Le contexte général de cette thèse est la simulation numérique de l'injection de carburant dans un moteur à combustion interne, afin d'améliorer son rendement et de limiter la production de polluants. De manière plus générale, ce travail s'applique à tout système industriel mettant en jeu un écoulement multiphasique constitué d'un carburant liquide injecté dans une chambre occupée initialement par du gaz, comme par exemple les moteurs automobiles ou aéronautiques, ou les turbomachines. Intrinsèquement, il est possible de simuler l'ensemble de l'écoulement avec les équations classiques de la dynamique des fluides sans avoir recours à des outils de modélisation supplémentaires liés au caractère diphasique. Mais, les tailles des structures générées pendant l'injection (gouttes de diamètre inférieur à 10 μm) conduisent à des temps de calculs prohibitifs pour une application industrielle. C'est pourquoi il est nécessaire d'introduire une modélisation diphasique. C'est dans ce contexte que deux régions sont formellement distinguées: le coeur liquide dense proche de l'injecteur, appelé écoulement à phases séparées, et le spray constitué d'une population de gouttes polydisperse (c'est-à-dire de tailles différentes) générées après le processus d'atomisation en aval de l'injecteur. Ce travail de thèse étudie les modèles Eulériens pour la description de spray évaporants et polydisperses, en vue d'applications industrielles. Ils représentent une alternative potentielle aux modèles Lagrangiens qui sont majoritairement utilisés en industrie mais présentant des inconvénients majeurs. Ainsi, le modèle multi-fluide est étudié dans un premier temps. Bien que prometteur, deux difficultés sont soulignées: le coût requis pour une description précise de la polydispersion, et son incapacité à décrire les croisements de gouttes (particle trajectory crossing, PTC, en anglais). La thèse propose des solutions à ces deux limitations. Ces solutions reposent chacune sur des méthodes de moments. Premièrement, le modèle appelé Eulerian Size Multi Size Moment (EMSM) permet de résoudre des sprays évaporants et polydisperses de manière bien plus efficace que le modèle multi-fluide. Des outils mathématiques sont utilisés pour fermer le système d'équations associé au modèle, et combinés à des schémas de types volumes finis appelés schémas cinétiques, afin de préserver la réalisabilité du vecteur de moments, pour le transport et l'évaporation. Une réponse à la seconde limitation est apportée avec le modèle appelé Eulerian Multi Velocity Moment (EMVM) basé sur le transport de moments en vitesse d'ordre élevé. Une distribution bimodale peut être localement reconstruite à partir des moments en utilisant une méthode de quadrature de moments (Quadrature Method of Moment, QMOM en anglais) en une ou plusieurs dimensions d'espace. De la même manière que précédemment, l'utilisation de schémas cinétiques permet de préserver la réalisabilité du vecteur de moment. De plus, une étude mathématique approfondie de la dynamique du système en une dimension d'espace en révèle toute la complexité et représente une étape indispensable en vue de l'élaboration de schémas de transport d'ordre élevé (supérieur ou égal à 2).Afin de les tester, ces deux modèles ainsi que les outils numériques associés sont implémentés dans MUSES3D, un code académique de simulation numérique directe (Direct Numerical Simulation DNS en anglais) dédié à l'évaluation des modèles de spray. Des résultats de grande qualité démontrent le potentiel des modèles. L'extension du modèle EMSM dans un contexte industriel est ensuite considérée, avec son implémentation dans IFP-C3D, un code résolvant des écoulements réactifs sur des maillages non structurés et mobiles (dû au mouvement du piston) dans un formalisme RANS (Reynolds Averaged Navier Stokes) en présence de sprays. Le formalisme ALE (Arbitrary Lagrangian Eulerian en anglais) est utilisé et le modèle EMSM réécrit dans ce formalisme afin de mener des calculs en maillage mobile. De plus, une étude numérique a permis d'étendre les propriétés de précision et de stabilité obtenues en maillage fixe. La robustesse du modèle EMSM est alors démontrée avec succès dans IFP-C3D sur un cas impliquant un mouvement de piston, ainsi que dans le cadre d'une comparaison avec le code MUSES3D. Enfin, des résultats très encourageants prouvent la faisabilité d'un calcul d'injection dans une chambre de combustion d'un spray polydisperse avec le modèle EMSM.

The impact dynamics and drying of paint films sprayed on steel were experimentally investigated. The rupture of sprayed liquid films was first photographed on different substrates. The critical film thickness, below which a film would break, was observed to increase with increasing advancing liquid-solid contact angle, and was unaffected by liquid viscosity for a given substrate. For viscous paint, it was observed that there is no rupture or splashing from a paint droplet impacting a solid substrate or another paint drop or film. For paint films drying at room temperature, mass fluxes were measured and correlated with a simple analytical model based on transient diffusion, and showed good agreement. The mass flux of sprayed paint films decreased slightly with time, and the volatile concentration decreased appreciably. For sprayed paint films cured with heat, there is a minimum stand-time in order to cure a film without any entrapped bubbles.

The impact dynamics and drying of paint films sprayed on steel were experimentally investigated. The rupture of sprayed liquid films was first photographed on different substrates. The critical film thickness, below which a film would break, was observed to increase with increasing advancing liquid-solid contact angle, and was unaffected by liquid viscosity for a given substrate. For viscous paint, it was observed that there is no rupture or splashing from a paint droplet impacting a solid substrate or another paint drop or film. For paint films drying at room temperature, mass fluxes were measured and correlated with a simple analytical model based on transient diffusion, and showed good agreement. The mass flux of sprayed paint films decreased slightly with time, and the volatile concentration decreased appreciably. For sprayed paint films cured with heat, there is a minimum stand-time in order to cure a film without any entrapped bubbles.

In this work the differences between and characteristics of water and high solids, heated black liquor sprays from air-assist atomizers are examined. Sprays were imaged with a high speed camera and the images analyzed with computer code to produce droplet size data and macroscopic spray characteristics such as mass distribution. Fluid flow rates were measured to allow relevant dimensionless groups for the spray to be calculated. A 1000 degree C tubular furnace was placed around the spray to determine the effect of industrially relevant temperatures on the droplet formation process, relative to room-temperature conditions. It was found that high solids black liquor forms long, thin ligaments rather than droplets. In high-temperature surroundings the size of these ligaments increases, which from a comparison with theory in the literature was attributed to enhanced skin-formation driven by heat transfer. The data suggest that this skin formation may prevent secondary breakup. All sprays for both fluids produced droplet size mass distributions that were well described by the square-root normal distribution. The normalized width (s*) of these distributions was similar for all sprays and consistent with literature data for other nozzle designs (0.24 < s* < 0.38). The image analysis method assumed droplets were spheres with the same projected area. When this assumption was changed for black liquor sprays to a cylindrical droplet assumption, the shape and normalized width of the resulting mass distributions remained the same, but the representative diameter (calculated from surface area to volume ratios) decreased. Based on the agreement between the normalized distribution width in this work and that in the black liquor spraying literature it was concluded that the addition of atomizing air cannot be considered a means to narrow a droplet size distribution independent of droplet size. The results also indicate the importance of including the effects of skin formation and temperature- and time-dependent fluid properties in spray modeling. It is intended that these results contribute to increased understanding of the black liquor atomization process and lead to improved computational modeling of the same.

No description available.

[pt] Técnicas ópticas não intrusivas foram utilizadas para obter imagens instantâneas e médias de chamas turbulentas não pré-misturadas de etanol e ar estabilizadas num queimador tipo bluff-body. O espalhamento Mie, PLIF (fluorescência induzida em um plano laser) e PIV (velocimetria por imagem de partículas) determinaram a distribuição da densidade de gotas no spray, intensidade da fluorescência do radical OH, para mapear a zona de reação da chama, e o campo da velocidade do ar, respectivamente. Inicialmente propriedades de sprays de água são comparadas quando se variam as vazões de combustível e de ar, mostrando assimetria de duas zonas de máxima intensidade de espalhamento Mie próximas do bocal de injeção, nos casos de maior vazão de jato, e nos casos de menor vazão, o fechamento do filme líquido ocorre na zona central. Em seguida foram estudados dois regimes de combustão que correspondem a duas vazões de combustível, para os quais foram caracterizadas, a partir do sinal de fluorescência do radical OH, a estrutura instantânea e a média das chamas turbulentas o que permitiu identificar zonas de extinção. A superposição das imagens médias do OH-PLIF e de espalhamento Mie permitem evidenciar, no caso de menor vazão, que o spray é completamente envolto pela chama, o que representa um comportamento clássico para o desenvolvimento e uso de modelos de combustão. No caso de maior vazão, o spray interage fortemente com o processo de combustão, sendo este um caso que se afasta das situações clássicas. / [en] A turbulent nonpremixed ethanol spray flame is characterized through experiments using laser diagnostics. The spray burner has been designed to generate a stable flame with the use of a bluff body. The experiments include spatially-resolved measurements of visualization of droplets distribution (Mie scattering), OH fluorescence intensity, which indicates the reaction zone (Planar laser-induced fluorescence, PLIF) and mean air-flow velocity (Particle imaging velocimetry, PIV). Initially, water sprays results are compared corresponding to different flow rates, showing two asymmetric maximum intensity zones of Mie scattering, which are found near the nozzle at jet velocities. For low flow velocity, coalescence of droplets occurs in the central zone. Then two combustion regimes have been studied, using OH PLIF that corresponds to two different fuel flow rates. The instantaneous and average structure of turbulent flames, allowed identifying local extinction regions. Combined Mie scattering/PLIF results allowed determining, in the case of smallest fuel flow rates, that the spray is surrounded by the flame, which represents a classical situation for the development of combustion models for turbulent flames. In the case of larger flow rate, discrepancies from the classical behavior were observed, since droplets interact strongly with the combustion process.

O foco principal do presente trabalho é estender uma metodologia numérica embasada no uso de uma técnica de refinamento adaptativo de malha (AMR - Adaptive Mesh Refinement) e no uso de esquemas temporais multipasso implícitos-explícitos (IMEX) a aplicações envolvendo escoamentos reativos com baixo número de Mach. Originalmente desenvolvida para escoamentos incompressíveis, a formulação euleriana daquela metodologia emprega as equações de Navier-Stokes como modelo matemático para descrever a dinâmica do escoamento e o Método da Projeção, baseado no divergente nulo da velocidade do escoamento, para tratar o acoplamento pressão-velocidade presente na formulação com variáveis primitivas. Tal formulação euleriana original é estendida para acomodar novas equações agregadas ao modelo matemático da fase contínua: conservação de massa, fração de mistura (para representar as concentrações de combustível e oxidante), e energia. Além disso, uma equação termodinâmica de estado é integrada ao modelo matemático estendido e é empregada juntamente com a equação de conservação de massa para produzir uma nova restrição (não nula desta vez) ao divergente do campo de velocidade. Assume-se que o escoamento ocorre a baixo número de Mach (hipótese principal). O Método de Diferença Finita é empregado na discretização espacial das variáveis eulerianas de estado, empregando-se uma malha AMR. As vantagens e dificuldades desta extensão são cuidadosamente investigadas e reportadas. Pela importância, do ponto de vista de aplicações práticas, alguns estudos numéricos preliminares envolvendo escoamentos incompressíveis turbulentos com sprays são realizados (as gotículas compõem a fase dispersa). Num primeiro momento, apenas sprays com gotículas inertes são considerados. Embora ainda apenas iniciais, tais estudos já se mostram importantes pois identificam com clareza, em primeira instância, algumas das dificuldades inerentes a serem enfrentadas ao se tratar dentro desta nova metodologia um conjunto relativamente grande de gotículas lagrangianas. No caso de escoamentos incompressíveis turbulentos com sprays, a integração temporal se dá com métodos IMEX para a fase contínua e com o Método de Euler Modificado para a fase dispersa. A turbulência, em todos os casos que a envolvem, é tratada pelo modelo de Simulação das Grandes Escalas (LES - Large Eddy Simulation). As simulações computacionais se dão em um domínio tridimensional, um parelelepípedo, e empregam uma extensão (resultante do presente trabalho) do código AMR3D, um programa de computador sequencial implementado em Fortran90, oriundo de uma colaboração de longa data entre o IME-USP e o MFLab/FEMEC-UFU (Laboratório de Dinâmica de Fluidos da Universidade Federal de Uberlândia). O processamento foi efetuado no LabMAP (Laboratório da Matemática Aplicada do IME-USP). / It is the main goal of the present work to extend a numerical methodology based on both the use of an adaptive mesh refinement technique (AMR) and the use of a multistep, implicit-explicit time-step strategy (IMEX) to applications involving low Mach number reactive flows. Originally developed for incompressible flows, the Eulerian formulation of that methodology employs the Navier-Stokes equations to model the flow dynamics and the Projection Method, based on the vanishing divergence of the velocity field, to tackle the pressure-velocity coupling present when using primitive variables. That Eulerian formulation is extended by adding a new set of equations to the original mathematical model, describing the various properties of the continuous phase: mass conservation, mixture fraction (to represent concentrations of fuel and oxidizer) and energy. Also, a thermodynamic equation of state is included into the extended mathematical model which is employed, along with the equation for the conservation of mass, to derive a new restriction (this time, different from zero) to the divergence of the velocity field. It is assumed that one is dealing with a low Mach number flow (the main hipothesis). The discretization in space employs the Finite Difference Method for the Eulerian variables on a AMR mesh. Advantages and difficulties of such an extension of the previous methodology are carefully investigated and reported. For its importance in the real-world applications, few preliminary numerical studies involving incompressible turbulent flows with sprays are performed (the droplets form what it is called the dispersed phase). Only sprays formed by inert droplets are considered. Even though initial yet, such studies are most important because they clearly identify, first hand, certain difficulties in handling relatively large sets of Lagrangian droplets in the context of this new AMR methodology. In the context of turbulent incompressible flows with sprays, the overall time-step scheme is given by IMEX methods for the continuous phase and by the Improved Euler Method for the dispersed phase. In all the cases in which it is considered, turbulence is modeled by the Large Eddy Simulation (LES) model. The computational simulations are held in a tridimensional domain given by a paralellepiped and all of them employ the extention (resulting of the present work) of the AMR3D code, a sequencial computer program implemented in Fortran90, whose origin is the collaborative work between IMEUSP and MFLab/FEMEC-UFU (Fluid Dynamics Laboratory, Federal University of Uberlândia). Computations were performed at LabMAP (Applied Mathematics Laboratory at IME-USP).

A busca por uma nova matriz energética intensificou significativamente o desenvolvimento de fontes renováveis de energia. Dentre essas, o álcool teve grande destaque, dado que sua utilização industrial já é intensa no Brasil. Em concordância com a tendência industrial, o comportamento do álcool em vários fenômenos físico-químicos tornou-se foco de intensa pesquisa e desenvolvimento, o que levou a parcerias entre faculdades e empresas. O presente trabalho objetiva ao desenvolvimento de simulações numéricas que consigam descrever com exatidão o comportamento de chamas de sprays turbulentos de etanol. Para a verificação da aderência da simulação a processos reais, serão utilizados dados experimentais. Esses dados serviram como base comparativa de validação do modelo numérico. A descrição do spray segue a abordagem Euler-Lagrange, na qual a fase líquida segue uma abordagem lagrangeana e a fase gasosa uma abordagem euleriana. Foi considerado o acoplamento completo entre as duas fases, considerando o impacto da fase líquida no regime turbulento da fase gasosa. A turbulência da fase gasosa é descrita pelo modelo RSM (Reynolds Stress Model), baseado na média temporal e decomposição da turbulência, enquanto o escoamento interno da fase líquida é negligenciado. A chama do spray caracterizada pelo modelo flamelet, o qual utilizará um mecanismo de cinética química reduzido da oxidação do etanol. / The research for a new energy source to substitute fossil fuels intensified the development of renewable energy sources. Among all, ethanol has been given great attention, once it has been widely used in many Brazilian industrial branches. In agreement with industrial trends, the behavior of ethanol became the center of innumerous researches and developments, which led to partnerships between academic and private companies. This thesis aims the development of numerical simulations that describe with accurately the behavior of ethanol turbulent spray flames. To verify correlation between model results and real process behavior, experimental data will be used for validation of the numerical model. The spray physical description is made through the use of the Euler- Lagrange approach, in which the liquid phase is described by the lagrangean approach and the gaseous phase is described by the eulerian approach. There is full coupling between both phases. The turbulent flow of the gaseous phase is modeled through the use of the Reynolds Stress Model (RSM), based on Reynolds decomposition and time averaging process. The liquid phase has no internal flow, the droplets behave as a solid sphere translating through the domain (gaseous phase). The spray flame is modeled by the flamelet model which will use a simplified chemical mechanism.

A busca por uma nova matriz energética intensificou significativamente o desenvolvimento de fontes renováveis de energia. Dentre essas, o álcool teve grande destaque, dado que sua utilização industrial já é intensa no Brasil. Em concordância com a tendência industrial, o comportamento do álcool em vários fenômenos físico-químicos tornou-se foco de intensa pesquisa e desenvolvimento, o que levou a parcerias entre faculdades e empresas. O presente trabalho objetiva ao desenvolvimento de simulações numéricas que consigam descrever com exatidão o comportamento de chamas de sprays turbulentos de etanol. Para a verificação da aderência da simulação a processos reais, serão utilizados dados experimentais. Esses dados serviram como base comparativa de validação do modelo numérico. A descrição do spray segue a abordagem Euler-Lagrange, na qual a fase líquida segue uma abordagem lagrangeana e a fase gasosa uma abordagem euleriana. Foi considerado o acoplamento completo entre as duas fases, considerando o impacto da fase líquida no regime turbulento da fase gasosa. A turbulência da fase gasosa é descrita pelo modelo RSM (Reynolds Stress Model), baseado na média temporal e decomposição da turbulência, enquanto o escoamento interno da fase líquida é negligenciado. A chama do spray caracterizada pelo modelo flamelet, o qual utilizará um mecanismo de cinética química reduzido da oxidação do etanol. / The research for a new energy source to substitute fossil fuels intensified the development of renewable energy sources. Among all, ethanol has been given great attention, once it has been widely used in many Brazilian industrial branches. In agreement with industrial trends, the behavior of ethanol became the center of innumerous researches and developments, which led to partnerships between academic and private companies. This thesis aims the development of numerical simulations that describe with accurately the behavior of ethanol turbulent spray flames. To verify correlation between model results and real process behavior, experimental data will be used for validation of the numerical model. The spray physical description is made through the use of the Euler- Lagrange approach, in which the liquid phase is described by the lagrangean approach and the gaseous phase is described by the eulerian approach. There is full coupling between both phases. The turbulent flow of the gaseous phase is modeled through the use of the Reynolds Stress Model (RSM), based on Reynolds decomposition and time averaging process. The liquid phase has no internal flow, the droplets behave as a solid sphere translating through the domain (gaseous phase). The spray flame is modeled by the flamelet model which will use a simplified chemical mechanism.

O foco principal do presente trabalho é estender uma metodologia numérica embasada no uso de uma técnica de refinamento adaptativo de malha (AMR - Adaptive Mesh Refinement) e no uso de esquemas temporais multipasso implícitos-explícitos (IMEX) a aplicações envolvendo escoamentos reativos com baixo número de Mach. Originalmente desenvolvida para escoamentos incompressíveis, a formulação euleriana daquela metodologia emprega as equações de Navier-Stokes como modelo matemático para descrever a dinâmica do escoamento e o Método da Projeção, baseado no divergente nulo da velocidade do escoamento, para tratar o acoplamento pressão-velocidade presente na formulação com variáveis primitivas. Tal formulação euleriana original é estendida para acomodar novas equações agregadas ao modelo matemático da fase contínua: conservação de massa, fração de mistura (para representar as concentrações de combustível e oxidante), e energia. Além disso, uma equação termodinâmica de estado é integrada ao modelo matemático estendido e é empregada juntamente com a equação de conservação de massa para produzir uma nova restrição (não nula desta vez) ao divergente do campo de velocidade. Assume-se que o escoamento ocorre a baixo número de Mach (hipótese principal). O Método de Diferença Finita é empregado na discretização espacial das variáveis eulerianas de estado, empregando-se uma malha AMR. As vantagens e dificuldades desta extensão são cuidadosamente investigadas e reportadas. Pela importância, do ponto de vista de aplicações práticas, alguns estudos numéricos preliminares envolvendo escoamentos incompressíveis turbulentos com sprays são realizados (as gotículas compõem a fase dispersa). Num primeiro momento, apenas sprays com gotículas inertes são considerados. Embora ainda apenas iniciais, tais estudos já se mostram importantes pois identificam com clareza, em primeira instância, algumas das dificuldades inerentes a serem enfrentadas ao se tratar dentro desta nova metodologia um conjunto relativamente grande de gotículas lagrangianas. No caso de escoamentos incompressíveis turbulentos com sprays, a integração temporal se dá com métodos IMEX para a fase contínua e com o Método de Euler Modificado para a fase dispersa. A turbulência, em todos os casos que a envolvem, é tratada pelo modelo de Simulação das Grandes Escalas (LES - Large Eddy Simulation). As simulações computacionais se dão em um domínio tridimensional, um parelelepípedo, e empregam uma extensão (resultante do presente trabalho) do código AMR3D, um programa de computador sequencial implementado em Fortran90, oriundo de uma colaboração de longa data entre o IME-USP e o MFLab/FEMEC-UFU (Laboratório de Dinâmica de Fluidos da Universidade Federal de Uberlândia). O processamento foi efetuado no LabMAP (Laboratório da Matemática Aplicada do IME-USP). / It is the main goal of the present work to extend a numerical methodology based on both the use of an adaptive mesh refinement technique (AMR) and the use of a multistep, implicit-explicit time-step strategy (IMEX) to applications involving low Mach number reactive flows. Originally developed for incompressible flows, the Eulerian formulation of that methodology employs the Navier-Stokes equations to model the flow dynamics and the Projection Method, based on the vanishing divergence of the velocity field, to tackle the pressure-velocity coupling present when using primitive variables. That Eulerian formulation is extended by adding a new set of equations to the original mathematical model, describing the various properties of the continuous phase: mass conservation, mixture fraction (to represent concentrations of fuel and oxidizer) and energy. Also, a thermodynamic equation of state is included into the extended mathematical model which is employed, along with the equation for the conservation of mass, to derive a new restriction (this time, different from zero) to the divergence of the velocity field. It is assumed that one is dealing with a low Mach number flow (the main hipothesis). The discretization in space employs the Finite Difference Method for the Eulerian variables on a AMR mesh. Advantages and difficulties of such an extension of the previous methodology are carefully investigated and reported. For its importance in the real-world applications, few preliminary numerical studies involving incompressible turbulent flows with sprays are performed (the droplets form what it is called the dispersed phase). Only sprays formed by inert droplets are considered. Even though initial yet, such studies are most important because they clearly identify, first hand, certain difficulties in handling relatively large sets of Lagrangian droplets in the context of this new AMR methodology. In the context of turbulent incompressible flows with sprays, the overall time-step scheme is given by IMEX methods for the continuous phase and by the Improved Euler Method for the dispersed phase. In all the cases in which it is considered, turbulence is modeled by the Large Eddy Simulation (LES) model. The computational simulations are held in a tridimensional domain given by a paralellepiped and all of them employ the extention (resulting of the present work) of the AMR3D code, a sequencial computer program implemented in Fortran90, whose origin is the collaborative work between IMEUSP and MFLab/FEMEC-UFU (Fluid Dynamics Laboratory, Federal University of Uberlândia). Computations were performed at LabMAP (Applied Mathematics Laboratory at IME-USP).