébricos para prever o comprimento de uma chama turbulenta têm sido foco de estudo de diversos grupos de pesquisa por suas aplicações na área de engenharia. O método experimental para obter o modelo varia desde visualizações simples, até técnicas fotográficas, este último com parâmetros fotográficos variando entre os autores. Técnicas fotográficas são usadas para estimar a altura de levantamento da base da chama, (Lift-Off) e o comprimento médio visível de chama (Visible Flame Length, VFL). Duas técnicas comuns que podem ser encontradas na literatura: por imagens de chama com baixo tempo de exposição e longo tempo de exposição, são comparados com um terceiro que se baseia na intensidade luminosa e na frequência de imagens de chama que ocupam um pixel. O melhor método foi utilizado para caracterizar o comportamento das chamas turbulentas de gás natural para diferentes regimes de velocidade do escoamento. Modelos algébricos que preveem o comprimento de chama, altura de levantamento e a velocidade crítica de extinção de chama são avaliados com os novos resultados experimentais. Logo após, os coeficientes numéricos dos melhores modelos algébricos são reajustados Finalmente, foram obtidos mapas de estabilidade relacionados à altura de levantamento e à velocidade crítica de extinção de chama para cada diâmetro em função da diluição com CO2 e do número adimensional de Reynolds. A terceira parte deste trabalho está focada no estudo da distribuição de radiação térmica. Em particular, foram consideradas três distâncias radiais medidas em comprimentos de chama (0,5 Lf, 1 Lf, 2 Lf) visando obter a distribuição do fluxo radiante experimental ao longo de um eixo vertical adjacente às chamas. Finalmente, os dados experimentais foram utilizados como dados de entrada em uma análise inversa com o objetivo de calcular os fatores de ponderação do modelo das múltiplas fontes ponderadas (por suas siglas em inglês WMPS). Nesta última parte, são apresentados frações radiantes e distribuições de fluxo de calor radiante de chamas de gás natural diluídas para diversas diluições com dióxido de carbono e diâmetros do queimador. / Predicting models for turbulent diffusion flame lengths have several applications driven the attention of many research groups. Since several studies use photographs to measure the flame length, with photographic parameters varying among authors, in other cases simple visualizations were used. It is important to explore possible discrepancies among measurement technics that could affect the results. Optical visualizations of turbulent diffusion flames are used to estimate the visible average flame length (VFL) and the lift-off. The study presents a study of three different methods to measure the VFL using optical techniques. The effect on the image of the main optic parameters such as focus, exposure time and ISO sensibility are analyzed. The VFL obtained with images in low exposure time and long exposure time are compared with a third optical method that is based on the luminous intensity and the frequency of flame images occupying a pixel. One method was used to characterize the behavior of turbulent diffusion flames of natural gas for a range of flames in function of the flow velocity. Universal non-dimensional models that describe the VFL, lift-off and the blow-out stability limit of gaseous jet diffusion flames in the still air have been compared with new experimental data. The numerical coefficients of the best models are adjusted. Finally, maps of stability related to lift-off and blow-out were obtained for each diameter in function of the dilution with CO2 and flow exit velocity expressed in non-dimensional Reynolds number The third part of this work focuses on the estimation of the thermal distribution of radiative flux from turbulent diffusion flames in laboratory-scale. The experimental measurements were gotten from the previous stability study. In particular, was considered three radial distances measured in flame lengths (0,5 Lf, 1 Lf, 2 Lf) aiming at obtaining the experimental radiant flux along a vertical axis adjacent to the flames. Finally, the experimental data was used as input data in an inverse analysis with the purpose of computing weight coefficients of the weighted multi-point source (WMPS) model. Then, experimental data that include: radiant fractions and radiative heat flux are presents for several flames with different dilutions with carbon dioxide and burner´s diameters.

Plus de 300 planètes extrasolaires ont été découvertes à ce jour. La variété et la diversité des caractéristiques qu'elles présentent sont extrêmement vastes. Et dans cette multitude, une population se distingue, les Jupiter-chauds. Ces planètes ne ressemblent, en effet, en rien à celles que l'on côtoie dans le Système Solaire. Elles ont une masse d'une à plusieurs fois celle de Jupiter et sont très proches de leur étoile parent, orbitant en seulement quelques jours. L'étude de cette population nous apporte beaucoup d'éléments quant à leur processus de formation et d'évolution. Certaines d'entre elles ont en effet la particularité de transiter devant leur étoile parent. Mes travaux de thèse m'ont mené à détecter et caractériser en densité 14 planètes extrasolaires, grâce aux programmes de recherche CoRoT et SuperWASP combinés au spectrographe SOPHIE (OHP). Parmi ces planètes figurent celles ayant la plus courte période orbitale et, autour d'une autre étoile, la plus importante masse jamais détectées en transit. Un programme novateur, que j'ai initié, nous a également permis d'explorer les capacités de détection d'un instrument de type multi-fibre (FLAMES/GIRAFFE et UVES). Nous avons montré que cet instrument peut être performant en terme de précision de mesure en vitesse radiale, et qu'il permet un très bon écrémage des cibles. L'instrument multi-fibre est utilisable également dans le cadre d'un suivi de candidats issus de programmes de recherche par photométrie, tels que la mission CoRoT , mais il nécessite dans ce cas, pour être eficace, d'un champ de vue significatif (de plusieurs degrés carrés par exemple). L'étude du transit spectroscopique de 3 systèmes planétaires m'a également permis d'apporter de fortes contraintes quant à leur processus de formation et d'évolution, et de mettre en évidence pour la première fois l'existence d'un système exotique : X0-3.