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The concurrent spreading of a boundary layer type diffusion flame is studied. The impossibility of obtaining a low velocity laminar flow without any perturbation induced by buoyancy has lead to the development of an experimental apparatus for use in micro-gravity facilities. Based on previous experimental observations, an original numerical approach has been developed showing, first the dominating role of the radiative heat transfer on the structure of the flame and second the major role of the soot on the extinction phenomenon at the flame trailing edge. The influence of the forced flow velocity, the fuel injection velocity and oxygen concentration on the geometry of the flame has been examined by imaging of CH* and OH* radicals spontaneous emission. Laser-Induced Incandescence (LII) is used to determine the soot field concentration in the flame. The soot formation has been studied by Laser Induced Fluorescence (LIF) of Polycyclic Aromatic Hydrocarbons (PAHs). The interaction between the reaction zone and the field of soot formation/oxidation is taken into account to analyze the flame length. These results can be used as the experimental input data for a future complete validation of numerical model simulating the soot formation and oxidation in this kind of flame.

A haute température, l’interdiffusion entre un superalliage et son revêtement protecteur (ß-NiAl ou ß- NiPtAl) dégrade à la fois la protection contre l’oxydation, par modification de la composition chimique du revêtement, et la microstructure du superalliage (3ième et 4ième générations) par formation de Zones de Réaction Secondaires (SRZ). Le but de cette étude a donc été (1) de développer des barrières de diffusion (BD) constituées d’une dense précipitation de phases a-W après traitement sous vide (BD simple) ou chromisation en phase vapeur (BD enrichie en chrome) (2) de mettre au point une méthode pour en étudier l’efficacité. Des mesures de concentration chimique (à partir de cartographies spectrales EDS), couplées à des ajustements des comportements en oxydation cyclique en utilisant le modèle « p-kp », et le développement d’un modèle « p-kp-ß » ont permis de montrer l’efficacité de la BD selon sa composition et la durée de vieillissement. Pour des longues durées de vieillissement, l’efficacité de la BD se réduit par la dissolution des précipités d’a-W dans les phases y’ et y formées à cause de la dégradation des propriétés protectrices du revêtement ß NiPtAl (augmentation de l’écaillage de l’oxyde formé et de la cinétique d’oxydation). Plusieurs causes probables de cette dégradation ont pu être déterminées, soit dues aux procédés (pollution au soufre) soit liées à la mise en place de la BD : augmentation de la transformation martensitique, enrichissement en tungstène et présence de précipités d’alpha chrome. Enfin, il a été montré que si l’initiation des SRZ est modifiée par l’ajout de la BD, leur cinétique de propagation ne l’est pas et est essentiellement dépendante de la composition de l’alliage. Un modèle de propagation des SRZ décrivant les évolutions chimiques locales de part et d’autres de l’interface « SRZ / superalliage » a été proposé. L’ajout de chrome à la BD permet d’inhiber la formation des SRZ, une couche riche en phases TCP remplace alors la SRZ. / At high temperature, interdiffusion between a superalloy and its protective coating (ß-NiAl or ß- NiPtAl) degrades the oxidation protection by modifying the chemical composition of the coating. It also degrades the 3rd et 4th generation superalloy microstructure due to the formation of Secondary Reaction Zones (SRZ). As a consequence, the aim of this study was (1) to develop diffusion barriers (DB) composed of a dense precipitation of a-W phases after a thermal treatment under vacuum (simple DB) or a vapour phase chromisation (Cr enriched DB), (2) to develop a method for quantifying the DB efficiency. Chemical concentration measurements (with EDS spectral maps) coupled with the « p-kp » modelling of the cyclic oxidation kinetics, and the development of the model « p-kp-ß » have permitted to study DB efficiency as a function of its composition and its high temperature ageing. For long ageing duration, the efficiency of the DB is reduced. Indeed, it is shown that the DB degrades the protection character of the ß-NiPtAl by increasing the oxide scale spallation and of its growth kinetic. This, in turns, accelerates the ß to y’ and y phases transformation and then increases the a-W precipitates dissolution. Some likely causes of this degradation have been determined, either due to the process (sulphur pollution) or intrinsic of the DB addition (increase of the martensitic transformation, enrichment in tungsten and a-Cr formation in the coating). Finally, it has been proved that DB addition modifies the SRZ initiation but not their propagation kinetic, which only depends on the superalloy local composition. A SRZ propagation model which describes local chemical evolutions on both sides of the « SRZ / superalloy » interface was proposed. The addition of chromium to the DB permits to inhibit the SRZ formation. In this case, a layer rich in TCP platelets replaces the SRZ.

Solid oxide fuel cells (SOFCs) are solid-state ceramic cells, typically operating between 1073 K and 1273 K. Because of high operating temperature, SOFCs are mostly applicable in stationary power generation. Among various configurations in which SOFCs exist, the planar configuration of solid oxide fuel cell (SOFC) has the potential to offer high power density due to shorter current path. Moreover, the planar configuration of SOFC is simple to stack and closely resemble the stacking arrangement of polymer electrolyte membrane (PEM) fuel cells. However, due to high operating temperature, there are problems associated with the development and commercialization of planar SOFCs, such as requirement of high temperature gas seals, internal stresses in cell components, and high material and manufacturing costs. Mathematical modeling is an essential tool for the advancement of SOFC technology. Mathematical models can help in gaining insights on the processes occurring inside the fuel cell, and can also aid in the design and optimization of fuel cells by examining the effect of various operating and design conditions on performance. A multi-component and multi-dimensional mathematical model of SOFCs has been developed in this thesis research. One of the novelties of the present model is its treatment of electrodes. An electrode in the present model is treated as two distinct layers referred to as the backing layer and the reaction zone layer. Reaction zone layers are thin layers in the vicinity of the electrolyte layer where electrochemical reactions occur to produce oxide ions, electrons and water vapor. The other important feature of the present model is its flexibility in fuel choice, which implies not only pure hydrogen but also any reformate composition can be used as a fuel. The modified Stefan-Maxwell equations incorporating Knudsen diffusion are used to model multi-component diffusion in the porous backing and reaction zone layers. The coupled governing equations of species, charge and energy along with the constitutive equations in different layers of the cell are solved for numerical solution using the finite volume method and developed code written in the computer language of C++. In addition, the developed numerical model is validated with various experimental data sets published in the open literature. Moreover, it is verified that the electrode in an SOFC can be treated as two distinct layers referred to as the backing layer and the reaction zone layer. The numerical model not only predicts SOFC performance at different operating and design conditions but also provides insight on the phenomena occurring within the fuel cell. In an anode-supported SOFC, the ohmic overpotential is the single largest contributor to the cell potential loss. Also, the cathode and electrolyte overpotentials are not negligible even though their thicknesses are negligible relative to the anode thickness. Moreover, methane reforming and water-gas shift reactions aid in significantly reducing the anode concentration overpotential in the thick anode of an anode-supported SOFC. A worthwhile comparison of performance between anode-supported and self-supported SOFCs reveals that anode-supported design of SOFCs is the potential design for operating at reduced temperatures. A parametric study has also been carried out to investigate the effect of various key operating and design parameters on the performance of an anode-supported SOFC. Reducing the operating temperature below 1073 K results in a significant drop in the performance of an anode-supported SOFC; hence ionic conductivity of the ion-conducting particles in the reaction zone layers and electrolyte needs to be enhanced to operate anode-supported SOFCs below 1073 K. Further, increasing the anode reaction zone layer beyond certain thickness has no significant effect on the performance of an anode-supported SOFC. Moreover, there is a spatial limitation to the transport of oxide ions in the reaction zone layer, thereby reflecting the influence of reaction zone thickness on cell performance.

Solid oxide fuel cells (SOFCs) are solid-state ceramic cells, typically operating between 1073 K and 1273 K. Because of high operating temperature, SOFCs are mostly applicable in stationary power generation. Among various configurations in which SOFCs exist, the planar configuration of solid oxide fuel cell (SOFC) has the potential to offer high power density due to shorter current path. Moreover, the planar configuration of SOFC is simple to stack and closely resemble the stacking arrangement of polymer electrolyte membrane (PEM) fuel cells. However, due to high operating temperature, there are problems associated with the development and commercialization of planar SOFCs, such as requirement of high temperature gas seals, internal stresses in cell components, and high material and manufacturing costs. Mathematical modeling is an essential tool for the advancement of SOFC technology. Mathematical models can help in gaining insights on the processes occurring inside the fuel cell, and can also aid in the design and optimization of fuel cells by examining the effect of various operating and design conditions on performance. A multi-component and multi-dimensional mathematical model of SOFCs has been developed in this thesis research. One of the novelties of the present model is its treatment of electrodes. An electrode in the present model is treated as two distinct layers referred to as the backing layer and the reaction zone layer. Reaction zone layers are thin layers in the vicinity of the electrolyte layer where electrochemical reactions occur to produce oxide ions, electrons and water vapor. The other important feature of the present model is its flexibility in fuel choice, which implies not only pure hydrogen but also any reformate composition can be used as a fuel. The modified Stefan-Maxwell equations incorporating Knudsen diffusion are used to model multi-component diffusion in the porous backing and reaction zone layers. The coupled governing equations of species, charge and energy along with the constitutive equations in different layers of the cell are solved for numerical solution using the finite volume method and developed code written in the computer language of C++. In addition, the developed numerical model is validated with various experimental data sets published in the open literature. Moreover, it is verified that the electrode in an SOFC can be treated as two distinct layers referred to as the backing layer and the reaction zone layer. The numerical model not only predicts SOFC performance at different operating and design conditions but also provides insight on the phenomena occurring within the fuel cell. In an anode-supported SOFC, the ohmic overpotential is the single largest contributor to the cell potential loss. Also, the cathode and electrolyte overpotentials are not negligible even though their thicknesses are negligible relative to the anode thickness. Moreover, methane reforming and water-gas shift reactions aid in significantly reducing the anode concentration overpotential in the thick anode of an anode-supported SOFC. A worthwhile comparison of performance between anode-supported and self-supported SOFCs reveals that anode-supported design of SOFCs is the potential design for operating at reduced temperatures. A parametric study has also been carried out to investigate the effect of various key operating and design parameters on the performance of an anode-supported SOFC. Reducing the operating temperature below 1073 K results in a significant drop in the performance of an anode-supported SOFC; hence ionic conductivity of the ion-conducting particles in the reaction zone layers and electrolyte needs to be enhanced to operate anode-supported SOFCs below 1073 K. Further, increasing the anode reaction zone layer beyond certain thickness has no significant effect on the performance of an anode-supported SOFC. Moreover, there is a spatial limitation to the transport of oxide ions in the reaction zone layer, thereby reflecting the influence of reaction zone thickness on cell performance.

O desempenho de uma unidade de flotação por ar dissolvido (FAD) em escala piloto, tratando água destinada ao abastecimento contendo 50 uC e 05 uT, foi investigada sob o ponto de vista das partículas presentes (micro-bolhas e flocos) e da hidrodinâmica. A análise centrou-se na zona de reação da unidade de FAD onde foi desenvolvido o modelo matemático proposto por Reali (1991). A determinação das partículas foi realizada por método que emprega a análise de imagem. A condição hidrodinâmica foi avaliada por meio de ensaios de estímulo e resposta, combinada com a análise tridimensional do escoamento feita através de mapeamento utilizando equipamento que emprega ultra-som (sonda micro-ADV). Para a consecução do trabalho foram desenvolvidos dois métodos. Um para aquisição, tratamento e obtenção da distribuição de tamanho de micro-bolhas e flocos utilizando a análise de imagem digital em instalação com escoamento contínuo, sem a extração de amostras. Outro, envolvendo o desenvolvimento de programa (VelDigital3D) para tratamento dos dados da sonda utilizada. Inicialmente, foram realizados os ensaios de validação da utilização da sonda micro-ADV em água contendo micro-bolhas de ar e os ensaios de mapeamento da unidade utilizando a referida sonda conjugada ao programa VelDigital3D. Posteriormente, foram investigados alguns possíveis agentes na aglutinação das micro-bolhas após a despressurização tais como: i) as condições de mistura expressas em termos da taxa de aplicação superficial (TAS) e tempo de detenção na zona de reação (Tdz.r), ii) razão de recirculação (p), iii) a variação de pH e, iv) a dosagem de coagulante (\'AL POT.+3\'). Finalmente, a sensibilidade do modelo matemático proposto por Reali (1991) para a zona de reação de unidades de FAD convencional foi verificada com base nos parâmetros de projeto: tempo de detenção da zona de reação (Tdz.r) e taxa de aplicação superficial na zona de clarificação TASap. (downflow); e com base nos parâmetros operacionais: distribuição de tamanho de partículas (micro-bolhas e flocos) e razão de recirculação (p). As principais conclusões foram: i) a sonda micro-ADV pode ser utilizada para obtenção do perfil de velocidade em águas contendo micro-bolhas de ar, nas vazões investigadas; ii) o escoamento no interior da zona de reação apresentou um padrão bem definido de recirculação ao longo da altura, confirmando os resultados obtidos com os ensaios estímulo e resposta; iii) a variação do diâmetro médio das micro-bolhas foi muito pequena (de 20 a 30 \'mü\'m), embora tenha havido ressalvas e iv) o modelo proposto por Reali (1991) foi sensível aos parâmetros investigados e responde coerentemente com o desempenho da unidade piloto de FAD. / This work investigated the performance of a dissolved air flotation (DAF) pilot plant, used to treat drinking water containing 05 Tu and 50 Cu. The particles (micro-bubbles and flocs) and hydraulic characteristics were taken into account. The focus was given in the reaction zone as hypothesized by Reali\'s mathematic model (REALI, 1991). An image analyses was used to assess particles distribution sizes. The hydraulics of the DAF tank was assessed by using a pulse stimulus-response test combined with the three-dimensional flow analyses carried-out with equipment that applied ultra sound (microADV). Therefore, two methods were developed; one for the acquisition, treatment and also to obtain the micro-bubbles and flocs sizes distribution, without the need of extracting samples and another, involved the development of a software (VelDigital3D) in order to treat the microADV data. Initially, the application of the microADV in the DAF process was evaluated. After, the data collection was made and the result was treated by using the VelDigital3D software. Sequentially, the effects of some agents for micro-bubbles coalescence after the releasing point were investigated: i) mixture conditions in terms of hydraulic loading rate (HLR) and detention time (DT); ii) recirculation rate (p); iii) pH variation and, iv) coagulant dosage (\'AL POT.+3\'). Finally, the mathematic model behavior proposed by Reali (1991) was investigated by varying some parameters in the design, such as: detention time in the reaction zone (DTr.z) and hydraulic loading rate in the clarification zone HLRc.z (dowflow); and by varying some operational parameters, such as: particles sizes distribution (micro-bubbles and flocs) and recirculation rate (p). The main conclusions were: i) the microADV probe can be used to obtain the velocity flow profile in water containing micro-bubbles; ii) the flow within the reaction zone showed a well defined pattern of recirculation throughout the height of the unit, confirming the obtained results by using the stimulus-response tests; iii) the variation of micro-bubbles medium size was low (from 20 to 30 \'mü\'m) but some points regarding this aspect were discussed and, iv) the behavior of the mathematical model proposed by Reali (1991) showed a significant adjustment to the experimental data proving that it can be applied to analyzed design parameters.

O desempenho de uma unidade de flotação por ar dissolvido (FAD) em escala piloto, tratando água destinada ao abastecimento contendo 50 uC e 05 uT, foi investigada sob o ponto de vista das partículas presentes (micro-bolhas e flocos) e da hidrodinâmica. A análise centrou-se na zona de reação da unidade de FAD onde foi desenvolvido o modelo matemático proposto por Reali (1991). A determinação das partículas foi realizada por método que emprega a análise de imagem. A condição hidrodinâmica foi avaliada por meio de ensaios de estímulo e resposta, combinada com a análise tridimensional do escoamento feita através de mapeamento utilizando equipamento que emprega ultra-som (sonda micro-ADV). Para a consecução do trabalho foram desenvolvidos dois métodos. Um para aquisição, tratamento e obtenção da distribuição de tamanho de micro-bolhas e flocos utilizando a análise de imagem digital em instalação com escoamento contínuo, sem a extração de amostras. Outro, envolvendo o desenvolvimento de programa (VelDigital3D) para tratamento dos dados da sonda utilizada. Inicialmente, foram realizados os ensaios de validação da utilização da sonda micro-ADV em água contendo micro-bolhas de ar e os ensaios de mapeamento da unidade utilizando a referida sonda conjugada ao programa VelDigital3D. Posteriormente, foram investigados alguns possíveis agentes na aglutinação das micro-bolhas após a despressurização tais como: i) as condições de mistura expressas em termos da taxa de aplicação superficial (TAS) e tempo de detenção na zona de reação (Tdz.r), ii) razão de recirculação (p), iii) a variação de pH e, iv) a dosagem de coagulante (\'AL POT.+3\'). Finalmente, a sensibilidade do modelo matemático proposto por Reali (1991) para a zona de reação de unidades de FAD convencional foi verificada com base nos parâmetros de projeto: tempo de detenção da zona de reação (Tdz.r) e taxa de aplicação superficial na zona de clarificação TASap. (downflow); e com base nos parâmetros operacionais: distribuição de tamanho de partículas (micro-bolhas e flocos) e razão de recirculação (p). As principais conclusões foram: i) a sonda micro-ADV pode ser utilizada para obtenção do perfil de velocidade em águas contendo micro-bolhas de ar, nas vazões investigadas; ii) o escoamento no interior da zona de reação apresentou um padrão bem definido de recirculação ao longo da altura, confirmando os resultados obtidos com os ensaios estímulo e resposta; iii) a variação do diâmetro médio das micro-bolhas foi muito pequena (de 20 a 30 \'mü\'m), embora tenha havido ressalvas e iv) o modelo proposto por Reali (1991) foi sensível aos parâmetros investigados e responde coerentemente com o desempenho da unidade piloto de FAD. / This work investigated the performance of a dissolved air flotation (DAF) pilot plant, used to treat drinking water containing 05 Tu and 50 Cu. The particles (micro-bubbles and flocs) and hydraulic characteristics were taken into account. The focus was given in the reaction zone as hypothesized by Reali\'s mathematic model (REALI, 1991). An image analyses was used to assess particles distribution sizes. The hydraulics of the DAF tank was assessed by using a pulse stimulus-response test combined with the three-dimensional flow analyses carried-out with equipment that applied ultra sound (microADV). Therefore, two methods were developed; one for the acquisition, treatment and also to obtain the micro-bubbles and flocs sizes distribution, without the need of extracting samples and another, involved the development of a software (VelDigital3D) in order to treat the microADV data. Initially, the application of the microADV in the DAF process was evaluated. After, the data collection was made and the result was treated by using the VelDigital3D software. Sequentially, the effects of some agents for micro-bubbles coalescence after the releasing point were investigated: i) mixture conditions in terms of hydraulic loading rate (HLR) and detention time (DT); ii) recirculation rate (p); iii) pH variation and, iv) coagulant dosage (\'AL POT.+3\'). Finally, the mathematic model behavior proposed by Reali (1991) was investigated by varying some parameters in the design, such as: detention time in the reaction zone (DTr.z) and hydraulic loading rate in the clarification zone HLRc.z (dowflow); and by varying some operational parameters, such as: particles sizes distribution (micro-bubbles and flocs) and recirculation rate (p). The main conclusions were: i) the microADV probe can be used to obtain the velocity flow profile in water containing micro-bubbles; ii) the flow within the reaction zone showed a well defined pattern of recirculation throughout the height of the unit, confirming the obtained results by using the stimulus-response tests; iii) the variation of micro-bubbles medium size was low (from 20 to 30 \'mü\'m) but some points regarding this aspect were discussed and, iv) the behavior of the mathematical model proposed by Reali (1991) showed a significant adjustment to the experimental data proving that it can be applied to analyzed design parameters.