Le rayonnement joue un rôle fondamental dans les problèmes d'incendie puisque c'est le mode dominant de transfert de chaleur entre la flamme et le milieu environnant. Il contrôle la pyrolyse, et donc la puissance de flamme, et la vitesse de croissance de l'incendie. Étudier les flammes de diffusion contrôlées par les forces de flottabilité est une première étape pour comprendre et de prédire les incendies. Le principal objectif de ce travail est de modéliser le transfert radiatif et les processus de production/destruction de la suie dans ce type de flammes. Premièrement, différents modèles de propriétés radiatives des gaz ont été comparés dans des configurations tests. Il est apparu que le modèle FSCK couplé avec le schéma de mélange de Modest et Riazzi est le meilleur compromis entre précision et temps de calcul, ce modèle étant un bon candidat pour être implémenté dans des codes CFD traitant des problèmes d'incendie. Dans un second temps, un modèle de formation/oxydation des suies semi-détaillé, considérant l'acétylène et le benzène comme précurseurs, a été validé dans des flammes de diffusion laminaires de type coflow sur une large gamme d'hydrocarbures (C1-C3) et pour différentes conditions. Ensuite, le FSCK et le modèle de formation/destruction ont été appliqués pour simuler des feux de nappe de méthane et de propane aux échelles du laboratoire et intermédiaire. Les structures de flamme prédites ainsi que les flux radiatif transférés au milieu environnant ont montré un bon accord avec les résultats expérimentaux disponibles. Finalement, les interactions entre le rayonnement et la turbulence ont été quantifiées. / The radiative heat transfer plays an important role in fire problems since it is the dominant mode of heat transfer between flames and surroundings. It controls the pyrolysis, and therefore the heat release rate, and the growth rate of the fire. In the present work a numerical study of buoyant diffusion flames is carried out, with the main objective of modelling the thermal radiative transfer and the soot formation/destruction processes. In a first step, different radiative property models were tested in benchmark configurations. It was found that the FSCK coupled with the Modest and Riazzi mixing scheme was the best compromise in terms of accuracy and computational requirements, and was a good candidate to be implemented in CFD codes dealing with fire problems. In a second step, a semi-empirical soot model, considering acetylene and benzene as precursor species for soot nucleation, was validated in laminar coflow diffusion flames over a wide range of hydrocarbons (C1-C3) and conditions. In addition, the optically-thin approximation was found to produce large discrepancies in the upper part of these small laminar flames. Reliable predictions of soot volume fractions require the use of an advanced radiation model. Then the FSCK and the semi-empirical soot model were applied to simulate laboratory-scale and intermediate-scale pool fires of methane and propane. Predicted flame structures as well as the radiant heat flux transferred to the surroundings were found to be in good agreement with the available experimental data. Finally, the interaction between radiation and turbulence was quantified.

This thesis is concerned with investigations on two related issues of turbulent jet diffusion flame, namely (a) stabilization at liftoff and (b) autoignition in a turbulent jet diffusion flame. The approach of Conditional Moment Closure (CMC) has been taken. Fully elliptic first order CMC equations are solved with detailed chemistry to simulate lifted H2/N2 flame in vitiated coflow. The same approach is further used to simulate transient autoignition process in inhomogeneous mixing layers. In Chapter 1, difficulties involved in numerical simulation of turbulent combustion problems are explained. Different numerical tools used to simulate turbulent combustion are briefly discussed. Previous experimental, theoretical and numerical studies of lifted jet diffusion flames and autoignition are reviewed. Various research issues related to objectives of the thesis are discussed. In Chapter 2, the first order CMC transport equations for the reacting flows are presented. Various closure models that are required for solving the governing equations are given. Calculation of mean reaction rate term for detailed chemistry is given with special focus on the reaction rates for pressure dependent reactions. In Chapter 3, starting with the laminar flow code, further extension is carried to include kε turbulence model and PDF model. The code is validated at each stage of inclusion of different model. In this chapter, the code is first validated for the test problem of constant density, 2D, axisymmetric turbulent jet. Further, validation of PDF model is carried out by simulating the problem of nonreacting jet of cold air issuing into a vitiated coflow. The results are compared with the published data from experiments as well as numerical simulations. It is shown that the results compare well with the data. In Chapter 4, numerical results of lifted jet diffusion flame are presented. Detailed chemistry is modelled using Mueller mechanism for H2/O2 system with 9 species and 21 reversible reactions. Simulations are carried out for different jet velocities and coflow stream temperatures. The predicted liftoff generally agrees with experimental data, as well as joint PDF results. Profiles of mean scalar fluxes in the mixture fraction space, for different coflow temperatures reveal that (1) Inside the flamezone, the chemical term balances the molecular diffusion term, and hence the structure is of a diffusion flamelet for both cases. (2) In the preflame zone, the structure depends on the coflow temperature: for low coflow temperatures, the chemical term being small, the advective term balances the axial diffusion term. However, for the high coflow temperature case, the chemical term is large and balances the advective term, the axial diffusion term being small. It is concluded that, liftoff is controlled (a) by turbulent premixed flame propagation for low cofflow temperature while (b) by autoignition for high coflow temperature. In Chapter 5, the numerical results of autoignition in inhomogeneous mixing layer are presented. The configuration consists of a fuel jet issued into hot air for which transient simulations are performed. It is found that the constants assumed in various modelling terms can severely influence the results, particularly the flame temperature. Hence, modifications to these constants are suggested to obtain improved predictions. Preliminary work is carried out to predict autoignition lengths (which may be defined by Tign × Ujet incase of jet- and coflowvelocities being equal) by varying the coflow temperature. The autoignition lengths show a reasonable agreement with the experimental data and LES results. In Chapter 6, main conclusions of this thesis are summarized. Possible future studies on this problem are suggested.

Ce travail de thèse porte sur l’analyse des effets de la recirculation des gaz de combustion, via l’étude de la dilution par le dioxyde de carbone et plus particulièrement de la vapeur d’eau sur des flammes méthane/air enrichi en dioxygène, dans le cas d’une combustion prémélangée pressurisée rencontrée dans les turbines à gaz. Des mesures de vitesses de flammes CH4/O2/H2O/N2 laminaires pressurisées ont été obtenues à l’aide d’une flamme sphérique se propageant librement dans une enceinte close. L’analyse des résultats expérimentaux a permis de vérifier la validité du schéma cinétique GRIMech ?3.0 au travers des calculs numériques de flammes libres monodimensionnelles. Des calculs complémentaires ont permis l’établissement d’une base de données (vitesse de flamme laminaire, longueur de Markstein et nombre de Lewis, température adiabatique de combustion et épaisseur de flamme) en fonction des paramètres d’entrées de la combustion (température, pression, X(H2O), richesse et enrichissement en dioxygène. L’étude expérimentale complémentaire en régime turbulent des flammes diluées à l’H2O ou au CO2 a permis de mettre en avant l’effet de la vitesse laminaire de flamme sur les structures moyennes et la stabilité des flammes turbulentes. Elle a également permis d’analyser les paramètres (température adiabatique, X(H2O), X(CO2), X(N2), Vitesse de flamme laminaire) ayant un rôle important sur la production des polluants CO et NO. / This PhD thesis deals with the analysis of the effects of exhaust gas recirculation (EGR) through the study of the dilution by carbon dioxide and more particularly of the water steam on dioxygen enriched methane/air flames, in the case of a premixed pressurized combustion encountered in gas turbines. CH4/O2/H2O/N2 pressurized laminar burning velocity measurement were obtained using a spherical flame propagating freely in a closed chamber. The analysis of the experimental results made it possible to check the validity of the kinetic scheme GRIMech.3.0 through numerical calculations of one-dimensional free flames. Further calculations allowed the establishment of a database (laminar burning velocity, Markstein length and Lewis number, adiabatic combustion temperature and flame thickness) as a function of combustion input parameters (temperature, pressure, X(H2O), equivalence ratio and dioxygen enrichment). The additional experimental study under turbulent regime, the flames diluted with H2O and CO2 allowed to highlight the effect of the laminar burning velocity on the average structures and the stability of turbulent flames. It also allowed to analyze the parameters (adiabatic flame temperature, pressure, X(H2O), X(CO2), X(N2), laminar burning velocity) which have an important role in the production of CO pollutants and NO.

Increasingly stringent pollution and emission controls have caused a rise in the use of combustors operating under lean, premixed conditions. Operating lean (excess air) lowers the level of nitrous oxides (NOx) emitted to the environment. In addition, concerns over climate change due to increased carbon dioxide (CO2) emissions and the need for energy independence in the United States have spurred interest in developing combustors capable of operating with a wide range of fuel compositions. One method to decrease the carbon footprint of modern combustors is the use of high hydrogen content (HHC) fuels. The objective of this research is to develop tools to better understand the physics of turbulent flame propagation in highly stretch sensitive premixed flames in order to predict their behavior at conditions realistic to the environment of gas turbine combustors. This thesis presents the results of an experimental study into the flame propagation characteristics of highly stretch-sensitive, turbulent premixed flames generated in a low swirl burner (LSB). This study uses a scaling law, developed in an earlier thesis from leading point concepts for turbulent premixed flames, to collapse turbulent flame speed data over a wide range of conditions. The flow and flame structure are characterized using high speed particle image velocimetry (PIV) over a wide range of fuel compositions, mean flow velocities, and turbulence levels. The first part of this study looks at turbulent flame speeds for these mixtures and applies the previously developed leading points scaling model in order to test its validity in an alternate geometry. The model was found to collapse the turbulent flame speed data over a wide range of fuel compositions and turbulence levels, giving merit to the leading points model as a method that can produce meaningful results with different geometries and turbulent flame speed definitions. The second part of this thesis examines flame front topologies and stretch statistics of these highly stretch sensitive, turbulent premixed flames. Instantaneous flame front locations and local flow velocities are used to calculate flame curvatures and tangential strain rates. Statistics of these two quantities are calculated both over the entire flame surface and also conditioned at the leading points of the flames. Results presented do not support the arguments made in the development of the leading points model. Only minor effects of fuel composition are noted on curvature statistics, which are mostly dominated by the turbulence. There is a stronger sensitivity for tangential strain rate statistics, however, time-averaged values are still well below the values hypothesized from the leading points model. The results of this study emphasize the importance of local flame topology measurements towards the development of predictive models of the turbulent flame speed.

L'influence de l'ajout d'hydrogène sur les flammes de premelange pauvre methane-air est simulée dans cette étude. Le modèle de la chambre a haute pression Orleans - ICARE (France), a été développé. Les propriétés du front de flamme sont examinées par deux modèles de combustion turbulente prémélangée, à savoir Zimont et Flamme Cohérente Model (CFM) modèles.Toutes les études de modélisation sont effectués avec le logiciel Fluent et les résultats sont comparés aux expériences. En suite, l'influence de la pression sur les statistiques de la front de flamme prémélangée a été examinée. Les simulations montrent que l'augmentation du ratio d'équivalence a diminué la hauteur des flammes et l'épaisseur de la flamme du méthane/air flames. D'autre part, l’ajout d’hydrogene de mélange pauvre méthane-air a modifié les propriétés de la flamme prémélangée. Lorsque le pourcentage volumique de l'hydrogène dans le mélange est augmenté, la position en hauteur de la flamme est réduite et l'épaisseur de la flamme devient plus mince. En outre, il a été observé que les propriétés de la flamme prémélangée ont été modifiées avec l'opération à des conditions de pression plus élevée. / Hydrogenated premixed methane/air flames under lean conditions are simulated in this study. The model of the high pressure chamber setup of Orleans - ICARE (France) has been developed. The flame front properties are investigated by two turbulent premixed combustion models, Zimont and Coherent Flame Model (CFM) models. All modeling studies are performed with Fluent software and compared to experiments. The influence of the pressure on the premixed flame front statistics has been examined as well. The simulations show that increasing the equivalence ratio decreases the flame tip height and the flame brush thickness for methane/air flames. In addition, enriching the methane-air mixture with hydrogen modifies the premixed flame front properties. When the volumetric percentage of hydrogen in the mixture is increased, the flame-end position is reduced and flame brush thickness becomes thinner. It is also observed that the premixed flame properties have been modified with operation at higher pressure conditions.

Soot prediction from combustion systems is still a major challenge in high-fidelity simulations of reactive flows, especially in turbulent conditions. Among the critical aspects, due to its slow characteristic formation times, soot sensitivity to strain rate and turbulence-chemistry interaction models for combustion closure can be found. Starting from the laminar problem, Soot Formation (SF) and Soot Formation Oxidation (SFO) counterflow flames are studied, allowing assessment of the roles of the different underlying phenomena concurring at soot formation with varying strain rates, depending on their relevance in each configuration. Attention is devoted to the inception model, which always regulates the onset of soot formation, and entirely determines the soot sensitivity to strain rate in the SF configuration through nucleation and condensation. Besides, surface growth and oxidation are analyzed in the SFO configuration, where they are predominant. The corresponding models are fine-tuned and generalized, and improved predictions are obtained in both configurations. Afterwards, a 2-points flame-controlling continuation method with soot module inclusion is developed to build a tool capable of flamelets generation inclusive of soot effects on the gas phase. The implementation is first tested discussing general features of the S-curve and verifying the consistency with previous works. The tool is finally used to compute the S-curve of ethylene pressurized sooting flames. The models and tools developed are incorporated into an OpenFOAM-based solver to perform Computational Fluid Dynamic (CFD) simulations of sooting turbulent flames. These are studied in pressurized, highly turbulent environments, to validate the soot model at a fundamental level but with practically relevant operative conditions. The numerical results are found to satisfactorily depict the soot volume fraction (SVF) formation, even though a few quantitative and qualitative discrepancies are discussed. Furthermore, soot intermittency and pressure scaling are analyzed. Finally, an alternative turbulence-chemistry interaction model for combustion closure is explored. A generalized partially-stirred reactor model is developed which accounts for all chemical times in a consistent manner. While the applicability of available models is confined to specific turbulence-chemistry interaction regimes, the incorporation of detailed chemistry description in the proposed approach improves synergistic predictions of all species and makes it suitable for systems with characteristic times very different from each other, such as soot and NOx.

<p>Numerical simulations have been done with the CHEMKINsoftware to study different aspects of wall effects in thecombustion of lean, laminar and premixed flames in anaxisymmetric boundary-layer flow.</p><p>The importance of the chemical wall effects compared to thethermal wall effects caused by the development of the thermaland velocity boundary layer has been investigated in thereaction zone by using different wall boundary conditions, walltemperatures and fuel/air ratios. Surface mechanisms include acatalytic surface (Platinum), a surface that promotesrecombination of active intermediates and a completely inertwall with no species and reactions as the simplest possibleboundary condition.</p><p>When hydrogen is the model fuel, the analysis of the resultsshow that for atmospheric pressure and a wall temperature of600 K, the surface chemistry gives significant wall effects atthe richer combustion case (f=0.5), while the thermal andvelocity boundary layer gives rather small effects. For theleaner combustion case (f=0.1) the thermal and velocityboundary layer gives more significant wall effects, whilesurface chemistry gives less significant wall effects comparedto the other case.</p><p>For methane as model fuel, the thermal and velocity boundarylayer gives significant wall effects at the lower walltemperature (600 K), while surface chemistry gives rather smalleffects. The wall can then be modelled as chemically inert forthe lean mixtures used (f=0.2 and 0.4). For the higher walltemperature (1200 K) the surface chemistry gives significantwall effects.</p><p>For both model fuels, the catalytic wall unexpectedlyretards homogeneous combustion of the fuel more than the wallthat acts like a sink for active intermediates. This is due toproduct inhibition by catalytic combustion. For hydrogen thisoccurs at atmospheric pressure, but for methane only at thehigher wall temperature (1200 K) and the higher pressure (10atm).</p><p>As expected, the overall wall effects (i.e. a lowerconversion) were more pronounced for the leaner fuel-air ratiosand at the lower wall temperatures.</p><p>To estimate a possible discrepancy in flame position as aresult of neglecting the axial diffusion in the boundary layerassumption, calculations have been performed with PREMIX, alsoa part of the CHEMKIN software. With PREMIX, where axialdiffusion is considered, steady, laminar, one-dimensionalpremixed flames can be modelled. Results obtained with the sameinitial conditions as in the boundary layer calculations showthat for the richer mixtures at atmospheric pressure the axialdiffusion generally has a strong impact on the flame position,but in the other cases the axial diffusion may beneglected.</p><p><strong>Keywords:</strong>wall effects, laminar premixed flames,platinum surfaces, boundary layer flow</p> / QC 20100504

Fully understanding soot formation in flames is critical to the development of practical combustion devices, which typically operate at high pressures, and fire suppression systems in space. Flames display significant changes under microgravity and high-pressure conditions as compared to normal-gravity flames at atmospheric pressure, but the exact causes of these changes are not well-characterized. As such, the effects of gravity and pressure on the stability characteristics and sooting behavior of laminar coflow diffusion flames were investigated. To study these effects, a new highly-scalable combustion modelling tool was developed specifically for use on large multi-processor computer architectures. The tool is capable of capturing complex processes such as detailed chemistry, molecular transport, radiation, and soot formation/destruction in laminar diffusion flames. The proposed algorithm represents the current state of the art in combustion modelling, making use of a second-order accurate finite-volume scheme and a parallel adaptive mesh refinement algorithm on body-fitted, multi-block meshes. An acetylene-based, semi-empirical model was used to predict the nucleation, growth, and oxidation of soot particles. Reasonable agreement with experimental measurements for different fuels and pressures was obtained for predictions of flame height, temperature and soot volume fraction. Overall, the algorithm displayed excellent strong scaling performance by achieving a parallel efficiency of 70% on 384 processors. The effects of pressure and gravity were studied for flames of two different fuels: ethylene-air flames between pressures of 0.5–5 atm and methane-air flames between 1–60 atm. Based on the numerical predictions, zero-gravity flames had lower temperatures, broader soot-containing zones, and higher soot concentrations than normal-gravity flames at the same pressure. Buoyant forces caused the normal-gravity flames to narrow with increasing pressure while the increased soot concentrations and radiation at high pressures lengthened the zero-gravity flames. Low-pressure flames at both gravity levels exhibited a similar power-law dependence of the maximum carbon conversion on pressure which weakened as pressure was increased. This dependence decayed at a faster rate in zero gravity when pressure was increased beyond 1–10 atm.

Os processos de conversão de energia tendem a considerar cada vez mais restrições econômicas e ambientais, tornando-se necessário o entendimento da interação entre combustão e turbulência. Esta tese tem como objetivo o desenvolvimento de soluções numéricas para chamas difusivas de metanol e etanol, sob forma de um jato turbulento considerando a formação de NO. Como resultado, obteve-se um mecanismo reduzido de 16 passos para combustão de metanol e outro de 21 passos para etanol, mediante aplicação de análise de sensibilidade para o mecanismo detalhado de Marinov (1999). Estes mecanismos reduzidos foram empregados na simulação, e os resultados foram satisfatórios quando comparados com valores encontrados na literatura. Para determinação da geração de NO na chama, empregou-se o mecanismo de Zel’dovich. Devido a cinética de formação de NO por este mecanismo ser consideravelmente mais lenta do que a taxa de oxidação do combustível principal, foi possível tratar os dois mecanismos separadamente. Os resultados obtidos para formação de NO comparam favoravelmente com dados da literatura. / The energy conversion processes increasingly tend to consider the economical and environmental constraints, making it necessary to understand the interaction between combustion and turbulence. This thesis aims the development of numerical solutions for diffusion flames of methanol and ethanol in the form of a turbulent jet, considering the formation of NO. As a result, a reduced mechanism of 16 steps for methanol combustion and another of 21 steps for etanol combustion was obtained by applying a sensitivity analysis to the detailed mechanism of Marinov (1999). These reduced mechanisms were used in the simulation, and the results were satisfactory when compared with data found in the literature. In order to determine the NO generation in the flame, the Zel’dovich mechanism was applied. Because the NO kinetics formation by this mechanism is considerably slower than the oxidation rate of the main fuel, it was possible to treat the two mechanisms separately. The results obtained for NO formation compare favorably with literature data.

Neste trabalho apresenta-se a modelagem de chamas difusivas na forma de jato, para baixo número de Mach e elevado número de Damköhler. O modelo é baseado na solução das equações na forma flamelet para a parte química e na fração de mistura para o fluxo. Este modelo descreve bem o comportamento de chamas difusivas, exceto na sua extremidade (ponta), onde geralmente surgem instabilidades. Resultados numéricos são apresentados para uma cinética química de uma e multietapas, utilizando a técnica LES (Large-Eddy Simulation) com o modelo de Smagorinsky para a viscosidade turbulenta. A discretização das equações governantes é feita em diferenças finitas, com a aplicação da técnica TVD (Total Variation Diminishing). Além disso, apresentamse mecanismos reduzidos multietapas para o metano, o metanol e o etanol, visando obter resultados realistas. A modelagem de chamas de metanol e etanol diferencia-se da modelagem de chamas de metano por ocorrer uma mudança de fase antes da combustão. Modela-se o efeito global das gotas usando uma descrição Lagrangeana que é incorporada à descrição Euleriana do escoamento, via termos fonte. Testes numéricos foram realizados para chamas difusivas de metano, metanol e etanol, e os resultados estão em concordância com os dados encontrados na literatura. / This work presents a model for a jet diffusion flame, for low Mach and high Damköhler numbers. The model is based on the solution of the flamelet equations for the chemistry and on the mixture fraction for the flow. This model describes well the behavior of diffusion flames, except at the flame tip, where instabilities can often occur. Numerical results are presented for an one-step and multi-step chemical kinetic models, using the LES (Large-Eddy Simulation) technique with the Smagorinsky model for the turbulent viscosity. The discretization of the governing equations follows the finite difference method, with the application of the TVD (Total Variation Diminishing) technique. Besides, multi-step reduced mechanisms for the methane, the methanol and the ethanol are employed, obtaining realistic results. The flame modeling of methanol and ethanol differs from the modeling of methane flames because of a phase change occurs before the combustion. The droplets global effect is modeled based on a Lagrangian description, which is incorporated into the Eulerian description of the flow through source terms. Numerical tests were carried out for methane, methanol and ethanol diffusion flames, and the results compare well with data in the literature.