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This thesis describes the first application of ultra-fast magnetic resonance imaging (MRI) towards the characterisation of bubbly flow systems. The principle goal of this study is to provide a hydrodynamic characterisation of a model bubble column using drift-flux analysis by supplying experimental closure for those parameters which are considered difficult to measure by conventional means. The system studied consisted of a 31 mm diameter semi-batch bubble column, with 16.68 mM dysprosium chloride solution as the continuous phase. This dopant served the dual purpose of stabilising the system at higher voidages, and enabling the use of ultra-fast MRI by rendering the magnetic susceptibilities of the two phases equivalent. Spiral imaging was selected as the optimal MRI scan protocol for application to bubbly flow on the basis of its high temporal resolution, and robustness to fluid flow and shear. A velocimetric variant of this technique was developed, and demonstrated in application to unsteady, single-phase pipe flow up to a Reynolds number of 12,000. By employing a compressed sensing reconstruction, images were acquired at a rate of 188 fps. Images were then acquired of bubbly flow for the entire range of voidages for which bubbly flow was possible (up to 40.8%). Measurements of bubble size distribution and interfacial area were extracted from these data. Single component velocity fields were also acquired for the entire range of voidages examined. The terminal velocity of single bubbles in the present system was explored in detail with the goal of validating a bubble rise model for use in drift-flux analysis. In order to provide closure to the most sophisticated bubble rise models, a new experimental methodology for quantifying the 3D shape of rising single bubbles was described. When closed using shape information produced using this technique, the theory predicted bubble terminal velocities within 9% error for all bubble sizes examined. Drift-flux analysis was then used to provide a hydrodynamic model for the present system. Good predictions were produced for the voidage at all examined superficial gas velocities (within 5% error), however the transition of the system to slug flow was dramatically overpredicted. This is due to the stabilising influence of the paramagnetic dopant, and reflects that while drift-flux analysis is suitable for predicting liquid holdup in electrolyte stabilised systems, it does not provide an accurate representation of hydrodynamic stability. Finally, velocity encoded spiral imaging was applied to study the dynamics of single bubble wakes. Both freely rising bubbles and bubbles held static in a contraction were examined. Unstable transverse plane vortices were evident in the wake of the static bubble, which were seen to be coupled with both the path deviations and wake shedding of the bubble. These measurements demonstrate the great usefulness for spiral imaging in the study of transient multiphase flow phenomena.

Submitted by Fabio Sobreira Campos da Costa (fabio.sobreira@ufpe.br) on 2016-03-30T13:38:45Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Tese_Mikele Cândida Sousa de Sant'Anna.pdf: 2684133 bytes, checksum: 61e61bb0edcf743d1f2af9c55724a92e (MD5) / Made available in DSpace on 2016-03-30T13:38:45Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Tese_Mikele Cândida Sousa de Sant'Anna.pdf: 2684133 bytes, checksum: 61e61bb0edcf743d1f2af9c55724a92e (MD5) Previous issue date: 2015-11-18 / CAPEs / O presente trabalho trata da simulação numérica de um gaseificador de leito fluidizado borbulhante fazendo uso da CFD para o sistema composto por gás-biomassa-areia. Inicialmente, simulou-se os sistemas gás-areia e gás-biomassa. O modelo computacional empregado foi validado empregando-se os resultados experimentais de Taghipuor et al. (2005).A seguir, foi realizado um planejamento fatorial 23, onde se variou a massa específica e o diâmetro da partícula e o percentual de biomassa na fase sólida. Para realizar as simulações foram utilizados os softwares ANSYS CFX 15.0 e ANSYS FLUENT 15.0, adotando-se a abordagem euleriana, com a Teoria Cinética de Escoamento Granular. As seguintes velocidades superficiais do gás foram testadas: 0,03, 0,1, 0,38 0,46 e 0,51 m.s-1. Para o sistema gás-areia, o leito permaneceu fixo nas velocidades de 0,03 e 0,10 m.s-1. Aos 2,50 s de simulação transiente, o leito encontrava-se fluidizado para as velocidades maiores ou iguais a 0,38 m.s-1 e assim permaneceu alcançando um estado pseudo-estacionário. No sistema gás-biomassa, o leito manteve-se fixo apenas na velocidade de 0,03 m.s-1. Dois sistemas foram testados com três componentes (gás-areia-biomassa) diferenciando-se entre si pelos tamanhos das partículas de areia e biomassa. Para grandes diferenças entre estes tamanhos, o sistema apresentou segregação durante a fluidização. No sistema com menor diferença nestes tamanhos, a fluidização ocorreu mais facilmente, uma vez que os efeitos de segregação foram atenuados. Foram obtidos perfis de fração volumétrica do gás, areia e biomassa para as 17 condições do planejamento fatorial, bem como um modelo que prediz a expansão do leito em sistemas fluidizados. O ensaio que apresentou maior altura final do leito (0,50 m), mantendo-se em regime borbulhante, foi aquele com 15% de partículas de biomassa com 375 m de diâmetro e 85% de areia, sendo, portanto, uma condição ótima para a fluidização. / This work has studied a bubbling fluidized bed gasifier though numerical simulation using computational fluid dynamics (CFD) for the system composed of gas - biomass - sand. Initially, gas-sand and gas-biomass systems were simulated. The computer model used was validated employing experimental results from Taghipuor et al. (2005) .Then, a 23 factorial design was employed, where the density, the particle diameter and the the fraction of biomass were varied. All simulations were carried out using ANSYS CFX 15.0 and ANSYS FLUENT 15.0. An eulerian approach coupled to the Kinetic Theory of Granular Flow were used. The following gas superficial velocities were tested: 0.03, 0.1, 0.38, and 0.46 0.51 m s-1. For gas-sand system, a fixed bed was obtained for gas velocities of 0.03 and 0.10 m s-1. After 2.50 s of transient simulation, the bed became fluidized for gas velocities greater or equal to 0.38 m s-1 staying in a pseudo-steady state. For the biomassgas system, the bed remained fixed only at the speed of 0.03 m s-1. Two systems were tested using the three components (gas, sand and biomass) differing from each other only by the size of sand and biomass particles. For high differences between these sizes, the system showed segregation during fluidization. In the system with lower size difference, the fluidization occurred more easily, since the segregation effects were attenuated. Volumetric fraction profiles of gas, sand and biomass were obtained for the 17 factorial design conditions used as well as a model that predicts the bed expansion in fluidized systems. The assay that showed higher final height of the bed (0.50 m) staying in a bubbling regime was one with 15% biomass particles with 375 mm in diameter and 85% of sand, being, therefore, a good condition to carry out fluidization.

Orientador: Teresa Massako Kakuta Ravagnani / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Quimica / Made available in DSpace on 2018-08-14T23:59:28Z (GMT). No. of bitstreams: 1 Domenico_MicheleDi_M.pdf: 2117165 bytes, checksum: 3837c602b224622b3736355d2e6b6e1c (MD5) Previous issue date: 2010 / Resumo: Com o propósito de aumentar a eficiência no processo de destilação convencional, e conseqüentemente diminuir o consumo excessivo de energia, inúmeras alternativas foram propostas na literatura. Uma destas metodologias, denominada paradestilação, é baseada na divisão da corrente gasosa em duas partes iguais no fundo de uma coluna de separação. Para este esquema de fluxo, uma divisória é colocada centralmente em toda a altura da coluna, e a corrente líquida contata, em estágios alternados, ambas as correntes do vapor. As vantagens deste processo incluem um aumento no número de estágios e uma diminuição na queda de pressão por unidade de altura da coluna convencional. Apesar das vantagens apresentadas em trabalhos computacionais, faz-se necessário o estudo experimental envolvendo os dois tipos de colunas sob as mesmas condições de operação. Assim, este trabalho apresenta um estudo comparativo entre uma coluna de destilação e outra de paradestilação, contendo respectivamente, 4 e 6 pratos perfurados, dotados de vertedores circulares. Para tanto, as colunas foram construídas de acordo com as recomendações de dimensionamento da literatura, de modo a fornecerem uma mesma eficiência de separação. Os experimentos foram realizados utilizando uma mistura de etanol-água, à pressão atmosférica e em condições de refluxo total e parcial. A comparação entre os processos foi realizada através da análise do comportamento hidrodinâmico e da eficiência de pratos de Murphree, avaliando-se parâmetros como a velocidade superficial da fase vapor, limites operacionais, tipo e altura da dispersão formada sobre o prato e perfil de composição da fase líquida ao longo das colunas. Verificou-se que as colunas de destilação e paradestilação apresentam uma faixa de operação muito semelhante. Sob condições de refluxo parcial, a coluna alternativa é menos sensível a uma variação na razão de refluxo. Ainda, as eficiências de Murphree foram fortemente influenciadas pelas propriedades do sistema destilante, composição da fase líquida e velocidades das fases líquida e gasosa. Os maiores valores de eficiência foram encontrados sob condições de refluxo total e em baixas velocidades da fase vapor. / Abstract: In order to increase the operation efficiency of conventional distillation and therefore reduce the excessive energy consumption, several techniques have been proposed in the literature. One of these methods, called parastillation, is based on the division of the vapor phase into two equal parts at the bottom of a separation column. For this configuration, an axial partition run the full height of the column, and the whole falling liquid stream contacts alternatively on a stage-by-stage basis with both parts of the vapor. The advantages of this process also include an increase in the number of stages and a decrease in pressure drop per unit of column height. Despite of the advantages presented in computational works, experimental studies involving distillation and parastillation columns operating under the same conditions are necessary. Therefore, this work presents a comparative study between a distillation and a parastillation column, using six and four sieve trays, respectively, with circular downcomers. The columns were assembled in agreement to the recommendations of literature design, in order to provide the same separation efficiency. The experiments were conducted using an ethanol-water system at atmospheric pressure and under total and partial reflux conditions. The comparison between the processes was carried out by analyzing the hydrodynamic behavior and the Murphree tray efficiency, evaluating parameters such as vapor phase velocity, operating limits, type and height of the dispersion formed above the trays and liquid composition profile along the columns. It was found that the distillation and parastillation columns have similar operating range. Under partial reflux conditions, the alternative column presented a lower sensitivity to a modification of the reflux ratio. In addition, the Murphree efficiency appears to be strongly dependant on system properties, composition of the liquid phase and vapor and liquid flow rates. The higher values of efficiency were found under total reflux conditions and at low vapor velocities. / Mestrado / Sistemas de Processos Quimicos e Informatica / Mestre em Engenharia Química

Numerical calculations were performed over a variety of two-dimensional rib roughness configurations in which the ratio of flow depth to roughness height was varied from 1.1 to 40. Periodically fully developed flow was achieved by employing periodic boundary conditions and the effect of turbulence was accounted for by a two-layer model. These calculations were used to test the hypothesis that any rough wall resistance may be reduced to an equivalent wall shear stress located on a plane wall. The position of the plane wall is determined by a novel method of prediction obtained by consideration of strearnwise force moments. The resistance is then determined by three dynamically significant length scales: the first (yo) specifies the position of the equivalent plane wall, the second is the depth of flow h and the third is similar to Nikuradse's sand grain roughness k,,. The latter length scale is however depth dependent and a universal relationship is postulated: ks y,, -,= F(Tkwhere ksw is the asymptotic value of ks at very large flow depths. For the calculation of friction factor, a resistance equation is proposed of the form typical of fully rough flows. These postulates are supported by the numerical model results though further work including physical experiments is required to confirm them. Before applying the two-layer model to this problem it was tested on smooth rectangular duct flows and Schlichting's (1936) long angle roughness experiments. The opportunity was taken to further explore these flows, and in addition calculations were carried out for Grass et al's ( 199 1) open channel rib roughness experiments. The periodic boundary conditions were also applied to a larninar counter-flow plate-fin heat exchanger.A novel source-sinka rrangemenfto r heat flux was developedi n order to implement these boundary conditions.

Aims. We numerically studied close encounters between a young stellar system hosting a massive, gravitationally fragmenting disk and an intruder diskless star with the aim of determining the evolution of fragments that have formed in the disk prior to the encounter. Methods. Numerical hydrodynamics simulations in the non-inertial frame of reference of the host star were employed to simulate the prograde and retrograde co-planar encounters. The initial configuration of the target system (star plus disk) was obtained via a separate numerical simulation featuring the gravitational collapse of a solar-mass pre-stellar core. Results. We found that close encounters can lead to the ejection of fragments that have formed in the disk of the target prior to collision. In particular, prograde encounters are more efficient in ejecting the fragments than the retrograde encounters. The masses of ejected fragments are in the brown-dwarf mass regime. They also carry away an appreciable amount of gas in their gravitational radius of influence, implying that these objects may possess extended disks or envelopes, as also previously suggested. Close encounters can also lead to the ejection of entire spiral arms, followed by fragmentation and formation of freely-floating objects straddling the planetary mass limit. However, numerical simulations with a higher resolution are needed to confirm this finding.

Lorsque de l’air est injecté à la base d’une couche de grains immergée, il traverse le système par percolation ou fracturation. Il forme ainsi plusieurs chemins qui atteignent la surface libre de la couche à des endroits différents. Nous avons réalisé l’étude expérimentale de ce processus (configuration 2D et 3D), ainsi que son analyse numérique et théorique. Dans un premier temps, nous nous sommes concentrés sur la dynamique d’invasion de l’air dans le milieu, à temps court et à temps long, lors de l’injection d’un flux d’air continu. À temps long, la taille typique de la région explorée par l’air (zone fluidifiée) peut être expliquée par un processus diffusif [1]. Nous avons également étudié l’effet de la gravité, en inclinant la cellule expérimentale. La comparaison des résultats avec des simulations numériques pour l’injection d’un volume d’air fixé permet de caractériser la morphologie de la zone d’invasion. Nous montrons que la hauteur et la largeur typique de la région explorée par l’air ne dépend pas uniquement du volume injecté, mais peut s’exprimer en fonction d’un paramètre χ sans dimension qui représente les effets relatifs de la gravité et de la capillarité [2]. Nous présenterons quelques résultats préliminaires montrant la dynamique d’évolution de la zone fluidifiée en fonction des différentsparamètres. Enfin, lorsqu’on augmente la hauteur d’eau au-dessus de la couche granulaire, l’advection et le dépôt successif des grains forment un cratère, composé de deux dunes qui croissent et s’éloignent du centre. La taille typique du cratère augmente de façon logarithmique dans le temps, indépendamment du processus d’émission du gaz [3].Références -[1] G.Varas, V. Vidal and J.-C. Géminard, Phys. Rev. E. 83, 011302 (2011).[2] G.Varas, V. Vidal and J.-C. Géminard, Phys. Rev. E. 83, 061302 (2011).[3] G.Varas, V. Vidal and J.-C. Géminard, Phys. Rev. E. 79, 021301 (2009). / When air is injected at the bottom of an immersed granular layer, it crosses the system by percolating or fracturing. It thus forms several paths that reach the free surface of the layer at different locations. In this thesis, we study this process experimentally (for a three and two dimensional setup), numerically and theoretically. First, we focus on the dynamics of the air invading the medium at short and long time scale, when injecting a continuous air flow. At long time, the typical size of the region explored by the air can be accounted for by a diffusion-like process [1]. We also investigate the effect of gravity by tilting the experimental cell. We contrast the results with numerical simulations for the injection of a fixed volume of air, and characterize the morphology of the invasion zone. We show that the typical height and width of the region explored by the air does not depend on the injected volume only, but also on a dimensionless parameter χ which accounts for the relative effects of the gravity and capillarity [2]. Finally, when increasing the water height above the granular layer, successive grain advection and deposition form a crater consisting of two dunes growing and moving apart one from the other. We observe that the typical size of the crater increases logarithmically with time, independently of the gas emission process [3].References -[1] G.Varas, V. Vidal and J.-C. Géminard, Phys. Rev. E. 83, 011302 (2011).[2] G.Varas, V. Vidal and J.-C. Géminard, Phys. Rev. E. 83, 061302 (2011).[3] G.Varas, V. Vidal and J.-C. Géminard, Phys. Rev. E. 79, 021301 (2009).

Les Céphéides sont des étoiles pulsantes utilisées pour calculer les distances dans l'univers (notamment dans le Groupe Local). Elle font partie de la bande d’instabilité du diagramme de Hertzsprung-Russell. Le mécanisme κ (où κ désigne l’opacité du milieu), proposé par Eddington en 1917 pour expliquer ces variations périodiques de rayon et luminosité, est encore mal connu dans lecas de Céphéides froides (bord rouge de la bande d’instabilité) présentant une zone convective en surface.Cette thèse consiste à effectuer des simulations 3D de ces étoiles afin d’étudier l’interaction entre leurs pulsations radiales acoustiques et la convection de surface. On se ramène à un cas simple : l’étude de la propagation de modes acoustiques dans une boite cartésienne dont une partie est convective.Nous utilisons le code Heracles développé au CEA par Édouard Audit, que nous avons complété (ajout d’une étape de dissipation, passage à l’ordre 2 en temps pour l’étape de conduction).Pour valider notre code et notre modèle de Céphéide, nous reproduisons les résultats de Gastine & Dintrans (2011b) à 1D (cas purement radiatifs) et 2D (avec convection) : certaines simulations instables au κ-mécanisme (avec une saturation non-linéaire aux temps longs à 1D) redeviennent stables à 2D à cause de l’influence de la convection. La bande d’instabilité théorique du diagramme de Hertzsprung-Russell est donc trop étroite à 2D.L’hypothèse à vérifier était alors que ces Céphéides retrouvent une certaine instabilité et une saturation non-linéaire lors de simulations 3D. En effet, il est connu que les simulations 2D et 3D de convection différent à bas nombre de Prandtl, ce qui est notre cas. Dès lors, nous montrons que les panaches de convection sont en effet moins forts à 3D et ne perturbent pas autant les pulsations. On retrouve ainsi des simulations instables. Le bord rouge de la bande d’instabilité théorique est donc plus proche du bord observé à 3D qu’à 2D.Enfin, nous montrons que les modèles 1D de convection dépendante du temps de Stellingwerf (1982) et de Kuhfuss (1986) donnent des résultats similaires lorsqu’il s’agit de reproduire le profil de flux convectif des simulations 3D, lorsque la simulation présente des mouvements d’ensemble de type κ-mécanisme. Les deux modèles peinent cependant à reproduire les valeurs de ce flux convectif de manière convaincante. Cela justifie l’importance des simulations 3D. / Cepheids are pulsating stars used to calculate distances in the universe (more precisely in the Local Group). They are part of the Hertzsprung-Russell diagram’s instability strip. Their periodic variations (of luminosity and radius) are well explained by κ-mechanism, first suggested by Eddington (1917). But cold Cepheids (red edge of the instability strip) have a convective zone near their surface that affects their pulsation properties.Therefore, this PhD. thesis aims at performing 3D simulations of simplified Cepheids to study the interaction between surface convection and radial pulsations. We actually study the propagation of acoustic modes in a cartesian box partially convective.For this, we use Heracles, a hydrodynamical code developed in CEA, France, by Edouard Audit. We had to complete the code with a dissipation step and a second order (in time) conduction step (which was already available as a first order method).To validate the code and the Cepheid model used, we reproduce Gastine & Dintrans (2011b) results in 1D (radiative cases) and 2D (with convection): some setups that are unstable for κ-mecanism (with nonlinear saturation in 1D simulations) are stable in 2D thanks to convection. The theoretical Hertzsprung-Russell diagram’s instability strip is then too narrow in 2D.The hypothesis for this work was that those Cepheids would be unstable again (with saturation) in 3D due to the fact that convection grows weaker when convective plumes are 3D. 2D and 3D simulations are indeed different when Prandtl number is low as it is in our simulations. We show that pulsations are indeed not quenched anymore and that simulations are unstable. Theoretical 3D instability strip is then closer to the observed one than 2D strip was.Finally, we show that the 1D models of time-dependant convection from Stellingwerf (1982) and Kuhfuß (1986) give similar results for the convective flux of 3D convection with κ-mecanism. But none of them is able to give the exact values. That means that 3D simulations are indeed precious.

Context. Whether or not a self-gravitating accretion disk fragments is still an open issue. There are many different physical and numerical explanations for fragmentation, but simulations often show a non-convergent behavior for ever better resolution. Aims. We aim to investigate the influence of different numerical limiters in Godunov type schemes on the fragmentation boundary in self-gravitating disks. Methods. We have compared the linear and non-linear outcomes in two-dimensional shearingsheet simulations using the VANLE ER and the SUPERBEE limiter. Results. We show that choosing inappropriate limiting functions to handle shock-capturing in Godunov type schemes can lead to an overestimation of the surface density in regions with shallow density gradients. The effect amplifies itself on timescales comparable to the dynamical timescale even at high resolutions. This is exactly the environment in which clumps are expected to form. The effect is present without, but scaled up by, self-gravity and also does not depend on cooling. Moreover it can be backtracked to a well known effect called oversteepening. If the effect is also observed in the linear case, the fragmentation limit is shifted to larger values of the critical cooling timescale.