Le but de l'étude présentée dans ce mémoire de thèse est de caractériser localement l'intensification des transferts thermiques observée le long d'une plaque chauffante lorsqu'elle est soumise à des ultrasons. Ces derniers induisent des effets hydrodynamique dans les fluides qu'ils traversent. Premièrement, la cavitation acoustique, qui permet de produire de forts effets mécaniques dans les liquides. Viens ensuite le courant acoustique qui génère une écoulement convectif sous l'action d'une dissipation visqueuse de l’énergie acoustique. Ce travaille à permis de mettre en évidence la relation existant entre ces effets hydrodynamiques et l'intensification des transferts thermiques observée. Ces résultats ont permis de définir quelles conditions expérimentales sont favorables à l'intégration d'ultrasons dans un échangeur de chaleur. / The aim of the study presented in this thesis is to characterize heat transfer enhancement by ultrasound observed along a hot plate in forced convection. These induced hydrodynamic effects in the fluids they cross. The first one is the acoustic cavitation, which can produce strong mechanical effects in liquids. The second one is the acoustic streaming which generates convective flows thanks to viscous dissipation of the acoustic energy. This work helped to highlight the relationship between ultrasound hydrodynamic effects and heat transfer enhancement. Thanks to those results, experimental conditions which allowed ultrasound integration in a heat exchangers, could be determined.

abstract: Microchannel heat sinks can possess heat transfer characteristics unavailable in conventional heat exchangers; such sinks offer compact solutions to otherwise intractable thermal management problems, notably in small-scale electronics cooling. Flow boiling in microchannels allows a very high heat transfer rate, but is bounded by the critical heat flux (CHF). This thesis presents a theoretical-numerical study of a method to improve the heat rejection capability of a microchannel heat sink via expansion of the channel cross-section along the flow direction. The thermodynamic quality of the refrigerant increases during flow boiling, decreasing the density of the bulk coolant as it flows. This may effect pressure fluctuations in the channels, leading to nonuniform heat transfer and local dryout in regions exceeding CHF. This undesirable phenomenon is counteracted by permitting the cross-section of the microchannel to increase along the direction of flow, allowing more volume for the vapor. Governing equations are derived from a control-volume analysis of a single heated rectangular microchannel; the cross-section is allowed to expand in width and height. The resulting differential equations are solved numerically for a variety of channel expansion profiles and numbers of channels. The refrigerant is R-134a and channel parameters are based on a physical test bed in a related experiment. Significant improvement in CHF is possible with moderate area expansion. Minimal additional manufacturing costs could yield major gains in the utility of microchannel heat sinks. An optimum expansion rate occurred in certain cases, and alterations in the channel width are, in general, more effective at improving CHF than alterations in the channel height. Modest expansion in height enables small width expansions to be very effective. / Dissertation/Thesis / M.S. Mechanical Engineering 2011

abstract: A relatively simple subset of nanotechnology - nanofluids - can be obtained by adding nanoparticles to conventional base fluids. The promise of these fluids stems from the fact that relatively low particle loadings (typically <1% volume fractions) can significantly change the properties of the base fluid. This research explores how low volume fraction nanofluids, composed of common base-fluids, interact with light energy. Comparative experimentation and modeling reveals that absorbing light volumetrically (i.e. in the depth of the fluid) is fundamentally different from surface-based absorption. Depending on the particle material, size, shape, and volume fraction, a fluid can be changed from being mostly transparent to sunlight (in the case of water, alcohols, oils, and glycols) to being a very efficient volumetric absorber of sunlight. This research also visualizes, under high levels of irradiation, how nanofluids undergo interesting, localized phase change phenomena. For this, images were taken of bubble formation and boiling in aqueous nanofluids heated by a hot wire and by a laser. Infrared thermography was also used to quantify this phenomenon. Overall, though, this research reveals the possibility for novel solar collectors in which the working fluid directly absorbs light energy and undergoes phase change in a single step. Modeling results indicate that these improvements can increase a solar thermal receiver's efficiency by up to 10%. / Dissertation/Thesis / Ph.D. Mechanical Engineering 2011

Em processos de combustão, uma determinação precisa dos parâmetros envolvendo transferência de calor influencia diretamente os demais fenômenos envolvidos. Dentre os mecanismos de transferência de calor presentes na combustão a radiação térmica é predominante, mas sua correta determinação impõe uma elevada complexidade, principalmente quando se trata da solução de meios participantes. O cálculo envolve propriedades de absorção que variam com a temperatura e o comprimento de onda, sendo então necessária a utilização de modelos espectrais para obter bons resultados com um baixo tempo computacional. Para o cálculo da transferência radiante, existem diversos modelos espectrais, desde modelos de simples implementação, como, por exemplo, o GG (gás cinza) e o WSGG (soma-ponderada-dos-gases-cinza), até modelos com um grau elevado de detalhamento, como o SLW (soma-ponderada-dos-gases-cinza baseado em linhas espectrais) e o CW (número de onda cumulativa). Como os modelos com maior grau de detalhamento são de complexa implementação, alguns autores preferem empregar modelos simplistas, como o GG (gás cinza), apenas por questões de conveniência, mesmo em detrimento da qualidade dos resultados. Uma forma de executar o cálculo da radiação térmica sem simplificações é levar em conta as absorções em cada comprimento de onda, sendo esses cálculos denominados integração linha-por-linha (LBL), por executar o cálculo da transferência radiante em cada linha de absorção, o que gera resultados benchmark, podendo ser utilizados para avaliar os diversos modelos existentes. Este trabalho tem por objetivo verificar e sintetizar a aplicação dos modelos espectrais, em configurações envolvendo concentração e temperatura não uniformes, onde são realizados cálculos em um meio contendo CO2, H2O e fuligem. São avaliados os modelos GG, WSGG, SLW e CW. Dentre os modelos avaliados, o que apresenta os melhores resultados para as condições apresentadas é o modelo WSGG. De forma a aprimorar o modelo WSGG, uma nova implementação para a solução de misturas é apresentada, a qual apresenta correlações para o H2O e para o CO2 geradas individualmente, possibilitando misturas com qualquer razão de concentração, mostrando que o modelo apresenta bons resultados em diversas situações e é uma boa opção para a solução de problemas de combustão. / In combustion processes a good determination of the heat transfer parameters are of great importance because of its direct influence in the computation of the chemical reactions rate in the process and, consequently, in the formation of the combustion products. Among the processes of heat transfer in combustion, thermal radiation is predominant, and their determination can be a very complex task, especially with participating medium. The analysis involves absorption properties that vary with the temperature and wavelength, and therefore it is necessary to use spectral models to ensure good results with low computational time. There are several spectral models developed along the years, since the simplistic models such as the GG (gray gas) and WSGG (weighted-sum-of-gray-gases), to more advanced methods such as the SLW (spectral line weighted-sum-of-gray-gases) and CW (cumulative wavenumber). Due advanced models are in general a hard task to implement, the option is to use simplified models, for example the GG, even working with considerably errors. In order to quantify these solutions, for temperature and concentration conditions of the absorbing species, it is necessary to implement the radiation heat transfer taking into account the absorption at each wavelength through line-by-line (LBL) integration, being this solution the exact one, or, the benchmark solution, which it is used to evaluate the spectral models. In this study, the LBL integration is carried out to evaluate some of the existing models in a non-isothermal and inhomogeneous medium containing CO2, H2O and soot. The work involves the GG, WSGG, SLW and CW spectral models. For the presented cases, the best results occur with WSGG model. In order to improve the WSGG model a new implementation for the mixture solution is presented, which solves the correlations for H2O and CO2 generated individually, enabling mixtures containing any concentration ratio, showing the good agreement of the spectral model at any condition, being the WSGG a good option to solve combustion problems.

La détermination des cinétiques réactionnelles est incontournable en génie de la réaction. Sans quoi on ne peut dimensionner convenablement un réacteur. Cependant, la complexité de certains systèmes réactionnels nécessite de fournir des efforts expérimentaux souvent rédhibitoires, en termes de moyens comme de temps. Des méthodologies de planification expérimentales pour la modélisation cinétique existent. Les objectifs de ces méthodes peuvent être la détermination précise des paramètres d’un modèle ou la discrimination de plusieurs modèles potentiels. Notre objectif est d’étudier des méthodologies pour discriminer entre des modèles et d’éprouver ces méthodologies sur des applications numériques et/ou expérimentales. Ces méthodologies s’appuient sur un processus itératif, qui, étape par étape, mène à la sélection d’un modèle, à la détermination de ses paramètres et à sa validation. Une fois la méthodologie précisée, un premier cas d’étude « numérique » concerne un système réactionnel, dont plusieurs modèles sont proposés, basés sur différents schémas réactionnels. L’objectif étant de trouver le bon schéma par application de la méthodologie. Ensuite un cas d’étude expérimentale est traité : l’acylation catalytique du ferrocène. Sa cinétique n’est pas connue. La méthodologie appliquée mènera à la proposition de plusieurs modèles cinétiques et à la sélection du plus adapté. Un dernier cas d’étude s’intéresse à la modélisation des réactions exothermique, en particulier en micro/milli réacteur continu. Un focus est d’abord fait sur une modélisation adaptée à ces systèmes réactionnels, à travers l’utilisation de nombres adimensionnels réduisant les degrés de liberté, et permettant une analyse synthétique du comportement du réacteur. Par la suite, plusieurs applications « numériques » de la méthodologie de discrimination de modèles sont proposés, dont l’objectif est la discrimination du comportement thermique des réacteurs. La méthodologie expérimentale utilise efficacement les données déjà accumulées sur les modèles, pour choisir au mieux chaque nouvelle expérience en fonction des objectifs ciblés. Elle permet de cibler efficacement les meilleures expériences pour atteindre l’objectif fixé. Les efforts expérimentaux sont ainsi optimisés, ainsi que la recherche de modèles cinétiques et thermiques. / Knowledge on kinetics is essential for chemical reactor modelling. Yet when chemical systems are very complex, development of good kinetic models leads to expensive and time consuming experiments, often prohibitive. Our goal is to develop efficient numerical methods to design the optimal experiments to select the best model among many possible candidates while precisely estimating its kinetics parameters. The gain is double: reduction of experiments and acquisition of more accurate information. Several study cases will enable the assessment of these methodologies. Optimal experimental design methodologies are based on iterative procedures,leading to the selection of the more accurate model, and to the identification of its parameters. In the present work, a first numerical study case is chosen as an illustration of the method, consisting on the discrimination of four synthesis pathway that are potential candidates to describe a reactional system. It is showed how the developed method can smartly choose experiments to lead to the choice of the accurate pathway. The second study case is the experimental study of the catalytic acetylation of ferrocene, for which any accurate kinetic models have not been found yet. Thanks to the iterative design of experiments, it is possible to characterize, very quickly, the order of reaction and how the catalyst effect has to be considered. The last part of this work deals with exothermic reactions and the coupling between thermal transfer and chemical reactions in milli/micro-reactors. The use of dimensionless numbers is proposed to reduce the number of parameters implied in such systems and to analyse the thermal behaviour of microreactors. Then, the aim is to illustrate how to discriminate thermal behaviours using the discriminatory methodology, through several study cases. Those examples demonstrate that iterative design of experiments is an efficient method to find the best experiments to solve the issues for selecting a model among others and for determining the associated parameters. This offers the advantages to reduce the experimental efforts in time and in matter, and thus to unlock modelling of many complex chemical systems.

abstract: Plasmon resonance in nanoscale metallic structures has shown its ability to concentrate electromagnetic energy into sub-wavelength volumes. Metal nanostructures exhibit a high extinction coefficient in the visible and near infrared spectrum due to their large absorption and scattering cross sections corresponding to their surface plasmon resonance. Hence, they can serve as an attractive candidate for solar energy conversion. Recent papers have showed that dielectric core/metallic shell nanoparticles yielded a plasmon resonance wavelength tunable from visible to infrared by changing the ratio of core radius to the total radius. Therefore it is interesting to develop a dispersion of core-shell multifunctional nanoparticles capable of dynamically changing their volume ratio and thus their spectral radiative properties. Nanoparticle suspensions (nanofluids) are known to offer a variety of benefits for thermal transport and energy conversion. Nanofluids have been proven to increase the efficiency of the photo-thermal energy conversion process in direct solar absorption collectors (DAC). Combining these two cutting-edge technologies enables the use of core-shell nanoparticles to control the spectral and radiative properties of plasmonic nanofluids in order to efficiently harvest and convert solar energy. Plasmonic nanofluids that have strong energy concentrating capacity and spectral selectivity can be used in many high-temperature energy systems where radiative heat transport is essential. In this thesis&#65292;the surface plasmon resonance effect and the wavelength tuning ranges for different metallic shell nanoparticles are investigated, the solar-weighted efficiencies of corresponding core-shell nanoparticle suspensions are explored, and a quantitative study of core-shell nanoparticle suspensions in a DAC system is provided. Using core-shell nanoparticle dispersions, it is possible to create efficient spectral solar absorption fluids and design materials for applications which require variable spectral absorption or scattering. / Dissertation/Thesis / M.S. Mechanical Engineering 2012

[pt] Muitas indústrias usam em seus processos materiais viscoplásticos. Esses materiais possuem propriedades que dependem fortemente da temperatura. Não é incomum encontrar processos envolvendo escoamentos não isotérmicos de materiais viscoplásticos. Nesses casos, informações sobre a transferência de calor são extremamente necessárias para um bom atendimento e aperfeiçoamento das operações. Fluidos de perfuração são tipicamente suspensões aquosas, e, por consequência, de natureza viscoplástica.Tais fluidos devem possuir densidade correta para manter a integridade física dos poços e evitar a produção prematura de hidrocarbonetos. Além disso, suas propriedades reológicas devem garantir a capacidade de arraste das partículas de rocha geradas durante o processo de perfuração, com um mínimo de potência de bombeamento. Tais particularidades requerem fluidos com baixas viscosidades a altas taxas de cisalhamento, que ocorrem em regiões próximas à parede, e altas viscosidades quando as taxas de deformação são baixas, o que ocorre na vizinhança do cascalho. Materiais viscoplásticos apresentam este tipo de comportamento. Portanto, o sucesso do processo de extração do petróleo depende do conhecimento e controle das propriedades reológicas dos fluidos de perfuração, as quais são fortemente dependentes da temperatura. Por esse motivo, a determinação do campo de temperatura no fluido de perfuração em escoamento faz-se necessária ainda em nível de projeto, o que só é possível com o conhecimento dos coeficientes de troca de calor. Estuda-se neste trabalho o problema da transferência de calor na região de entrada de escoamentos laminares de fluidos viscoplásticos através de espaços anulares. O comportamento do material é representado pelo modelo do fluido Newtoniano generalizado, com a função viscosidade descrita pela equação de Herschel-Bulkley. As equações de conservação são resolvidas numericamente via o método de volumes finitos. Investigam-se os efeitos (no coeficiente de troca de calor) da tensão limite de escoamento, índice power-law, razão de aspecto e dos números adimensionais de Reynolds e Peclet. Dentre outras conclusões, mostra-se que o números de Nusselt é uma função muito fraca das propriedades reológicas, desviando-se muito pouco dos valores Newtonianos. Surpreendentemente, esta conclusão contrasta-se fortemente com o comportamento observando em escoamentos de materiais viscoplásticos através de tubos. Convém enfatizar a importância desse fato no que tange a projetos de processos. / [en] There are many industries that use in their processes viscoplastic materials. These materials have properties that strongly depend on temperature. It is not uncommon to find processes involving the non-isothermal flow of viscoplastic materials. For these cases, heat transfer information is needed to allow reliable process designs. Drilling muds are typically aqueous suspensions and, consequently, viscoplastic in nature. They must have the correct density to provide the pressure needed for well integrity, and for avoiding premature production of hydrocarbons. Their rheological properties must be such as to aloe carrying the drill chips with a minimum of pumping power. This requires a highly shear-thinning rheological behavior. Also, the success of a well cementing operation depends to a great extent on the knowledge and control of cement rheological properties, which are also temperature dependent. In this work, heat transfer in the entrance-region flow of viscoplastic materials through annular spaces is analyzed. The flow is laminar, and the material is assumed to behave as a Generalized Newtonian fluid, with a Herschel-Bulkley viscosity function. The conservation equations are solved numerically via a finite volume method. The effect on heat transfer of yield stress, power-law exponent, aspect ratio and dimensionless Peclet and Reynolds numbers is investigated. Among other findings, it is shown that the Nusselt number is a rather weak function of the rheological properties, deviating very little from the Newtonian values. Surprisingly, this stands in strong contrast to the behavior observed for flows of viscoplastic materials through tubes. It is worth noting that this finding has important consequences in process design.

[pt] Uma das características das alternativas consideradas para o desenvolvimento de reatores nucleares com características avançadas de segurança consiste na utilização de resfriamento do vaso de contenção com um fluxo de água vindo de seu topo. Este sistema forma um escoamento de um filme líquido evaporando sobre uma superfície metálica aquecida com ar escoando em contra-corrente. O presente trabalho consiste no desenvolvimento de um modelo analítico e numérico da transferência de calor e massa que ocorre na interface entre o filme líquido e a mistura ar-vapor, neste tipo de escoamento e também em escoamentos co-correntes. Devido à ausência de dados experimentais, o modelo é validado através de comparações com resultados analíticos disponíveis na literatura para situações próximas aos casos de interesse. A influência de diversos parâmetros geométricos e operacionais é analisada e são fornecidos resultados para os números de Nusselt e de Sherwood observados na interface do escoamento das fases. / [en] The design of so called Advanced Nuclear reactors includes many features of passive safety, one of them being a water flow system for cooling the containment vessel. This system generates a counter-current flow pattern with a downward liquid film evaporating over the heated vessel surface and na upward stream of air. This work presents an analytical and numerical model for analysis of heat and mass transfer occuring in the interface formed by water film surfece anda ir-vapor mixture in counter-curent and parallel flows. Due to the lack of experimental data, the model is validated throught comparisons with analytical results found in the flow analysed and the results for Nusselt and Sherwood numbers are presented.

[pt] O objetivo deste trabalho é analisar as transferências de momentum, calor e massa num canal bidimensional inclinado, onde escoam um filme líquido descendente e uma corrente turbulenta de ar de sentido oposto, ou de mesmo sentido oposto, ou de mesmo sentido. O filme líquido, suposto laminar, consiste numa solução fraca, ou degenerada, de trietileno glicol em água. A placa inferior do canal é mantida aquecida numa temperatura uniforme, de modo a facilitar a remoção de água do líquido para a fase gasosa. A corrente de ar deve ser turbulenta, para garantir taxas convenientes da massa de água transferida. Considera-se, não obstante, a possibilidade do ar também escoar liminarmente. A placa superior do canal, ou cobertura é adiabática e ambas as placas são impermeáveis à transferência de água. A parte hidrodinamica do problema é resolvida separadamente. Atribui-se maior importância à determinação dos perfis de temperatura e concentração de águas nas duas fases. Pretende-se que a aparelhagem acima descrita opere como um regenerador da substância líquida higroscópica, tendo este sido previamente usado num secador de ar, em aplicação de fim industrial ou agrícola. A tarefa proposta pelo problema é a simulação das condições operativas do trocador de massa. Como resultado desta simulação, tenciona-se predizer os valores dos coeficientes de transferências de calor e massa, variando amplamente as taxas de escoamento, tanto da fase gasosa, quanto da líquida. Na verdade, diversos pesquisadores têm revelado, nos últimos anos, um grande interesse no estudo de regeneradores do tipo aqui analisado. Toda vez que se tem disponibilidade de energia a temperaturas moderadas e baixo custo, como energia solar ou calor de rejeito industrial, parece indicado regenerar desta forma o desumidificante líquido nas instalações de condicionamento de ar por resfriamento evaporativo. Estabelecidas as equações diferenciais parciais do problema e as condições de contorno pertinentes, elas são resolvidas através de algoritmos obtidos por diferenças finitas, dentro do enfoque de volumes de controle. O procedimento numérico é interativo, usando-se o computador digital na obtenção da solução. Verifica-se que os resultados se mantêm dentro da analogia entre transferência de calor e massa, conforme era esperado. A partir dos resultados, podem-se estabelecer algumas correlações para os principais parâmetros do problema. Propõe-se, por fim, uma metodologia para o projeto do equipamento. Para isto, não é necessário fazer uso direto do método numérico, pois existem algumas poucas equações analíticas, simples, que podem ser facilmente manipuladas num microcomputador ou, até mesmo, numa calculadora eletrônica. Estas equações são deduzidas a partir da aplicação da teoria de penetração ao problema. As correlações dos resultados numéricos são, entretanto, essenciais ao bom emprego da teoria penetração. / [en] The combined momentum, heat and mass transfer is analysed in a two domensional inclined channnel for a countercurrent, or co-current turbulent air strem flowing past a liquid falling film. The film flow, supposed to be laminar, consists of a weak, or co-current tubulent air stream flowing past a liquid falling film. The film flow, supposed to be laminar, consists of a weak, or degenerate, solution of triethylene glycol and water. The film flow, supposed to be laminar, consists of a weak, or degenerate, solution of triethylene glycol and water. The lower plate of the channel is maintained at a constant, relatively high from the liquid to the gas phase. The stream of air is usually turbulent, thus assuring convenient rates of tranferred mass of water. Nevertheless, the possibility of laminar flow in the gas is not avoided. The second plate of the channel is considered as adiabatic and both plates are impervious to water. The hydrodynamic part of the problem is solved separately, and the determination of temperature and concentration of water profiles in the two phases is of major importance. The apparatus above described is intended to be a regenerator of the hygroscopic liquid, previously used in an air dryer, for industrial or agricultural purposes. The tash suggested by the problem is then to simulate the conditions, under which this mass exchanger will operate. Following the simulation, heat and mass transfer coefficientes can be predicted for a large range of flow rates of both gas an liquid phases. In fact, in recent years several investigators have manifested an increasing interest in developing studies of such equipment. In situations where a source of energy at low temperature is freely available, and this is the case of solar energy or industrial rejects, the employment of liquid dehumidifier regenerators is particularly attractive for evaporative cooling air conditioning systems. The partial differential equations of the problem, accompanied by suitable boundary conditions, are solved by a finite difference scheme, based on the volume of control approach. There are iterative procedures involved and solutions is reached in a mainframe computer. The results seem to be in accordance with the expected analoggy between heat and mass transfer. Some correlations are presented for the principal parameters of the problem. Lastly, a methodology is proposed for the design of the equipment. In spite of the complexity of the problem, it is possible to provide the user with a few simple analytic equations, which can be solved in any micro-computer or even in a pocket calculation. Theses equations arise from the employment of the penetration theorym briefly discussed and compared with numerical results. Indubitably, the use of this theory must be in compliance with the previously obtained numerical correlations.

A parametric study of oil-jet lubrication in gear wheels is conducted using Computational Fluid Dynamics (CFD) to study the effect of the different design parameters on the cooling performance in a gearbox. Flow in oil jet lubrication is found to be complex with the formation of oil ligaments and droplets. Various hole radii of 1.5, 2 and 2.5 mm along with five oil velocities is analyzed and it is found that at lower volumetric rates, velocity has more effect on the cooling and at higher volumetric rates, hole size has more effect on the cooling. At higher velocities, the heat transfer is much greater than the actual heat production in the gear wheel, hence these velocity ratios are considered less suitable for jet lubrication. At low velocity ratios of below 2, the oil doesn’t fully impinge the gear bottom land and the sides leading to low cooling. Based on the cooling, impingement length and amount of oil lost to the casing surface, 2 mm hole with a velocity ratio of 2.225 is selected for a successful oil jet lubrication. Varying the inlet position in X, Y and Z directions (horizontal, vertical and lateral respectively) is found to have no improvement on the cooling. Making the oil jet hit the gear wheel surface at an angle is found to increase the cooling. Analysis with the use of a pipe to supply oil was conducted with circular and square inlet and it was found that the heat transfer decreases in both cases due to the splitting of oil jet caused by the combination of the effects of high pressure from the pipe and vorticity in the air field. A method has been developed for two gear analysis using overset meshes which can be used for further studies of jet lubrication in multi-gear systems. Single inlet is found to be better for cooling two gear wheels as it would require a reduced volumetric flow rate compared to double inlets. Oil system requirements for jet lubrication was studied and it was concluded that larger pumps have to be used to provide the high volumetric rates and highly pressurized oil required. On comparing the experimental losses from dip lubrication and the analytical losses for jet lubrication, dip lubrication is found to have lesser loses and more suitable for this case. Good quality lubrication would reduce the fuel consumption and also increase the longevity of gearboxes and hence more research into analyzing alternate lubrication systems can be carried out using the results from this thesis.