Cette thèse apporte une contribution à la simulation numérique de la construction de dépôts dans le cadre de la projection plasma type APS (Atmospheric Plasma Spraying). Ce travail est focalisé sur la construction d’un volume représentatif du revêtement en prenant en compte l’ensemble des phénomènes propres au procédé, tels que la nature de l’écoulement de gaz, la cinétique (multiphasique, mouillabilité) et la thermique (transferts thermiques, résistance thermique de contact, solidification) durant l’impact et l’empilement de particules. Une méthode numérique particulière, appelée VOF-SM (Volume Of Fluid - Sub Mesh), est développée. La simulation de l’impact d’un jet instationnaire et turbulent de plasma ArH2 sur un substrat a été réalisée pour définir la nature de l’écoulementen proche paroi et le transfert thermique entre cet écoulement et le substrat. Les phénomènes propres à l’impact de particules ont été intégrés au code de calcul Thétiset validés indépendamment par comparaison à des solutions analytiques et combinés par comparaison à un cas d’étude expérimentale millimétrique. Enfin, une étude d’impacts successifs de particules de Zircone Yttriée sur un substrat en acier a été menée, par une approche en similitudes thermique et cinétique pour pallier la difficulté de la résolution à petites échelles. / This PhD thesis is a contribution to the numerical simulation of the plasma sprayedcoating build-up by APS process (Atmospheric Plasma Spraying). This work focuses onthe build-up of a representative volume of the coat considering a great range of phenomenonappearing in APS process such as gas flow properties, kinetic (multiphase flow,wettability) and thermal (heat transfers, thermal contact resistance, solidification) duringthe impact and steaking of particles. An original numerical method, named VOF-SM(Volume Of Fluid - Sub Mesh) is developped. The simulation of the impact of an unsteadyand turbulent ArH2 plasma flow is carried out in order to define the gas flow closeto the wall and heat transferred to the substrate by the plasma. Specific phenomena of theimpact of particles were incorporated into the CFD code (Thétis) and validated independentlyby caparison with analytical solutions, then together combined by the comparisonto a millimeter size impact experimental data. Finally, a study of successive impacts ofYttria-Stabilized Zirconia particles onto a steel substrate was carried out by thermal andkinetic approach similarities to overcome the difficulty of resolving small scales.

L'objectif de cette thèse est de développer un nouveau code pour prédire l'équilibre final d'un processus chimique complexe impliquant beaucoup de produits, plusieurs phases et plusieurs processus chimiques. Des méthodes numériques ont été développées au cours des dernières décennies pour prédire les équilibres chimiques finaux en utilisant le principe de minimisation de l'enthalpie libre du système. La plupart des méthodes utilisent la méthode des « multiplicateurs de Lagrange » et résolvent les équations en employant une approximation du problème de Lagrange et en utilisant un algorithme de convergence pas à pas de type Newton-Raphson. Les équations mathématiques correspondantes restent cependant fortement non linéaires, de sorte que la résolution, notamment de systèmes multiphasiques, peut être très aléatoire. Une méthode alternative de recherche du minimum de l’énergie de Gibbs (MCGE) est développée dans ce travail, basée sur une technique de Monte-Carlo associée à une technique de Pivot de Gauss pour sélectionner des vecteurs composition satisfaisant la conservation des atomes. L'enthalpie libre est calculée pour chaque vecteur et le minimum est recherché de manière très simple. Cette méthode ne présente a priori pas de limite d’application (y compris pour las mélanges multiphasiques) et l’équation permettant de calculer l’énergie de Gibbs n’a pas à être discrétisée. Il est en outre montré que la précision des prédictions dépend assez significativement des valeurs thermodynamiques d’entrée telles l'énergie de formation des produits et les paramètres d'interaction moléculaire. La valeur absolue de ces paramètres n'a pas autant d’importance que la précision de leur évolution en fonction des paramètres du process (pression, température, ...). Ainsi, une méthode d'estimation cohérente est requise. Pour cela, la théorie de la « contribution de groupe » est utilisée (ceux de UNIFAC) et a été étendue en dehors du domaine d'interaction moléculaire traditionnel, par exemple pour prédire l'énergie de formation d’enthalpie libre, la chaleur spécifique... Enfin, l'influence du choix de la liste finale des produits est discutée. On montre que la prédictibilité dépend du choix initial de la liste de produits et notamment de son exhaustivité. Une technique basée sur le travail de Brignole et Gani est proposée pour engendrer automatiquement la liste des produits stable possibles. Ces techniques ont été programmées dans un nouveau code : CIRCE. Les travaux de Brignole et de Gani sont mis en œuvre sur la base de la composition atomique des réactifs pour prédire toutes les molécules « réalisables ». La théorie de la « contribution du groupe » est mise en œuvre pour le calcul des propriétés de paramètres thermodynamiques. La méthode MCGE est enfin utilisée pour trouver le minimum absolu de la fonction d'enthalpie libre. Le code semble plus polyvalent que les codes traditionnels (CEA, ASPEN, ...) mais il est plus coûteux en termes de temps de calcul. Il peut aussi être plus prédictif. Des exemples de génie des procédés illustrent l'étendue des applications potentielles en génie chimique. / The objective of this work is to develop a new code to predict the final equilibrium of a complex chemical process with many species/reactions and several phases. Numerical methods were developed in the last decades to predict final chemical equilibria using the principle of minimizing the Gibbs free energy of the system. Most of them use the “Lagrange Multipliers” method and solve the resulting system of equations under the form of an approximate step by step convergence technique. Notwithstanding the potential complexity of the thermodynamic formulation of the “Gibbs problem,” the resulting mathematical formulation is always strongly non-linear so that solving multiphase systems may be very tricky and having the difficult to reach the absolute minimum. An alternative resolution method (MCGE) is developed in this work based on a Monte Carlo technique associated to a Gaussian elimination method to map the composition domain while satisfying the atom balance. The Gibbs energy is calculated at each point of the composition domain and the absolute minimum can be deduced very simply. In theory, the technique is not limited, the Gibbs function needs not be discretised and multiphase problem can be handled easily. It is further shown that the accuracy of the predictions depends to a significant extent on the “coherence” of the input thermodynamic data such the formation Gibbs energy of the species and molecular interaction parameters. The absolute value of such parameters does not matter as much as their evolution as function of the process parameters (pressure, temperature, …). So, a self-consistent estimation method is required. To achieve this, the group contribution theory is used (UNIFAC descriptors) and extended somewhat outside the traditional molecular interaction domain, for instance to predict the Gibbs energy of formation of the species, the specific heat capacity… Lastly the influence of the choice of the final list of products is discussed. It is shown that the relevancy of the prediction depends to a large extent on this initial choice. A first technique is proposed, based on Brignole and Gani‘s work, to avoid omitting species and another one to select, in this list, the products likely to appear given the process conditions. These techniques were programmed in a new code name CIRCE. Brignole and Gani-‘s method is implemented on the basis of the atomic composition of the reactants to predict all “realisable” molecules. The extended group contribution theory is implemented to calculate the thermodynamic parameters. The MCGE method is used to find the absolute minimum of the Gibbs energy function. The code seems to be more versatile than the traditional ones (CEA, ASPEN…) but more expensive in calculation costs. It can also be more predictive. Examples are shown illustrating the breadth of potential applications in chemical engineering.

Boost dc-to-dc converters have very good source interface properties. The input inductor makes the source current smooth and hence these converters provide very good EMI performance. On account of this good property, the boost converter is also the preferred converter for off-line UPF rectifiers. One of the issues of concern in these converters is the large size of the storage capacitor on the dc link. The boost converter suffers from the disadvantage of discontinuous current injected to the load. The size of the capacitor is therefore large. Further, the ripple current in the capacitor is as much as the load current; hence the ESR specification of the tank capacitor is quite demanding. This is specially so in the emerging application areas of automotive power conversion, where the input voltage is low (typically 12V) and large voltage boost (4 to 5) are desired. The first part of this thesis suggests multi-phase boost converter to overcome the disadvantages of large size storage capacitor in boost converter. Comparison between the specification of single stage and multi-stages is thoroughly examined. Besides the average small signal analysis of N converters in parallel and obtaining an equivalent second order system are discussed. By paralleling the converters the design of closed loop control is a demanding task. To achieve proper current sharing among the stages using current control method is inevitable. Design and implementation of closed loop control of multi-phase boost converter both in analog and digital is the topic of next part of the thesis. Comparison between these two approaches is presented in this part and it will be shown that digital control is more convenient for such a topology on account of the requirement of synchronization, phase shifted operation, current balancing and other desired functions, which will be discussed later in detail. A new direct digital control method, which is simple and fast, is developed. Two different realizations with DSP controller and FPGA controller are considered. In the last part of the thesis a novel soft switching circuit for boost converter is presented. It provides Zero Voltage Switching (ZVS) for the main switch and Zero Current Switching (ZCS) for the auxiliary switch. The paper presents the idealized analysis giving all the circuit intervals and the equations necessary for the design of such a circuit. The proposed soft switching circuit is particularly suited for the tapped-inductor boost circuit with a minimum number of extra components. Extension of the method to tapped inductor boost converter addresses the application of Zero Voltage Transition (ZVT) to high conversion ratio converters. Extension of the method to multiphase boost converter shows that with less number of auxiliary switches soft switching operation can be achieved for all interleaved switching devices. Several laboratory prototype boost converters have been built to confirm the theoretical results and design methods are matching with both simulation and experimental results.

Of all the strategies to reduce carbon emissions, carbon dioxide (CO₂) geological sequestration is an immediately available option for removing large amounts of the gas from the atmosphere. However, our understanding of the transition behavior between Forchheimer and Darcy flow through porous media during CO₂ injection is currently very limited. In addition, the kinetic mass transfer of SO₂ and CO₂ from CO₂ stream to the saline and the fully coupling between the changes of porosity and permeability and multiphase flow are two significant dimensions to investigate the brine acidification and the induced porosity and permeability changes due to SO₂ co-injection with CO₂. Therefore, this dissertation develops a multiphase flow, contaminant transport and geochemical model which includes the kinetic mass transfer of SO₂ into deep saline aquifers and obtains the critical Forchheimer number for both water and CO₂ by using the experimental data in the literature. The critical Forchheimer numbers and the multiphase flow model are first applied to analyze the application problem involving the injection of CO₂ into deep saline aquifers. The results show that the Forchheimer effect would result in higher displacement efficiency with a magnitude of more than 50% in the Forchheimer regime than that for Darcy flow, which could increase the storage capacity for the same injection rate and volume of a site. Another merit for the incorporation of Forchheimer effect is that more CO₂ would be accumulated in the lower half of the domain and lower pressure would be imposed on the lower boundary of the cap-rock. However, as a price for the advantages mentioned above, the injection pressure required in Forchheimer flow would be higher than that for Darcy flow. The fluid flow and contaminant transport and geochemical model is then applied to analyze the brine acidification and induced porosity and permeability changes due to SO₂ co-injection. The results show that the co-injection of SO₂ with CO₂ would lead to a substantially acid zone near the injecting well and it is important to include the kinetic dissolution of SO₂ from the CO₂ stream to the water phase into the simulation models, otherwise considerable errors would be introduced for the equilibrium assumption. This study provides a useful tool for future analysis and comprehension of multiphase Darcy-Forchheimer flow and brine acidification of CO₂ injection into deep saline aquifers. Results from this dissertation have practical use for scientists and engineers concerned with the description of flow behavior, and transport and fate of SO₂ during SO₂ co-injection with CO₂ in deep saline aquifers.

Oil well blow outs are investigated to determine methods to quickly and accurately respond to an emergency situation. Flow rate is needed to guide containment and dispersal operations. The Stratified Integral Multiphase Plume, SIMP, model was used to investigate the range of initial conditions available to integral modeling. Sensitivity to initial conditions is modest, but without experimental data at the appropriate scale the most accurate condition is unable to be determined. Flow rates are difficult to directly measure in blow out situations, so another method must be determined; therefore, sensitivity of several parameters to flow rate was also evaluated. Methane concentration in the first intrusion can be used in conjunction with velocity and trap height measurements to determine flow rate using an integral model. Plume width and temperature were determined to have little sensitivity. Separately, a containment dome was tested in the laboratory to determine if a full scale dome can be used to contain an oil leak in the field. The dome was found to have satisfactory entrapment in the designed position.

This thesis presents a numerical method for the phase-resolving simulation of rigid particles and deformable bubbles in viscous, magnetohydrodynamic flows. The presented approach features solid robustness and high numerical efficiency. The implementation is three-dimensional and fully parallel suiting the needs of modern high-performance computing. In addition to the steps towards magnetohydrodynamics, the thesis covers method development with respect to the immersed boundary method which can be summarized in simple words by From rigid spherical particles to deformable bubbles. The development comprises the extension of an existing immersed boundary method to non-spherical particles and very low particle-to-fluid density ratios. A detailed study is dedicated to the complex interaction of particle shape, wake and particle dynamics. Furthermore, the representation of deformable bubble shapes, i.e. the coupling of the bubble shape to the fluid loads, is accounted for. The topic of bubble interaction is surveyed including bubble collision and coalescence and a new coalescence model is introduced. The thesis contains applications of the method to simulations of the rise of a single bubble and a bubble chain in liquid metal with and without magnetic field highlighting the major effects of the field on the bubble dynamics and the flow field. The effect of bubble coalescence is quantified for two closely adjacent bubble chains. A framework for large-scale simulations with many bubbles is provided to study complex multiphase phenomena like bubble-turbulence interaction in an efficient manner.

Οι ηλεκτρονικοί αρμονικοί ταλαντωτές είναι βαθμίδες με ευρεία εφαρμογή σε τηλεπικοινωνιακά συστήματα, σε συστήματα επεξεργασίας σήματος και σε ηλεκτρονικά ισχύος. Στην εργασία αυτή, προτείνεται μια νέα γενική τοπολογία ταλαντωτή πολλαπλών φάσεων, η οποία λειτουργεί στο πεδίο του υπερβολικού ημιτόνου. Κύρια πλεονεκτήματα είναι η δυνατότητα λειτουργίας σε περιβάλλον χαμηλής τάσης τροφοδοσίας και η ηλεκτρονική ρύθμιση της συμπεριφοράς του ταλαντωτή. Ως παράδειγμα σχεδίασης δίνεται ένας αρμονικός ταλαντωτής πολλαπλών φάσεων εξόδου 3ης/6ης τάξης, που η εξομοίωση της λειτουργίας του έγινε με τη χρήση του λογισμικού Analog Design Environment της Cadence. / Electronic harmonic oscillators are stages with wide application in telecommunication systems, in signal processing systems and power electronics. In this project, a new multiphase oscillator topology is proposed, designed to operate in sinh domain. Main advantages are the ability to operate at low voltage and the electronic tuning of the oscillator’s behavior. A multiphase sinusoidal oscillator, 3rd/6th order, is given as an example. The validity of the proposed methods is verified through simulation results using the Cadence Analog Design Environment software.

[pt] Acidentes com influxo descontrolado de hidrocarbonetos em um poço de petróleo (blowouts) são eventos com baixa probabilidade de ocorrência na indústria, porém têm impactos catastróficos. O evento de Macondo, com uma sonda afretada pela British Petroleum (BP) no Golfo do México (GoM), mostrou que um acidente dessa proporção tem um impacto significativo nas pessoas, no meio ambiente, nos ativos e na imagem da empresa. Portanto, uma resposta rápida e definitiva para o problema se mostra necessária. Dentre as possibilidades de se conter um derramamento de óleo ocasionado por um blowout, a mais efetiva para cessar o vazamento com segurança e abandonar definitivamente o poço em descontrole é o poço de alívio. Trata-se de um poço direcional perfurado a uma determinada distância do poço em blowout, respeitando-se critérios mínimos de segurança, com o objetivo de interceptar este no ponto estabelecido em projeto. Após a interceptação, injeta-se fluido de alta densidade que, quando preenche o poço que estava em blowout, gera uma contra-pressão capaz de cessar o influxo de hidrocarbonetos do reservatório. Quando é feita essa contra-pressão e o poço que estava em blowout estiver estável e sem influxo, injeta-se cimento pelo poço de alívio para que seja tamponado o reservatório e o poço possa ser abandonado de forma definitiva. No presente trabalho o enfoque é no amortecimento do poço em blowout através do poço de alívio e, portanto, parte-se do pressuposto que a detecção e interceptação do poço em influxo foi feita com sucesso no ponto desejado. A detecção, a interceptação e o abandono no poço em blowout não são estudados com detalhes. O trabalho é desenvolvido tomando como base um poço exploratório de gás a ser simulado, trabalhando os requisitos de pressão e vazão que melhor se adequam ao proposto. O objetivo do amortecimento do poço em blowout pelo poço de alívio é cessar o influxo descontrolado de forma eficaz, otimizando os parâmetros de pressão, vazão e volume de fluido às capacidades de sondas e embarcações disponíveis no mercado. Caso não seja possível o amortecimento variando a densidade do fluido injetado e a pressão nas bombas de injeção, parte-se para mudanças na estratégia de amortecimento como, por exemplo, variação no número de poços de alívio a serem perfurados para amortecer o poço em blowout ao mesmo tempo. Não será objetivo do presente trabalho a modificação no projeto do poço com o objetivo de facilitar o amortecimento. / [en] Accidents with uncontrolled influx of hydrocarbons into an oil well (blowouts) are events with a low probability of occurrence in the industry. Although, if they occur, the impact may be catastrophic. The Macondo event, with British Petroleum s rig (BP) in the Gulf of Mexico (GoM), showed that an accident of this proportion has a significant impact on people, environment and an image of the company. Therefore, a quick and definitive response for the problem is necessary. There are several possibilities to contain an oil spill caused by a blowout. However, the most effective way to safely plug and abandon (P and A) the blowout well is the relief well. This technique consists of the construction of a directional well at a certain distance from the blowout well, respecting minimum safety criteria, in order to intercept this at the point established in the project. After the interception, kill mud is pumped at high rates what makes a backpressure capable of killing the blowout well. When this backpressure is enough to kill the blowout well, cement is pumped through the relief well to plug and abandon permanently the well. In the present work, the objective is to kill the blowout well and we consider that detection and interception of the blowout well was successfully been made at the target point. Detection, interception and P and A of the blowout well are not covered in detail. This work is developed based on an exploratory gas well to be simulated, working the pump rate and pump pressure requirements that best fit in the proposed one. The purpose of well killing is to kill the well optimizing the parameters of pressure, flow and volume of fluid considering the rig and vessels available in the market. If well killing is not possible by varying the density of the kill mud and pump pressure it will be tried new strategies like changing the number of relief wells at the same time to be drilled killing the blowout well. It s not a purpose of this study to change the well design.

Multiphase flows are a common phenomenon. Rains, sediment transport in rivers, snow and dust storms, mud slides and avalanches are examples of multiphase flows occurring in nature. Blood flow is an example of multiphase flow in the human body, which is of vital importance for survival. Multiphase flows occur widely in industrial applications from hydrocarbon extrac-tion to fuel combustion in engines, from spray painting to spray drying, evaporators, pumps and pneumatic conveying. Predicting multiphase flows is of vital importance to understand natural phenomenon and to design and improve industrial processes. Separated flows and dispersed flows are two types of multiphase flows, which occur together in many industrial applications. Physical features of these two classes are different and the transition from one to another involves complex flow physics. Experimental studies of multiphase flows are not easy, as most real world phenomenon cannot be scaled down to laboratory models. Even for those phenomenon that can be demonstrated at lab-oratory scale, rescaling to real world applications requires mathematical models. There are many challenges in experimental measurements of multiphase flows as well. Measurement techniques well suited for single phase flows have constraints when measuring multiphase phenomenon. Un-certainty in experimental measurements poses considerable difficulties in validating numerical models developed for predicting these flows. Owing to the computational effort required, direct simulation of multiphase flows, even for small scale real world applications is out of present scope. Numerical methods have been developed for dealing with each class of flow separately, that in-volves use of models for phenomenon that is computationally demanding. Reynolds Averaged Navier-Stokes (RANS) methods for predicting multiphase flows place strong requirements on turbulence models, as information about fluctuating quantities in the field, that have significant effects on dispersed phase, is not available. Large Eddy Simulation (LES) gives better predictions than RANS as the instantaneous field data is available and large scale unsteadiness that effects the dispersed phase can be captured. Recent LES studies of multiphase flows showed that the sub-grid-scale (SGS) model used for the continuous phase has an effect on the evolution of the dispersed phase. In this work, LES of multiphase flows is performed using Explicit Filtering Large Eddy Sim-ulation method. In this method, spatial derivatives are computed using higher order compact schemes that have spectral-like resolution. SGS modeling is provided by the use of a filter with smoothly falling transfer function. This method is mathematically consistent and converges to a DNS as the grid is refined. It has been successfully applied to combustion and aero-acoustics and this work is the first application of the method to multiphase flows. Study of dispersed multiphase flows was carried out in this work. Modeling of the dispersed phase is kept simple since the in-tention was to evaluate the capability of explicit filtering LES method in predicting multiphase flows. Continuous phase is solved using a compressible formulation with explicit filtering method. Spatial derivatives are computed using fourth and sixth order compact schemes that use derivative splitting method proposed by Hixon & Turkel (2000a) and second order Runge-Kutta (RK2) time stepping. The grid is stretched as needed. Non-reflecting boundary conditions due to Poinsot & Lele (1992) are used to avoid acoustic reflections from boundaries. Buffer zones (Bogey & Bailly (2002)) are employed at outflow and lateral boundaries to damp vortical structures. The code developed for continuous phase is evaluated by studying round jets at Re =36,000 and comparing with experimental measurements of Hussein et al. (1994) and Panchapakesan & Lumley (1993). Simulations showed excellent agreement with experimental results. Rate of decay of axial velocity and the evolution of turbulence intensities on the centerline matched very well with measurements. Radial profiles of mean and fluctuating components of velocities exhibit self-similarity. A set of studies were then performed using this code to assess the effect of numerical scheme, grid refinement & stretching and simulation times on the predictions. Results from these simulations showed good agreements with experiments and established the code for use in multiphase flows under various simulation conditions. To assess the prediction of multiphase flows using this LES method, an evaporating spray ex-periment by Chen et al. (2006) was simulated. The experiment uses a nebuliser for generating a finely atomized spray of acetone, which avoids complex breakdown phenomenon associated with air blast atomizers and provides well defined boundary conditions for model evaluation. The neb-uliser sits upstream in a pipe carrying air and droplets travel along with air for a distance of 10 diameters before exiting into a wind tunnel with co-flowing air. Droplet breakdown, if any, takes place inside the pipe and the spray is finely atomized by the time it reaches pipe exit. One of the experimental cases at Re =31,600, with a mass loading of 1.1% and a jet velocity of 56 m/s is simulated. Particle size has a χsquared distribution with a Sauter mean diameter of 18µm. In the self-similar region, decay of centerline velocity and turbulence intensities matched well with ex-perimental results. Continuous phase exhibits self-similar behavior. A series of simulations were then performed to match the initial region of the spray by altering the inflow conditions in the sim-ulation. Simulation that matched the breakdown location of the experiment revealed the presence of a relaxation zone with a higher initial spreading rate, followed by a lower asymptotic spreading rate. Studies were performed to understand the effect of various phenomenon like evaporation and droplet size on this behavior. A study of breakdown region of particle-laden jets was performed to understand the presence of relaxation zone post breakdown. Flow conditions were similar to evaporating spray experiment except that particles do not evaporate, mass loading is 2% and jet Reynolds number Re =2000. A series of grid refinements were performed and on the largest grid, gird spacing Δy =7.5η, where ηis an estimate of the Kolmogorov length scale based on flow conditions. Decay of axial velocity on the centerline showed variations with grid refinement, tending to the experimentally measured value as the grid is refined. Variation of turbulence intensities along the centerline revealed a jump in axial velocity fluctuations at the breakdown location, while radial and azimuthal velocities showed a smooth increase to their asymptotic value. This jump was resolved on grid refinement and on fine grids axial velocity fluctuations followed the other two quantities closely in their rise to asymptotic state. Comparison of these quantities with a jet without particles revealed that the flow features are same for a jet with and without particles, and at the mass loading studied, particles have negligible effect on jet breakdown. Another study performed at a higher Reynolds number of Re =11,000, under similar flow conditions showed similar behavior. To assess the ability of predicting dispersed phase, simulations of particle-laden flows at low Stokes number were performed and compared against an experiment by Lau & Nathan (2014). The experiment studies variation of velocity and particle concentration along the centerline, and half widths of a jet velocity and concentration. Particles are injected into a pipe along with air, and the two phase flow is fully developed by the time it exits the pipe into a wind tunnel along with a co-flow. Particles are mono-disperse with a density of 1200 kg/m3. Mass loading is 40% so that particles have a significant effect on the continuous phase. Two cases at particle Stokes number of 1.4, one with Re =10,000, bulk velocity of 12 m/s and particle diameter of 20µm and another with Re =22,500, bulk velocity of 36 m/s and particle diameter of 10µm were simulated. Simulations of both the cases showed good match with experimental measurements of centerline decay for the continuous phase. For the dispersed case, simulations with larger particles showed good match with experimental results, while smaller particles showed differences. This was understood to be the effect of lateral migration which is prominent in case of smaller particles, the models for which have not been used in the present simulation study.

The production and transportation of hydrocarbon fluids in multiphase pipelines could be severely hindered by particulate solids deposit such as produced sand particles which accompany hydrocarbon production. Knowledge of the flow characteristics of solid particles in fluids transported in pipelines is important in order to accurately predict solid particles deposition in pipelines. This research thesis presents the development of a three-dimensional (3D) Computational Fluids Dynamics (CFD) modelling technique for the prediction of liquid-solids multiphase flow in pipes, with special emphasis on the flow in V-inclined pipe bends. The Euler-Euler (two-fluid) multiphase modelling methodology has been adopted and the multiphase model equations and closure models describing the liquid-solids flow have been implemented and calculated using the finite volume method in a CFD code software. The liquid phase turbulence has been modelled using a two-equation k−ε turbulence model which contains additional terms to account for the effects of the solid-particles phase on the multiphase turbulence structure. The developed CFD numerical framework has been verified for the relevant forces and all the possible interaction mechanisms of the liquid-solids multiphase flow by investigating four different numerical frameworks, in order to determine the optimum numerical framework that captures the underlying physics and covers the interaction mechanisms that lead to sand deposition and the range of sand transport flow regimes in pipes. The flow of liquid-sand in pipe has been studied extensively and the numerical results of sand concentration distribution across pipe and other flow properties are in good agreement with published experimental data on validation. The numerical framework has been employed to investigate the multiphase flow in V-inclined pipe bends of ±4o−6o, seemingly small inclined bend angles. The predicted results which include the sand segregation, deposition velocity and flow turbulence modulation in the pipe bend show that the seemingly small pipe bends have significant effect on the flow differently from that of horizontal pipes. The pipe bend causes abrupt local change in the multiphase flow characteristic and formation of stationary sand deposit in the pipe at a relatively high flow velocity. The threshold velocity to keep sand entrained in liquid in pipe bends is significantly higher than that required for flow horizontal pipes. A critical implication of this is that the correlations for predicting sand deposition in pipelines must account for the effect of pipe bend on flow characteristics in order to provide accurate predictions of the critical sand transport velocity (MTV) in subsea petroleum flowlines, which V-inclined pipe bends are inevitable due to seabed topology.