L’engouement pour l’originalité des propriétés physiques des nanoparticules s’est accompagné d’un développement de nombreuses méthodes de synthèse depuis un demi siècle. Parmi elles, l’ablation laser en liquide permet de produire des nanoparticules avec des surfaces libres de tout contaminant et ce pour une multitude de combinaisons de matériaux et de solvants. Cependant, la simplicité apparente de cette technique dissimule la complexité des mécanismes physico-chimiques, ce qui entraîne actuellement un manque de contrôle des objets synthétisés. Tout d’abord, nous nous sommes intéressés à la cinétique des bulles pour laquelle les conditionsextrêmes d’ablation laser en liquide présentent des cas originaux de cinétique dans le domaine de la mécanique des fluides. Puis, ce travail de thèse vise à donner de plus amples perspectives quant à une meilleure maîtrise de la structure cristalline des nanoparticules et de la stabilité colloïdale. Une manière plus directe de contrôler la taille, la phase cristalline et la stabilité colloïdale des solutions contenant des nanoparticules est d’ajouter des ligands. Nous avons donc étudié les mécanismes de stabilisation de ces solutions en utilisant des ions qui se complexent aux nanoparticules d’or. Nous avons aussi réussi à synthétiser des nanoparticules de rubis (alumine dopée chrome). La stabilisation de ces nanoparticules dans une phase métastable en utilisant des ligands organiques a été expliquée par une étude théorique / Laser generation of nanoparticles in liquids : new insights on crystal structure control and colloidal stability The great interest of nanoparticles for their original physical and an chemical properties has been supported by the development of numerous methods of synthesis. In the nineties, laser generation of nanoparticles in liquids appeared, including Pulsed Laser Ablation in Liquids (PLAL). The PLAL technique enables to produce surface free particles for plenty of material and solvent combinations. However, the apparent simplicity of its implementation hides complex physico-chemical mechanisms resulting in a lack of control of the final products. We firstly investigated the dynamics of the laser-generated bubbles for which the PLAL extreme conditions present new studied cases of bubbles dynamics not encountered in the field of fluid mechanics. Then, we aim to bring new insights into better control of the nanoparticles morphology and their colloidal stability. A straight way to tune sizes, crystal structures and the colloidal stability consists in the addition of stabilizing agents. Hence, we investigated the mechanisms of stabilization of colloidal gold using complexing ions. We also succeed to synthesis nano-rubies, i.e. chromium doped corundum alumina nanoparticles, unexpected at nanoscale. The stabilization of the metastable crystal structure using ligands is explained thanks to a comprehensive theoretical approach

Characterization of local boiling trends, in addition to the typically reported area-averaged trends, is essential for the robust design and implementation of phase change technologies to sensitive heat transfer applications such as electronics cooling. Obtaining the values of heat fluxes corresponding to locally varying surface temperatures has been a challenge limiting most investigations to area-averaged results. This thesis illustrates the importance of a spatially local heat transfer analysis during boiling. Pool and submerged jet impingement boiling scenarios on a silicon surface are considered at the macroscale (27.5 mm heater with multiple nucleation sites) and microscale (1000 ��m heater for isolated bubble generation), by the use of two thin film serpentine heater geometries. The macroscale heater highlights the effect of spatial variations in imposed heat flux on boiling heat transfer with a circumferentially uniform but radially non-uniform heat flux distribution. The microscale heater simulates a local hot-spot for spot cooling on an electronic device. Spatial variation in boiling heat transfer and bubble dynamics with and without a jet flow are documented using thin film voltage sensors along with qualitative and quantitative high speed imaging and infra-red thermography. Unique to this study is the documentation of local boiling curves for different radial locations on the heat transfer surface and their comparison with the corresponding area-averaged representations. It is shown here that sectionally averaged representations of boiling curves over regions of like-imposed heat flux can substantially simplify the interpretation of data while retaining important information of the local variations in heat transfer. The radial influence of the convective jet flow on the bubble dynamics and boiling heat transfer is assessed for a single circular submerged jet configuration. Varied parameters include jet exit Reynolds numbers, nozzle geometry, test fluid (deionized water and FC-72), fluid subcooling and the supplied heat flux. Distinct modifications of the surface temperature distribution imposed by the impinging jet flow are highlighted by comparing radial temperature profiles during pool and jet impingement boiling. It is demonstrated that in contrast with pool boiling, thermal overshoots during jet impingement boiling for a highly wetting fluid like FC-72 are highest in regions farthest from the impingement point. The effect of jet inertia on bubble departure characteristics are compared with pool boiling under subcooled conditions for FC-72. Qualitative high speed visualization indicates the presence of two modes of bubble generation during jet impingement boiling (a) bubble departure from the surface and (b) bubble separation from the source resulting in sliding bubbles over the surface. The effect of jet flow on bubble entrainment is depicted. Quantitative results indicate that in general departure diameters for pool and jet impingement boiling increase and plateau at a maximum value with increasing power input while no notable trends were observed in the corresponding departure frequencies. The largest departure diameters for jet impingement boiling at fixed fluid subcoolings of 10��C and 20��C were found to be smaller than that for the corresponding pool boiling test by a factor of 1.6 and 2.3, respectively. / Graduation date: 2013

Carbon dioxide hydrates were synthesized in a 0.10m I.D. and 1.22m tall bubble column equipped with a cooling jacket for heat removal. Visual observations at different driving forces (pressures between 2.75 and 3.60 MPa and temperatures between 0 and 8ºC) were recorded with a digital camera through a sight glass of 118.8 by 15.6 mm. The superficial gas velocity was varied from 20 to 50 mm/s to attain different levels of turbulence in the liquid. The growth rate was found to be dependent on the sequence/method used to reach the operating temperature and pressure. A greater supersaturation was obtained when the system temperature and pressure were reached with very low or no bubble-induced mixing. As a result, hydrates nucleated and grew immediately when starting the gas flow with the reactor volume being quickly filled with hydrates. Moreover, the hydrate growth rate and solution final density were higher when operating conditions partially condensed CO2 resulting in greater interphase mass transfer rates. In parallel, since hydrate formation is an exothermic process and the reaction is often limited by the rate of heat removal, heat transfer measurements were achieved in a simulated hydrate environment. The instantaneous heat transfer coefficient and related statistics gave insight on the role of bubbles on heat transfer and hydrodynamics.

Carbon dioxide hydrates were synthesized in a 0.10m I.D. and 1.22m tall bubble column equipped with a cooling jacket for heat removal. Visual observations at different driving forces (pressures between 2.75 and 3.60 MPa and temperatures between 0 and 8ºC) were recorded with a digital camera through a sight glass of 118.8 by 15.6 mm. The superficial gas velocity was varied from 20 to 50 mm/s to attain different levels of turbulence in the liquid. The growth rate was found to be dependent on the sequence/method used to reach the operating temperature and pressure. A greater supersaturation was obtained when the system temperature and pressure were reached with very low or no bubble-induced mixing. As a result, hydrates nucleated and grew immediately when starting the gas flow with the reactor volume being quickly filled with hydrates. Moreover, the hydrate growth rate and solution final density were higher when operating conditions partially condensed CO2 resulting in greater interphase mass transfer rates. In parallel, since hydrate formation is an exothermic process and the reaction is often limited by the rate of heat removal, heat transfer measurements were achieved in a simulated hydrate environment. The instantaneous heat transfer coefficient and related statistics gave insight on the role of bubbles on heat transfer and hydrodynamics.

Carbon dioxide hydrates were synthesized in a 0.10m I.D. and 1.22m tall bubble column equipped with a cooling jacket for heat removal. Visual observations at different driving forces (pressures between 2.75 and 3.60 MPa and temperatures between 0 and 8ºC) were recorded with a digital camera through a sight glass of 118.8 by 15.6 mm. The superficial gas velocity was varied from 20 to 50 mm/s to attain different levels of turbulence in the liquid. The growth rate was found to be dependent on the sequence/method used to reach the operating temperature and pressure. A greater supersaturation was obtained when the system temperature and pressure were reached with very low or no bubble-induced mixing. As a result, hydrates nucleated and grew immediately when starting the gas flow with the reactor volume being quickly filled with hydrates. Moreover, the hydrate growth rate and solution final density were higher when operating conditions partially condensed CO2 resulting in greater interphase mass transfer rates. In parallel, since hydrate formation is an exothermic process and the reaction is often limited by the rate of heat removal, heat transfer measurements were achieved in a simulated hydrate environment. The instantaneous heat transfer coefficient and related statistics gave insight on the role of bubbles on heat transfer and hydrodynamics.

Conselho Nacional de Desenvolvimento Científico e Tecnológico / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Fundação de Amparo a Pesquisa do Estado de Minas Gerais / Petrobrás / Em estudos de escoamentos do tipo colunas de bolhas o uso de ferramentas computacionais desempenham um importante papel por permitir que, através de experimentos virtuais, a comunidade científica agregue mais conhecimento sobre as forças que atuam nesse tipo de escoamento. Considerando a confiabilidade dos resultados, torna-se possível o aperfeiçoamento de modelos constitutivos de forças fluidodinâmicas como de arrasto, sustentação e massa virtual. O código AMR3D trabalha com malhas de refinamento localizado bloco-estruturadas que se adaptam dinamicamente às características de um dado escoamento. Utiliza-se um método de projeção para o acoplamento pressão-velocidade e o método do Volume de Fluido (VOF) para a representação das fases e interfaces do escoamento bifásico. As equações do balanço da quantidade de movimento linear são discretizadas no tempo por um esquema semi-implícito de segunda ordem (IMEX). A presente tese contribuiu com o código com a implementação de um novo critério de refinamento que captura as estruturas turbilhonares do escoamento e com um algoritmo para identificação de bolhas para que seja possível a realização de operações sobre uma mesma bolha em meio à uma população. O código é verificado e validado com problemas da literatura. Resultados de simulações de bolhas ascendentes isoladas em meio quiescente são apresentados e comparados com o diagrama de Grace indicando boa concordância. Análises de forças de resistência fluidodinâmicas atuantes nas bolhas foram apresentadas e coeficientes de arrasto, sustentação e massa virtual calculados. Através de comparações com correlações da literatura, foi possível observar concordância dos resultados. Simulações da interação entre duas bolhas de volumes conhecidos são realizadas e comparações são realizadas com resultados da literatura. Em um dos casos, o algorítimo de identificação é verificado ao rastrear consistentemente os volumes e centroides das bolhas. Simulações com população de bolhas foram apresentadas e, através do algoritmo de identificação, estima-se a força de resistência fluidodinâmica total que age em uma bolha em meio à população. Conclui-se que, com base nos resultados, que o código AMR3D, com as implementações realizadas neste trabalho, possui potencial para a investigação de escoamentos borbulhantes. / In bubble dynamics studies computational tools plays an important role as it allows, through virtual experimentations, the increase of knowledge about forces that act in this kind of flow. Considering results reliability, it is common to observe constitutive force models improvement, as in drag, lift and virtual mass correlations. The AMR3D code, developed in the Fluid Mechanics Laboratory - MFlab, uses locally block-structured meshes that dynamically adapts itself to characteristics of a given flow. A projection method is used for the pressure-velocity coupling, and the Volume of Fluid - VOF method for the phases representation, including interface dynamics. The equations of linear momentum balance are discretized with a second order semi-implicit scheme (IMEX). The present thesis contributes to the code with the implementation of a new refinement criteria that captures the main structures of the flow, and an identification algorithm that allows users to perform operations on a single bubble between a swarm of bubbles. The present code is verified and validated against well known literature problems. Results of isolated ascending bubbles in a quiescent media are presented and compared with Grace’s diagram, showing good agreement. Fluid-dynamic forces are computed and analyzed, as well as fluid-dynamics forces coefficients for drag, lift and virtual mass, showing good agreement with literature. Simulations of a pair of bubbles interacting were presented and comparisons were realized against results of literature, showing good proximity. In one of the cases, the bubble identification algorithm was tested as it calculates bubbles known volumes and centroid positions consistently. Bubble swarm simulations were presented, and the identification algorithm were also applied to track a bubble as it ascend though the domain to expose the fluid-dynamics forces acting on it. With the presented results, one can conclude that the AMR3D code has a potential for investigative approaches in bubbly flows. / Tese (Doutorado)

A cavitação e a dinâmica de bolhas são tópicos bastante recorrentes na literatura, devido sobretudo a seus efeitos em diversos tipos de fenômenos, como transferência de calor e escoamento em tubos. Considerando fases líquidas, sabe-se que estas estruturas de cavidade estão normalmente associadas ao equilíbrio metaestável, alcançado devido a quedas locais de pressão ou ao superaquecimento de uma substância pura (ou quase). Nestes casos é necessária a inicialização da mudança de fase através de algum mecanismo adequado, o qual gera uma sequência rápida de fenômenos. Apesar de comumente associado a danos, recentemente vários estudos vêm mostrando aplicações práticas deste tema, além de um campo ainda pouco explorado, que é o das macro cavidades. Essas cavidades podem ser geradas através do aquecimento de água a baixa pressão, sob condições específicas, criando sequências explosivas e formando movimentos como pistão para a água no interior de um invólucro convenientemente dimensionado. Este fenômeno mostra-se semelhante em diversos aspectos às micro cavidades, mais especificamente às cavidades próximas a superfícies livres, embora, sem dúvida, em escala muito maior. Os aspectos mencionados foram filmados com câmeras de alta velocidade e as características observadas foram comparadas com aquelas observadas em micro escala. Vários testes foram desenvolvidos de forma a melhor entender a dinâmica da formação e colapso dessas estruturas, sobretudo levando em conta um comportamento mais unidimensional para a evolução da bolha. Através de várias aproximações e análise de diferentes hipóteses para a variação de pressão e para a força de resistência, soluções analíticas e numéricas foram obtidas para a força exercida no fundo do contêiner e para a expansão e colapso das bolhas ao longo do tempo. As soluções propostas, em comparação com os dados experimentais, mostraram boa concordância entre si, sugerindo que os aspectos fundamentais da dinâmica da cavidade foram devidamente considerados e quantificados. / Cavitation and bubble dynamics are fairly recurring topics in literature, mostly due to their effects in various types of phenomena such as heat transfer and flow in pipes. Considering liquid phases, it is known that these cavity structures are normally associated with the metastable equilibrium, reached due to local pressure drop or overheating of a pure substance (or nearly so). In these cases, the phase change require a startup via some appropriate mechanism, which generates a fast sequence of phenomena. Although commonly associated with damage, recently several studies shown practical applications of these topics, and a still little explored field emerged, which is the field of macro cavities. These cavities can be generated by heating water at a low pressure, under specific conditions, creating an explosive sequences and forming piston like movements for the water inside a properly scaled casing. This phenomenon appears to be similar in many aspects to micro cavities, more specifically for cavities near free surfaces, although, without doubt, on a much larger scale. The mentioned aspects were filmed with high-speed cameras and the main features were compared with those observed in micro scale. Several tests have been developed to better understand the dynamics of the formation and collapse of these structures, especially taking into account a more one-dimensional behavior to the evolution of the bubble. Through various approximations, and analysis of different assumptions for the variation of pressure and the resistance force, analytical and numerical solutions were obtained for the force exerted on the bottom of the container and the expansion and collapse of bubbles over time. The proposed solutions in comparison with experimental data showed good agreement between each other suggesting that the fundamental aspects of the dynamics of the cavity were properly considered and quantified.

No description available.

L’ébullition est un mode de transfert de chaleur très efficace utilisé dans de nombreux systèmes technologiques comme les centrales nucléaires ou refroidissement de micro-électronique. La prédiction des échanges thermiques par ébullition reste actuellement très délicate, en raison de la complexité du phénomène, malgré des décennies de recherche sur le sujet. Le coefficient de transfert thermique est intimement lié à la dynamique de bulles (nucléation des bulles, croissance et détachement) ainsi qu’à des facteurs tels la densité de sites de nucléation ou les interactions entre bulles voisines et successives. La présente étude porte sur l’ébullition saturée sur un site de nucléation artificiel unique (ou deux sites voisins) sur une paroi en cuivre poli. La dynamique de croissance des bulles a été caractérisée pour différentes surchauffes de paroi et une loi expérimentale de croissance a été établie. Les interactions entre bulles successives issues du même site ont été étudiées, montrant qu’elles peuvent provoquer des oscillations de la bulle en croissance. Les forces agissant sur une bulle en croissance ont été clairement définies, et un bilan de quantité de mouvement a été réalisé à tous les stades de la croissance d’une bulle. La courbure le long de l’interface a été mesurée, ce qui a permis de mieux saisir le mécanisme de détachement de la bulle. L’ascension d’une bulle après son détachement a été analysée, et la vitesse maximale atteinte avant un changement de direction a été estimée et comparée aux modèles existants dans la littérature. L’interaction entre bulles croissant côte à côte a été étudiée. La génération et la propagation d’une onde lors de la coalescence a été mise en évidence. Dans le contexte de travaux de recherche sur des techniques d’intensification des échanges thermiques, cette étude se penche particulièrement sur l’intensification par électrohydrodynamique. Des expériences d’ébullition ont été réalisées en présence de champs électriques, et leurs effets sur les transferts thermiques et sur la dynamique des bulles ont été analysés. Bien que le volume au détachement des bulles et la relation entre la fréquence et la surchauffe reste inchangées, la courbe de croissance des bulles est modifiée. Les bulles sont allongées dans la direction du champ électrique, et cette élongation a été estimée et comparée à d’autres résultats de la littérature. La vitesse d’ascension des bulles est réduite en présence de champs électriques, et les interactions de bulles voisines sont modifiées: il s’avère qu’en présence de champs électriques les bulles ont tendance à se repousser. Ces résultats, obtenus dans un environnement parfaitement contrôlé apportent la preuve que la présence de champs électriques modifie la dynamique des bulles et par conséquents les transferts thermiques associés. / Since boiling heat transfer affords a very effective means to transfer heat, it is implemented in numerous technologies and industries ranging from large power generation plants to micro-electronic thermal management. Although having been a subject of research for several decades, an accurate prediction of boiling heat transfer is still challenging due to the complexity of the coupled mechanisms involved. It appears that the boiling heat transfer coefficient is intimately related to bubble dynamics (i.e. bubble nucleation, growth and detachment) as well as factors such as nucleation site density and interaction between neighbouring and successive bubbles. In order to contribute to the understanding of the boiling phenomenon, an experimental investigation of saturated pool boiling from a single or two neighbouring artificial nucleation sites on a polished copper surface has been performed. The bubble growth dynamics has been characterized for different wall superheats and a experimental growth law has been established. The interaction between successive bubbles from the same nucleation site has been studied, showing the bubble shape oscillations that can be caused by these interactions. The forces acting on a growing bubble has been reviewed, and a complete momentum balance has been made for all stages of bubble growth. The curvature along the interface has been measured, and indications concerning the mechanism of bubble detachment have been suggested. The rise of bubble after detachment has been investigated, and the maximum velocity reached before a change of direction has been estimated and compared to existing models from the literature. The interaction between bubbles growing side by side has been studied: the generation and propagation of a wave front during the coalescence of two bubbles has been highlighted. As boiling heat transfer enhancement techniques are being imagined and developed, this study also focuses on the electrohydrodynamic enhancement technique. Boiling experiments have been performed in the presence of electric fields, and their effects on heat transfer and bubble dynamics have been characterized. Although the volume of the bubbles at detachment and the relationship between the bubble frequency and the wall superheat were not affected, the bubble growth curve was modified. The bubbles were elongated in the direction of the electric field, and this elongation was estimated and compared to other studies from the literature. The rising velocity of the bubble was reduced in the presence of electric field, and the behaviour of bubbles growing side by side was modified, the electric field causing the bubbles to repeal each other. These results, obtained in a fully controlled environment, provide compelling evidence that electric fields can be implemented to alter the bubble dynamics and subsequently heat transfer rates during boiling of dielectric fluids.

Cette thèse évalue d'abord le potentiel thermodynamique du cycle au CO2 supercritique (sc-CO2) pour une large gamme de température de source chaude et étudie son couplage aux applications nucléaires, 45.7% d’efficacité thermique étant obtenu pour un réacteur à neutrons rapides refroidi au sodium. Des simulations CFD sont réalisées sur un compresseur à échelle réduite et confrontées à une expérience, apportant des éléments de qualification. Des simulations sur un compresseur à échelle 1:1 révèlent des particularités liées à la compression du sc-CO2 au comportement gaz réel, offrant un retour d’expérience pour la conception. Dans ce cadre, une approche de cartes de performance est proposée et validée à l'aide de simulations. Enfin, une étude de la collapse d’une bulle dans le CO2 liquide au voisinage du point critique est réalisée et indique l'absence d’effet destructif de cavitation, ouvrant la voie au fonctionnement du compresseur en phase liquide, lieu optimum de l'efficacité du cycle. / This study first evaluates the thermodynamic performance of the supercritical CO2 (sc-CO2) cycle in a large range of heat source temperature, with a focus on the nuclear applications; a thermal efficiency of 45.7% is reported for a Sodium-cooled Fast Reactor. Second, CFD simulations have been performed on a small scale sc-CO2 compressor and results have been confronted positively with the experimental data. Simulation results on a real scale compressor have then revealed some particularities during the compression of a real fluid, providing feedbacks for the component design. In addition, a reliable performance maps approach has been proposed for the sc-CO2 compressor and validated using the CFD results. Finally, an investigation of bubble collapse in the liquid CO2 near the critical point has disclosed the likely absence of detrimental effects. As such, risks of cavitation damage should be low, favoring the compressor operation in the liquid region for cycle efficiency improvement.