A compreensão dos mecanismos de estabilidade de chamas é de extrema importância tanto para o projeto/dimensionamento de queimadores utilizados em fornos e fornalhas industriais, câmaras de combustão de turbinas a gás e flares; como para a substituição de combustíveis em queimadores existentes. Há um intervalo de condições (como velocidade de descarga do jato e concentração de combustível na mistura gasosa) na qual a combustão estável pode ser mantida, sendo limitada por dois fenômenos denominados como liftoff (descolamento da base da chama do bocal e posterior estabilização desta a certa distância do bocal) e blowout (desprendimento e extinção da chama). Por razões de segurança, operações próximas às condições em que o blowout pode ocorrer devem ser evitadas. Muitas teorias têm sido publicadas para descrever as características de liftoff e blowout de chamas de difusão turbulenta. Este trabalho apresenta algumas destas teorias, bem como as hipóteses assumidas e os processos físicos considerados responsáveis por estes fenômenos (liftoff e blowout). Correlações para a previsão da velocidade de blowout e resultados experimentais disponíveis na literatura também são apresentados. Uma nova correlação para a velocidade de blowout é proposta, a qual se baseia nos movimentos de grande escala observados em jatos turbulentos e no adimensional número de Damköhler (relação entre o tempo de cinética química e o tempo de mistura dos reagentes e destes com os produtos da reação). Comparações entre as previsões da correlação proposta com resultados experimentais e com previsões de outras correlações disponíveis na literatura foram realizadas, para diferentes combustíveis e diâmetros de bocais. A correlação proposta apresentou boa concordância com os resultados experimentais. A partir das análises desenvolvidas neste trabalho, verificou-se que a velocidade de blowout de chamas de difusão turbulenta é função das propriedades do combustível, das características do bocal, das condições do ambiente e do adimensional número de Damköhler. / The study of flame stability is very important to the design of burners used in industrial ovens and furnaces, combustion chambers of gas turbines and flares; and fuel substitution in burners. There is a range of conditions (for example gas velocity at the nozzle exit and jet fuel concentration in the gas mixture) at which stable combustion can be maintained, being limited by two phenomena called liftoff and blowout. Lift-off is the detachment of the flame from the fuel nozzle, and blowout its detachment and extinction. Operating conditions close to stability limits should be avoided for security reasons. Many theories have been published to describe the blowout and lifted characteristics of turbulent jet diffusion flames. This document presents some theories, as well as the assumptions and physical processes considered responsible for these phenomena (liftoff and blowout). Correlations for predicting the blowout velocity and experimental results available in the literature are also shown. A new correlation is proposed, which is based on large-scale motions observed in turbulent jets and the dimensionless Damköhler number (ratio of the characteristic chemical reaction time and the time associated with the mixing of reentrained hot products into fresh reactants). Comparisons between the predictions of the proposed correlation with experimental results and predictions of other correlations available in the literature were performed for different fuels and nozzle diameters. The proposed correlation showed good agreement with the experimental results. The analyses developed in this work allow us to conclude that the blowout velocity of the turbulent diffusion flame depends on the fuel properties, characteristics of the nozzle, the environmental conditions and the Damköhler number.

A new Trapped Vortex Combustor (TVC) concept has been studied for applications such as those in Unmanned Aerial Vehicles (UAVs) as it offers potential for superior flame stability and low pressure loss. Flame stability is ensured by a strong vortex in a physical cavity attached to the combustor wall, and low pressure loss is due to the absence of swirl. Earlier studies on a compact combustor concept showed that there are issues with ensuring stable combustion over a range of operating conditions. The present work focuses on experimental studies and numerical simulations to study the stability issues and performance optimization in this compact single-cavity TVC configuration. For performing numerical simulations, an accurate and yet computationally affordable Modified Eddy Dissipation Concept combustion model is built upon the KIVA-3V platform to account for turbulence-chemistry interactions. Detailed validation with a turbulent non-premixed CH4/H2/N2 flame from literature showed that the model is sufficiently accurate and the effect of various simulation strategies is assessed. Transient flame simulation capabilities are assessed by comparison with experimental data from an acoustically excited oscillatory H2-air diffusion flame reported in literature. Subsequent to successful validation of the model, studies on basic TVC flow oscillations are performed. Frequencies of flow oscillations are found to be independent of flow velocities and cavity length, but dependent on the cavity depth. Cavity injection and combustion individually affect the magnitude of flow oscillations but do not significantly alter the resonant frequencies. Reacting flow experiments and flow visualization studies in an existing experimental TVC rig with optical access and variable cavity L/D ratio show that TVC flame stability depends strongly on the cavity air velocity. A detailed set of numerical simulations also confirms this and helps to identify three basic modes of TVC flame stabilization. A clockwise cavity vortex stabilized flame is formed at low cavity air velocities relative to the mainstream, while a strong anticlockwise cavity vortex is formed at high cavity air velocities and low L/Ds. At intermediate conditions, the cavity vortex structure is found to be in a transition state which leads to large scale flame instabilities and flame blow-out. For solving the flame instability problem, a novel strategy of incorporating a flow guide vane is proposed to establish the advantageous anticlockwise vortex without the use of cavity air. Experimental results with the modified configuration are quite encouraging for TVC flame stability at laboratory conditions, while numerical results show good stability even at extreme operating conditions. Further design optimization studies are performed in a multi-parameter space using detailed simulations. From the results, a strategy of using inclined struts in the main flow path along with the flow guide vane seems most promising. This configuration is tested experimentally and results pertaining to pressure drop, pattern factor and flame stability are found to be satisfactory.

The objectives of this paper focus upon composing recommendations with respect to burners design while observing aerodynamic, together with present and advanced developments in the field of flame technology principles. Specifically the aims are set at flame stability and pollutions. Important part of this paper is a novel design procedure dealing with premixed burners. Further, by exploiting the results of testing program enabled one to design burners exhibiting low level of nitrogen oxides emissions. On the basis of aerodynamic analysis of premixed burners flow path several recommendations on the geometry resulted in proposing two ejectors, which transport the combustion air. In the first ejector the fuel jet, generated by flow through a nozzle, draws the specified part of the combustion air, and then the mixture is a driving agent of the second ejector. The remaining part of combustion air flows first through the radial curved vanes, and second its tangential velocity component is augmented by axial vanes prior it enters mixing part of the second ejector. For the additional contribution to the rotating motion the inclined axial vanes built inside the nozzle of the second ejector are responsible. The resulting swirling number reaches the values up to 2.6. For the burner so arranged fire stability and low NOx emissions in the wide operating conditions can be expected. Within the frame of extensive testing program in the field of diffusion burners several modifications have been carried out such as setting of axial vanes which secure the swirling motion, changes of combustion air flows path, and modifications of secondary nozzles, these resulted in extending zone of stabilized flame and reducing the formation of NOx, e.g. NOx emissions of 70 ppm, prior the modifications, have dropped to 30 ppm (dry flue gas 3% O2). The main contribution of this paper stands in applications of development activities results in aerodynamic and flame within their implementation in the field of burners design. Further, it is the proposed design of burners which are capable of simultaneously firing products of the biomass microwave pyrolysis, namely syngas and bio-oils.

L'application de champ électrique est connue pour avoir la capacité d'améliorer significativement la stabilité des flammes. A ce sujet, un modèle mathématique permettant de modéliser la combustion en présence d'un champ électrique a été développé. Les équations de l'aérothermochimie sont couplées à des équations de bilan pour les densités des espèces chargées, et une équation de Poisson pour le potentiel électrique est résolue. Une situation principale est étudiée pendant la thèse ; elle concerne la stabilisation de flammes de diffusion par application d’un champ électrique continu ou alternatif.Les résultats obtenus montrent que la présence du champ électrique améliore significativement la stabilisation de la flamme. L’ampleur de cette amélioration dépend de l’intensité et de la polarité de la tension appliquée. Si la tension appliquée est alternative, un facteur supplémentaire s’ajoute pour influencer la stabilisation ; il s’agit de la fréquence. Une interprétation des mécanismes permettant la stabilisation est proposée. / The application of electric field is known to have the ability to improve significantly the flame stability. In this regard, a mathematical approach to model combustion in the presence of an electric field was developed. The Navier-Stokes equations along with transport equations for charged species and the electric potential Poisson’s equation are solved. A main situation, that concerns the stabilization of diffusion flames by applying a direct or alternating electric field, is studied. The results show that the presence of the electric field improves the flame stabilization. The magnitude of this improvement depends on the intensity and polarity of the applied voltage. If the applied voltage is alternating, an additional factor, which is the frequency of the electric current, influences also the extent of the flame stabilization improvement. An interpretation of the stabilization mechanisms is proposed.

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No description available.

<p> Lifted non-premixed turbulent jet flames in the Transverse Instability Combustor (TIC) have been analyzed using qualitative and quantitative methods. Lifted flames in the TIC have been observed to stabilize about zero to five injector exit diameters downstream of the dump plane into the chamber and exhibit pulsating, unsteady burning. Anchored flames immediately begin reacting in the injector recess and burn evenly in a uniform jet from the injector exit through the entire optically accessible region. Statistically significant, repeatable behavior lifted flames are observed. It is shown that the occurrence of lifted flames is most likely for an injector configuration with close wall-spacing, second greatest for a configuration with close middle-element spacing, and lowest for a configuration with even element-spacing. For all configurations, of those elements that have been observed to lift, the center element is most likely to lift while the second element from the wall was likely. Flames at the wall elements were never observed to lift. Evidence is shown to support that close injector element spacing and stronger transverse pressure waves aid lateral heat transfer which supports flame stability in the lifted position. It is hypothesized that the stability of lifted flames is influenced by neighboring ignition sources, often a neighboring anchored flame. It is also shown that instances of lifted flames increase with the root-mean-squared magnitude of pressure fluctuation about its mean (P’ RMS) up to a threshold, after which flames stabilize in the anchored recess position.</p> <p>Dynamic mode decomposition (DMD) and proper orthogonal decomposition (POD) analyses of CH* chemiluminescence data is performed. It is found that lateral ignition of the most upstream portion of lifted flames is dominated by the 1W mode. Furthermore, it is shown that low-frequency high energy modes with spatial layers resemble intensity-pulses, possibly attributable to ignition. These modes are trademarks of CH* chemiluminescent intensity data of lifted flames. It was also shown that the residence time in the chamber may be closely associated with those low-frequency modes around 200 Hz. DMD and POD were repeated for a downstream region on the center element, as well as a near-wall element, highlighting differences between the lifted flame dynamics in all three regions. </p> <p>It is shown that lifted flames are best characterized by their burning behavior and in rare cases may stabilize in the recess, while still being “lifted”. Furthermore, it is shown that flame position differentiation can extend into an initial period of highly stable combustor operation. Dynamic mode decomposition is explored as potential method to understand physical building blocks of proper orthogonal spatial layers. Non-visual indicators of lifted flames within the high-frequency (HF) pressure signal are sought to seek a method that allows for observation of lifted flames in optically inaccessible combustors, such as those in industry. Some attributes of power-spectral diagrams and cross-correlations of pressure signals are provided as potential indicators. </p>

Combustion systems utilizing bluff bodies to stabilize the combustion processes can experience oscillatory heat release due to the alternate shedding of coherent, von Kármán vortices under certain operating conditions. This phenomenon needs to be understood in greater detail, since unsteady burning due to vortex shedding can lead to combustion instabilities and flame extinction in practical combustion systems. The primary objective of this study was to elucidate the influence of combustion process heat release upon the Bénard-von Kármán (BVK) instability in reacting bluff body wakes. For this purpose, spatial and temporal heat release distributions in bluff body-stabilized combustion of liquid Jet-A fuel with high-temperature, vitiated air were characterized over a wide range of operating conditions. Upon comparing the spatial and temporal heat release distributions, the fuel entrainment and subsequent heat release in the near-wake were found to strongly influence the onset and amplitude of the BVK instability. As the amount of heat release in the near-wake decreased, the BVK instability increased in amplitude. This was attributed to the corresponding decrease in the local density gradient across the reacting shear layers, which resulted in less damping of vorticity due to gas expansion. The experimental results were compared to the results of a parallel, linear stability analysis in order to further understand the influence of the combustion processes in the near-wake upon the wake instability characteristics. The results of this analysis support the postulate that oscillatory heat release due to BVK vortex shedding is the result of local absolute instability in the near-wake, which is eliminated only if the temperature rise across the reacting shear layers is sufficiently high. Furthermore, the results of this thesis demonstrate that non-uniform fuelling of the near-wake reaction zone increases the likelihood of absolutely unstable, BVK flame dynamics due to the possibility of near-unity products-to-reactants density ratios locally, especially when the reactants temperature is high.

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.

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.