De manière à respecter les normes européennes en matière d’émission de gaz polluants, les constructeurs automobiles pratiquent le downsizing. Cette pratique consiste en une réduction de la cylindrée des moteurs tout en maintenant un bon niveau de performance. Il s’en suit des puissances spécifiques moteur importantes, l’objectif cible étant de l’ordre de 100 kW/l. Pour de telles puissances, les températures atteintes au niveau des pistons sont élevées, ce qui pose alors le problème de leur tenue thermomécanique. Le refroidissement du piston devient donc un acteur important de la fiabilité du moteur. Le procédé le plus répandu actuellement est le refroidissement par jet d’huile. Le piston est alors refroidi par un écoulement turbulent diphasique incompressible (Air/Huile) dans un environnement mobile. Il s’agit de phénomènes physiques complexes qui sont pour l’instant mal connus. L’optimisation du refroidissement nécessitant une bonne compréhension des phénomènes physiques concernés, nous nous proposons dans ce mémoire de le modéliser au moyen du modèle 1-Fluide diphasique couplé à l’équation de l’énergie. / The present work aims at studying the cooling system used in cars engine to ensure piston thermo mechanical resistance by numerical simulation. Most of actual engines use an oil jet cooling system coupled with 'cocktail shaking' to extract heat from piston. This cooling method brings into play a two-phase incompressible turbulent flow in a mobile environment, due to motion of pistons in the cylinder. The need today for more effective cooling of pistons involves an accurate understanding of the physical mechanisms which are concerned. Modeling could be a good way to achieve it. The idea is to support the engine design process to account for advanced technologies to improve turbine or engine performances, less fuel burn and green house gases. In the present work, a numerical model dedicated to the simulation at small scale of oil/air two-phase flows and related heat transfers is proposed to characterize the cooling of engine elements under fragmented jet impact.

Le travail réalisé dans cette thèse a consisté en le développement et l'utilisation des modèles numériques pour étudier les interactions multi-échelles entre une éolienne offshore et la dynamique locale océanique et sédimentaire. Dans une première partie, les interactions entre le système couplé océan-sédiment et le sillage atmosphérique généré par une turbine éolienne offshore sont étudiées à l'aide d'un modèle numérique 2D développé au cours de la thèse et écrit en fortran. Ce modèle résout les équations de Barré-De-Saint-Venant pour l'océan et l'équation d'Exner pour le sédiment. Dans un seconde partie, le phénomène d'affouillement 3D autour d'un cylindre vertical est étudié à l'aide d'un modèle diphasique eulérien-eulérien, sedFoam, implémenté dans la boîte à outils numériques OpenFOAM. L'approche diphasique permet de tenir compte des processus de petite échelle en s'affranchissant des hypothèses classiquement faites pour la modélisation du transport sédimentaire, notamment la corrélation locale entre le flux de sédiments et la contrainte de cisaillement fluide sur le fond.Concernant l'impact du sillage atmosphérique généré par une turbine, nous avons montré que celui-ci peut générer des allées tourbillonnaires dans l'océan. La dynamique turbulente océanique est alors contrôlée par le paramètre de sillage S=Cd D/H, où D est le diamètre du sillage au point d'impact sur la surface de l'océan, Cd est le coefficient de la loi de friction quadratique entre l'océan et le fond et H la profondeur de l'océan. Une paramétrisation des flux turbulents basée sur S est proposée pour modéliser la dynamique océanique dans des modèles à plus grande échelle de type RANS (Reynolds Averaged Navier-Stokes). Les résultats montrent que la dynamique océanique a une rétro-action sur la puissance du vent disponible. Les résultats montrent également que la dynamique sédimentaire instantanée est couplée à la dynamique océanique. Cependant, les variations de l'élévation du fond marin sont faibles (mm/mois) et l'impact morphodynamique du sillage est négligeable.Concernant la simulation diphasique de l'affouillement, après une validation du modèle sur des configurations 1D et 2D, des simulations tridimensionnelles autour d'une pile cylindrique sont présentées. Dans un premier temps, une configuration sans sédiments est réalisée afin de valider la capacité du modèle de turbulence URANS (Unsteady Reynolds Averaged Navier-Stokes) développé dans ce travail de thèse à reproduire les structures tourbillonnaires responsables de l'affouillement comme le tourbillon en fer à cheval et le lâché tourbillonnaire à l'aval du cylindre. Ensuite, les premières simulations diphasiques 3D de l'affouillement autour du cylindre ont été réalisées en régime de transport de type lit-mobile. Ces simulations constitue un véritable challenge en terme calcul numérique à haute performance. La comparaison favorable des résultats de simulations avec les résultats expérimentaux de la littérature apporte la preuve de concept que l'approche diphasique est pertinente pour étudier des configurations d'écoulements complexes instationnaire et tridimensionnelle. Les résultats de simulation sont ensuite analysés pour étudier la relation entre le flux local de transport de sédiments, la valeur de la contrainte fluide sur le fond et la pente locale du lit sédimentaire. La déviation par rapport aux résultats obtenus en écoulement uniforme permet d'identifier les mécanismes prépondérant de transport associées au tourbillon en fer à cheval, à la pente de fond et aux tourbillons lâchés dans le sillage du cylindre. Les résultats obtenus montrent une sensibilité à la résolution numérique en particulier à l'aval du cylindre illustrant le besoin de réaliser des simulations des grandes échelles turbulentes diphasiques. / The work undertaken in this PhD thesis was to develop and use numerical models to investigate the multi-scale interactions between an offshore wind turbine and the local ocean and sediment dynamics. First, the interactions between the coupled ocean-sediment system and the atmospheric wake generated by an offshore wind turbine are investigated using an idealized two-dimensional model developed during this Phd thesis and written in fortran. The model integrates the shallow water equations for the ocean together with the Exner equation for the sediment bed. In a second part, the 3D scour phenomenon around a vertical cylinder in a steady current is studied using a two-phase flow eulerian-eulerian solver, sedFoam, written within the framework of the numerical toolbox OpenFOAM. The two-phase flow approach accounts for small-scale processes by avoiding the traditional assumptions made for sediment transport modeling, such as a local corre- lation between the sediment flux and the fluid bed shear stress.Regarding the atmospheric wake generated by a turbine, the results shows that its impact on the ocean’s surface can generate vortices. The resulting turbulent ocean dynamics is controlled by the wake parameter S = CdD/H, where D is the wake diameter at the impact location on the ocean surface, Cd is the quadratic friction coefficient between the ocean and the sediment and H is the oceanic layer depth. A turbulence parameterization based on S is proposed, allowing for upscaling simulations in larger scales Reynolds Averaged Navier-Stokes (RANS) models. It is shown that the ocean dynamics has an effect on the available wind power. The results also show that the instantaneous sediment dynamics is strongly coupled with the ocean one but that the overall seabed elevation variations remain small (a few millimeters/month). The morphodynamic impact of the wake is thus negligible.Concerning the two-phase flow simulation of scour, sedFoam is first validated on 1D and 2D configurations. Then, 3D simulations around a vertical cylindrical pile are presented. At first, a validation of the Unsteady Reynolds Averaged Navier-Stokes (URANS) turbulence model developed in this work is performed on a configuration without sediment. The results show that the vortices structures responsible for scouring, the Horse Shoe Vortex (HSV) and the vortex-shedding in the lee of the cylinder are correctly reproduced. Then, 3D two-phase flow simulations of the scour around a cylindrical pile have been carried out in a live-bed configuration. This work is the first attempt to model 3D scour phenomenon using the two-phase flow approach. Such simulations represent a real challenge in terms of high performance computing. The good agreement between the numerical predictions and the literature experimental results provide the proof of concept that the two-phase flow approach can be used to study complex 3D and unsteady flow configurations. The relationship between the local bed shear stress, the sediment flux and the local sediment bed slope is further investigated. The deviation of the results from a uniform flow configuration is further analyzed to identify the relevant sediment transport mechanisms associated with the HSV, the slope in the scour mark and the vortex-shedding downstream of the cylinder. Finally, the numerical results show a grid sensitivity of the morphological predictions in the lee of the cylinder that are most probably related to small-scale resolved vortical structures. This highlights the need for two-phase flow Large Eddy Simulations on this configuration in the future.

L’écoulement de Taylor-Couette entre deux cylindres concentriques (cylindre interne en rotation et cylindre externe fixe) est actuellement mis à profit au CEA pour étudier les performances d’extraction d’une colonne liquide/liquide pour le retraitement du combustible nucléaire. Ces performances étant fortement liées au mélange, il est important de le quantifier. En monophasique, les propriétés de mélange ont été étudiées dans une thèse précédente, à la fois expérimentalement et numériquement, et ont été reliées aux paramètres hydrodynamiques de l’écoulement. L’effet du nombre de Reynolds, du régime d’écoulement et de la taille des rouleaux (longueur d’onde axiale) ont notamment été prouvés. Le but de ce travail est d’étendre les précédentes études aux écoulements de Taylor-Couette diphasiques. Pour des raisons pratiques et afin de s’affranchir des phénomènes de coalescence et de rupture, des billes de PMMA de diamètres 800 µm à 3 mm sont choisies pour simuler la phase dispersée, en suspension dans une solution aqueuse de Dimethylsulfoxyde (DMSO) et de Thiocyanate de Potassium (KSCN). Le montage expérimental couple les méthodes de PIV et de PLIF afin d’obtenir en simultané les informations concernant l’hydrodynamique de l’écoulement et le mélange. Cependant la mise en place du diphasique impose un certain nombre de contraintes qui doivent être prises en compte. Bien que les deux phases soient soigneusement choisies afin d’être adaptées en indice et en densité, le recours à une deuxième chaine d’acquisition PLIF est nécessaire afin d’améliorer la qualité des mesures. Ainsi, une première voie de PLIF classique suit l’évolution au cours du temps de la concentration de Rhodamine WT, injectée au centre de la colonne au début de l’expérience. La voie supplémentaire visualise un autre fluorophore, de la Fluorescéine répartie de manière homogène dans la colonne, permettant ainsi de créer un masque dynamique des billes. Grâce à ce montage expérimental, une étude paramétrique (taille, rétention des billes) a été menée. Un double effet des billes sur le mélange a ainsi été observé. D’une part, la présence d’une phase dispersée modifie les propriétés hydrodynamiques de l’écoulement : les régimes (Couette, Taylor Vortex Flow et Wavy Vortex Flow) sont d’autant plus déstabilisés que la rétention ou la taille des billes augmente. De plus un régime supplémentaire, inhabituel dans le cas du cylindre externe fixe, apparait, forcé par la phase dispersée : le régime Spiral Vortex Flow, dans lequel le mélange est très efficace. D’autre part, une influence propre des billes sur le mélange a été mise en évidence, en fonction de leur taille et de leur concentration. Ces deux effets se combinent pour expliquer une forte augmentation du mélange en présence de la phase dispersée. Les mécanismes physiques liés à ces résultats sont ensuite discutés, et leur influence relative est comparée. Enfin, le rôle du mélange local sur le coefficient de dispersion global, paramètre classiquement utilisé en génie chimique afin de prédire les performances des colonnes d’extraction, est discuté. / In the scope of the nuclear fuel reprocessing, Taylor-Couette flows between two concentric cylinders (the inner one in rotation and the outer one at rest) are used at laboratory scale to study the performances of new liquid/liquid extraction processes. Separation performances are strongly related to the mixing efficiency, the quantification of the latter is therefore of prime importance. A previous Ph.D. work has related the mixing properties to the hydrodynamics parameters in single-phase flow, using both experimental and numerical investigations. The Reynolds number, flow state and vortices height (axial wavelength) impacts were thus highlighted. This Ph.D. work extends the previous study to two-phase configurations. For experimental simplification, and to avoid droplets coalescence or breakage, spherical solid particles of PMMA from 800 µm to 1500 µm diameter are used to model rigid droplets. These beads are suspended in an aqueous solution of dimethyl sulfoxide (DMSO) and potassium Thiocyanate (KSCN). The experimental setup uses coupled Particle Image Velocimetry (PIV) and Planar Laser-Induced Fluorescence (PLIF) to access simultaneously the hydrodynamic and the mixing properties. Although the two phases are carefully chosen to match in density and refractive index, these precautions are not sufficient to ensure a good measurement quality, and a second PLIF channel is added to increase the precision of the mixing quantification. The classical PLIF channel monitors the evolution of Rhodamine WT concentration, while the additional PLIF channel is used to map a Fluorescein dye, which is homogeneously concentrated inside the gap. This way, a dynamic mask of the bead positions can be created and used to correct the Rhodamine WT raw images. Thanks to this experimental setup, a parametric study of the particles size and concentration is achieved. A double effect of the dispersed phase is evidenced. On one hand, the particles affect the flow hydrodynamic properties : the more the particles size and concentration grows, the more the studied flow regimes (Couette, Taylor Vortex Flow and Wavy Vortex Flow) are destabilized. In addition, a new flow state appears in presence of a dispersed phase, that is unusual in the configuration we use where the outer cylinder is at rest. This Spiral Vortex Flow is characterized by an enhanced mixing. On the other hand, for given hydrodynamic properties and depending on the particles size and concentration, a specific effect of the particles on mixing is highlighted. Both the “hydrodynamic” and “intrinsic“ effects are responsible for the significant increase of the global mixing observed in two-phase configuration. Possible physical mechanisms are proposed to analyze these results, and their relative influence is compared. At last, an attempt is made to relate the local mixing properties to a global dispersion coefficient of the flow, data commonly used in chemical engineering to predict the performances of extraction columns.

Les mécanismes d'interaction entre une interface fluide et une paroi solide en situation de mouillage sont encore des problèmes ouverts. Parmi les nombreuses interrogations traitées dans la littérature, le "problème de la ligne de contact en mouvement" est très étudié depuis les années 1970, lorsque le paradoxe lié au mouvement de la ligne de contact a été identifié. En quelques mots ce paradoxe est le suivant : les modèles hydrodynamiques macroscopiques utilisant une condition de non-glissement à la paroi prédisent un cisaillement infini au niveau de la ligne de contact. Des études prometteuses pour aborder ce problème se sont appuyées sur des résultats fournis par les simulations dynamiques moléculaires. Elles confirment la présence de glissement au niveau de la ligne de contact. Malheureusement, les simulations de type dynamique moléculaire sont limitées à de très petites échelles à la fois temporelles et spatiales de sorte que les modèles hydrodynamiques et les simulations numériques des équations de Navier-Stokes restent nécessaires. Dans ce type de simulation, la méthode Continuum Surface Force pour traiter le terme capillaire entraine une vitesse et un cisaillement au niveau de la ligne de contact dépendant de la résolution, problème qui est abordé dans ce travail. L'écoulement au voisinage de la ligne de contact est analysé théoriquement dans la limite des écoulements de Stokes et l'effet des conditions limites à la paroi est exploré. Une des conditions proposées dans la littérature permet de lever la divergence du cisaillement et rend possible l'observation de tourbillons de Moffatt au voisinage de la ligne de contact ce qui reste encore à observer que ce soit expérimentalement ou numériquement. Cette possibilité est explorée de manière théorique puis numérique à l'aide du code JADIM. Sur le plan numérique, la présence de courants parasites est apparue comme limitante si la méthode VoF est utilisée. Pour remédier à cet obstacle numérique, une version très prometteuse de la méthode front-tracking utilisant des markers Lagrangien a été implémentée et améliorée pour permettre de traiter des distributions nonuniformes de markers sans perdre les performance de réduction significative des courants parasites. De nombreux tests ont été réalisés pour valider la méthode développée et montre la réduction à la précision machine des courants parasites. La méthode est également validée pour la simulation des lignes de contact statiques et dynamiques avant d'être utilisée pour l'étude de tourbillons induits par la mise en mouvement d'une ligne de contact. Finalement, s'appuyant sur les développements théoriques de ce travail, un nouveau modèle de sous maille est proposé pour permettre la simulation de lignes de contact aux échelles macroscopiques. Il est implémenté dans la nouvelle méthode front-tracking introduite dans JADIM. Les premiers résultats montrent une amélioration partielle de l'effet du maillage sur la vitesse de la ligne de contact mais une maitrise totale du cisaillement. Les simulations de l'étalement de gouttes permet de retrouver de manière très satisfaisante les résultats théoriques et expérimentaux de référence. / The exact mechanism with which a fluid interface interacts dynamically with a solid surface during wetting is still open to research. Among the many subjects addressed in this field in the literature, the "moving contact line problem" is one that has been ubiquitous since at least the 1970s, where a paradox in the description of the contact line was found to exist. The paradox in a few words is the next: macroscopic hydrodynamic models using the no-slip boundary condition will predict infinite shear stress close to the contact line. The most promising studies to tackle the problem come from information provided by molecular dynamics simulations. They have confirmed that close to the contact line, the no-slip boundary condition is relaxed to some form of slip. Unfortunately, molecular simulations are still limited to very small scales in space and time, so hydrodynamic models and numerical simulations based on Navier-Stokes equations are still needed. In these simulations, the Continuum Surface Force model CSF for the calculation of the capillary contribution introduces a grid dependent contact line velocity and shear at the wall, which is a problem we proposed to solve here. In this work, we analyze the flow close to the moving contact line in the context of corner stokes-flow and explore the effects of the boundary conditions at the wall. One of these conditions offered in the literature, provides relief to the shear divergence and also opens the possibility to observe Moffatt vortices in the vicinity of the contact line, not yet seen in experiments or numerical simulations. We explore this possibility analytically and then numerically using the code JADIM. The latter task is constrained by the contamination of the velocity field by the so-called spurious velocities if the VOF method is used. To solved this inconvenient, a very promising version of the front-tracking method with lagrangian markers is implemented and enhanced to handle non-uniform distribution of markers without losing its spurious velocities elimination features. Numerical tests are conducted to validate the implementation, spurious velocities are reduce close to machine precision and comparison to benchmark data is performed obtaining good agreement. Tests including contact lines are then compared with exact solutions for shape analyzing the effect of the Bond number, showing remarkable results. Numerical experiments with this implementation close to a contact line show the existence of vortical patterns during of spreading. Finally, and based on the theoretical background developed in this work, a new sub-grid model method is proposed for macroscopic numerical simulations and implemented in the new front-tracking method of JADIM. Quantitative data is obtained and compared to numerical and experimental spreading cases revealing improvement of grid convergence and excellent agreement.

Severe accidents arising from the fusion of a nuclear reactor core must be anticipated to enhance the efficiency of their mitigation. Such accidents have occurred at TMI-2 (1979) and Fukushima (2011). Following a loss of coolant accident, core heating and oxidation of the fuel cladding followed by reflooding (injection of water) may lead to the collapse of fuel rods and formation of porous debris bed in the core. Steam produced upon reflooding may activate the exothermic oxidation of Zircaloy leading to partial melting of materials. Such evolution generates zones with reduced porosity limiting coolant penetration and/or impermeable blocked zones. In this situation, the efficiency of injecting water into the core to stop the progress of degradation and prevent the reactor core melting may be significantly reduced. In this scope, IRSN launched PEARL program to investigate the thermal hydraulics of reflooding of hot debris beds surrounded by a more permeable zone simulating the presence of intact or less damaged zones in the core. The PEARL experiments were modeled and simulated using ICARE/CATHARE code to assess the evolution of a bottom reflooding of a superheated debris bed surrounded by a bypass of larger permeability. The thermal hydraulics of the quenching process has been analyzed and the effect of each of the initial conditions on the reflooding behavior was assessed. The effect of pressure was investigated and related to the entrainment of injected water at quench front level into the bypass. An analytical model was then developed to investigate thoroughly the reflooding of a superheated heterogeneous porous medium, composed of two layers of contrasting permeability and porosity, and to describe the water entrainment in the bypass. This model computes the main variables characterizing the reflooding process such as quench front velocity, water-to-steam conversion ratio, and the flow rate of water entrained in the bypass. It provides good qualitative and quantitative results for the two-phase flow redistribution as compared to experimental results. This model has several advantages. It is written in a rather general form including the Forchheimer correction terms and non-zero cross-terms in the generalized Darcy-Forchheimer momentum equation. Variations of proposed momentum equations including changes in correlations andinterfacial friction laws can be tested easily and efficiently. Comparison of the calculations against experimental results indicated that it is necessary to include an interfacial friction law to obtain good predictions. This model allows performing fast evaluations of the efficiency of cooling bycomputing the fraction of the injected flow rate that participates in cooling. Upscaling to the reactor scale is straightforward and calculations were performed to assess the impact of geometric parameters of the debris bed (particle size, porosity, dimensions) as well as thermal hydraulic conditions (temperature, pressure, injection flow rate) on the reflooding process. Thus the model is very useful to estimate the total quenching time and the maximum temperature that could be reached by the hot debris bed at large scales. This allows assessing the probability of a successful quenching of a hot debris bed formed during a hypothetical accidental scenario.

Made available in DSpace on 2014-06-11T19:23:39Z (GMT). No. of bitstreams: 0 Previous issue date: 2006-05-26Bitstream added on 2014-06-13T18:51:01Z : No. of bitstreams: 1 dias_jp_me_ilha.pdf: 1702007 bytes, checksum: 8fc663dc91ce05f27bfdce5cb7c35609 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Este trabalho apresenta um estudo numérico do escoamento bifásico com formação de espuma de uma mistura composta por óleo lubrificante sintético Freol a10 com pequenas quantidades de refrigerante R134a, usualmente encontrada em compressores de sistemas de refrigeração e ar condicionado. A geometria considerada é um tubo longo horizontal de seção circular constante. Em razão da queda de pressão inicialmente causada pelas forças de atrito viscoso, a solubilidade do refrigerante no óleo diminui e o refrigerante evapora da mistura líquida formando bolhas de gás que escoam com a fase líquida. A formação de bolhas de refrigerante pode ser tão intensa a ponto de o escoamento assumir a forma de espuma quando a fração de vazio atinge valores superiores à aproximadamente 70%. Um ponto importante com relação ao escoamento de espuma é que o seu comportamento é muito diferente dos escoamentos bifásicos convencionais, apresentando quedas de pressão bem superiores. Os resultados gerados para as distribuições de pressão e temperatura ao longo do escoamento são confrontados com dados experimentais para a validação do modelo numérico. Será verificado que o modelo representa satisfatoriamente os resultados experimentais para várias condições do escoamento. / This work presents a numerical study of the two-phase flow with foam formation of a mixture composed by synthetic lubricant oil Freol a10 with small amounts of refrigerant R134a, usually encountered in compressors of refrigeration and air conditioning systems. A straight horizontal tube with circular cross section was chosen to accomplish the simulation. Due to the pressure drop initially caused by the viscous friction forces, the refrigerant solubility in the oil reduces and the refrigerant evaporates from the liquid mixture (outgassing) forming gas bubbles that flow with the liquid phase. In this type of flow, the bubble formation can be so large that foam is formed as the void fraction reaches values above 70%. An important point with reference to foam is that the foam flow behavior is much different from the conventional two-phase flows, presenting larger pressure drops. Results for pressure and temperature distributions along the flow are compared with experimental data in order to validate the numerical model. It will be seen that the model represents satisfactorily the experimental results for many flow conditions.

Made available in DSpace on 2014-10-09T12:43:02Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:08:23Z (GMT). No. of bitstreams: 1 06171.pdf: 4113918 bytes, checksum: f80c43f20a8b10129dced614d40007ba (MD5) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Made available in DSpace on 2014-10-09T12:53:58Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:09:27Z (GMT). No. of bitstreams: 0 / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energéticas e Nucleares - IPEN-CNEN/SP

Orientador: José Luiz Gasche / Banca: André Luiz Seixlack / Banca: Adriano da Silva / Resumo: Este trabalho apresenta um estudo numérico do escoamento bifásico com formação de espuma de uma mistura composta por óleo lubrificante sintético Freol a10 com pequenas quantidades de refrigerante R134a, usualmente encontrada em compressores de sistemas de refrigeração e ar condicionado. A geometria considerada é um tubo longo horizontal de seção circular constante. Em razão da queda de pressão inicialmente causada pelas forças de atrito viscoso, a solubilidade do refrigerante no óleo diminui e o refrigerante evapora da mistura líquida formando bolhas de gás que escoam com a fase líquida. A formação de bolhas de refrigerante pode ser tão intensa a ponto de o escoamento assumir a forma de espuma quando a fração de vazio atinge valores superiores à aproximadamente 70%. Um ponto importante com relação ao escoamento de espuma é que o seu comportamento é muito diferente dos escoamentos bifásicos convencionais, apresentando quedas de pressão bem superiores. Os resultados gerados para as distribuições de pressão e temperatura ao longo do escoamento são confrontados com dados experimentais para a validação do modelo numérico. Será verificado que o modelo representa satisfatoriamente os resultados experimentais para várias condições do escoamento. / Abstract: This work presents a numerical study of the two-phase flow with foam formation of a mixture composed by synthetic lubricant oil Freol a10 with small amounts of refrigerant R134a, usually encountered in compressors of refrigeration and air conditioning systems. A straight horizontal tube with circular cross section was chosen to accomplish the simulation. Due to the pressure drop initially caused by the viscous friction forces, the refrigerant solubility in the oil reduces and the refrigerant evaporates from the liquid mixture (outgassing) forming gas bubbles that flow with the liquid phase. In this type of flow, the bubble formation can be so large that foam is formed as the void fraction reaches values above 70%. An important point with reference to foam is that the foam flow behavior is much different from the conventional two-phase flows, presenting larger pressure drops. Results for pressure and temperature distributions along the flow are compared with experimental data in order to validate the numerical model. It will be seen that the model represents satisfactorily the experimental results for many flow conditions. / Mestre

O estudo de fluxos bifásicos é essencial para a modelagem de diversos sistemas. Neste contexto, técnicas de processamento de imagens tem sido cada vez mais utilizadas, pois possibilitam a identificação e avaliação visual de importantes características por meio de soluções relativamente simples para projetar/implementar e que não causam distúrbios no fluxo. Este trabalho propõe uma metodologia para medir os parâmetros de interesse baseada em métodos Level Set e em transformações de morfologia matemática. Devido a sua acurácia e versatilidade, tais métodos vêm sendo amplamente considerados, sendo uma de suas principais vantagens a capacidade de tratar eficazmente mudanças topológicas e/ou descontinuidades que podem eventualmente ocorrer durante a propagação de interfaces. A partir da segmentação precisa da imagem (ou seja, da delimitação das regiões pertencentes a cada fase), os principais parâmetros relacionados à distribuição das fases para o regime de escoamento denominado golfada foram calculados. A comparação entre os parâmetros estimados e aqueles obtidos manualmente por um especialista comprova o bom desempenho da abordagem proposta. Tal segmentação também possibilita a extração de características que permitem classificar uma imagem como pertencente a um determinado regime de escoamento. Os testes de classificação, que consideram padrões de escoamento bifásico horizontal gás-líquido, também conduziram a resultados promissores, encorajando o desenvolvimento de trabalhos futuros. / The study of two-phase flows has important applications in an increasing numbers of areas, since reliable measurements of the flow characteristics are crucial for the accurate modeling of two-phase systems. In this context, image processing techniques have been extensively used, being relatively simple to design and implement while enabling the identification and visual evaluation of important features. This work proposes a methodology to measures flowing parameters based on the Level-Set method and on mathematical morphology transformations. Level Set methods have been largely considered, mainly due to its precision and capability to deal with topological changes and/or discontinuities that may arise during the interface evolution. Based on an accurate image segmentation (that is, on the region delimitation of each phase), we compute the parameters related to the phase distribution for the slug pattern. The comparison between the estimated parameters and that determined by a specialist comproves the good results of the proposed approach. This segmentation also enables the extraction of characteristics that can classify an image as belonging to a specific flow pattern. Classification tests considering four horizontal gas-liquid flow patterns also conduced to accurate measurements that encourage future works.