A radiação térmica é o principal mecanismo de transferência de calor em fenômenos que envolvem meios participantes em temperaturas elevadas, tais como em processos de combustão. A dependência fortemente irregular do coeficiente de absorção em relação ao número de onda torna desafiador o estudo de situações em que a radiação é apenas parte de um problema mais complexo. A exatidão do cálculo da radiação fica condicionada à solução da equação da transferência radiativa (RTE) por meio da integração linha-por-linha (LBL), sendo, muitas vezes, impraticável, em virtude do esforço computacional requerido para contabilizar as centenas de milhares ou milhões de linhas espectrais do coeficiente de absorção. Alternativamente, modelos espectrais, como a soma-ponderada-de-gases-cinza (WSGG), têm sido empregados de maneira eficaz na obtenção de resultados em substituição à integração LBL. Nessa dissertação, o modelo WSGG é aplicado na solução da transferência de calor radiativa em um sistema unidimensional, formado por duas placas planas paralelas infinitas e preenchido por uma mistura homogênea de dióxido de carbono e vapor de água, considerando-se perfis distintos de temperatura. Diferentemente da maioria dos estudos da literatura que empregam a mesma geometria, mas com paredes negras, o presente trabalho supõe superfícies cinzas e não cinzas. O objetivo central é, portanto, avaliar o erro em se assumir fronteiras negras quando estas não apresentam esse comportamento. Os resultados para o modelo WSGG aplicado a superfícies não cinzas, cinzas e negras são comparados com a solução linha-por-linha para paredes não cinzas. As análises dos desvios entre as soluções pelo modelo da soma-ponderada-de-gases-cinza e pela integração LBL mostram que a suposição de paredes negras, para casos em que as superfícies deveriam ser consideradas não cinzas, pode levar a erros de até 50% nos resultados para o fluxo de calor e para o termo fonte radiativo. / Thermal radiation is the main heat transfer mechanism in phenomena that involves high temperatures, such as in combustion processes. The strongly irregular dependence of the absorption coefficient on the wavenumber makes challenger the study of situations in which the radiation is only part of a more complex problem. The accuracy of the calculation of the radiation is conditioned to the solution of the radiative transfer equation (RTE) by line-by-line (LBL) integration, being frequently impracticable, due to the computational effort required to account for the hundreds of thousands or millions spectral lines of the absorption coefficient. Alternatively, spectral models, such as the weighted-sum-of-gray-gases (WSGG) model, have been used with success to obtain results in comparison to LBL integration. In this study, the WSGG model is applied to solve the radiative heat transfer in a one-dimensional system, formed by two infinite flat parallel plates and filled by a homogeneous mixture of carbon dioxide and water vapor, for different temperature profiles. Unlike most studies of the literature that employ the same geometry, but with black walls, the present work supposes gray and non-gray surfaces. The central objective is, therefore, to evaluate the error in assuming black boundaries when they do not present this behavior. The results for the WSGG model applied to non-gray, gray and black surfaces are compared with the line-by-line solution for non-gray walls. Analyzes of the deviations between the solutions by the weighted-sum-of-gray-gases model and the LBL integration show that the assumption of black walls, for cases where the surfaces should be considered as non-gray, may lead to errors of up to 50% in results for the heat flux and the radiative source term.

A radiação térmica é frequentemente considerada um mecanismo de transferência de calor muito importante em processos de combustão em alta pressão, devido à presença de meios participantes e às altas temperaturas envolvidas. Resolver a radiação térmica em meios participantes é um problema complexo devido à natureza integro-diferencial da equação governante e à dependência espectral altamente irregular das propriedades de radiação. Atualmente, o método mais preciso para resolver a integração espectral é o método linha-porlinha (LBL), que possui um custo computacional muito elevado. Para contornar essa dificuldade, o problema espectral é geralmente resolvido usando modelos espectrais e, consequentemente, a equação da transferência radiativa (RTE) é simplificada. Um destes modelos é o da soma-ponderada-de-gases-cinza (WSGG), que substitui o comportamento espectral altamente irregular do coeficiente de absorção, por bandas de coeficientes de absorção uniforme e tem mostrado um bom desempenho em diversas aplicações, mesmo sendo um modelo bastante simplificado. Entretanto, recentemente alguns autores não obtiveram bons resultados ao tentar aplicar o WSGG a problemas de combustão em alta pressão. Este artigo desenvolve um modelo WSGG para CO2 e H2O em condições de alta pressão. Para validar o modelo, a emitância total é calculada usando os coeficientes WSGG e comparada à solução do LBL obtida usando o banco de dados espectrais HITEMP 2010. Os resultados mostraram grande convergência entre os valores de emitância de ambos os métodos, mesmo para valores de alta pressão, tanto para o CO2 quanto para H2O, provando que o método WSGG é aplicável a condições de alta pressão. O modelo também foi validado pelo cálculo do fluxo de calor e termo fonte radiativo, e comparando-os com os obtidos através do método LBL. O H2O teve melhores resultados para baixas pressões, enquanto o CO2 apresentou melhores resultados para pressões mais altas. O efeito da pressão total sobre a solução de LBL foi maior para o H2O, o que pode ser um dos motivos pelo qual os desvios foram maiores para os casos de alta pressão. / Thermal radiation is often a very important heat transfer mechanism in high pressure combustion processes due to the presence of participating media and the high temperatures involved. Solving thermal radiation in participating media is a tough problem due to the integro-differential governing equation and the complex spectral dependence of radiation properties. Currently, the most accurate method to solve the spectral integration is the line-byline (LBL) method, which has a very high computational cost. In order to avoid this drawback the spectral problem is usually solved using spectral models, and as a consequence the radiative transfer equation (RTE) is simplified. One of the models is the weighted-sum-ofgray- gases (WSGG) which replaces the highly irregular spectral behavior of the absorption coefficient by bands of uniform absorption coefficients, and has shown great performance a lot of applications even though it is a very simple model. However, recently some authors didn’t have good results when trying to apply the WSGG to high pressure combustion problems. This thesis develops a WSGG model for both CO2 and H2O on high pressure conditions. In order to validate the model the total emittance is calculated using the WSGG coefficients and compared to the LBL solution which was obtained using the HITEMP 2010 spectral emissivity database. The results showed that the emittance values from both methods were very close even for high pressure values for both CO2 and H2O proving that the WSGG method is applicable to high pressure conditions. The model was also validated by calculating the radiative heat flux and source, and comparing them with the LBL method. H2O had better results for low pressures while CO2 had better results for higher pressures. The effect of total pressure on the LBL solution was higher for H2O, which might be the reason why deviations were higher at high pressure values.

Neste trabalho são obtidos os coeficientes do modelo da Soma-Ponderada-dos-gases-cinza (WSGG) a partir do banco de dados HITEMP 2010, permitindo o uso do modelo com os dados mais precisos disponíveis atualmente. Neste trabalho também se faz uma comparação dos valores de emitância total obtidos a partir do modelo WSGG com valores benchmark, obtidos nesse trabalho, mostrando uma excelente concordância. Com os coeficientes obtidos, problemas unidimensionais de transferência de calor radiante são resolvidos de modo a comparar a solução obtida pelo modelo WSGG com a solução obtida pela integração LBL (solução benchmark). Nas comparações, diferentes perfis de temperatura, comprimentos de trajeto, gradientes de temperatura e concentrações de espécies são utilizadas. Em todos os casos é possível verificar uma boa concordância entre os resultados WSGG e LBL. Para comparações com perfil de temperatura parabólico, verifica-se erros locais abaixo de 8%. Para perfis de temperatura cossenoidais, é possível observar erros de até 18% para alguns casos, porém com erros médios menores que 1,6%. / In this work the coefficients for the Weighted Sum-of-Gray-Gases model (WSGG) are determined from HITEMP 2010 database, allowing the use of the model with the most accurate data available nowadays. This study also makes a comparison of the total emittance values obtained from the model with benchmark values, obtained in this work, showing an excellent agreement. With the obtained coefficients, one-dimensional radiant heat transfer problems are solved in order to compare the solution obtained by the WSGG model with the solution obtained by the LBL integration (benchmark solution). In the comparisons, different temperature profiles, path lengths, temperature gradients and species concentrations are used. In all cases it is possible to verify the good agreement of the WSGG and LBL results. For comparisons with parabolic temperature profile, the local error is below 8%. For cosine temperature profile, the local error is about 18% for some cases, but with average errors less than 1,6%.

Neste trabalho são obtidos os coeficientes do modelo da Soma-Ponderada-dos-gases-cinza (WSGG) a partir do banco de dados HITEMP 2010, permitindo o uso do modelo com os dados mais precisos disponíveis atualmente. Neste trabalho também se faz uma comparação dos valores de emitância total obtidos a partir do modelo WSGG com valores benchmark, obtidos nesse trabalho, mostrando uma excelente concordância. Com os coeficientes obtidos, problemas unidimensionais de transferência de calor radiante são resolvidos de modo a comparar a solução obtida pelo modelo WSGG com a solução obtida pela integração LBL (solução benchmark). Nas comparações, diferentes perfis de temperatura, comprimentos de trajeto, gradientes de temperatura e concentrações de espécies são utilizadas. Em todos os casos é possível verificar uma boa concordância entre os resultados WSGG e LBL. Para comparações com perfil de temperatura parabólico, verifica-se erros locais abaixo de 8%. Para perfis de temperatura cossenoidais, é possível observar erros de até 18% para alguns casos, porém com erros médios menores que 1,6%. / In this work the coefficients for the Weighted Sum-of-Gray-Gases model (WSGG) are determined from HITEMP 2010 database, allowing the use of the model with the most accurate data available nowadays. This study also makes a comparison of the total emittance values obtained from the model with benchmark values, obtained in this work, showing an excellent agreement. With the obtained coefficients, one-dimensional radiant heat transfer problems are solved in order to compare the solution obtained by the WSGG model with the solution obtained by the LBL integration (benchmark solution). In the comparisons, different temperature profiles, path lengths, temperature gradients and species concentrations are used. In all cases it is possible to verify the good agreement of the WSGG and LBL results. For comparisons with parabolic temperature profile, the local error is below 8%. For cosine temperature profile, the local error is about 18% for some cases, but with average errors less than 1,6%.

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.

Philosophiae Doctor - PhD / In this thesis, we consider some nonlinear differential models that govern unsteady magneto-hydrodynamic convective flow and mass transfer of viscous, incompressible,electrically conducting fluid past a porous plate with/without heat sources. The study focusses on the effect of a combination of a number of physical parameters (e.g., chemical reaction, suction, radiation, soret effect,thermophoresis and radiation absorption) which play vital role in these models.Non dimensionalization of these models gives us sets of differential equations. Reliable solutions of such differential equations can-not be obtained by standard numerical techniques. We therefore resorted to the use of the singular perturbation approaches. To proceed, each of these model problems is discretized in time by using a suitable time-stepping method and then by using a fitted operator finite difference method in spatial direction. The combined methods are then analyzed for stability and convergence. Aiming to study the robustness of the proposed numerical schemes with respect to change in the values of the key parame- ters, we present extensive numerical simulations for each of these models. Finally, we confirm theoretical results through a set of specificc numerical experiments.

Le rayonnement thermique joue un rôle important dans un large éventail d'applications de génie thermique comprenant des écoulements turbulents. La motivation principale de cette thèse est le besoin croissant de précision et fiabilité dans les simulations numériques appliqué à ce domaine.Cette thèse s’intéresse tout particulièrement à la compréhension physique de l’impact du rayonnement thermique sur la dynamique des fluides et le transfert thermique, ainsi que de l’influence des fluctuations turbulentes sur le transfert radiatif dans les écoulements à couche de cisaillement.L'objectif de cette thèse est de fournir des données haute-fidélités de jets libres turbulents couplés au rayonnement thermique afin de développer et de valider des modèles turbulents d’écoulements à couche de cisaillement prenant en compte les interactions de couplage. À cette fin, les jets libres turbulents sont décrits par des simulations numériques directes (DNS) couplées à une méthode de Monte-Carlo réciproque pour résoudre l'équation de transfert radiatif. La dépendance spectrale des propriétés radiatives est prise en compte avec la méthode Correlated-k (ck). L'étude numérique est réalisée avec la plus grande fidélité pour être aussi représentative que possible d'un jet réel dans un milieu participatif. La simulation est optimisée en termes de temps de calcul en tirant parti d'une méthode d'accélération appelée Acoustic Speed Reduction et en injectant de la turbulence artificielle pour améliorer les conditions d'entrée.Deux simulations directes de jets chauffés couplés au rayonnement thermique sont réalisées. D'une part, un jet chauffé avec un rayonnement modéré a été simulé et l’analyse de ses données DNS couplées a permis de dériver une nouvelle loi d’échelle pour la décroissance du profil de température. Cette mise à l'échelle rend compte des effets de la densité modifiée due à un rayonnement modéré. De plus, cela permet de distinguer si le rayonnement thermique modifie ou non la nature des mécanismes de transfert thermique dans la région développée du jet. D'autre part, un jet libre fortement chauffé a été calculé afin de quantifier les effets du rayonnement sur les champs de température et de vitesse moyens ainsi que sur les moments de second ordre.Outre les données DNS couplées, un solver RANS pour les écoulements à densité variable couplé au rayonnement thermique a été développé au cours de cette thèse. L'objectif était de quantifier directement la précision des modèles turbulents existants et d'identifier les paramètres clés pour une modélisation plus poussée des interactions de couplage. / Radiation plays an important role in a broad range of thermal engineering applications comprising turbulent flows. The growing need for accurate and reliable numerical simulations to support the design stages of such applications is the main motivation of this thesis.Of special interest in this work are the free-shear flows and the fundamental understanding of how radiation can modify their fluid dynamics and heat trans- port as well as how their turbulence fluctuations can alter radiative transfer. The goal of this thesis is to provide high-fidelity data of turbulent free jets coupled with thermal radiation in order to develop and validate free-shear turbulent models accounting for coupling interactions. To this end, turbulent free jets are described by direct numerical simulations (DNS) coupled to a reciprocal Monte- Carlo method to solve the radiative transfer equation. The spectral dependency of the radiative properties is accounted for with an accurate Correlated-k (ck) method. The numerical study is carried out with state-of-the-art fidelity to be as representative as possible of an actual jet in a participating medium. The simulation is optimized in terms of processing time taking advantage of an acceleration method called Acoustic Speed Reduction and by injecting artificial turbulence to enhance inlet boundaries.Two direct simulations of heated jets coupled with thermal radiation are carried out. On the one hand, a heated jet with moderate radiation is simulated. The analysis of its high-fidelity coupled DNS data has allow to derive a new scaling law for the decay of the temperature profile. This scaling accounts for the effects of modified density due to moderate radiation. Moreover, it allows for distinguishing whether thermal radiation modifies the nature of heat transfer mechanisms in the jet developed region or not. On the other hand, a strongly heated free jet is computed in order to quantify the effects of radiation on mean temperature and velocity fields as well as on second order moments.Besides the coupled DNS data, a RANS solver for variable-density flows coupled with thermal radiation has been implemented during the course of this thesis. The goal is to directly quantify the accuracy of the existing turbulent models, and to identify key parameters for further modeling of coupling interactions.

<p dir="ltr">Thermal radiation is a naturally abundant form of light that is continuously emitted from objects above absolute zero. Because this form of electromagnetic radiation is invisible to the human eye, much of human and machine perception neglects the rich information that is present in infrared energy. By harvesting the spectral and polarimetric characteristics of thermal signals, thermal imaging can deliver an enormous impact to remote sensing, machine perception, autonomous navigation, and biomedical applications. The goal of this thesis is to present numerous techniques that enable the extraction of the vast information available via thermal radiation.</p><p dir="ltr">This thesis investigates a more robust and approachable method of providing spectral and polarimetric resolution to short-wave infrared cameras. Through the application of a liquid crystal interferometer, this research demonstrates an electrically-tunable spectral imaging platform that is compact, robust, cost-effective, and accurate, offering a durable solution for remote sensing and autonomous navigation. This thesis also examines the design of filters specific to the short-wave infrared signature of greenhouse gasses, enabling aerial detection and measurement of greenhouse gas sources via a single filtered image, which can drastically improve the speed and accuracy of monitoring greenhouse gas emissions. In the long-wave infrared regime, this research explores a four-color imaging thermometer, capitalizing on the resolution provided by four spectral bands—in conjunction with the <i>TeX-</i><i>Vision</i><i> </i>temperature-estimation algorithm—to yield unprecedented temperature estimation accuracy that can advance current medical diagnostic practices.</p><p dir="ltr">The examples described in this thesis reveal the breadth of untapped information that is present in thermal radiation, which carries the ability to enhance the way we perceive our surroundings.</p>

In this thesis, I use the new generation of radio interferometer along with X-ray observations to investigate the dynamics and energetics of radio-loud active galaxies which are key to understanding AGN feedback and the evolution of galaxies as a whole. I present new JVLA observations of powerful radio source and use innovative techniques to undertake a detailed analysis of JVLA observations of powerful radio galaxies. I compare two of the most widely used models of spectral ageing, the Kardashev-Pacholczyk and Jaffe-Perola models and also results of the more complex, but potentially more realistic, Tribble model. I find that the Tribble model provides both a good fit to observations as well as providing a physically realistic description of the source. I present the first high-resolution spectral maps of the sources and find that the best-fitting injection indices across all models take higher values than has previously been assumed. I present characteristic hot spot advance speeds and compare them to those derived from dynamical ages, confirming that the previously known discrepancy in speed remains present in older radio sources even when ages are determined at high spectral and spatial resolutions. I show that some previously common assumptions made in determining spectral ages with narrow-band radio telescopes may not always hold. I present results from a study of the powerful radio galaxy 3C223 at low frequencies with LOFAR to determine its spectrum on spatially small scales and tightly constrain the injection index, which I find to be consistent with the high values found at GHz frequencies. Applying this new knowledge of the low energy electron population, I perform synchrotron / inverse-Compton model fitting and find that the total energy content of the radio galaxy lobes increases by a factor greater than 2 compared to previous studies. Using this result to provide revised estimates of the internal pressure, I find the northern lobe to be in pressure balance with the external medium and the southern lobe to be overpressured. I go on to present the first large sample investigation of the properties of jets in Fanaroff and Riley type I radio galaxies (FR-I) at X-ray energies based on data from the Chandra archive. I explore relations between the properties of the jets and the properties of host galaxies in which they reside. I find previously unknown correlations to exist, relating photon index, volume emissivity, jet volume and luminosity, and find that the previously held assumption of a relationship between luminosities at radio and X-ray wavelengths is linear in nature when bona fide FR-I radio galaxies are considered. In addition, I attempt to constrain properties which may play a key role in determination of the diffuse emission process. I test a simple model in which large-scale magnetic field variations are primarily responsible for determining jet properties; however, we find that this model is inconsistent with our best estimates of the relative magnetic field strengths in my sample.

Sintering is an important processing step for obtaining the necessary mechanical stability and rigidity of ceramic bulk materials. Both mass and heat transfer are essential in the sintering process. The importance of radiation heat transfer is significantly enhanced at high temperatures according to the well-known Stefan-Boltzmann’s law. In this thesis, we modified the pressure-less spark plasma sintering set-up to generate intense thermal radiation, aiming at rapid consolidation of ceramic bulk materials. This approach was named as “Sintering by Intense Thermal Radiation (SITR)” as only thermal radiation contributed. Firstly, the heat and mass transfer mechanisms during the SITR process were studied by choosing zirconia ceramics as references. The results revealed that the multiple scattering and absorption of radiation by the materials contributed to the heat diffusion. The observed enhanced densification and grain growth can be explained by a multiple ordered coalescence of zirconia nanocrystals using high heating rates. Secondly, the temperature distribution during the SITR process was investigated by both numerical simulation and experimental verifications. It showed that the radiator geometry, sample geometry and radiating area were influencing factors. Besides, the change of material and geometry of the radiators resulted in an asymmetric temperature distribution that favored the formation of SiC foams. The foams had gradient structures with different open porosity levels and pore sizes and size distributions. Finally, ceramic bulk materials were successfully fabricated by the SITR method within minutes. These materials included dense and strong ZrO2 ceramics, Si3N4 foams decorated with one-dimensional nanostructures, and nasal cavity-like SiC-Si3N4 foams with hierarchical heterogeneities. Sufficient densification or formed strong necks were used for tailoring these unique microstructures. The SITR approach is well applicable for fast manufacture of ceramic bulk materials because it is clean and requires low energy consumption and properties can be controlled and tuned by selective heating, heating speed or temperature distribution. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted. Paper 4: Manuscript.</p><p> </p>