O presente trabalho tem por objetivo estudar as Interações Turbulência-Radiação em um escoamento não reativo para meios participantes. Estas interações caracterizam-se por um complexo fenômeno transiente, devido à combinação de dois fenômenos, unindo as características das flutuações da turbulência e da elevada não linearidade do fenômeno da radiação térmica. O estudo consiste em análise numérica do problema por dinâmica de fluidos computacional, através da utilização do Fire Dynamics Simulator (FDS), um software Open-Source, na qual a modelagem da turbulência é feita através da Simulação de Grandes Escalas. Como se trata de um software novo, bem como sendo introduzido no grupo de pesquisa, primeiramente é realizada a simulação de um caso benchmark para verificação e avaliação da formulação numérica. A análise do TRI é realizada em um problema proposto baseado em trocadores de calor reais utilizados em máquinas térmicas, como por exemplo, geradores de vapor ou coletores de escapamento de motores, envolvendo transferência combinada de convecção forçada e radiação térmica. A metodologia de avaliação consiste em comparar o fluxo radiante médio nas fronteiras obtido através da simulação transiente e compará-lo com o fluxo obtido por meio do campo médio temporal de temperaturas. São avaliadas a influência da intensidade de turbulência na entrada do escoamento, assim como a da espessura óptica, ambos relevantes para os efeitos do TRI. Conforme descrito pela literatura, neste tipo de problema as interações podem ser negligenciadas, confirmando os resultados obtidos, da ordem de 2% para o fluxo radiante. / This dissertation has the objective of analyzing the Turbulence-Radiation Interaction for a non-reactive flow with a participating media. These interactions are characterized by complex transient effects, due to the combination of two phenomena, coupling the scalar fluctuations of the turbulence and the highly non-linearity of thermal radiation. The study consists in a numerical analysis through Computational Fluid Dynamics, using the Fire Dynamics Simulator (FDS), an Open-Source software, which employs the Large Eddy Simulation method. Because the software is under development and new in the research group, it will be performed the simulation of a benchmark case for verification and evaluation of the numerical methodology. The TRI analysis will be performed in a proposed problem, based on real heat exchangers, as an example, steam generators or exhaust manifold of combustion engines, involving combined heat transfer between forced convection and radiative heat transfer. The methodology consists in evaluating the radiative mean heat flux obtained by the transient simulation and compare it with the flux obtained with the time-averaged temperature field. It will be evaluated the influence of the turbulence intensity at the inlet and the optical thickness, both very important for the TRI effects. According to the literature, in this case the TRI effects could be neglected, confirming the obtained results, around 2% for the radiative heat flux.

A radiação térmica é um mecanismo de transferência de calor muito importante em processos que envolvem gases participantes, como CO2 e H2O, em temperaturas elevadas. A dependência altamente irregular do coeficiente de absorção em relação ao número de onda torna a integração linha-por-linha (LBL) da equação da transferência radiativa (RTE) impraticável, sobretudo em situações onde a radiação pode ser apenas parte de um problema mais complexo. Modelos espectrais globais, como a soma-ponderada-de-gases- cinza (WSGG) e a soma-ponderada-de-gases-cinza baseada em linhas espectrais (SLW), representam alternativas à integração LBL. Entretanto, alguns modelos requerem o uso da aproximação de escala (que assume que as dependências espacial e espectral da seção transversal de absorção são separáveis). Essa aproximação pode produzir erros consideráveis, principalmente quando existem gradientes elevados de temperatura e de concentração. Neste trabalho, os resultados de alguns modelos espectrais globais foram comparados com os da solução LBL. E, a partir de algumas proposições dos modelos WSGG e SLW (integração espectral através de gases cinza e intervalos espectrais fixos por meio da definição de uma temperatura de referência), foi desenvolvida a soma-ponderada-de-gases-cinza com coeficientes não constantes (NCC-WSGG). Nesse modelo, os coeficientes de absorção e de emissão para cada gás cinza são funções polinomiais da temperatura. O modelo NCC-WSGG foi aplicado em problemas unidimensionais e bidimensionais, envolvendo CO2, H2O e mistura dessas espécies químicas. Comparações com a solução LBL mostraram que o NCC-WSGG pode fornecer resultados muito satisfatórios para o fluxo de calor e para o termo fonte radiativos em problemas não isotérmicos e não homogêneos. / The thermal radiation is a very important mechanism of heat transfer in processes that embody participating gases, as CO2 and H2O, at high temperature. The highly irregular dependence of the absorption coefficient with respect to the wavenumber makes the application of line-by-line (LBL) integration of the radiative transfer equation (RTE) prohibitive, principally in situations where the radiation can be only part of a more complex problem. Global spectral models, like the weighted-sum-of-gray-gases (WSGG) and the spectral line based weighted-sum-of-gray-gases (SLW), are alternatives to the LBL integration. However, some models demand the application of the scaling approximation (which assumes that the spatial and spectral dependences of the absorption cross-section are separable). This approximation can lead to pronounced errors, mainly under high gradients of temperature and concentration. In this work, some results obtained from global models were compared with the LBL solution. And, applying some proposals from WSGG and SLW models (spectral integration through gray gases and fixed spectral intervals by the definition of a reference condition), it was developed the nonconstant coefficient weighted-sum-of-gray-gases (NCC-WSGG). In this model, the absorption and emission coefficients of each gray gas are polynomials functions of the temperature. The model was applied to solve one and two dimensional problems, which were comprise of CO2, H2O and mixtures of these chemical species. Comparisons with the LBL solution showed that the NCC-WSGG can provide very good results for the heat flux and for the radiative heat source under nonisothermal and nonhomogeneous conditions.

A integração da equação da transferência radiante (RTE) é uma tarefa complexa devido a forte variação do coeficiente de absorção com relação ao número de onda. O modelo da soma ponderada dos gases cinza (WSGG) evita a integração linha por linha da RTE reduzindo o esforço computacional na resolução de problemas que envolvam gases participantes. Com a atualização dos coeficientes do WSGG, obtidos através do banco de dados HITEMP 2010, este trabalho se propôs a validá-los por meio de problemas unidimensionais de transferência de calor radiante. Os problemas são resolvidos pelo modelo WSGG e comparados com a solução obtida pela integração LBL (solução benchmark). Nas comparações foram utilizados diferentes perfis de temperatura, distâncias características, gradientes de temperatura e concentrações de espécies. Nos casos analisados é possível verificar uma boa concordância geral entre os resultados WSGG e LBL. O modelo também é testado na resolução de perfis advindos de seções de uma câmara de combustão cilíndrica que apresentaram condições diferentes para os quais os coeficientes WSGG foram propostos. Mesmo assim os resultados obtidos apresentaram uma boa concordância para o termo fonte radiante e para o fluxo de calor, sendo que os maiores erros foram observados na entrada da câmera onde os gradientes de temperatura são mais significativos. / The spectral integration of the radiative transfer equation (RTE) is still a complex task due to the strong variation of the absorption coefficient with the wavenumber. The Weighted-Sumof- Gray-Gases (WSGG) model avoids the Line-by-Line (LBL) integration of RTE. The aim of this study is to evaluate the updated WSGG coefficients, obtained from the database HITEMP 2010, in one-dimensional problems of radiative heat transfer. The problems are solved by the WSGG model and compared with the solution obtained by the LBL integration (benchmark solution). Various temperature and concentration profiles were evaluated and showed a good overall agreement between the WSGG and LBL results. The model was also tested by solving profiles arising from cylindrical combustion chamber and the obtained results showed good agreement for the radiative heat source term and the heat flux. The largest errors were observed near the chamber entrance where the temperature gradients are most significant.

Made available in DSpace on 2014-10-09T12:26:30Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:04:10Z (GMT). No. of bitstreams: 1 13881.pdf: 3129908 bytes, checksum: d9b68275e6de8a29cb87899a8943a443 (MD5) / Dissertacao (Mestrado Profissionalizante em Lasers em Odontologia) / IPEN/D-MPLO / Intituto de Pesquisas Energeticas e Nucleares, IPEN/CNEN-SP; Faculdade de Odontologia, Universidade de Sao Paulo

Dans le moyen infrarouge, il n’existe pas à l’heure actuelle de source bon marché, compacte et modulable rapidement en amplitude. L’émission thermique est souvent écartée à cause des propriétés du rayonnement de corps noir : il est large spectralement, isotrope, non polarisé et la fréquence de modulation en intensité est limitée à quelques hertz par l’inertie thermique des émetteurs.Cependant, aucune limite fondamentale n’impose ces inconvénients. L’objectif de cette thèse est de concevoir, fabriquer et caractériser des sources infrarouges incandescentes, de spectre et polarisation contrôlés, modulables au-delà du mégahertz. Les dispositifs que nous présentons reposent sur la modulation rapide de la température d’un émetteur de faible épaisseur, posé sur un substrat qui demeure froid : en effet, la conduction permet de le refroidir en un temps qui dépend quadratiquement de l’épaisseur.Dans un premier temps, nous présentons une source émettant en bande II (3 – 5 microns) fondée sur le principe de l’écran de Salisbury ; sa réponse en fréquence est caractérisée jusqu’à la dizaine de mégahertz.Puis nous modifions cette structure pour utiliser un réseau métallique sub-longueur d’onde et faisons ainsi la démonstration d’une source en bande II modulable et polarisée linéairement.Enfin, nous proposons plusieurs dispositifs pouvant rayonner avec une polarisation circulaire ainsi qu’une source en bande III (8 – 12 microns) constituée d’une métasurface de nano-émetteurs chauds couplés à des nano-antennes froides. / Currently, there is no available source in the mid-infrared range which can be cheap, compact, and whose intensity can be modulated at high frequency. For this purpose, thermal radiation is often considered irrelevant because of the blackbody properties: it is intrinsically broadband, isotropic, unpolarized and the intensity modulation rate is usually limited to a few hertz by thermal inertia.However, there is no fundamental limit that imposes these properties. The goal of this thesis is to design, fabricate and experimentally characterize infrared incandescent sources with a controlled spectrum and polarization and with an intensity that can be modulated faster than 10 megahertz. We present devices which rely on fast temperature modulation of a thin emitter placed on a cold substrate. Indeed, thanks to heat conduction, this emitter can cool down within a characteristic time which varies as the square of its thickness.Firstly, we show a device emitting in MWIR (mid-wave infrared, 3 – 5 microns) based on the Salisbury screen’s principle. We characterize its frequency response up to 10 MHz.Then, we modify this structure and use instead a sub-wavelength metallic grating, thus demonstrating a MWIR source linearly polarized with the same modulation properties.Finally, we propose several devices which can emit circularly polarized infrared radiation and a source operating in LWIR (long-wave infrared, 8 – 12 microns) consisting in a metasurface of hot nano-emitters coupled to cold nano-antennas.

<p>Understanding of thermal transport in small scales gains more importance with increasing demand in microelectronics and advancing fabrication technologies. In addition, scarce in energy sources adds more pressure with increasing expectations on research in energy conversion devices and renewable energies. In parallel to these, new phenomena observable only in small scales are discovered with the research, bringing more opportunities for engineers to solve real-world problems by applying the discoveries and more questions to answer. Thermal radiation as a thermal transport phenomenon is the epicenter of this research. Recent developments such as near-field radiative heat transfer exceeding blackbody radiation or control of radiative cooling via biasing grows the attraction on thermal radiation because these examples challenge our long-lasting understanding of nature. Exploring nature further in the small scale may help us meet the expectations mentioned above.</p> <p> </p> <p>In this thesis work, first, we carry out analyses on radiative heat transfer of natural hyperbolic material, calcite, and compare to that of a polar material SiC. Our study reveals that the high- modes within the hyperbolic bands are responsible for the substantial enhancement in near field radiation. Comparison of calcite with SiC illustrates the significance of the high- modes in calcite vs. surface polariton modes in SiC in their contributions to near-field radiation enhancement, for temperature differences ranging from 1 K to 400 K. We also noticed that the contributions of high- modes in calcite to near-field radiation is comparable to that of surface polaritons in SiC. The results of these analyses will be helpful in the search of hyperbolic materials that can enhance near field radiative transfer.</p> <p> </p> <p>Second, we demonstrate an experimental technique to measure near-field radiative heat transfer between two parallel plates at gap distances ranging from a few nanometers to far-field. A differential measurement circuit based on resistive thermometry to measure the defined temperatures are explained. To predict the defined temperatures, a computational method is utilized. We also detail an alignment technique that consists of a coarse and fine alignment in the relevant gap regions. This technique presents a method with high precision for gap measurement, dynamic gap control, and reliable sensitivity for extreme near-field measurements. Finally, we report experimental results that shows 18,000 times enhancement in radiative heat transfer between two parallel plates.</p> <p> </p> <p>Third, we analyze near-field radiative transfer due to hyperbolic phonon polaritons, driven by temperature gradient inside the bulk materials. We develop a mesoscale many-body scattering approach to account for the role of hyperbolic phonon polaritons in radiative transfer in the bulk and across a vacuum gap. Our study points out the equivalency between the bulk-generated mode and the surface mode in the absence of a temperature gradient in the material, and hence provide a unified framework for near-field radiative transfer by hyperbolic phonon polaritons. The results also elucidate contributions of the bulk-generated mode and the bulk temperature profile in the enhanced near-field radiative transfer.</p> <p> </p> <p>Forth, we study radiative heat transfer in hyperbolic material, hyperbolic boron nitride (hBN), and show a major contribution to energy transport arising from phonon polaritons supported in Reststrahlen bands. This contribution increases spectral radiative transfer by six orders of magnitude inside Reststrahlen bands compared to that outside Reststrahlen bands. The equivalent radiative thermal conductivity increases with temperature increase, and the radiative thermal conductivity can be of the same order of the phonon thermal conductivity. Experimental measurements are discussed. We showed the radiative contribution can account for as much as 27 % of the total thermal transport at 600 K. Hence, in hBN the radiative thermal transport can be comparable to thermal conduction by phonons. We also demonstrate contribution of polaritons to thermal transport in MoO₃. To calculate radiative heat transfer in three principal coordinates separately, we modify and apply the derived many-body model. Our analysis shows that radiative thermal conductivity in both in- and out-of-plane directions increases with temperature and contribution to energy transport by polaritons exceeds that by phonons.</p> <p> </p> Fifth, we build an experimental setup to examine near-field properties of materials using an external thermal source. The nanospectroscopy setup combines near-field microscopy technique, near-field scanning optical microscopy (NSOM), and Fourier-transform infrared (FTIR) spectroscopy. We further explain challenges in building a nanospectroscopy setup using a weak thermal source and coupling two techniques. This method enables us to investigate spectral thermal radiation and local dielectric properties in nanoscale.

The study whose results are presented here was carried out in support of an ongoing larger effort to investigate and understand the impact of coherence and polarization on the performance of instruments intended to monitor the Earth's radiant energy budget. The visibility of fringes produced by a Michelson interferometer is known to be sensitive to the degree to which the incident light beam is monochromatic. Therefore, the Michelson interferometer has significant potential as a tool for quantifying the degree of temporal coherence of a quasi-monochromatic light beam. Simulation of the performance of an optical instrument using the Monte-Carlo ray-trace (MCRT) method has been shown to be an efficient method for transferring knowledge of the coherence state of a beam of light from one instrument to another. The goal of the effort reported here is to create and experimentally validate an MCRT model for the optical performance of a Michelson interferometer. The effort is motivated by the need to consolidate the knowledge and skills of the investigator in the realm of physical optics, and by the need to make a useful analytical tool available to other investigators in the larger effort. / Master of Science

The Thermal Radiation Group at Virginia Polytechnic Institute and State University has been working closely with scientists and engineers at NASA's Langley Research Center to develop accurate analytical and numerical models suitable for designing next-generation thin-film thermal radiation detectors for earth radiation budget measurement applications. The current study provides an analytical model of the notional thermal radiation detector that takes into account thermal transport phenomena, such as the contact resistance between the layers of the detector, and is suitable for use in parameter estimation. It was found that the responsivity of the detector can increase significantly due to the presence of contact resistance between the layers of the detector. Also presented is the effect of doping the thermal impedance layer of the detector with conducting particles in order to electrically link the two junctions of the detector. It was found that the responsivity and the time response of the doped detector decrease significantly in this case. The corresponding decrease of the electrical resistance of the doped thermal impedance layer is not sufficient to significantly improve the electrical performance of the detector. Finally, the "roughness effect" is shown to be unable to explain the decrease in the thermal conductivity often reported for thin-film layers / Master of Science

The Thermal Radiation Group, a laboratory in the Department of Mechanical Engineering at Virginia Polytechnic Institute and State University, is currently working to develop a new technology for thermal radiation detectors. The Group is also studying the viability of replacing current Earth Radiation Budget radiometers with this new concept. This next-generation detector consists of a thermopile linear array thermal radiation detector. The principal objective of this research is to develop an optical model for the detector and its cavity. The model based on the Monte-Carlo ray-trace (MCRT) method, permits parametric studies to optimize the design of the detector cavity and the specification of surface optical properties. The model is realized as a FORTRAN program which permits the calculation of quantities related to the cross-talk among pixels of the detector and radiation exchange among surfaces of the cavity. An important capability of the tool is that it provides estimates of the discrete Green's function that permits partial correction for optical cross-talk among pixels of the array. / Master of Science

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.