Measuring void fraction distribution in two-phase flow has been a challenging task for many decades because of its complex and fast-changing interfacial structure. In this study, a non-intrusive X-ray measurement system is developed and calibrated to mitigate this challenge. This approach has several advantages over the conventional methods such as the multi-sensor conductivity probe, wire-mesh sensor, impedance void meter, or direct optical imaging. The X-ray densitometry technique is non-intrusive, insensitive to flow regime changes, capable of measuring high temperature or high-pressure flows, and has reasonable penetration depth. With the advancement of detector technology, the system developed in this work can further achieve high spatial resolution (100 micron per pixel) and high temporal resolution (1000 frames per second). This work mainly focuses on the following aspects of the system development: establishing a geometrical model for the line detector system, conducting spectral analysis for X-ray attenuation in two-phase flow, and performing calibration tests. The geometrical model has considered the measurement plane, geometry of the test-section wall and flow channel, relative position of the X-ray source and detector pixels. By assuming axisymmetry, an algorithm has been developed to convert void fraction distribution along the detector pixels to the radial void profile in a circular pipe. The X-ray spectral analysis yielded a novel prediction model for non-chromatic X-rays and non-uniform structure materials such as the internal two-phase flow which contains gas, liquid and solid wall materials. A calibration experiment has been carried out to optimize the detector conversion factor for each detector pixels. Finally, the data measured by the developed X-ray system are compared with the double-sensor conductivity probe and gas flow meter for sample bubbly flow and slug flow conditions. The results show reasonable agreement between these different measuring techniques. / Master of Science / Two-phase flow is a widely observed phenomenon in a nuclear reactor operation and thermal hydraulic applications during thermal energy transfer process. Hence, precise understanding of two-phase flow model is essential to a thermal hydraulic design and safe operation of nuclear reactor operation systems. However, two-phase flow analysis, via measuring void fraction distribution of a two-phase flow, has been a challenging task for many decades because of its complex and dynamical interfacial characteristics. In this study, a nonintrusive X-ray measuring technique is developed to mitigate some of the conventional challenges of void fraction measurement of a two-phase flow. The void fraction imagery via X-ray densitometry technique is insensitive to flow regime changes at high temperature or high pressure flows conditions with reasonable penetration depth capabilities. Together, with the advanced detector technology and spectral analysis of the X-ray attenuation in two-phase flow, this study delivers both qualitative and quantitative geometrical model for the line detector system to provide a radial void profile of a circular pipe. Moreover, the X-ray spectral analysis yielded a novel prediction model of a non-chromatic X-rays and non-uniform structure materials such as the internal two-phase flow which contains gas, liquid, and solid pipe materials. A calibration experiment has been carried out to optimize the detector conversion factor for each detector pixels. Finally, the data measured by the developed X-ray system are compared with the double-sensor conductivity probe and gas flow meter for sample bubbly flow and slug flow conditions. The results show reasonable agreement between these different measuring techniques.

A presente dissertação de mestrado envolve o estudo da perda da pressão e dos padrões para escoamentos água/ar em um canal retangular segundo inclinações a partir do plano horizontal de -90° a 90°. Também foi avaliado o efeito nestes parâmetros de rotacionar a seção de testes em torno de seu eixo axial segundo ângulos de 45° e 60°, determinados a partir da condição de suas faces superior e inferior posicionadas horizontalmente. O texto se inicia com a apresentação de um amplo estudo da literatura sobre métodos de previsão de padrões de escoamento e perda de pressão durante escoamentos bifásicos no interior de dutos. Em seguida é descrito o aparato experimental projetado e construído para este estudo. Tal descrição inclui o detalhamento da seção de testes, que consiste em um canal retangular de seção transversal com dimensões de 6,0 x 6,5 mm². Resultados foram levantados para vazões mássicas entre 90 e 760 kg/m²s, correspondendo a velocidades superficiais entre 0,03 a 19,42 m/s e 0,1 a 0,76 m/s, para, respectivamente as fases gás e líquido. Mapas de escoamento foram desenvolvidos com base em imagens capturadas por câmera de alta velocidade, e também com base na técnica de agrupamento de dados k-means. Os escoamentos foram classificados segundo os padrões bolhas, intermitente e anular. As características hidrodinâmicas e as transições entre estes padrões foram significativamente alteradas pela inclinação do canal. Constatou-se também significativa influência da rotação em torno do eixo do canal, favorecendo efeitos de estratificação no escoamento. As transições obtidas experimentalmente foram comparadas com os métodos de previsão disponíveis na literatura. Os métodos de Taitel e Dukler (1976), para escoamento horizontal, e Taitel et al. (1980) para escoamento vertical ascendente, apresentaram as melhores previsões dos dados experimentais. Com o objetivo de estimar a parcela gravitacional da perda de pressão, levantou-se resultados para a fração de vazio superficial avaliada com base na velocidade média de bolhas alongadas e no tratamento de imagens no caso do escoamento em bolhas. Resultados de perda de pressão por atrito foram comparados com vinte métodos de previsão da literatura. As correlações de Mishima e Hibiki (1996) e Zhang et al. (2010) proporcionaram as melhores previsões para escoamento horizontal. Para escoamentos inclinados, comparou-se 25 métodos da literatura para a determinação da perda de pressão por atrito com os resultados de perda de pressão experimentais, com a perda de pressão gravitacional estimada considerando 20 métodos para a fração de vazio superficial. Desta análise constatou-se que a combinação dos métodos de Mishima e Hibiki (1996) e Zhang et al. (2010) para a previsão da parcela de perda de pressão devido ao atrito e o método de Spedding e Chen (1984) para previsão da fração de vazio superficial, utilizado para a determinação da parcela gravitacional, proporcionam previsões satisfatórias dos dados experimentais. / The present dissertation concerns a study on pressure drop and flow patterns for air/water flows inside a rectangular channel positioned according to inclination angles, relative to the horizontal plane, from -90° to 90°. The effects on flow patterns and pressure drop of rotating the test section relative to its longitudinal axis according to angles of 45° and 60° are also evaluated. Initially, a broad review of the literature concerning experimental studies and predictive methods for flow pattern and pressure drop inside channels is presented. Then, the experimental facility developed for this study is described. In this description, the test section, consisting of a rectangular channel with cross-sectional area of 6.0 x 6.5 mm² is detailed. Experimental data were obtained for mass velocities from 90 to 760 kg/m²s, corresponding to gas and liquid superficial velocities from 0.03 to 19.4 m/s and from 0.1 to 0.76 m/s, respectively. Flow patterns maps were developed based on the following approaches: analyses of two-phase flow images from a high speed video camera; and using the k-means clustering algorithm based on pressure drop and optical signals. The bubbly, intermittent and annular flow patterns were characterized. From the analyses of the data, it was found that the flow pattern transitions are significantly affected by the flow inclination and channel rotation. Two-phase flow stratification effects are enhanced by rotating the channels. Among the flow pattern predictive method evaluated in the present study, Taitel and Dukler (1976), for horizontal channels, and Taitel et al. (1980), for upward flow, provided the best predictions of the data obtained in the present study. In order of estimating the gravitational parcel of the pressure drop, superficial void fraction results were obtained based on the mean velocity of elongated bubbles, for intermittent flow, and on the image processing of bubbles, for bubbly flow. Experimental results for frictional pressure drop were compared against 20 predictive methods available in the literature. The methods of Mishima e Hibiki (1996) and Zhang et al. (2010) performed the best for horizontal flows. The frictional pressure drop predictions were also evaluated for inclined flows by comparing the measured total pressure drop against the corresponding calculated values based on the combination of 25 frictional pressure drop predictive methods and the gravitational parcel of pressure drop estimated according to 20 predictive methods for superficial void fraction. From this analysis, it was found that the combination of Mishima e Hibiki (1996) and Zhang et al. (2010) methods for frictional pressure drop with the method of Spedding e Chen (1984), for void fraction, used to determine the gravitational parcel, provide satisfactory predictions of the experimental data.

Submitted by CARLA MARIA GOULART DE MORAES (carlagm) on 2015-06-24T17:27:45Z No. of bitstreams: 1 Hoff_Alexandre.pdf: 2065929 bytes, checksum: 98c0be0d6bc2dd6c614a2379999cb6f6 (MD5) / Made available in DSpace on 2015-06-24T17:27:45Z (GMT). No. of bitstreams: 1 Hoff_Alexandre.pdf: 2065929 bytes, checksum: 98c0be0d6bc2dd6c614a2379999cb6f6 (MD5) Previous issue date: 2013-01-18 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / PROSUP - Programa de Suporte à Pós-Gradução de Instituições de Ensino Particulares / Este trabalho apresenta o desenvolvimento de uma metodologia experimental para medição de fração de vazio em um escoamento bifásico ar-água, utilizando sensor óptico em um minicanal horizontal de 2,6 mm de diâmetro interno. O sistema de medição consiste de um diodo emissor de luz (LED) de alto brilho, uma resistência dependente de luz (LDR), um circuito amplificador e um circuito para aquisição de dados. Os experimentos são realizados com um tubo de vidro que fica entre o LED e o LDR, no qual passa a mistura ar-água e conforme o padrão de escoamento a luz é atenuada no LDR, que altera sua resistência variando a tensão. Os testes são realizados com vazões volumétricas do líquido de 160, 123 e 63 ml/min e volume de gás de 0,05 e 0,1 ml e os padrões de escoamento testados e observados foram do tipo bolhas, pistonados e golfadas. Nos ensaios são registradas imagens do escoamento, ou dos padrões de bolhas, com uma câmera de alta velocidade e o processamento destas imagens é utilizado para valid ar a técnica do sensor óptico. As áreas das bolhas, em pixels, são comparadas com as áreas das curvas do sinal do sensor óptico e uma boa correlação foi encontrada. Uma curva de calibração do sensor é apresentada para padrão de escoamento tipo bolhas, permitindo encontrar uma relação entre a fração de vazio e a tensão medida pelo sensor. Os resultados obtidos mostraram que a técnica do sensor óptico pode ser usada com bons resultados para a medição de fração de vazio média para um escoamento bifásico ar-água e para a identificação de padrões de escoamento em tubos de diâmetro reduzido. / This work presents the development of an experimental methodology for measuring void fraction in an air-water two-phase flow, using optical sensor in a horizontal mini channel of 2.6 mm internal diameter. The measurement system consists of a light emitting diode (LED) of high brightness, light dependent resistor (LDR), amplifier circuit and data acquisition circuit. The experiments are performed with the air-water flow passing through a glass tube section positioned between the LED and LDR . According to the flow pattern, light is attenuated and impinges on LDR, changing its resistance and thus varying the voltage measured by data acquisition circuit. The tests are conducted with liquid volumetric flow of 160, 123 and 63 ml/min and gas volume of 0.05 and 0.1 ml and flow patterns tested and observed were bubbly and plug. During the tests images of the flow are registered, or the bubbles flow patterns, with a high-speed camera and the processing of these images is used to validate the optical sensor technique. The bubble areas, in pixels, are compared to area under the curve produced from optical sensor signal and a good correlation was found. A calibration curve of the sensor is obtained for bubbly flow pattern, and a correlation between void fraction and the tension is proposed. The results show that the optical sensor technique is adequate for average void fraction measurements in air-water two-phase flow and for flow patterns identification in mini channels.

Packed beds find extensive application in a wide variety of industries. The objective of the present work is to analyze and evaluate the effects of the wall on structural characteristics, hydrodynamics and heat transfer in packed beds of spheres. As a first attempt, spheres of uniform size are considered. The cylindrical wall of the bed confines the location of the particles thus leading to significant radial variations in void fraction and specific lateral surface area. The two characteristics at any given radial position r* are estimated by defining a concentric cylindrical channel (CCC) of an arbitrary thickness such that its boundaries are equidistant from the cylindrical surface passing through r* and accounting for the solid volumes or lateral surface areas of the segments of spheres (cap, slice, rod and annular ring) contained in the CCC and with centers lying within a distance of a particle radius from r*.The curved boundaries of the sphere segments are rigorously accounted for. The low aspect ratio beds (aspect ratio less than or equal to 2) show three distinct types of behavior. In beds of aspect ratio 2, the void fraction starts from a value of unity at the wall and decreases to a minimum and then increases to unity at the center of the bed. In beds with aspect ratio between l\/¯3/2, there is a continuous decrease in void fraction from unity at the wall to a fairly low value towards the axis and then a slight increase followed by another decrease. The profiles for aspect ratio less than l\/¯3/2 show a continuous decrease from a value of unity at the wall to zero towards the axis. In contrast, beds of high aspect ratio show heavily damped oscillations in the void fraction up to about five particle diameters from the wall and then a constant value. The lateral surface area variations in low aspect ratio beds show a steep fall from a very high value near the wall, and in high aspect ratio beds an oscillatory nature, though not as strong as in the corresponding void fraction profiles. The distribution of flow in packed beds for steady flow of an incompressible Newtonian fluid under isothermal conditions is modeled by using Ergun equation with Brinkman-type correction to account for the viscous effects in the region close to the wall. The confining effect of the wall is incorporated through the radial variations in void fraction and specific lateral surface area. The hydraulic radius in the region next to the wall is modified to take into account the resistance of the wall surface to flow. The resulting model equations with appropriate boundary conditions are solved numerically by collocation technique. The influence of aspect ratio in the range 1.25 to 20.3 and Reynolds number from 0.1 to 1000, the two most important factors affecting the flow behavior, is evaluated. The velocity profiles show a peak in the region close to the wall thus indicating severe channeling effect in this region. The magnitude and location of the peak depend on aspect ratio and Reynolds number. The model predictions agree remarkably with reported experimental data on velocity profiles in a bed of aspect ratio 10.7, and on the effect of Reynolds number on friction factors in beds of low aspect ratio. The radial variations in void fraction, velocity and effective thermal conductivity are incorporated in the two-dimensional pseudo-homogeneous steady-state model to analyze the wall effects on heat transfer in packed beds. Both constant wall temperature and constant wall flux boundary conditions are adopted. The equations are solved numerically using finite difference technique. The radial temperature profiles are seen to be fairly uniform in beds of low aspect ratio thus showing that the often made assumption of complete radial thermal mixing in low aspect ratio beds is valid. Beds of high aspect ratio show strong radial gradients. For constant heat flux condition the slope of the temperature profile remains constant after a small distance from the Inlet thus leading to thermally fully-developed flow. For this condition the heat transfer equations are solved analytically to obtain expressions for Nusselt number and the radial temperature profiles. There is a significant difference in the temperature profiles evaluated in the presence and absence of wall effects. Good agreement is found between the Nusselt numbers obtained from the model and reported experimental data.

A presente dissertação de mestrado envolve o estudo da perda da pressão e dos padrões para escoamentos água/ar em um canal retangular segundo inclinações a partir do plano horizontal de -90° a 90°. Também foi avaliado o efeito nestes parâmetros de rotacionar a seção de testes em torno de seu eixo axial segundo ângulos de 45° e 60°, determinados a partir da condição de suas faces superior e inferior posicionadas horizontalmente. O texto se inicia com a apresentação de um amplo estudo da literatura sobre métodos de previsão de padrões de escoamento e perda de pressão durante escoamentos bifásicos no interior de dutos. Em seguida é descrito o aparato experimental projetado e construído para este estudo. Tal descrição inclui o detalhamento da seção de testes, que consiste em um canal retangular de seção transversal com dimensões de 6,0 x 6,5 mm². Resultados foram levantados para vazões mássicas entre 90 e 760 kg/m²s, correspondendo a velocidades superficiais entre 0,03 a 19,42 m/s e 0,1 a 0,76 m/s, para, respectivamente as fases gás e líquido. Mapas de escoamento foram desenvolvidos com base em imagens capturadas por câmera de alta velocidade, e também com base na técnica de agrupamento de dados k-means. Os escoamentos foram classificados segundo os padrões bolhas, intermitente e anular. As características hidrodinâmicas e as transições entre estes padrões foram significativamente alteradas pela inclinação do canal. Constatou-se também significativa influência da rotação em torno do eixo do canal, favorecendo efeitos de estratificação no escoamento. As transições obtidas experimentalmente foram comparadas com os métodos de previsão disponíveis na literatura. Os métodos de Taitel e Dukler (1976), para escoamento horizontal, e Taitel et al. (1980) para escoamento vertical ascendente, apresentaram as melhores previsões dos dados experimentais. Com o objetivo de estimar a parcela gravitacional da perda de pressão, levantou-se resultados para a fração de vazio superficial avaliada com base na velocidade média de bolhas alongadas e no tratamento de imagens no caso do escoamento em bolhas. Resultados de perda de pressão por atrito foram comparados com vinte métodos de previsão da literatura. As correlações de Mishima e Hibiki (1996) e Zhang et al. (2010) proporcionaram as melhores previsões para escoamento horizontal. Para escoamentos inclinados, comparou-se 25 métodos da literatura para a determinação da perda de pressão por atrito com os resultados de perda de pressão experimentais, com a perda de pressão gravitacional estimada considerando 20 métodos para a fração de vazio superficial. Desta análise constatou-se que a combinação dos métodos de Mishima e Hibiki (1996) e Zhang et al. (2010) para a previsão da parcela de perda de pressão devido ao atrito e o método de Spedding e Chen (1984) para previsão da fração de vazio superficial, utilizado para a determinação da parcela gravitacional, proporcionam previsões satisfatórias dos dados experimentais. / The present dissertation concerns a study on pressure drop and flow patterns for air/water flows inside a rectangular channel positioned according to inclination angles, relative to the horizontal plane, from -90° to 90°. The effects on flow patterns and pressure drop of rotating the test section relative to its longitudinal axis according to angles of 45° and 60° are also evaluated. Initially, a broad review of the literature concerning experimental studies and predictive methods for flow pattern and pressure drop inside channels is presented. Then, the experimental facility developed for this study is described. In this description, the test section, consisting of a rectangular channel with cross-sectional area of 6.0 x 6.5 mm² is detailed. Experimental data were obtained for mass velocities from 90 to 760 kg/m²s, corresponding to gas and liquid superficial velocities from 0.03 to 19.4 m/s and from 0.1 to 0.76 m/s, respectively. Flow patterns maps were developed based on the following approaches: analyses of two-phase flow images from a high speed video camera; and using the k-means clustering algorithm based on pressure drop and optical signals. The bubbly, intermittent and annular flow patterns were characterized. From the analyses of the data, it was found that the flow pattern transitions are significantly affected by the flow inclination and channel rotation. Two-phase flow stratification effects are enhanced by rotating the channels. Among the flow pattern predictive method evaluated in the present study, Taitel and Dukler (1976), for horizontal channels, and Taitel et al. (1980), for upward flow, provided the best predictions of the data obtained in the present study. In order of estimating the gravitational parcel of the pressure drop, superficial void fraction results were obtained based on the mean velocity of elongated bubbles, for intermittent flow, and on the image processing of bubbles, for bubbly flow. Experimental results for frictional pressure drop were compared against 20 predictive methods available in the literature. The methods of Mishima e Hibiki (1996) and Zhang et al. (2010) performed the best for horizontal flows. The frictional pressure drop predictions were also evaluated for inclined flows by comparing the measured total pressure drop against the corresponding calculated values based on the combination of 25 frictional pressure drop predictive methods and the gravitational parcel of pressure drop estimated according to 20 predictive methods for superficial void fraction. From this analysis, it was found that the combination of Mishima e Hibiki (1996) and Zhang et al. (2010) methods for frictional pressure drop with the method of Spedding e Chen (1984), for void fraction, used to determine the gravitational parcel, provide satisfactory predictions of the experimental data.

Les récentes avancées technologiques des matériels d’acquisition de données ont permis de réduire le temps d’acquisition d’image en tomographie électrique, ce qui offre des opportunités pour l’étude des écoulements diphasiques transitoires. Parmi les nombreux atouts de cette technique d’imagerie d’écoulements diphasiques, on peut citer son caractère non-intrusif, sa haute fréquence d’acquisition et son faible coût. Un ensemble d’électrodes placées sur le pourtour d’une conduite servent à transmettre une excitation électrique au milieu et à le sonder. Ainsi, la distribution des phases perturbe les champs électriques de manière caractéristique. L’objectif de cette thèse est d’évaluer le potentiel de la tomographie d’impédance électrique rapide. La première étape consiste au développement d’un prototype de capteur et à l’évaluation de sa performance par des essais simplifiés. L’architecture du système utilise un contrôle en potentiel du signal d’excitation et ne nécessite donc pas d’implémenter un module de conversion tension-courant. La seconde étape est la reconstruction de l’image à partir des données mesurées. L’approche qui a été considérée est de supposer une image approchée de la distribution des phases grâce à une identification du régime d’écoulement. Ainsi, le défi de résoudre un problème inverse fortement non-linéaire est simplifié. Une méthode d’identification de régimes d’écoulements horizontaux eau-air a été élaborée avec un module de tomographie de capacitance électrique et une boucle d’essais hydrauliques déjà éprouvés. Cette technique est en cours d’adaptation au prototype de tomographie d’impédance électrique rapide et en amélioration grâce à l’inclusion des régimes d’écoulements verticaux. En parallèle, une méthode de reconstruction d’image a été développée, basée sur l’algorithme NOSER et un postulat pseudo-2D. L’analyse des images reconstruites à partir d’un set d’expériences de référence procure un aperçu des avantages et des défauts de la méthode et du prototype. / Recent developments with data acquisition equipment have reduced the time required for image acquisition with electrical tomography, thereby bringing new opportunities for the study of fast-evolving two-phase flows. Amongst the numerous advantages of this imaging technique for multiphase flow related research are non-intrusiveness, high acquisition rates, low-cost and improved safety. A set of electrodes placed on the periphery of the pipe to be imaged is used to impose an electrical excitation and measure the system response. The distribution of phases inside the study volume distorts the electrical field in a characteristic manner. The objective of this thesis is to assess the potential of electrical impedance tomography at high acquisition rate. The first stage consists in developing a prototype sensor and assessing its performance with simplistic experiments. The system architecture employs voltage control of the excitation and therefore does not require the implementation of the conventional voltage-to-current converter module. A novel data collection method, the full scan strategy, is considered and provides correcting factors for the parasitic impedances in the system. The second stage is the image reconstruction from the measurement data. The approach considered in the thesis is to assume that flow regime identification techniques may provide valuable information on the phase distribution that can be injected in the inverse problem for imaging, thereby tackling the challenge of the non-linearity of the inverse problem. A method for horizontal air-water flow regime identification has been elaborated with an electrical capacitance tomography sensor and multiphase flow rig tried and tested. It is being adapted to the fast electrical impedance tomography prototype and upgraded to include vertical flow regimes. In parallel, an image reconstruction method has been developed based on the NOSER algorithm and a pseudo-2D postulate. The analysis of the reconstructed images for a set of benchmark experiments provide insights on the merits and deficiencies of the algorithm and of the prototype.

<div>With the rapid development of the advanced two-phase flow experimental technologies, more experimental databases with extended measurement ranges have been established to support the two-phase flow model development. The advantage of the Two Fluid model in modeling the complex two-phase flow phenomena over the mixture models stands out. One key aspect in the Two Fluid model development is the accurate modeling of the interfacial area between phases, which is strongly related to the interfacial mass, momentum, and energy transfer. As a closure relation of interfacial area concentration (interfacial area per unit volume) for the Two Fluid model, the Interfacial Area Transport Equation (IATE) provides dynamic predictions on the interfacial area change. It substantially solves the shortcoming of using flow-regime-dependent empirical correlations that can introduce numerical discontinuities between flow regimes. </div><div><br></div><div>The IATE has been extensively developed over the past twenty-five years. Many studies targeted on improving its prediction capability by developing bubble interaction source terms based on their experimental data. </div><div>The existing models are usually based on medium and large flow channels, yet the models may not be physically fit the small flow channels. The major reason is that the wall effect can have a larger influence on the two-phase flow in a small flow channel, as the surface area to volume ratio greatly increases. Therefore, the primary objectives of this study are to physically investigate the wall effect on two-phase flow and develop a generalized IATE by extending the application range of existing IATE from large and medium flow channels to small flow channel.</div><div><br></div><div>To achieve the objective, this study established a rigorous database of air-water two-phase flows in a small diameter pipe with its inner diameter of 12.7 mm, focusing on the bubbly-to-slug transition regime. The experimental analysis was performed on the pipe wall effect on the interfacial characteristics, based on the current experimental database and the existing experimental database collected on vertical pipes of different sizes. It is observed that 1) the pipe wall effect can alter the non-uniform radial two-phase distribution; 2) the bubbly-to-slug flow regime transition in a small diameter pipe happens in a smaller void fraction than in a large diameter pipe; 3) the bubble coalescence phenomenon can be more dominant for small pipe flow, and an intensive intergroup transfer can happen for the two-group interfacial area transport in two-phase flows. </div><div>As the interfacial area transport is directly related to the two-phase geometrical configuration, the two-phase geometrical parameters, void fraction and relative bubble size, are identified as the key parameters for modeling.</div><div><br></div><div>In the modeling of IATE source terms, the high geometrical scalability of the model is realized by properly including the wall effect into the modeling consideration. The following major improvements on the existing models are: 1) the inertia subrange assumption on the turbulent-driven interaction is properly improved; 2) the bubble-induced turbulent-driven interactions such as wake entrainment is revised by considering the wall effect on the wake region. In summary, models of bubble interaction due to random collision, wake entrainment, turbulent impact, and shearing-off are revised based on the existing studies on the IATE source terms development. The newly proposed interfacial area transport models are evaluated against an experimental database with 112 test conditions in total from a wide range of experimental pipe diameters from 12.7 mm to 304.8 mm. The new models can accurately capture the drastic intergroup transfer of void fraction and interfacial area concentration between two groups in transition flows. Overall, the relative error of void fraction and interfacial area concentration comparing with the experimental data are within ±15\% and ±10\%, respectively.</div>

京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第21887号 / エネ博第388号 / 新制||エネ||75(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 齊藤 泰司, 教授 横峯 健彦, 准教授 伊藤 啓 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

Fluidized beds have widespread industrial applications ranging from chemical industries to power plants. The flow inside a fluidized bed system consists of two main phases, a particle phase and the fluid phase. The two phases are strongly coupled to each other through various forces like drag and pressure. Capturing this multiphase phenomenon requires modeling strategies that possess good fidelity over a range of scales. Discrete Element Modeling (DEM) coupled with Computational Fluid Dynamics (CFD) provides a good platform to analyze the complex coupled multiphase hydrodynamics inside fluidized bed systems. Conventional DEM-CFD framework suffers from contradictory spatial resolution requirements for the particle and fluid phases, respectively. This prevents the conventional DEM-CFD method to be applied to geometries that have features comparable to the particle diameter of the solid phase. The novelty of this work lies in the development and validation of a two-grid formulation that removes the resolution restrictions of the conventional DEM-CFD framework. The results obtained from this new framework agree reasonably well with the experiments showing the capability of the new scheme to simulate conditions not possible with conventional DEM-CFD framework. In addition, this research also focuses on performing both 2D and 3D jetting fluidized bed simulations having millions of particles; validate/compare results with experiments and to perform heat transfer studies in a jetting fluidized bed system. The results suggest convective and diffusive mixing for a single jet at higher superficial velocity to be better than the mixing obtained in a multiple jet framework. The comparison with experimental results obtained in a multiple jetting setup shows that a 2D simulation captures the essential jet characteristics near the distributor plate reasonably well while a 3D simulation is needed to capture proper bubble dynamics near the freeboard of the bed. These results give insight into the detailed dynamics of fluidized bed systems and provide a foundation for a better design of these systems. / Ph. D.

A Agência Internacional de Energia Atômica (IAEA - \"International Atomic Energy Agency\") vem incentivando o desenvolvimento de sistemas passivos de refrigeração em plantas nucleares visando a simplificação e o incremento da confiabilidade em funções essenciais de segurança nos projetos de uma próxima geração de reatores nucleares refrigerados a água. O principal fundamento desses sistemas é o emprego da circulação natural como sistema de segurança aplicável em operações de desligamento do reator para manutenção ou na ocorrência de acidentes. A circulação natural é um fenômeno que surge em virtude do gradiente de temperatura em pontos diferentes do circuito de refrigeração. Em condições extremas de estabilidade têm-se o estabelecimento do escoamento bifásico gás/líquido podendo configurar-se segundo diferentes regimes. A fração de vazio é reconhecida como um dos parâmetros chave na predição da ocorrência de instabilidades do escoamento bifásico. Apresenta-se neste trabalho uma inovadora metodologia para estimativa da fração de vazio a partir de imagens digitais capturadas diretamente de circuitos experimentais que geram o escoamento bifásico. O método é baseado na aquisição de imagens, com controle da profundidade de campo, de uma seção do Circuito de Circulação Natural (CCN) presente no IPEN/CNEN-SP. A imagem é segmentada com base na inferência fuzzy de diferentes parâmetros de segmentação e ajustada ao foco utilizado na sua aquisição. Ela é varrida de um modo inédito e iterativo, utilizando máscaras de diferentes tamanhos integrando um conjunto de redes neurais com a Transformada Randomizada de Hough. Cada diferente tamanho de máscara é escolhido de acordo com os tamanhos das bolhas que são os objetos de interesse. O volume da bolha é estimado baseado em sua projeção plana capturada nas imagens digitais. O cálculo da fração de vazio considera o volume da seção geométrica do escoamento no tubo de vidro cilíndrico e a profundidade de campo utilizada e nos parâmetros geométricos inferidos para cada bolha detectada. Os resultados mostraram que a integração entre o conjunto de redes neurais e a Transformada Randomizada de Hough aumentaram a robustez das estimativas do sistema. / The International Atomic Energy Agency (IAEA) has been encouraging the use of passive cooling systems in new designs of nuclear power plants. Next nuclear reactor generations are intended to possess simpler and robust safety functions. Natural circulation based systems hold an undoubtedly prominent position among these. Natural circulation phenomenon occurrence depends only on the existence of refrigerant liquid temperature gradient in different sections of the plant refrigerator circuit. The study of limit conditions for these systems has led to instability behavior analysis where many different two-phase flow patterns are present. Void fraction is a key parameter in thermal transfer analysis of theses flow instability conditions. This works presents a new method to estimate void fraction from digital images captured at an experimental two-phase flow circuit. The method is primarily based on depth-of-field controlled image acquisition of a section of a closed loop of natural circulating water through cylindrical glass tubes. This loop is called Natural Circulation Facility (NCF) and is located at Nuclear and Engineering Research Institute in Brazil (IPEN/CNEN-SP). Image is segmented based on fuzzy inference of different segmentation parameters and adjusted to image acquisition focus. The image is then scanned in an inedited way using different-sized masks integrating a set of different artificial neural networks with a modified Randomized Hough Transform. Each different mask size is chosen in accordance to bubble sizes which are objects of interest. The bubble volume is estimated based on two-dimensional projection sizing based on digitally acquired images. Void fraction calculation takes into account the volume of the geometrical section of flow inside cylindrical glass tube considering used depth-of-field. It is also based on the summed bubble geometrical parameters inferred for each detected bubble. The results have shown that integration between artificial-neural-net sets and Randomized Hough Transforms increase system estimations robustness.