A presente tese envolve um estudo experimental da ebulição convectiva no interior de um dissipador de calor baseado em multi-microcanais. Resultados experimentais para perda de pressão e coeficiente de transferência de calor foram levantados para os hidrocarbonetos R600a (isobutano), R290 (propano) e R1270 (propileno), fluidos com reduzido GWP (Global Warming Potential) e ODP (Ozone Depletion Potential) nulo. O desempenho termo-hidráulico destes fluidos foi avaliado em um dissipador de calor de cobre, contendo cinquenta canais paralelos com seção transversal retangular de 123x494 µm2 , 15 mm de comprimento e área de base de 15x15 mm2. Os experimentos foram realizados para fluxos de calor de até 400 kW/m2, velocidade mássica variando entre 165 e 823 kg/m2s, graus de sub-resfriamento do líquido na entrada da seção de testes de 5, 10 e 15°C e temperaturas de saturação de 21 e 25°C. Os dados experimentais foram amplamente analisados e discutidos, focando o efeito do fluido refrigerante. Oscilações dos sinais de temperatura e pressão foram analisadas parametricamente visando caracterizar efeitos de instabilidades térmicas. Adicionalmente, realizou-se análise comparativa de desempenho dos refrigerantes baseada na 2ª Lei da Termodinâmica. Os dados para hidrocarbonetos foram comparados com resultados de trabalhos prévios para o refrigerante R134a levantados na mesma seção de testes e utilizando a mesma bancada experimental. A partir destes dados, conclui-se que os hidrocarbonetos proporcionam coeficientes de transferência de calor superiores ao R134a. Em geral, o coeficiente de transferência de calor apresenta a seguinte ordem decrescente: R290, R1270, R600a e R134a. No entanto, o R290 necessitou superaquecimentos da parede superiores ao R1270 para iniciar o processo de ebulição. O refrigerante R1270 proporcionou perdas de pressão totais inferiores aos demais fluidos segundo a seguinte ordem decrescente: R600a, R134a, R290 e R1270. O refrigerante R1270 apresentou frequências de oscilação inferiores na temperatura da câmara de saída. Baseado na análise de desempenho da 2ª Lei da Termodinâmica, conclui-se que, as irreversibilidades devido ao processo de transferência de calor foram predominantes quando comparadas àquelas devido à perda de pressão. Através desta análise também constatou-se o melhor desempenho para o refrigerante R290. / The present thesis concerns an experimental study on flow boiling inside a microchannel array. Experimental results for two-phase pressure drop and heat transfer coefficient were acquired for the hydrocarbons R600a (isobutane), R290 (propane) and R1270 (propylene). These fluids present low Global Warming Potential (GWP) and null Ozone Depletion Potential (ODP). The cooling performance of these hydrocarbons were evaluated for a copper heat sink containing fifty parallel microchannels. The microchannels are rectangular with cross section of 123x494 µm2, 15 mm length and a footprint area of 15x15 mm2. The experimental evaluation was performed in a test facility located at the Laboratory of Thermal and Fluid Engineering of School of Engineering of São Carlos, University of Sao Paulo. The experiments were performed for heat fluxes up to 400 kW/m2, mass velocities from 165 to 823 kg/m2s, degrees of liquid subcooling at the test section inlet of 5, 10 and 15°C and saturation temperatures of 21 and 25°C. The experimental data were carefully analyzed and discussed focusing on the effects of the fluid on the heat sink thermal hydraulic performance. Fluctuations in the temperature and pressure were analyzed parametrically in order to evaluate thermal instability effects. Additionally, an exergy analysis was performed to evaluate the refrigerant efficiency during convective evaporation. Subsequently, the parametric effects and performance of hydrocarbons were compared with previous results for refrigerant R134a obtained in the same test facility and under the same experimental conditions. The refrigerant R290 provided heat transfer coefficients higher than R600a and R1270. However, R290 needed a degree of wall superheating for the onset of nucleate boiling higher than R1270. Based on the exergy analysis it was concluded that, the irreversibility associated to the heat transfer process are predominant compared with the irreversibility due to the pressure drop. According to the Second Law analyses it was also concluded R290 as the fluid providing the best performance.

A presente pesquisa teve por objetivo o estudo da ebulição convectiva dos novos refrigerantes halogenados no interior de tubos horizontais de cobre. Uma busca na literatura pertinente ao assunto mostrou que a determinação do coeficiente de transferência de calor pode ser determinado por correlações que, aqui, foram classificadas como: 1- Estritamente Convectivas, 2- Superposição de Efeitos e 3- Estritamente Empíricas. Essas correlações mostraram-se inadequadas a generalizações. Uma bancada experimental foi concebida e construída, o que permitiu a obtenção de dados experimentais envolvendo uma ampla faixa de condições operacionais. Efeitos de parâmetros físicos como a velocidade mássica, fluxo de calor, temperatura de saturação e título, foram investigados. Os dados experimentais obtidos foram utilizados no desenvolvimento de uma correlação para o coeficiente de transferência de calor na ebulição convectiva que satisfizesse adequadamente esses dados obtidos para condições operacionais típicas de aplicações frigoríficas. A partir da análise efetuada foi proposto um modelo no qual foram introduzidos os adimensionais que envolvem os principais efeitos relacionados à mudança de fase. O modelo proposto apresentou resultados bastante adequados não apenas na correlação dos resultados experimentais obtidos na presente pesquisa como também em dados encontrados na literatura. / Present research has aimed at the study of convective boiling of recently introduced halocarbon refrigerants inside horizontal copper tubes. A comprehensive literature survey has revealed that the correlations for the convective boiling heat transfer coefficient can be divided into three main categories: (1) strictly convective; (2) superposition of effects; (3) strictly empirical. As a general rule these correlations are not fitted for generalizations. An experimental set up has been developed and constructed in order to raise data involving a relatively wide range of operational conditions. These data have been used to investigate effects of such parameters as heat flux, mass velocity, quality and evaporating temperature. In addition gathered data have been used in the development of a correlation for the heat transfer coefficient under convective boiling conditions typical of refrigeration applications. The model has been developed in terms of the main intervening non dimensional groups. The proposed equation correlated very well not only experimental data from present investigation but data obtained else where.

Durante um acidente em uma usina nuclear, a integridade do vaso pressurizado deve ser assegurada. Em resposta a um possível derretimento do combustível nuclear, a atual geração de usinas possui um sistema para a injeção de água potável na cavidade do vaso pressurizado com intuito de resfriar sua parede, prevenindo danos a sua estrutura e evitando o vazamento de material radioativo. Esse estudo considerou o uso de água marinha como refrigerante para inundar a cavidade do vaso pressurizado combinado com a fixação de um estrutura porosa em forma de grade em sua parede externa como meio de aprimorar a margem de segurança durante a contenção de acidentes. Experimentos de longa duração para a ebulição em piscina de água marinha artificial foram conduzidos em uma superfície circular de cobre plana com 30 mm de diâmetro. Foi encontrado um fluxo de calor crítico de 1; 6 MW/m2 sob pressão atmosférica. Esse valor é significantemente maior que aquele obtido (1; 0 MW/m2) nas mesmas condições experimentais. Foi verificado que os depósitos de sais marinhos podem aumentar a molhabilidade e a capilaridade da superfície de teste, aprimorando assim o fluxo crítico. Combinando a água marinha e a fixação da estrutura porosa sobre a superfície de teste, verificou-se um melhora no coeficiente de transmissão de calor e no fluxo de calor crítico de até 110 % (2; 1 MW/m2), quando comparado a água destilada na superfície limpa, sem a instalação da estrutura. Após os experimentos, foi identificado que muitos dos poros presentes nas superfícies da estrutura porosa encontravam-se bloqueados devido ao aglutinamento de sais marinhos. Isso levou a conclusão que o aumento no valor do fluxo crítico observado para a água marinha artificial ocorreu devido, principalmente, a separação das fases líquida e gasosa do fluido na região próxima a superfície de teste, efeito proporcionado pela forma de grade da estrutura porosa, e ao aumento da molhabilidade e capilaridade da superfície devido a formação dos depósitos marinhos. / During a severe nuclear power plant accident, the integrity of the reactor pressure vessel must be assured. In response to a possible fuel meltdown, operators of the current generation of nuclear power plants are likely to inject water into the reactor pressure vessel to cool down the reactor vessel wall, preserving its integrity and avoiding leakage of radioactive material. This study considers the use of seawater to flood a reactor pressure vessel combined with the attachment of a honeycomb porous plate (HPP) on the vessel outer wall as a way to improve the safety margins for in-vessel retention of fuel. In long-duration experiments, saturated pool boiling of artificial seawater was performed with an upward-facing plain copper heated surface 30 mm in diameter. The resulting value for critical heat flux (CHF) was 1; 6 MW/m2 at atmospheric pressure, a value significantly higher than the CHF obtained when the working fluid was distilled water (1; 0 MW/m2). It was verified that sea-salt deposits could greatly improve surface wettability and capillarity, enhancing the CHF. The combination of artificial seawater and an HPP attached to the heated surface improved the boiling heat transfer coefficient and increased the CHF up to 110% (2; 1 MW/m2) as compared to distilled water on a bare surface. After the artificial seawater experiments, most of the wall micropores of the HPP were clogged because of sea-salt aggregation on the HPP top and bottom surfaces. Thus, the CHF enhancement observed in this case was attributed mainly to the separation of liquid and vapor phases provided by the HPP channel structure and improvement of surface wettability and capillarity by sea-salt deposition.

Les écoulements diphasiques liquide-gaz et liquide-vapeur sont présents dans de nombreuses problématiques industrielles. De fait, ils sont rencontrés dans des configurations diverses, tant en termes de dimensions que d’orientation et présentent des caractéristiques variées. Pour autant, il peut être distingué dans la littérature deux cas limites de modélisation de l’écoulement diphasique liées à son confinement : l’échelle micro et l’échelle macro. Cependant, la caractérisation de la transition entre ces deux échelles reste un enjeu majeur de compréhension de l’écoulement diphasique. De plus, les comportements thermiques en ébullition convective sont impactés par le phénomène de nucléation, fortement influencé par la géométrie de l’application et les conditions de saturations. De ce fait, l’objectif principal de la thèse est de contribuer à la compréhension des écoulements diphasiques adiabatiques et non adiabatiques à une échelle intermédiaire entre les échelles micro et macro, où les comportements tant dynamiques que thermiques sont mal définis. Pour ce faire, dans le travail présent, les effets du confinement et de l’inclinaison de l’écoulement diphasique sur les régimes d’écoulement, les chutes de pression et les échanges thermiques sont étudiés et comparés avec les observations et modèles de la littérature. Une section test inclinable de 1,6 mm de diamètre intérieur a été installée sur le banc d’essais de Charnay (2014), permettant l’étude de l’ébullition convective du R-245fa. Celle-ci permet de visualiser l’écoulement à l’intérieur de l’évaporateur ainsi que d’acquérir simultanément les chutes de pression et le champ de température de paroi extérieure. Pour ce faire, un tube en saphir chauffé par un dépôt d’ITO a été employé. Le champ de température a été obtenu par une procédure d’acquisition et de post-traitement infrarouge. Au préalable, une campagne d’essais a été menée en conditions adiabatiques sur la section de Charnay (2014). La section test présente, quant à elle, a permis deux campagnes d’essais en conditions adiabatiques et en ébullition convective. Des études menées en conditions adiabatiques est apparu un fort effet de l’inclinaison et du confinement sur les régimes d’écoulement ainsi que sur les chutes de pression. L’effet de l’inclinaison s’apparente à ce qui a pu être observé en macro-canal dans la littérature. De plus, l’inclinaison n’affecte que peu les échanges thermiques au regard des incertitudes inhérentes au dispositif infrarouge. Enfin, il est observé un effet de l’inclinaison sur le régime d’assèchement. Dans cette configuration, l’évolution temporelle du champ de température de paroi extérieure apparaît synchrone avec l’évolution dynamique de l’écoulement diphasique. / Liquid-gas and liquid-vapour two-phase flows are encountered in a large range of industrial applications. They are observed in multiple configurations, in terms of dimension and orientation and then have various characteristics. In the literature, it can be distinguished two limit cases of the two-phase flow linked to its confinement: micro and macro-scale. Nonetheless, characterisation of the micro-to-macro scale transition is still a main issue for the comprehension of two-phase flow. In addition, thermal behaviours of flow boiling are affected by the nucleation phenomenon, which is strongly influenced by the geometry of the application and the saturation conditions. The main objective of the thesis is to contribute to the comprehension of adiabatic and diabatic two-phase flows in mini-scale, where the general behaviours are not well definite. In the present study, the effects of confinement and orientation of two-phase flows on flow patterns, pressure drops and heat exchanges are studied and compared with observations and models of the literature. An 1.6 mm inner diameter inclinable test section was installed on the test bench of Charnay (2014), which enabled to study R-245fa flow boiling. This test section enables to visualize the flow directly in the evaporator and the simultaneous acquisition of the pressure drops and the outside wall temperature field. A sapphire tube, heated by a transparent ITO coating, is unemployed to insure the transparency. The temperature field is obtained by an IR image processing. Previously, tests were led on the section of Charnay (2014) in adiabatic conditions. The present test section was used for two series of tests, led in both adiabatic and diabatic conditions. It appeared a strong effect of confinement and orientation on the flow patterns and pressure drops. The effect of the orientation is likely the same that one observed in macro-scale. In addition, the orientation slightly affects heat exchanges in comparison with the uncertainties of the IR dispositive. Finally, it is observed an effect of orientation on dryout flow pattern. In this configuration, the temperature field evolution with time is synchronized with the dynamic evolution of the two-phase flow.

"Electrohydrodynamic (EHD) pumping relies on the interaction between electrical and flow fields in a dielectric fluid medium. Advantages such as simple and robust design as well as negligible vibration and noise during operation make EHD conduction pumping suitable for various applications. This work investigates meso-scale EHD conduction pumping used as an active flow distribution control mechanism for thermal management systems. Two different scenarios are considered for this purpose: alteration of uniform flow distribution and flow maldistribution correction. Its capability of actively controlling the flow distribution is examined in terms of the value of applied potential for initiation of flow divergence or flow equalization and the flow rate difference between each branch. Experimental results confirm that the reverse pumping direction configuration of EHD pumping is more effective than the same pumping direction configuration. A fundamental explanation of the heterocharge layer development is provided for the effect of flow direction on EHD conduction pumping performance. This study also involves a macro-scale EHD conduction pump used as an alternative mechanism of mixing liquid within a storage tank, for example under low-g condition. A numerical analysis of a simplified model of the experimental setup is provided to illustrate the liquid mixing and thermal homogenization process. The experimental and numerical study provide fundamental understanding of liquid mixing and thermal homogenization via EHD conduction pumping. Liquid-vapor phase change phenomena are used as effective mechanisms for heat transfer enhancement and have many applications such as HVAC&R systems. With this in mind, two detailed studies in pool boiling and in-tube flow condensation are carried out. Specifically, nucleate pool boiling on nano-textured surfaces, made of alumina ceramic substrate covered by electrospun nanofiber, is experimentally investigated. Also, the role of surface roughness and orientation in pool boiling is experimentally characterized. The in-tube convective condensation of pure water in mini-channels under sub-atmospheric pressure is also experimentally explored. This study provides valuable information for the design of condensers in a vapor compression cycle of HVAC&R systems using water as the refrigerant, this process has zero global warming potential. "

Direct numerical simulations, where both the large and small scales in the flow are fully resolved, provide an excellent instrument to validate multiphase flow processes and also further our understanding of it. Three multiphase systems are studied using a finite difference/front-tracking method developed for direct numerical simulations of time-dependent system¬¬s. The purpose of these studies is to demonstrate the benefit in developing accurate sub-grid models that can be coupled with the direct numerical simulations to reduce the computational time. The primary reason to use the models is that the systems under consideration are sufficiently large that resolving the smallest scales is impractical. The processes that are examined are: (1) droplet motion and impact (2) nucleate boiling and (3) convective mass transfer. For droplet impact on solid walls and thin liquid films, the splash characteristics are studied. The collision of a fluid drop with a wall is examined and a multiscale approach is developed to compute the flow in the film between the drop and the wall. By using a semi-analytical model for the flow in the film we capture the evolution of films thinner than the grid spacing reasonably well. In the nucleate boiling simulations, the growth of a single vapor from a nucleation site and its associated dynamics are studied. The challenge here is the accurate representation of the nucleation site and the small-scale motion near the wall. To capture the evaporation of the microlayer left behind as the base of the bubble expands we use a semi-analytical model that is solved concurrently with the rest of the simulations. The heat transfer from the heated wall, the evolution of the bubble size and the departure diameter are evaluated and compared with the existing numerical results. The mass transfer near the interface, without fully resolving the layer by refining the grid is accommodated by using a boundary layer approximation to capture it. The behavior of the concentration profile is taken to be self-similar. A collection of potential profiles is tested and the accuracy of each of these models is compared with the full simulations.

O presente trabalho envolve a análise experimental do efeito da deposição de nanopartículas por meio da ebulição em piscina na molhabilidade e na textura da superfície. Inicialmente, este estudo apresenta uma análise da literatura sobre métodos de avaliação do ângulo de contato, preparo de nanofluidos, procedimentos de avaliação da rugosidade e possíveis efeitos que a deposição de nanopartículas tem sobre a textura da superfície. Verificou-se para as superfícies recobertas com nanopartículas ângulos de contato próximos a zero e comportamento dinâmico para gotas de água depositadas sobre elas. Desta forma, optou-se por avaliar a molhabilidade qualitativamente através da análise da velocidade de espalhamento de uma gota depositada sobre a superfície recoberta. Caracterizou-se também a massa de nanopartículas depositadas, a morfologia e a rugosidade das superfícies. Efetuou-se o recobrimento das superfícies por meio da ebulição em piscina de nanofluidos a base de água deionizada contendo nanopartículas de Al2O3 (10, 20-30 e 40-80 nm), Cu (25 nm) e SiO2 (15 e 80 nm) para concentrações volumétricas de 0,001, 0,01, 0,1 e 0,5%, submetidos a tempos de ebulição de 15, 30, 45 e 180 minutos em superfícies de alumínio e aço inoxidável. Como resultado final deste estudo concluiu-se que a rugosidade superficial e a molhabilidade se elevam com a deposição das nanopartículas. Além disso, a molhabilidade aumenta com o incremento da área da superfície recoberta com aglomerados. / The present study concerns an investigation on the wettability and the surface texture behavior of flat aluminum and stainless steel plates covered with porous thin-films of nanoparticles obtained through pool boiling of nanofluids. Since the contact angle of the obtained surfaces is small and in many cases the deposited droplet exhibits a dynamic behavior, dynamic top-down analyses of spreading droplets were performed. Evaluations were performed of nanoparticles mass deposition on the sample, surface roughness and micro-structural with an SEM (Scanning Electron Microscopy). Experiments were performed for nanofluids containing nanoparticles of Al2O3 (10, 20-30 and 40-80 nm), Cu (25nm) and SiO2 (15 and 80 nm) for volumetric concentrations of 0.001, 0.01, 0.1 and 0.5% for pool boiling time set to 15, 30, 45 and 180 minutes over aluminum and stainless steel plates. As a final result of this study it was found that surface roughness and wettability increase with the deposition of the nanoparticles. In addition, the wettability increases with increasing of the surface area covered with clusters.

Submitted by JOSIANE SANTOS DE OLIVEIRA (josianeso) on 2018-09-24T18:18:53Z No. of bitstreams: 1 Lucas Ezequias da Silva Silveira_.pdf: 2681603 bytes, checksum: bdf77c94ace5cc8dc7bd13716275dc35 (MD5) / Made available in DSpace on 2018-09-24T18:18:53Z (GMT). No. of bitstreams: 1 Lucas Ezequias da Silva Silveira_.pdf: 2681603 bytes, checksum: bdf77c94ace5cc8dc7bd13716275dc35 (MD5) Previous issue date: 2018-05-11 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / A utilização de trocadores de calor com canais de diâmetro reduzido vem crescendo devido à demanda por trocadores de calor compactos, que permitam altas taxas de transferência de calor, baixa perda de pressão e redução da carga de fluido refrigerante e de custo de materiais. O presente trabalho analisa a ebulição convectiva através de canais de pequeno diâmetro e a influência de alguns parâmetros operacionais no coeficiente de transferência de calor e na queda de pressão. São apresentados os resultados de três estudos experimentais para avaliar a influência do fluido de trabalho, do diâmetro do canal e da temperatura de saturação. Na primeira análise, é estudada a ebulição do isobutano, R600a, e do propano, R290, em um canal com 1,0 mm de diâmetro interno, com fluxo de massa na faixa de 240 a 480 kg m-2 s-1, fluxo de calor de 5 a 60 kW m-2 e temperatura de saturação de 25°C. Na segunda análise, é estudada a ebulição do R600a em canais com diâmetros de 1,0 e 2,6 mm, temperatura de saturação de 22°C, com variações do fluxo de massa de 188 a 377 kg m-2s-1 e do fluxo de calor de 28 a 56 kW m-2. Por fim, na terceira análise, a ebulição do R600a em um canal de 1,0 mm de diâmetro foi estudada com duas temperaturas de saturação, 20 e 30°C, com fluxo de massa de 240 a 480 kg m-2 s-1 e fluxo de calor de 20 a 60 kW m-2. São avaliados os efeitos do fluxo de massa, do fluxo de calor e do título de vapor, sobre o comportamento do coeficiente de transferência de calor e da perda de pressão por atrito. A análise do fluido de trabalho mostrou que, de modo geral, o R600a apresenta os maiores coeficientes de transferência de calor e perdas de pressão por atrito. Da análise da influência do diâmetro do canal observou-se que, para todas as condições experimentais, o coeficiente de transferência de calor foi superior no canal de 1,0 mm. Os resultados mostraram também que os maiores coeficientes de transferência de calor foram obtidos com a temperatura de saturação de 30°C. Complementando as análises, foram avaliados os padrões de escoamento, através das imagens registradas, e os modelos de mapas de padrões comparando os dois fluidos. Os modelos que melhor se ajustaram às curvas foram os de Revellin e Thome (2007b) e Ong e Thome (2011). Algumas correlações para transferência de calor propostas para mini e micro canais foram avaliadas com dados experimentais, de onde observou-se que, de modo geral, a correlação de Kim e Mudawar (2013b) apresentou o melhor ajuste, dentre as correlações avaliadas. / The use of heat exchangers with reduced diameter channels has been growing due to the demand for compact heat exchangers which allow high rates of heat transfer, low pressure drop and reduction of refrigerant fluid charge and material cost. The present work analyzes the convective boiling through small diameter channels and the influence of some parameters on the heat transfer coefficient and the pressure drop. The results of three experimental studies are presented to evaluate the influence of the working fluid, the channel diameter and the saturation temperature. In the first analysis, the boiling of isobutane, R600a, and propane, R290, in a channel with a 1 mm internal diameter, with mass velocity from 240 to 480 kg m-2 s-1, heat flux from 5 to 60 kW m-2 and saturation temperature of 25°C. In the second analysis, it’s studied the boiling of R600a in channels with diameters of 1.0 and 2.6 mm, saturation temperature of 22°C, with mass velocity from 188 to 377 kg m-2 s-1 and heat flux from 28 to 56 kW m-2. Finally, the boiling of R600a in a 1.0 mm diameter channel was studied with two saturation temperatures, 20 and 30°C, with a mass velocity from 240 to 480 kg m-2 s-1 and heat flux from 20 to 60 kW m-2. The influence of mass flow, heat flux and vapor quality on the behavior of the heat transfer coefficient and the frictional pressure drop are evaluated. The analysis of the working fluid showed that, in general, R600a presents the highest coefficients of heat transfer and frictional pressure drop. From the analysis of the influence of the channel diameter, it was observed that, for all experimental conditions, the heat transfer coefficient was higher in the 1.0 mm channel. The results also showed that the higher heat transfer coefficients were obtained with the saturation temperature of 30°C. Complementing the analyzes, the flow patterns were evaluated through the recorded images, and the flow pattern maps, comparing the two fluids. The map that best fit the curves were those of Revellin and Thome (2007b) and Ong and Thome (2011). Some proposed heat transfer correlations for mini and micro channels were evaluated with the experimental data, from which it was observed that, in general, the correlation of Kim and Mudawar (2013b) presented a better adjustment, among the evaluated correlations.

La croissance et le détachement des bulles formées sur une paroi par injection ou ébullition sont présents dans de nombreuses situationsindustrielles (les contacteurs gaz/liquide, les évaporateurs ou des dispositifs d’impression à jet d’encre). Dans la plupart des applications, les bulles grossissent dans un écoulement cisaillé ou au repos et la plupart des études visent à prévoir le rayon de détachement par une analyse de l’équilibre des forces. Ainsi, une bonne modélisation des forces capillaires et hydrodynamiques est nécessaire pour prédire correctement le rayonau détachement. Dans un premier temps, la croissance quasi-statique d’une bulle d’air formée sur la paroi d’une cuve et d’un canal rectangulaire est étudiée. Trois substrats différents sont utilisés pour observer l’influence des conditions de mouillage. La dynamique de la croissance de la bulle et le détachement sont alors enregistrés différentes configurations : canal horizontal, canal incliné, avec et sans écoulement liquide. Ensuite,le comportement des bulles de vapeurs en croissance sur une paroi en ébullition en vase et convective est étudié. Les expériences sonteffectuées en gravité terrestre et en microgravité dans un liquide sous-refroidi (HFE 7000) à basse pression (1-2bars). Des bulles de vapeurssont nuclées sur la paroi par l’intermédiaire d’un générateur de bulles. Ce dernier chauffe le liquide environnant au-dessus de sa température de saturation. Les bulles croient sur le site de nucléation dans un écoulement cisaillé et se détachent soit directement soit en glissant le long de la paroi. Une copie de la cellule d’essai appelée RUBI développée par l’ESA est implémentée dans le dispositif expérimental pour effectuer lespremières expériences en microgravité avant son lancement dans la station spatiale ISS. Toutes les acquisitions sont filmées à l’aide d’une caméra rapide et l’extraction du contour de la bulle est réalisée par traitement d’images. Différentes forces agissant sur une bulle sont calculées à partir des paramètres géométriques de la bulle. Un modèle prédictif de détachement de bulles est proposé. Enfin, un bilan thermique diphasique est effectué. / The growth and departure of bubbles nucleated on a wall are of particular interest in industrial situations such as gas/liquid contactors in chemicalprocessing, vapor nucleation in evaporators or inkjet printing devices. In most of these industrial applications, the bubbles grow in a shear flow and most studies aim topredict the radius at detachment by a force balance analysis. Thus a good modeling of both the hydrodynamic and capillary forces is needed to predict correctly this radius. First, the quasi-static growth of an air bubble nucleated on the wall of a tank and a rectangular channel is investigated. Three different substrates are used to observe the wettability effect. The dynamics of the bubble growth and detachment is then recorded for different configurations : horizontal channel, inclined channel, with and without a shear flow. Then, behavior of vapor bubbles growing at a wall in pool boiling and flow boiling is investigated. Experiments are performed in normal and microgravity environments with a subcooled test liquid HFE-7000 at low pressure (1-2 bars). Vapor bubbles are nucleated on the wall by a bubble generator that heats the liquid above its saturation temperature. This bubble generator was designed to provide nucleation on an isolated site. These bubbles grow on this nucleation site under an imposed shear flow and depart downstream either along the wall or perpendicularly. A copy of the test cell called RUBI developed by ESA is included in our experiment to perform the first experiments in microgravity before its launch to the ISS. Geometric and kinematic features of the air bubbles and vapor bubblesare measured by processing images obtained by high-speed video recordings. Different forces acting on the bubbles are calculated from these measurements, using a point force approach. The validity of this mechanistic approach to the bubble dynamics is discussed. At last, heat transfer balance is estimated on the bubble.

Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 1981. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Bibliography: leaves 333-336. / by Hsueh-Rong Chang. / Sc.D.