Tesi sul tema "Porous Media Combustion"

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Tecnologias avançadas para a geração de energia usando combustíveis não convencionais xisto betuminoso e seu semi-coque, areias betuminosas, petróleo extra-pesado e biomassa proveniente de resíduos sólidos urbanos e de lodo de esgoto - têm em comum processos termoquímicos compostos de complexas reações químicas. Este trabalho trata da formulação e otimização de mecanismos químicos normalmente envolvidos na pirólise do xisto betuminoso e na combustão do xisto betuminoso e seu semi-coque. Problemas inversos (usando o algoritmo de Levenberg-Marquardt) foram empregados para minimizar o erro entre os valores estimados e os dados de termogravimétria para os mecanismos de reação de 3 passos para a pirólise do xisto betuminos, e mecanismos de 4 e 3 passos para o xisto betuminoso e seu semi-coque, respectivamente. Os parâmetros cinéticos, tais como ordem de reação, fator pré-exponencial, energia de ativação e os coeficientes estequiométricos que afetam a secagem, as reações de oxidação, pirólise e descarbonatação foram estimadas com sucesso. Além disso, os erros estatísticos e residuais foram avaliados, resultando em um valor razoável para todas as estimativas e o mecanismo cinético proposto e estimado para a combustão do semi-coque foi aplicado em um código em meios porosos. Um estudo paramétrico entre o perfil de temperatura e a velocidade do ar, e o perfil de temperatura e a concentração de carbono fixo foi desenvolvido. Este estudo mostra que o perfil de temperatura é extremamente influenciado por estes parâmetros, confirmando que a propagação da frente é controlada pela injeção de O2. Palavras-chave: Xisto Betuminoso, Semi-Coque, Pirólise, Combustão, Estimação de Parâmetros, Problemas Inversos, Levenberg-Marquardt, Meios Porosos.

In the effort to introduce fuel cell technology in the field of decentralized and mobile power generators, a hydrocarbon reformer to syngas seems to be the way for the market uptake. In this thesis, a potential technology is developed and investigated, in order to convert commercial liquid fuel (diesel, kerosene and biodiesel) to syngas. The fundamental concept is to oxidise the fuel in a oxygen depleted environment, obtaining hydrogen and carbon monoxide as main products of the reaction. In order to extend the flammability limit of hydrocarbon/air mixtures, the rich combustion experiments have been carried out in a two-layer porous medium combustor, which stabilises a flame at the matrix interface and recirculates the enthalpy of the hot products in order to enhance the reaction rates at ultra-rich equivalence ratio. This thesis demonstrates the feasibility of the concept, by exploring characteristic parameters for a compact, reliable and cost effective device. Specifically, a range of equivalence ratios, thermal loads and porous materials have been examined. n-heptane was successfully reformed up to an equivalence ratio of 3, reaching a conversion efficiency (based on the lower heating value of H2 and CO over the fuel input) up to 75% for a packed bed of alumina beads. Thermal loads from P=2 to 12 kW at phi=2.0 demonstrated that heat losses can be reduced to 10%.Similarly, diesel, kerosene and bio-diesel were reformed to syngas in a Zirconia foam burner with conversion efficiency over 60%. The effect of different burners, thermal loads and equivalence ratios have also been assessed for these commercial fuels, leading to equivalent conclusions. A preliminary attempt to reduce the content of CO and hydrocarbons in the reformate has been also performed using commercial steam reforming and water-gas shift reaction catalysts, obtaining encouraging results. Finally, soot emission has been assessed, demonstrating particle formation for all the fuels above phi=2.0, with biodiesel showingthe lowest soot formation tendency among all the fuels tested.

This study investigates peculiarities of the filtration combustion (FC) of the gaseous fuel-air mixtures in a porous inertia media (PIM). Combustion wave velocities and temperatures were measured for hydrogen-air, propane-air and methane-air mixtures in the PIM at different mixture filtration velocities. It is shown that the dependences of the combustion wave velocities on the equivalence ratio are V-shaped, It was further confirmed that the FC in the PIM has more contrasts than similarities with the normal homogeneous combustion. One of the interesting observations in the present study, which is not common in normal homogenous combustion, is the shifting of the fuel-air equivalent ratio at the minimum combustion wave velocity from the stoichiometric condition (¢ = 1). For a hydrogen-air mixture, the fuel-air equivalence ratio at the minimum combustion velocity shifts from the stoichiometric condition to the rich region, while for the propane-air and methane-air mixtures the fuel-air equivalence ratio at the minimum combustion velocity shifts toward fuel-leaner conditions. The measured maximum porous media temperatures in the combustion waves are found to be weakly dependent on the mixture filtration velocities. In general, the effects of the mixture filtration velocities on the measured maximum porous media temperatures are not significant.

This work presents one and two dimensional numerical results for combustion of an air/methane mixture in inert porous media, using both laminar and turbulence models, and radiation. Comparisons with experimental data are reported. The burner used as reference is composed by a preheating section followed by a combustion region. Macroscopic equations for mass, momentum and energy are obtained based on the volume average concept. Distinct energy equations are considered for the solid phase and the flowing gas. The numerical technique employed for discretizing the governing equations was the control volume method with a boundary-fitted non-orthogonal coordinate system. The SIMPLE algorithm was used to relax the entire equation set. Inlet velocity, excess air ratio, porosity and solid thermal conductivity were varied in order to investigate their effect on temperature profiles and flame front position. Results indicate that higher inlet velocities result in higher gas temperatures, pushing the flame front towards the exit of the burner, following a similar trend observed in the experimental data used for comparisons. Burning mixtures close to the stoichiometric conditions also increased temperatures, as expected, and brings the flame front to preheating region, next to inlet. Increasing the thermal conductivity of the preheating section reduced peak temperature in combustion region. The use of porous material with very high thermal conductivity on the combustion region did not affect significantly temperature levels or flame front profiles.

This research investigated the potential of smouldering combustion to be employed as a remediation approach for soil contaminated by non-aqueous phase liquids (NAPLs). Small-scale (~15 cm), proof-of-concept experiments were the first to demonstrate that organic liquids embedded within an inert soil matrix can be successfully smouldered. Intermediate-scale (~30 cm) column experiments examined in detail the behaviour of the combustion process including its relationship to mass and energy balance and the evolution of temperature profiles. In addition, detailed evaluations of environmental parameters (e.g., soil concentrations, gas emissions) were conducted. For the first time, it was demonstrated that NAPL smouldering combustion can be self-sustaining (i.e., propagation of the smouldering front after termination of the igniter) and self-terminating (i.e., natural extinction of the reaction after all of the NAPL is destroyed). More than 30 column sensitivity experiments quantified the broad range of process parameters - including contaminant type, contaminant mass, soil type, and oxidizer flow rates - within which the process was self-sustaining and essentially complete remediation was achieved (i.e. contaminant mass removal in excess of 99.5%). Maximum burning temperatures were observed in the range 600-1100 C. Average propagation velocities varied between 0.7e-4 and 1.2e-4 m/s. Intensity and velocity of the process were shown to be controlled by the rate at which oxidizer is delivered. Contaminant type and mass was observed to affect peak temperatures and propagation velocity by influencing the energy balance at the reaction front. Moreover, mass and energy balance models were demonstrated to provide reasonable predictions of the observed propagation velocities. Overall, this research introduced an entirely new approach to the remediation of NAPL-contaminated soils and, further, advanced the understanding of the mechanisms that control the underlying process of smouldering combustion of liquids.

The combustion of ultra-lean fuel to air mixtures provides an efficient way to convert the chemical energy of hydrocarbons into useful power. Conventional burning techniques of a mixture have defined flammability limits beyond which a flame cannot self-propagate due to heat losses. Matrix stabilized porous medium combustion is an advanced technique in which a solid porous matrix within the combustion chamber accumulates heat from the hot gaseous products and preheats incoming reactants. This heat recirculation extends the standard flammability limits and allows the burning of ultra-lean fuel mixtures, conserving energy resources, or the burning of gases of low calorific value, utilizing otherwise wasted resources. The heat generated by the porous burner can be harvested with thermoelectric devices for a reliable method of generating electricity for portable electronic devices by the burning of otherwise noncombustible mixtures. The design of the porous media burner, its assembly and testing are presented. Highly porous (~80% porosity) alumina foam was used as the central media and alumina honeycomb structure was used as an inlet for fuel and an outlet for products of the methane-air combustion. The upstream and downstream honeycomb structures were designed with pore sizes smaller than the flame quenching distance, preventing the flame from propagating outside of the central section. Experimental results include measurements from thermocouples distributed throughout the burner and on each side of the thermoelectric module along with associated current, voltage and power outputs. Measurements of the burner with catalytic coating were obtained for stoichiometric and lean mixtures and compared to the results obtained from the catalytically inert matrix, showing the effect on overall efficiency for the combustion of fuel-lean mixtures.
ID: 030646196; System requirements: World Wide Web browser and PDF reader.; Mode of access: World Wide Web.; Thesis (M.S.A.E.)--University of Central Florida, 2011.; Includes bibliographical references (p. 105-119).
M.S.A.E.
Masters
Mechanical and Aerospace Engineering
Engineering and Computer Science
Aerospace Engineering; Thermofluid Aerodynamic Systems Track

Ce travail de thèse, réalisé en collaboration avec l’IMFT et TOTAL, traite de la modélisation de la combustion in-situ appliquée à une huile lourde Vénézuéliène. Il a été initié suite à une observation simple : même si le procédé est étudié depuis plusieurs décénies, on ne peut pas encore le modéliser correctement. Des résultats expérimentaux, issus d’expérience à l’échelle du laboratoire (tubes à combustions), ne sont pas reproductibles avec des outils numériques commerciaux de types simulateurs réservoirs thermiques. Par conséquent, et face à ce constat, nous avons été contraint d’explorer plusieurs pistes pour améliorer la modélisation du procédé : – La chimie et les méthodes de détermination de mécanismes réactionnels. – La description thermodynamique d’une huile lourde et le calcul d’équilibre triphasique. – Le transport de masse et de chaleur dans un milieu poreux, en situation multiphasique, réactive et miscible. – La conception d’un modèle mathématique et numérique d’un modèle complet. Nous pensons que le problème pluridisciplinaire et fortement complexe peut trouver une réponse si l’ensemble des mécanismes et leurs liens sont traités de façon adéquate. Une campagne expérimentale (expériences de cellules cinétiques), portant sur l’étude des effets de l’eau sur les réactions chimiques de l’huile, a permis de mettre en évidence des effets inattendus et nouveaux. Ces données, complétées par des expériences de types tubes à combustion, fournissent une importante base de données expérimentale. Pour modéliser les expériences de cellules cinétiques, nous avons tout d’abord développer un nouvel outil de simulation directe, reposant sur une description compositionnelle de l’huile où les comportements de toutes les phases sont prédits par les équations d’états. Le calcul d’équilibre est fait grâce à un flash diphasique. Afin de déterminer un mécanisme réactionnel paramétré, nous avons couplé ce dernier outil à un algorithme génétique. Finalement, dans le but de simuler les expériences de tubes à combustion, un nouveau simulateur compositionel, triphasique, thermique et réactif a été développé. Il est spécialement adapté à la simulation de ce genre d’expérience. Le calcul d’équilibre de phase est réalisé grâce à un nouvel outil développé pour l’occasion. Ce dernier repose sur l’hypothèse free water et repose sur une formulation originale et novatrice
The study of this PhD, realized jointly with IMFT and TOTAL, deals with modeling of in-situ combustion applied to a Venezuelan heavy oil. It has begun with a relatively simple observation: even if the process has been extensively studied since some decades, we cannot correctly model it. Experiment data provided by lab scale experiments (combustion tubes) mismatches numerical results obtained from commercial thermal simulator, especially for wet experiments. The need to better understand the process related to this issue forced us to explore multiple tracks for various scientific fields. Thus, one can cite: • The chemistry and methods of reduction of reactive mechanisms. • The thermodynamic description of the heavy oil and the calculations of three-phase equilibrium. • Heat and mass transport in multiphase, reactive and miscible porous medium. • Mathematical and numerical design of a full model. The problem exceedingly complex can find a complete and consistent answer if one takes into account the whole mechanisms and links between them. We have followed this way in order to determine a robust reactive scheme using both theoretical numerical and experimental developments. A whole set of kinetic cell manipulations was conducted to better understand and discriminate the effects of water on chemistry on a certain type of heavy oils. New interactions and effects on steam on heavy oil combustion have been discovered and studied. These manipulations, supplemented by a set of some combustion tubes provide a large set of experimental data. This will compose our base case that we will try to match later using some new tools devised during this study. To model kinetic experiments, we firstly developed a new simulation tool based on a compositional description and a full equation of state formulation. Equilibrium calculation is made by a two-phase flash. To determine consistent kinetic parameters, we used a genetic algorithm coupled with the new tool. Finally, in order to validate the kinetic model and simulate combustion tube experiment, a new threephase compositional simulator has been developed. It is especially fitted to take into account characteristic of the experimental device. Three-phase equilibrium calculation is computed by a new free-water

This report details an approach to using metal foam heat exchangers inside an integrated thermal management system on a variable cycle engine. The propulsion system of interest is a variable cycle engine with an auxiliary, variable flow rate fan. The feasibility of utilizing an open-celled metallic foam heat exchanger in the ducting between the constant and variable-fans on this variable cycle engine to cool the avionics was explored using an experimental approach. Two heat exchangers, 6.3 inch width by 6.3 inch length by 0.5 inch thickness, were constructed from 20 and 40 pores per inch (PPI) metal foam and tested. Both were constructed using 6061-T6 aluminum open-cell metal foam with a relative density of 8% and brazed using 4047 aluminum braze to 0.02 inch thick sheet metal made of 6061-T6 aluminum. Both models were subjected to internal forced convection using heated air with flow rates of 4, 8, 12, 16, and 20 standard cubic feet per minute (SCFM). They were also subjected to external forced convection using blowers to supply cooling air to simulate the variable cycle engine’s fans. One duct was supplied with a constant 34 ft/s cooling flow, while the other cooling flow velocity was varied between 0% and 100% of this 34 ft/s, in 25% increments. The temperature and pressure of the flow internal to the metal foam, as well as the heat exchanger external surface and cold flow temperatures, were recorded. A hot-flow Reynolds number range of 1,300 to 6,400 was tested. Results showed expected trends for the hydraulic performance of both heat exchangers. The form factors were 50.4 and 54.8 ft^-1 and the permeabilities were 9.11E-7 and 6.32E-7 ft^2 for the 20 and 40 PPI heat exchangers, respectively. Due to a defect on one side of the 40 PPI heat exchanger, the thermal results are based only on the 20 PPI heat exchanger. While the present study examines a different metal foam heat transfer configuration than most other studies, the metal foam Nusselt numbers were comparable to past studies. In addition, the pumping power required was not excessive and would allow the thermal management system to be realized without an unreasonable energy input. Therefore, a metal foam heat exchanger integrated within the ducting of a variable cycle engine is deemed feasible. The pumping power and thermal resistance were used to create a performance predicting model of the 20 PPI heat exchanger. From this model, the optimized 20 PPI heat exchanger has a hot-flow rate of 10.5 SCFM. The resulting pumping power and thermal resistance are estimated to be 6.7 BTU/hr and 0.036 °R/(BTU/hr), respectively.

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O diÃxido de estanho (SnO2) em forma de filme fino pode ser produzido com grande transparÃncia à luz visÃvel e boa condutividade elÃtrica. Os filmes finos de SnO2 possuem muitas aplicaÃÃes tecnolÃgicas na indÃstria, principalmente em aparelhos eletrÃnicos que utilizam display de visualizaÃÃo, como em dispositivos em laboratÃrios de pesquisa. Uma das aplicaÃÃes mais promissoras à a sua utilizaÃÃo como Ãxido condutor transparente em cÃlulas solares fotovoltaicas. Devido as suas caracterÃsticas de transparÃncia Ãtica no espectro visÃvel e baixa resistividade, os filmes finos de diÃxidos de estanho sÃo empregados como componente constituinte de cÃlulas solares fotovoltaicas. A proposta deste trabalho à a utilizaÃÃo de um forno, que utiliza a tecnologia de CombustÃo em Meios Porosos para fabricaÃÃo de filmes finos de diÃxido de estanho sobre substratos de vidro, utilizando a tÃcnica de spray pirÃlise. O forno utilizado nesse projeto possui uma cÃmara, onde os filmes de SnO2 sÃo sinterizados, e uma antecÃmara, onde a soluÃÃo precursora dos filmes à aplicada sobre substratos de vidro. Uma pistola spray foi adaptada ao forno, acoplada a antecÃmara, para a aspersÃo da soluÃÃo de estanho sobre substratos de vidro. Foi utilizada a tÃcnica de dopagem dos filmes finos com flÃor com o intuito de reduzir a resistÃncia à corrente elÃtrica. Os filmes finos de SnO2 foram caracterizados em relaÃÃo a transmitÃncia Ãtica ao espectro visÃvel e em relaÃÃo a resistÃncia elÃtrica. TambÃm foram realizadas medidas de difraÃÃo de raios-X e microscopia de forÃa atÃmica para a revelaÃÃo e estudo da estrutura dos filmes de Ãxido de estanho.
The tin dioxide (SnO2) as thin film can be produce with high transparency to visible light and good electrical conductivity. The SnO2 thin films have many technological applications in industry, mainly in electronic devices that use preview display, such as devices in research laboratories. One of the most promising applications is its use as a transparent conductive oxide in photovoltaic solar cells. Due to its transparency in the visible spectrum and low resistivity, the films of tin dioxide are used as a constituent component of photovoltaic solar cells. The present work aims to use a heat furnace, which uses combustion in porous media technology for the production of thin films of tin dioxide (SnO2) on glass substrates, using the technique of spray pyrolysis. The furnace used has a chamber where the films of SnO2 are sintered and a pre-chamber, where the precursor solution is applied films on glass substrates. Spray gun was adapted to furnace and coupled to the antechamber for the spraying of the solution of tin on glass substrates. The technique of doping thin films of Fluoride was used in order to reduce the resistance to electrical current. The thin films of SnO2 was characterized by their optical transmittance spectrum and electrical resistance. The structure of films of tin oxide was study by x-ray diffraction and atomic force microscopy.

La transition vers une économie neutre en carbone est confrontée à deux défis majeurs : le stockage de l'excès d'énergie provenant des énergies renouvelables et la décarbonation des processus de combustion dans les secteurs difficiles à électrifier. La stratégie Power to Gas (P2G) propose de résoudre ces problèmes en substituant partiellement l'hydrogène dans le réseau actuel de gaz naturel. Cependant, cela nécessite le développement de brûleurs flexibles capables de s'adapter à des niveaux variables d'hydrogène dans le réseau. Cela est compliqué à cause des différences entre les propriétés de la flamme d’hydrogène et celles des combustibles hydrocarbonés. Les brûleurs poreux (PMBs) sont considérés comme une technologie prometteuse en raison de leurs propriétés uniques. Les PMBs utilisent la recirculation de chaleur pour stabiliser les flammes à l'intérieur de matrices poreuses inertes, incrémentant le taux de consommation de la flamme et atteignant des températures locales superadiabatiques. Cela permet des densités de puissance plus élevées et l’extension des limites d'inflammabilité, ce qui se traduit par des dispositifs compacts et une faible émission de NOx avec des efficacités radiatives élevées.Le mécanisme fondamental de fonctionnement des brûleurs poreux à l'échelle macroscopique, la recirculation de la chaleur, est bien compris. Cependant, il existe encore une connaissance limitée sur certains phénomènes à l'échelle des pores et de leur influence sur le comportement du système global. En raison de la non-linéarité de la combustion et du transfert de chaleur, la stabilisation de la flamme et les performances du brûleur dépendent fortement des détails à l'échelle des pores. Les modèles de bas ordre actuels n'incluent pas la modélisation des interactions flamme-paroi et des effets de diffusion préférentielle, ce qui entraîne une faible précision. Les diagnostics non intrusifs avancés pourraient être utilisés pour étudier la structure locale de la flamme et guider l'amélioration des modèles de bas ordre. Cependant, les mesures expérimentales dans les PMBs sont entravées par le manque d'accès optique à l'intérieur de la matrice poreuse. Malgré les efforts récents, l'application de diagnostics optiques et non intrusifs dans les PMBs est encore très rare. Cette thèse présente une étude expérimentale sur la combustion en milieu poreux et est consacrée au développement de diagnostics optiques. Des PMBs optiquement accessibles sont produits en combinant des topologies définies par ordinateur avec des techniques de fabrication additive. La méthodologie actuelle offre un accès optique étendu dans une configuration de brûleur 3D sans perturber la structure de la matrice. L'accès optique est utilisé pour appliquer une série de diagnostics optiques, y compris la chimiluminescence CH*, l'imagerie de diffusion de Mie et la micro-PIV. Nos résultats montrent les limites des VAMs actuels et de leurs méthodes de validation. La mise en œuvre de diagnostics novateurs a également révélé différentes tendances de stabilisation dans les flammes enrichies en H2, soulignant l'effet des mécanismes d'ancrage local sur les limites de fonctionnement du brûleur. Enfin, l'accès optique est exploité pour effectuer des diagnostics laser et étudier la structure de la flamme à l'échelle des pores. Nos résultats révèlent différents modes de stabilisation et mettent en évidence l'impact de l’écoulement interstitiel sur les performances du brûleur. Cette thèse ouvre de nouvelles voies pour l'application de diagnostics non intrusifs et plaide pour un développement supplémentaire des techniques expérimentales avancées dans les brûleurs poreux
Porous Media Burners (PMBs) are a combustion technology based on heat recirculation where a flame is stabilized within the cavities of an inert porous matrix. In PMBs, heat is transferred upstream from the burned to the unburned gas through the solid matrix yielding a preheating of the reactants.This increases their burning rate allowing for more compact combustion devices and the operation beyond conventional flammability limits. As a result, the stabilization of flames at ultra-lean equivalence ratios is possible, with the subsequent reduction of the flame temperature and NOx emissions. In these burners, a substantial fraction of the power is radiated by the hot solid phase, with radiated power fractions ranging between 20-30 %. This, together with their elevated efficiency and low pollutant emissions, has motivated their commercial use in various infrared heating applications.In the past years, PMBs have received renewed interest owing to their potential as fuel flexible burners. Their ability to stabilize flames over a wide range of burning rates makes them promising candidates to handle the uneven flame properties of hydrogen and hydrocarbon fuels.The mechanism of heat recirculation in PMBs is well understood. However, there is still limited knowledge about many pore-scale phenomena that have a critical impact on the macroscopic behavior of the system and its performance.Advanced nonintrusive diagnostics could be used to study local flame stabilization mechanisms and improve current models. However, experimental measurements in PMBs are hindered by the lack of optical access to the interior of the porous matrix.This dissertation presents an experimental study on porous media combustion and is devoted to the application of optical diagnostics. Optically accessible PMBs are produced by combining computer-defined topologies with additive manufacturing techniques. This methodology provides an extensive optical access in a 3D burner configuration without altering the matrix structure. Optical access is leveraged to apply CH* chemiluminescence, Mie-scattering imaging and micro PIV. Topology tailoring is exploited to analyze the influence of the geometrical parameters of the porous matrix. Direct flame visualization enables the tracking of the reaction region as a function of the operating conditions, which can be used for model validation. The present results bring to light several limitations of current low order models and highlight the influence of the pore size on flame stabilization. Flame-front tracking is also used to investigate the effect of H2-enrichment on the behavior of the flame. This technique reveals different stabilization trends in H2-enriched flames that are not well retrieved by current models. Mie-scattering permits the quantification of the re-equilibration distance and the analysis of the flame shape. Micro PIV measurements show the influence of the topology on the interstitial flow and on the contribution of hydrodynamic effects to flame stabilization.This PhD seeks to open new paths for the application of non-intrusive diagnostics in PMBs and to improve the current understanding of flame stabilization mechanisms

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Apresentamos um modelo para a injeção de ar em um meio poroso que contém combustível sólido levando em conta as perdas térmicas para rocha circundante. Em trabalhos anteriores, o modelo foi simplificado e todas as sequências de ondas para a solução de problemas de Riemann foram obtidas sem levar em conta as perdas térmicas. Nesse trabalho, é feito um primeiro passo para entender o efeito das perdas de calor, que são importantes especialmente em experimentos de laboratório. Para provar a existência e unicidade da solução de ondas viajantes, os efeitos de difusão e a dependência da densidade do gás na temperatura são desconsiderados. Também são apresentadas simulações numéricas que validam os resultados obtidos, bem como simulações numéricas para um sistema mais geral que considera termos difusivos. Por fim são comparadas as soluções numéricas para ambos sistemas e um exemplo numérico com valores típicos dos parâmetros para um modelo de combustão é apresentado.
We present a model for the injection of air into an underground porous medium that contains a solid fuel. In previous works the model was simplified and all wave sequences for the Riemann problem solution were obtained without taking into account thermal losses to the surrounding rock. In that work the first step was made to understand the effect of heat losses, which are important especially in laboratory experiments. In order to prove of the existence and uniqueness of the traveling wave solution, diffusion effects and the dependence of gas density on temperature were disregarded. We will also present numerical simulations that validate the results obtained, as well as numerical simulations for a more general system that considers diffusive terms. Furthermore, we will compare the numerical solutions for both systems and a numerical example with typical values of the parameters for a combustion model is presented.

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É crescente o interesse na utilização de métodos térmicos para recuperação de óleo de média e alta viscosidade. Um desses métodos é a combustão in situ, que consiste na liberação de calor no interior do reservatório através da combustão do ar injetado. As componentes mais pesadas do óleo atuam como combustível para as reações exotérmicas e o calor gerado reduz a viscosidade do óleo, estimulando o fluxo em direção aos poços de produção. Os modelos matemáticos para este método de recuperação em geral são complexos. Portanto, a obtenção de soluções analíticas para tais modelos é inviável, sendo necessária a utilização de simulações computacionais. Diversos trabalhos apresentam estudos analíticos e numéricos de modelos unidimensionais para a combustão em meios porosos. Em trabalhos anteriores, estimativas analíticas para modelos unidimensionais foram obtidas. Neste trabalho, tais estimativas são ligeiramente generalizadas através da inclusão da pressão prevalecente. É proposto um modelo bidimensional para o processo de combustão in situ em meios porosos heterogêneos que considera pressão variável. Soluções numéricas são obtidas utilizando o método de elementos finitos para a discretização espacial, o esquema de diferenças finitas de Crank-Nicolson para discretização no tempo e o método de Newton para resolução das equações não lineares resultantes. Estimativas analíticas para a temperatura e velocidade da onda de combustão são obtidas através de um modelo unidimensional simplificado. Tais estimativas são validadas com sucesso para o modelo geral através das simulações. Uma outra simplificação unidimensional do modelo geral é simulada numericamente através de duas abordagens: a primeira é similar à utilizada para a solução do modelo geral; e a segunda é escrita como um problema de complementaridade. Os problemas de complementaridade não-linear são resolvidos pelo algoritmo FDA-NCP. As duas abordagens numéricas utilizadas são comparadas com uma estimativa analítica para a onda térmica e mostram bons resultados.
There is a growing interest in using thermal methods for the recovery of medium and high viscosity oil. One of these methods is the in-situ combustion, which consists in release heat within the reservoir through combustion of the injected air. The heavier oil components are used as fuel for exothermic reactions and the generated heat reduces the oil viscosity, stimulating the flow towards the production well. In general, the mathematical models for this recovery method are complex. Therefore, the analytical solutions for such models are impossible, requiring numerical simulations. Several works present analytical and numerical studies of one-dimensional models for combustion in porous media. In previous works analytical estimates for one dimensional models were obtained. Here these estimates are slightly generalized by including the prevailing pressure. We propose a two-dimensional model for the in-situ combustion process in heterogeneous porous media, considering variable pressure. Numerical results are obtained using the finite element method for spatial discretization, Crank-Nicolson finite difference scheme for time discretization and Newton’s method for the arising nonlinear equations. Analytical estimates for combustion wave speed and combustion wave temperature are obtained using one-dimensional simplified model. These estimates are successfully validated in the general model through the simulation results. Another one-dimensional simplification of the general model is numerically simulated by two approaches: the first is similar to the one previously described; and the second one is written as a complementarity problem. The arising nonlinear complementarity problems are solved by the FDA-NCP algorithm. Both numerical approaches are compared to the analytical estimate for the thermal wave, showing good agreement.

CoordenaÃÃo de AperfeiÃoamento de Pessoal de NÃvel Superior
The use of biogas, with high concentrations of carbon dioxide (CO2) in its composition, in thermal systems of conventional combustion can result in combustion instabilities, leading to a decrease of the flame front propagation velocity, resulting even to the flame extinction. In addition, this contaminant can increase greenhouse gas levels in the exhaust, such as carbon monoxide (CO), unburned hydrocarbons (UHC), nitrogen oxides (NOx) among others. Thus, this research aims to demonstrate the effectiveness of "Filtration Combustion" (FC) to deal with fuels of low heat content, such as biogas. CF is a non-conventional technology capable of producing ultra-low emissions of CO, HC and NOx. The experimental apparatus used in this research consists of a porous burner constituted of ceramic spheres (alumina) that fill the combustion chamber, where heat exchangers are inserted at the porous matrix ends. The FC allows even the application of a reciprocating gas flow system, which periodically switches the direction of the gas flow in the chamber. The reciprocal filtration combustion allows the operation with several fuels and providing a stable combustion process with temperature distribution on trapezoidal profile, with temperature peaks between 1300 and 1600 K. In this context, the present experimental study tries to identify and to analyze the effects of carbon dioxide in FC, which covers energy extraction efficiency, emissions, reaction stability, and flammability limits using several air-fuel mixtures, altering both the CO2 concentration in the biogas composition as the equivalence ratio (ER), in which the technical methane is taken as the reference gas. The results have pointed out significant benefits of the reversal on the combustion process, allowing operation in a wide equivalence ratio range (0.1 <Ф <1.0), and achieving energy extraction efficiencies above 90%, with ultra-low CO and NOx emissions (below 1 ppm). However, when the burner operates on only flow direction, it is possible to realize a drastic reduction of the flammability limit, as the CO2 content in the biogas composition is increased.
A utilizaÃÃo do biogÃs, com elevadas concentraÃÃes de diÃxido de carbono (CO2) em sua composiÃÃo, em sistemas tÃrmicos de combustÃo convencionais pode resultar em instabilidades na reaÃÃo, levando a uma diminuiÃÃo da velocidade de propagaÃÃo da frente de chama (onda de combustÃo), ocasionando inclusive a sua extinÃÃo. AlÃm disso, este contaminante pode aumentar os Ãndices de gases poluentes na exaustÃo, tais como: monÃxido de carbono (CO), hidrocarbonetos nÃo queimados (HC), Ãxidos de nitrogÃnio (NOx), dentre outros. Por esta razÃo, esta pesquisa tem como objetivo demonstrar a eficÃcia da "CombustÃo de FiltraÃÃo" (CF) em lidar com os combustÃveis de baixo poder calorÃfico, como o biogÃs. CF à uma tecnologia nÃo-convencional capaz de produzir emissÃes ultrabaixas de CO, HC e NOx. O aparato experimental empregado nessa pesquisa consistiu de um queimador poroso, constituÃdo de esferas cerÃmicas (alumina) que preenchem a cÃmara de combustÃo, onde trocadores de calor estÃo inseridos nas extremidades dessa matriz porosa. A CF possibilitou, inclusive, a aplicaÃÃo de um sistema de escoamento recÃproco, que alternou periodicamente a direÃÃo do escoamento dos gases na cÃmara. A combustÃo de filtraÃÃo recÃproca permitiu a operaÃÃo com diversos combustÃveis e proporciona um processo de combustÃo estÃvel com a distribuiÃÃo de temperatura em perfil trapezoidal, com picos de temperatura entre 1300 e 1600 K. Neste contexto, o presente estudo experimental buscou identificar e analisar os efeitos do diÃxido de carbono na CF, o que engloba eficiÃncia de extraÃÃo de energia, emissÃes, estabilidade da reaÃÃo, e limites de inflamabilidade, utilizando vÃrias misturas ar-combustÃvel, alterando tanto a concentraÃÃo de CO2 na composiÃÃo do biogÃs como a razÃo de equivalÃncia (RE), tendo como gÃs de referÃncia o metano tÃcnico. Os resultados apontaram benefÃcios significativos da reversÃo sobre o processo de combustÃo, permitindo a operaÃÃo em uma ampla faixa de razÃo de equivalÃncia (0,1<Ф<1,0), e alcanÃando uma eficiÃncia de extraÃÃo de energia acima de 90%, com emissÃes ultrabaixas de CO e NOx (abaixo de 1 ppm). Em contrapartida, quando o queimador operou em apenas um sentido do escoamento, foi possÃvel perceber uma reduÃÃo no limite de inflamabilidade à medida que foi incrementado o teor de CO2 na composiÃÃo do biogÃs.

Under certain circumstances pulse combustors have been shown to improve both heat transfer and drying rate when compared to steady flow impingement. Despite this potential, there have been few investigations into the use of pulse combustor driven impingement jets for industrial drying applications. The research presented here utilized experimental and numerical techniques to study the heat transfer characteristics of these types of oscillating jets when impinging on solid surfaces and the heat and mass transfer when drying porous media. The numerical methods were extensively validated using laboratory heat flux and drying data, as well as correlations from literature. As a result, the numerical techniques and methods that were developed and employed in this work were found to be well suited for the current application. It was found that the pulsating flows yielded elevated heat and mass transfer compared to similar steady flow jets. However, the numerical simulations were used to analyze not just the heat flux or drying, but also the details of the fluid flow in the impingement zone that resulted in said heat and mass transport. It was found that the key mechanisms of the enhanced transfer were the vortices produced by the oscillating flow. The characteristics of these vortices such as the size, strength, location, duration, and temperature, determined the extent of the improvement. The effects of five parameters were studied: the velocity amplitude ratio, oscillation frequency, the time-averaged bulk fluid velocity at the tailpipe exit, the hydraulic diameter of the tailpipe, and the impingement surface velocity. Analysis of the resulting fluid flow revealed three distinct flow types as characterized by the vortices in the impingement zone, each with unique heat transfer characteristics. These flow types were: a single strong vortex that dissipated before the start of the next oscillation cycle, a single persistent vortex that remained relatively strong at the end of the cycle, and a strong primary vortex coupled with a short-lived, weaker secondary vortex. It was found that the range over which each flow type was observed could be classified into distinct flow regimes. The secondary vortex and persistent vortex regimes were found to enhance heat transfer. Subsequently, transition criteria dividing these regimes were formed based on dimensionless parameters. The critical dimensionless parameters appeared to be the Strouhal number, a modified Strouhal number, the Reynolds number, the velocity amplitude ratio, and the H/Dh ratio. Further study would be required to determine if these parameters offer similar significance for other configurations.

碩士
國立成功大學
航空太空工程學系碩博士班
100
This research investigates the characteristic of hydrogen diffusion flame and the practicability of the reverse water gas shift reaction in the porous media burner. Experimental parameters included the flow rate of the fuel (3~8 L/min), equivalence ratio (0.25~2), stacked number (1~4 piece), the pore size of the porous media (15, 30 PPI), and the H2/CO2 ratio (1, 3, 5). The measurements of the temperature variation of axial direction of burner and the radial direction of the interface between two porous media, the CO2 conversion efficiency were carried out. Moreover, the flame front in the porous media burner was observed to understand the influence of porous media on the flame characteristic. It was found that the operable range of the hydrogen diffusion flame in the porous media burner was larger than that of the premixed flame. The temperature becomes higher when the equivalence ratio (0.25~2) was more close to the lean-burn conditions. However, effects of hydrogen flow rate were not significant when the air flow rate was fixed. From the test of the reverse water gas shift reaction, decreasing the hydrogen flow rate or increasing the H2/CO2 ratio and the equivalence ratio increased the CO2 conversion efficiency. The highest CO2 conversion efficiency was 31.6% when hydrogen flow rate was 4 L/min and H2/CO2 ratio was 5. The result shows that it can be found at some particular conditions of the hydrogen diffusion flame with porous media burner, but it needs to be further studied in order to apply it to a present combustion system.

碩士
國立中央大學
機械工程研究所
91
The combustion characteristics of premixed hydrogen- methane-air mixture in a porous media burner with catalysts are studied experimentally. La2O3 and MgO are used as the catalysts in this work. They are supported on highly porous SiC and Al2O3 forms. Flow rates are controlled by mass flow meters. Gas temperature within the burner and CO, NOX, THC emissions are monitored throughout the experiments. Results show that the flame temperature is increased by adding catalysts due to the increased reaction rate. Flame positions are different for different material burners. For Al2O3 porous burners, flame is usually stabilized in the middle of the burner. But for SiC burners, flame is stabilized at near the inlet due to higher thermal conductivity of SiC. In addition, the stable region is wider for SiC burner and the flame temperature is higher due to better preheating effects. Addition of hydrogen in the methane-air mixture does not change the flame temperature very much. Although hydrogen does promote chemical reaction, but it also has higher thermal conductivity and cooling by convection is enhanced due to increase flame speed. For burners without catalysts, CO emission increases with the equivalence ratio and flame speed slightly because of shorter residence time. But the concentrations are all less than 15ppm. For burners with catalysts, the CO emissions are all reduced to below 5ppm. The catalysts do promote CO to CO2 conversion. On the other hands, NOX emissions are all in the range of 10-15 ppm.

碩士
國立成功大學
航空太空工程學系碩博士班
101
In this study, propane is taken as fuel in the investigation of the combustion of a self-designed porous media burner. Three porous media, OB-SiC with a pores size distribution of 15 PPI, Al2O3 with pores size distribution of 15 PPI and 30 PPI were used in this work. The observation of the axial temperature distribution and the analysis of the exhaust gas were carried out at different fuel flow rates, equivalence ratios and addition of hydrogen. Experimental results show that the maximum temperature of propane-air flame can be maintained, and that isn’t sensitive to the equivalence ratio. Meanwhile, the concentration analysis of exhaust discloses that CO, HC and NOx concentrations are related with the equivalence ratio, position of the high temperature area and the heat release rate of a self-made porous media burner. From the experiment of propane-air flame mixed with hydrogen, it can be seen that not only the range of operation, but also the emission of CO and HC is improved under proper operational parameters. 　　Using Al2O3 as both the upstream and downstream porous media at the fuel flow rate of 1.25 L/min and the equivalence ratio of 0.6. Under those conditions, interior combustion can be produced, and a stable flame temperature up to 1200 oC at the interface of two porous media can be achieved. In addition, a CO concentration of 16.08 ppm, a NOx concentration of 24 ppm and a HC concentration of almost zero were measured. It is concluded that a combustion environment of high-temperature operation and low-pollution emission can be obtained using the porous media combustor.

博士
國立成功大學
機械工程學系
104
This study investigated the heat recovery rates of hydrogen flame modes in porous medium combustion. The porous medium was oxide-bonded silicon carbide (OB-SiC), aluminum oxide (Al2O3) or zirconia (ZrO2) with 60 or 30 PPI. The results indicated that the reaction temperature of a flame mode was controlled by the equivalence ratio (Flame velocity), thermal load and solid medium thermal properties (k and CP). The operation region of the flame modes was controlled by the equivalence ratio and dimensionless velocity (V*). Under ultra-lean conditions (=0.2-0.25), the flame was blown out when the dimensionless velocity was above 4.5 for OB-SiC and Al2O3 settings. In contrast no blow out occurred for the ZrO2 setting and under a high equivalence ratio (＞0.4), and the flame mode was a conical flame when the dimensionless velocity was above unity. The heat recovery mechanism of surface and interior combustion was based on the conduction and radiation of the porous medium. The dimensionless temperature (*) is defined as the ratio of the reaction temperature over the adiabatic flame temperature. When the dimensionless temperature was unity, the reaction temperature approached the adiabatic flame temperature. Under interior combustion, the maximum dimensionless temperature was 0.994 for the OB-SiC (=0.3) setting. Furthermore, the maximum dimensionless temperature was 0.942 for Al2O3 and 0.969 for ZrO2 under operation at =0.3. The heat recovery rate of hydrogen combustion under surface and interior combustion was thus higher than that of the conical flame mode.

Combustion with a high surface area continuous solid immersed within the flame, referred to as combustion in porous media, is an innovative approach to combustion as the solid within the flame acts as an internal regenerator distributing heat from the combustion byproducts to the upstream reactants. By including the solid structure, radiative energy extraction becomes viable, while the solid enables a vast extension of flammability limits compared to conventional flames, while offering dramatically reduced emissions of NOx and CO, and dramatically increased burning velocities. Efforts documented within are used for the development of a streamlined set of design principles, and characterization of the flame's behavior when operating under such conditions, to aid in the development of future combustors for lean burn applications in open flow systems. Principles described herein were developed from a combination of experimental work and reactor network modeling using CHEMKIN-PRO. Experimental work consisted of a parametric analysis of operating conditions pertaining to reactant flow, combustion chamber geometric considerations and the viability of liquid fuel applications. Experimental behavior observed, when utilizing gaseous fuels, was then used to validate model outputs through comparing thermal outputs of both systems. Specific details pertaining to a streamlined chemical mechanism to be used in simulations, included within the appendix, and characterization of surface area of the porous solid are also discussed. Beyond modeling the experimental system, considerations are also undertaken to examine the applicability of exhaust gas recirculation and staged combustion as a means of controlling the thermal and environmental output of porous combustion systems. This work was supported by ACS PRF "51768-ND10 and NSF IIP 1343454.
M.S.M.E.
Masters
Mechanical and Aerospace Engineering
Engineering and Computer Science
Mechanical Engineering; Thermo-Fluids Track