Nesse trabalho, é investigada a combustão pré-misturada de metano e ar em um queimador poroso radiante com área de seção transversal variável. A estabilidade de chama e a eficiência de radiação neste tipo de queimador são analisadas através de simulação numérica. O problema considerado para análise é de um queimador unidimensional de área variável com perdas de calor por radiação em suas extremidades. A metodologia numérica usada para resolver o conjunto de equações de conservação é o método dos volumes finitos com o sistema de coordenadas cartesianas. Os resultados mostram que, o aumento da área da seção transversal proporciona um aumento na faixa de estabilidade de chama do queimador quando comparado com um de área constante. A eficiência de radiação também é influenciada positivamente pela variação de área, ou seja, um aumento na área da seção de saída do dispositivo resulta em um aumento na eficiência. Também é testada uma modelagem alternativa baseada no método de curvas de nível (level-set method). Nesse modelo as equações das espécies químicas são substituídas pela equação-G, a qual descreve a dinâmica de uma frente de chama infinitamente fina. Os resultados numéricos das simulações bem como as vantagens e as limitações do modelo de curvas de nível são discutidas. Em linhas gerais os resultados do modelo de curvas de nível não foram capazes de reproduzir adequadamente o comportamento obtido com o modelo convencional. / In this work, the premixed combustion of methane and air in a porous radiant burner with a variable cross-sectional area is investigated. The flame stability and the radiant efficiency in this kind of burner are analyzed through numerical simulation. The problem considers a variable area one-dimensional burner with radiation heat losses in its extremities. The numerical method used to solve the set of conservation equations is the finite volume method with the Cartesian coordinate system. The results show that an increase of the cross-sectional area promotes an increase of the flame stability range when compared with a constant area burner. The radiant efficiency is also positively influenced by the area variation, i.e., the increase of the outlet area results in an enhanced efficiency. An alternative modelling based on the level-set method is also tested. In this model the equations of chemical species are replaced by the G-Equation, which describes the dynamics of an infinitely thin flame front. The numerical results of simulations as well as advantages and limitations of the level-set model are discussed. In general the results of the level-set model were not able to reproduce in a suitable way the behavior obtained with the conventional model.

Ce travail de recherche est consacré à l'étude des processus d'allumage et de stabilisation des flammes turbulentes en configuration aéronautique. Cette thèse, entièrement expérimentale, se base dans une première partie sur l'étude et la compréhension de l'effet des paramètres locaux (vitesse et concentration en combustible) et de l'historique du noyau de flamme au cours de sa propagation sur les statistiques d'allumage. Pour expliquer ces statistiques, des scénarios sont proposés et validés pour différentes configurations opératoires. La deuxième partie de la thèse est dédiée à l'amélioration de la stabilité des flammes kérosène/air par dopage en hydrogène. Nous avons montré qu'à même structure de flamme, l'amplitude des fluctuations de pression était abaissée par la présence d'hydrogène. / This research is dedicated to the study of turbulent flames ignition and stabilization processes in representatives' aircraft combustion chambers. This PHD thesis, fully experimental, is based on studying and understanding local parameters (velocity and fuel concentration) and historical effects of the flame kernel's environment during its propagation on ignition statistics. To explain this statistics, various scenarios arre proposed and discussed. In this way, we justify the existence of high and low efficiency areas thanks to a time development criterion of the flame kernel. The second part of this thesis is dedicated to flame stabilization improvment by using H2 enrichment. Turbulent kerosene/air flames have been doped in gaseous hydrogen. We demonstrate a reduction of pressure oscillations' amplitude for the same flame structure.

Flame-holding in a recessed cavity is investigated experimentally in a Mach 2.5 preheated cross-flow for both stable and unstable combustion, with a relatively low preheating. Self-sustained combustion is investigated for stagnation pressures and temperatures reaching 1.4 MPa and 750 K. In particular, cavity blowout is characterized with respect to cavity aspect ratio (L/D =2.84 - 3.84), injection strategy (floor - ramp), aft ramp angle (90 deg - 22.5 deg) and multi-fuel mixture (CH₄-H₂ or CH₄-C₂H₄ blends). The results show that small hydrogen addition to methane leads to significant increase in flame stability, whereas ethylene addition has a more gradual effect. Since the multi-fuels used here are composed of a slow and a fast chemistry fuel, the resulting blowout region has a slow (methane dominant) and a fast (hydrogen or ethylene dominant) branch. Regardless of the fuel composition, the pressure at blowout is close to the non-reacting pressure imposed by the cross-flow, suggesting that combustion becomes potentially unsustainable in the cavity at the sub-atmospheric pressures encountered in these supersonic studies. The effect of preheating is also investigated and results show that the stability domain broadens with increasing stagnation temperature. However, smaller cavities appear less sensitive to the cross-flow preheating, and stable combustion is achieved over a smaller range of fuel flow rate, which may be the result of limited residence and mixing time. The blowout data point obtained at lower fuel flow rate fairly matches the empirical model developed by Rasmussen et al. for floor injection phi = 0.0028 Da^-.8, where phi is the equivalence ratio and Da the Damkohler number. An alternate model is proposed here that takes into account the ignition to scale the blowout data. Since the mass of air entrained into the cavity cannot be accurately estimated and the cavity temperature is only approximated from the wall temperature, the proposed scaling has some uncertainty. Nevertheless the new phi-Da scaling is shown to preserve the subtleties of the blowout trends as seen in the current experimental data.

Indiana University-Purdue University Indianapolis (IUPUI) / In order to conform to pollutant-control regulations and minimize NOx emissions, modern household boilers and central heating systems are moving toward premixed combustors. These combustors have been successful with regards to emissions along with efficiency. However, their implementation has been associated with acoustical instability problems that could be solved through precise optimization in design rather than trial and error experimentation. This thesis introduces an experimental apparatus, which is designed to investigate the acoustic instability problem at the flame level. The goal is an experimental determination of the flame transfer function and comparison of the experimental data with a theoretical model of the flame. An experimental procedure is designed to diagnose the origins of the combustion instabilities by measurement of the flame transfer function. This research is carried out in three steps. The first step is to understand the acoustic instability problem through study of the theoretical models of the flame transfer function and selection of a model, which is most functional in industrial applications. A xiii measurement technique for the flame transfer function is developed according to the required accuracy in measurements, repeatability, and configurability for a wide range of operating conditions. Subsequently, an experimental apparatus is designed to accommodate the flame transfer function measurement technique. The components of the acoustic system are carefully sized to achieve precise measurement of the system parameters such as flows, pressures, and acoustic responses, and the apparatus is built. The apparatus is operated to measure the flame transfer function at several operating conditions. The experimentally measured flame transfer function is compared with a theoretical model for further verification. The experimental apparatus provides an improved assessment of the acoustic instability problem for industrial applications.

This thesis aims at improving our knowledge on swirl combustors. The work presented here is based on Large Eddy Simulations (LES) coupled to an advanced combustion model: the Conditional Moment Closure (CMC). Numerical predictions have been systematically compared and validated with detailed experimental datasets. In order to analyze further the physics underlying the large numerical datasets, Proper Orthogonal Decomposition (POD) has also been used throughout the thesis. Various aspects of the aerodynamics of swirling flames are investigated, such as precession or vortex formation caused by flow oscillations, as well as various combustion aspects such as localized extinctions and flame lift-off. All the above affect flame stabilization in different ways and are explored through focused simulations. The first study investigates isothermal air flows behind an enclosed bluff body, with the incoming flow being pulsated. These flows have strong similarities to flows found in combustors experiencing self-excited oscillations and can therefore be considered as canonical problems. At high enough forcing frequencies, double ring vortices are shed from the air pipe exit. Various harmonics of the pulsating frequency are observed in the spectra and their relation with the vortex shedding is investigated through POD. The second study explores the structure of the Delft III piloted turbulent non-premixed flame. The simple configuration allows to analyze further key combustion aspects of combustors, with further insights provided on the dynamics of localized extinctions and re-ignition, as well as the pollutants emissions. The third study presents a comprehensive analysis of the aerodynamics of swirl flows based on the TECFLAM confined non-premixed S09c configuration. A periodic component inside the air inlet pipe and around the central bluff body is observed, for both the inert and reactive flows. POD shows that these flow oscillations are due to single and double helical vortices, similar to Precessing Vortex Cores (PVC), that develop inside the air inlet pipe and whose axes rotate around the burner. The combustion process is found to affect the swirl flow aerodynamics. Finally, the fourth study investigates the TECFLAM configuration again, but here attention is given to the flame lift-off evident in experiments and reproduced by the LES-CMC formulation. The stabilization process and the pollutants emission of the flame are investigated in detail.