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[ES] Este proyecto de tesis tiene como objetivo el estudio de flujos monofásicos y multifásicos en una tubería equipada con elementos estáticos, centrándose específicamente en los elementos tipo Kenics. Su diseño consta de un plano retorcido 180° ensamblado con diferentes configuraciones, cambiando el número y orientación de los elementos. El estudio pretende contribuir al conocimiento existente sobre los elementos estáticos, dadas sus ventajas y aplicaciones extendidas en la industria química y de procesos. Optar por elementos estáticos puede proporcionar una solución de alta eficiencia y bajo costo para mezclar o separar fases dentro de líneas de transporte. Se utilizó Velocimetría de Imagen de Partículas (PIV) para medir el campo de velocidad del flujo monofásico turbulento completamente desarrollado. El experimento comenzó con una tubería horizontal donde el escenario monofásico se analizó más detalladamente mediante Tomografía de Resistencia Eléctrica (ERT), observando la distribución de un trazador en varios planos de sección transversal a lo largo de la tubería. Además, se utilizó un transductor de presión diferencial para medir la caída de presión a través de los elementos estáticos. En los experimentos de flujo bifásico, la tubería se orientó verticalmente, donde se investigó el flujo ascendente de aire y agua. Con la ayuda de una cámara de alta velocidad y la técnica ERT mencionada anteriormente, se evaluó la distribución de la fracción volumétrica de gas aguas arriba y aguas abajo de los elementos estáticos, evaluando así la idoneidad de la configuración del elemento seleccionado para la separación gas-líquido. Finalmente, la combinación de técnicas ópticas y tomográficas ha permitido seleccionar el algoritmo de reconstrucción más fiable para aplicar en el tratamiento de datos de ERT. Además, el resultado del proyecto ayudará a conocer las principales características globales y locales del flujo y servirá como punto de referencia para la validación de sim, [EN] This thesis project is aimed at studying single-phase and multi-phase flows in a pipeline equipped with static elements, focused specifically on the Kenics-type elements. Its design consists of a flat plane twisted 180°, assembled with different configurations by changing the number and orientation of the elements. The study intends to contribute to the existing knowledge about static elements, given their advantages and extended applications in the chemical and process industry. Opting for these static elements can provide a high-efficiency and low-cost solution when trying to achieve mixing or separation inside transport pipelines. Particle Image Velocimetry (PIV) was used to measure the velocity field of the fully developed turbulent single-phase flow. The experiment started with a horizontal pipe where the single-phase scenario was further analysed using Electrical Resistance Tomography (ERT), observing the distribution of a tracer in various cross-sectional planes along the pipe. Additionally, a differential pressure transducer was used to measure the pressure drop across the static elements. In the two-phase flow experiments, the pipeline was orientated vertically, where the ascendant flow of air and water was investigated. With the aid of a high-speed camera and the previously mentioned ERT technique, the gas volume fraction distribution was evaluated upstream and downstream of the static elements, thus assessing the suitability of the selected element configuration for gas-liquid separation. Finally, the combination of optical and tomographic techniques has allowed the selection of the most reliable reconstruction algorithm to be applied for the ERT data treatment. Moreover, the outcome of the project will help gain insight into the main global and local characteristics of the flow and will serve as a benchmark for the validation of Computational Fluid Dynamics (CFD) simulations carried out in a parallel investigation.

Melt infiltration into porous media is an intriguing phenomenon that holds immense significance across various sciences and technologies. In this work, the problem of metallic melt infiltration in particulate beds is investigated for understanding and prediction of severe accident progression associated with a molten pool penetrating through an underlying debris bed which may form in the reactor core or in the lower head of a light water reactor. The present study aims to quantify the effect of particle surface's wettability on melt infiltration kinetics. For this purpose, two categories of experiment are conceived and carried out to measure the wettability of different material surfaces by melt and to characterize melt infiltration kinetics in one-dimensional particulate beds, respectively. The melt material is tin–bismuth eutectic alloy with a melting point of 139 °C. Copper (Cu), stainless steel (SS), Tin (Sn) and tin-coated stainless steel (Sn-coated SS) are chosen as materials of substrates and particles in wettability measurement and melt infiltration study. The particulate beds, packed with 1.5 mm spheres, are preheated to 200 °C before the melt infiltration begins. The experimental data of wettability measurement shows that the contact angles of liquid Sn-Bi eutectic on the above-mentioned material surfaces range from 79° to 135°. The results of melt infiltration tests confirm the significant effect of wettability on melt penetration kinetics. The capillary force plays a significant role in the initial infiltration of particulate beds. Specifically, a wettable particulate bed enhances the initial melt infiltration, whereas non-wettable beds hinder it., QC 20240702

A novel mixed enriched finite element model is developed for coupled non-linear thermo-hydro-mechanical simulation of fractured porous media with three-phase flow and thermal coupling. Simulation of induced acoustic emission (AE) and microseismic emission (ME) due to tensile fracturing and shear slip instability of pre-existing fracture interfaces is carried out and the numerical results of the emitted signals are analysed. The mathematical model is based on the generalized Biot's theory for coupled interaction of solid and fluid phases. A computationally robust non-linear solver is developed to handle the severe non-linearities arising from fluid saturations, relative permeabilities of fluids, constitutive models of interfaces and convective thermal coupling. To model pre-existing natural fractures and faults, discrete fracture propagation and nucleation of cracks (micro-cracking) independently of the original mesh topology, a local Partition-of-Unity (PU) finite element method, namely, the Phantom Node Method (PNM) is implemented. The cohesive fracture modelling scheme is implemented to account for the non-linear behaviour of fracturing and localization, and to rectify the non-physical stress singularity condition at the fracture tip. Effects of different system parameters on fracturing, shear-slip instability and the associated induced AEs and MEs are investigated through various numerical results.

Among the different thermo-chemical recycling routes for plastic waste valorization, gasification is one of the most promising, converting plastic waste into syngas (H2+CO) and energy in the presence of an oxygen-rich gas. Plastic waste gasification is associated with many different complexities due to the multi-scale nature of the process, the feedstock complexity (mixed polyolefins with different contaminations), intricate reaction mechanisms, plastic properties (melting behavior and molecular weight distribution), and complex transport phenomena in a multi-phase flow system. Hence, creating a reliable model calls for an extensive understanding of the phenomena at all scales, and more advanced modeling approaches than those applied today are required. Indeed, modeling of plastic waste gasification (PWG) is still in its infancy today. Our review paper shows that the thermophysical properties are rarely properly defined. Challenges in this regard together with possible methodologies to decently define these properties have been elaborated. The complexities regarding the kinetic modeling of gasification are numerous, compared to, e.g., plastic waste pyrolysis, or coal and biomass gasification, which are elaborated in this work along with the possible solutions to overcome them. Moreover, transport limitations and phase transformations, which affect the apparent kinetics of the process, are not usually considered, while it is demonstrated in this review that they are crucial in the robust prediction of the outcome. Hence, possible approaches in implementing available models to consider these limitations are suggested. Finally, the reactor-scale phenomena of PWG, which are more intricate than the similar processes—due to the presence of molten plastic—are usually simplified to the gas-solid systems, which can result in unreliable modeling frameworks. In this regard, an opportunity lies in the increased computational power that helps improve the model’s precision and al

This study develops robust numerical algorithms for the simulation of three-phase (solid-liquid-gas) boiling and bubble formation problems in rectangular channels. The numerical algorithms are based on the Finite Volume Methods (FVM) and implement both the volume-of-fluid (VOF) methods for liquid-gas interface tracking as well as the volume-fraction methods to account for the concentration of embedded solid nano-particles in the liquid phase. Water is used as the base-liquid and the solid phase is modelled via metallic nano-particles (both aluminium oxide and titanium oxide nano-particles are considered) that are homogeneously mixed within the liquid phase. The gas phase is considered as a vapour arising from the bolling processes of the liquid-phase. The finite volume methodology is implemented on the OpenFOAM software platform, specifically by careful modification and manipulation of existing OpenFOAM solvers. The governing fluid dynamical equations, for the three-phase boiling problem, take into account the thermal conductivity effects of the solid (nano-particle), the momentum and energy equations for both the liquid-phase and the gas-phase, and finally the decoupled mass conservation equations for the liquid- and gas- phases. The decoupled mass conservation equations are specifically used to model the phase change between the liquid- and gas- phases. In addition to the FVM and VOF numerical methodologies for the discretization of the governing equations, the pressure-velocity coupling is resolved via the PIMPLE algorithm, a combination of the Pressure Implicit with Splitting of Operator (PISO) and the Semi-Implicit Method for Pressure-Linked Equations (SIMPLE) algorithms. The computational results are presented graphically with respect to variations in time as well as in the nano-particle volume fractions. The simulations and results accurately capture the formation of vapour bubbles in the two-phase (particle-free) liquid-gas flow and additionally the computat

Zonal modeling is a common technique for the numerical certification of fire-extinguishing systems, however it is not valid to simulate the complex physical phenomena that occurs near the agent injection. We present a multi-scale method for the accurate generation of inflow boundary conditions valid for zonal modeling based on the description of the phase change of a rapidly depressurised mist of a fire suppression system. The generation of accurate boundary conditions includes the characterization of the injection of the fire suppression agent from atomization to evaporation and mixing. The multi-scale methodology is based on the use of a high fidelity multiphase conservative level set LES for the characterization of the nozzle to develop an empirical model for primary breakup. Secondly, a low fidelity particle-based method with phase change and unsteady RANS is used for parametric studies. This multi-scale approach requires an affordable computational effort. The multi-scale methodology is tested in a system consisting of a pressurised fire extinguishing agent (Novec-1230) that is injected into the ambient through a nozzle that produces the atomization of the agent. The accuracy of the developed approach is compared with the experimental data., This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the grant agreement No 785549 (FireExtintion: H2020-CS2-CFP06-2017-01)., Peer Reviewed, Postprint (author's final draft)

This study investigates the potential of a newly released multi-phase solver to simulate atomisation in an air-blast type atomiser. The 'VOF-to-DPM' solver was used to simulate primary and secondary atomisation in an atomiser with a coaxial injector-like geometry. The solver uses a hybrid Eulerian/Eulerian-Lagrangian formulation with geometric transition criteria between the two models. In this study isothermal, non-reacting flow at room temperature was assumed. The primary focus was predicting Sauter mean diameter and droplet velocity data at a sampling plane downstream of the injector. The solver produces the expected data and predicts trends similar to those found in experimental measurements. The accuracy of the produced droplet diameters was roughly a factor 2 off compared to experiment. This is attributed primarily to mesh resolution. It was concluded that the solver has the potential to predict atomisation at a reasonable computational cost, but further study is needed to confirm its full capabilities., Accepted Author Manuscript, Fluid Mechanics, Space Systems Egineering

Extremely small pore size, low porosity, and ultra-low permeability are among the characteristics of shale rocks. In tight shale reservoirs, the nano-confinement effects that include large gas-oil capillary pressure and critical property shifts could alter the phase behaviors, thereby affecting the oil or gas production. In this research, two in-house simulation models, i.e., a compositionally extended black-oil model and a fully composition model are developed to examine the nano-pore confinement effects on convective and diffusive multicomponent multiphase fluid transport. Meanwhile, the effect of nano-confinement and rock intrinsic properties (porosity and tortuosity factor) on predicting effective diffusion coefficient are investigated. First, a previously developed compositionally extended black-oil simulation approach is modified, and extended, to include the effect of large gas-oil capillary pressure for modeling first contact miscible (FCM), and immiscible gas injection. The simulation methodology is applied to gas flooding in both high and very low permeability reservoirs. For a high permeability conventional reservoir, simulations use a five-spot pattern with different reservoir pressures to mimic both FCM and immiscible displacements. For a tight oil-rich reservoir, primary depletion and huff-n-puff gas injection are simulated including the effect of large gas-oil capillary pressure in flow and in flash calculation on recovery estimations. A dynamic gas-oil relative permeability correlation that accounts for the compositional changes owing to the produced gas injection is introduced and applied to correct for changes in interfacial tension (IFT), and its effect on oil recovery is examined. The results show that the simple modified black-oil approach can model well both immiscible and miscible floods, as long as the minimum miscibility pressure (MMP) is matched. It provides a fast and robust alternative for large-scale reservoir simulation with the purpose

Over the past decades, attention has been brought to the importance of indoor air quality and the serious threat of bio-aerosol contamination towards human health. A novel idea to transport hazardous particles away from sensitive areas is to automatically control bio-aerosol concentrations, by utilising airflows from ventilation systems. Regarding this, computational fluid dynamics (CFD) may be employed to investigate the dynamical behaviour of airborne particles, and data-driven methods may be used to estimate and control the complex flow simulations. This thesis presents a methodology for machine-learning based control of particle concentrations in turbulent gas-solid flow. The aim is to reduce concentration levels at a 90 degree corner, through systematic manipulation of underlying two-phase flow dynamics, where an energy constrained inlet airflow rate is used as control variable. A CFD experiment of turbulent gas-solid flow in a two-dimensional corner geometry is simulated using the SST k-omega turbulence model for the gas phase, and drag force based discrete random walk for the solid phase. Validation of the two-phase methodology is performed against a backwards facing step experiment, with a 12.2% error correspondence in maximum negative particle velocity downstream the step. Based on simulation data from the CFD experiment, a linear auto-regressive with exogenous inputs (ARX) model and a non-linear ARX based neural network (NN) is used to identify the temporal relationship between inlet flow rate and corner particle concentration. The results suggest that NN is the preferred approach for output predictions of the two-phase system, with roughly four times higher simulation accuracy compared to ARX. The identified NN model is used in a model predictive control (MPC) framework with linearisation in each time step. It is found that the output concentration can be minimised together with the input energy consumption, by means of tracking specified target trajector

The conducted study investigates the potential of a newly released multi-phase solver to simulate atomization in liquid rocket injectors. The "VOF-to-DPM" solver was used to simulate primary and secondary atomization in an air-blast atomizer with a coaxial injector-like geometry. The solver uses a hybrid Eulerian/Eulerian-Lagrangian formulation with a geometric transition criteria between the two models. The conducted study assumed isothermal, non-reacting flow at room temperature. The primary focus was predicting Sauter Mean Diameter and droplet velocity data at a sampling plane downstream of the injection site. The results showed that the solver is able to produce the expected data and to predict trends similar to those found in experimental measurements. The accuracy of the produced droplet diameters was roughly a factor 2 off compared to experiment. This is attributed to mesh resolution. Measurements were obtained via a cooperative agreement between TU Delft and The University of Sydney. It was concluded that the solver has the potential to predict atomization at a reasonable computational cost, but further study is needed to confirm its full capabilities., Aerospace Engineering

Energetic Variational Approach is used to derive a novel thermodynamically consistent three-phase model of a mixture of Newtonian and visco-elastic fluids. The model which automatically satisfies the energy dissipation law and is Galilean invariant, consists of coupled Navier-Stokes and Cahn-Hilliard equations. Modified General Navier Boundary Condition with fluid elasticity taken into account is also introduced for using the model to study moving contact line problems. Energy stable numerical scheme is developed to solve system of model equations efficiently. Convergence of the numerical scheme is verified by simulating a droplet sliding on an inclined plane under gravity. The model can be applied for studying various biological or biophysical problems. Predictive abilities of the model are demonstrated by simulating deformation of venous blood clots with different visco-elastic properties and experimentally observed internal structures under different biologically relevant shear blood flow conditions.

This report concerns the addition of a new well control option ‘Flow Through Surface Choke’ (CHK) to AD-GPRS, the Automatic Differentiation General Purpose Research Simulator developed at Stanford University. The report provides background information on the modelling of fluid flow through surface chokes and gives details of the implementation in AD-GPRS., This research was carried out within the context of the Integrated Systems Approach to Petroleum Production (ISAPP) knowledge centre. ISAPP is a joint project of TNO, Delft University of Technology, ENI, Statoil and Petrobras., Petroleum Engineering

Geological Carbon Storage (GCS) is considered as one of the key techniques to reduce the rate of atmospheric emissions of CO2 and thereby to contribute to controlling the global warming. A successful application of a GCS project requires the capability of the formation to trap CO2 for a long term. In this context, processes related to CO2 trapping and also possible leakage of CO2 to the near surface environment need to be understood. The overall aim of this thesis is to understand the flow and transport of CO2 through porous media in the context of geological storage of CO2. The entire range of scales, including the pore scale, the laboratory scale, the field experiment scale and the industrial scale of CO2 injection operation are addressed, and some of the key processes investigated by means of experiments and modeling. First, a numerical model and laboratory experimental setup were developed to investigate the CO2 gas flow, mimicking the system in the near-surface conditions in case a leak from the storage formation should occur. The system specifically addressed the coupled flow and mass transport of gaseous CO2 both in the porous domain as well as the free flow domain above it. The comparison of experiments and modelling results showed a very good agreement indicating that the model developed can be applied to evaluate monitoring and surface detection of potential CO2 leakage. Second, the field scale CO2 injection test carried out in a shallow aquifer in Maguelone, France was analyzed and modeled. The results showed that Monte Carlo simulations accounting for the heterogeneity effects of the permeability field did capture the key observations of the monitoring data, while a homogeneous model could not represent them. Third, a numerical model based on phase-field method was developed and model simulations carried out addressing the effect of wettability on CO2-brine displacement at the pore-scale. The results show that strongly water-wet reservoirs provide a bet

Multi bubbles interaction and merger in a micro-channel flow boiling has been numerically studied. Effects of mass flux (56, 112, 200, and 335 kg/m2 ∗ s), wall heat flux (5, 10, and 15 kW/m2) and saturated temperature (300.15 and 303.15 K) are investigated. The coupled level set and volume of fluid (CLSVOF) method and non-equilibrium phase model are implemented to capture the two-phase interface, and the lateral merger process. It is found that the whole transition process can be divided to three sub-stages: sliding, merger, and post-merger. The evaporation rate is much higher in the first two stages due to the boundary layer effects in. Both the mass flux and heat flux affect bubble growth. Specifically, the bubble growth rate increase with the increase of heat flux, or the decrease of mass flux., Funding text: The support of the Swedish Energy Agency through its research program Effsys Expand is gratefully acknowledged. QC 20170221

The dynamics and heat transfer characteristics of flow boiling bubble train moving in a micro channel is studied numerically. The coupled level set and volume of fluid (CLSVOF) is utilized to track interface and a non-equilibrium phase change model is applied to calculate the interface temperature as well as heat flux jump. The working fluid is R134a and the wall material is aluminum. The fluid enters the channel with a constant mass flux (335 kg/m(2)*s), and the boundary wall is heated with constant heat flux (14 kW/m(2)). The growth of bubbles and the transition of flow regime are compared to an experimental visualization. Moreover, the bubble evaporation rate and wall heat transfer coefficient have been examined, respectively. Local heat transfer is significantly enhanced by evaporation occurring vicinity of interface of the bubbles. The local wall temperature is found to be dependent on the thickness of the liquid film between the bubble train and the wall., QC 20171106

This paper presents a coupled air-water flow analysis to evaluate the performance of a three-layer capillary barrier for controlling water infiltration into and gas emission from a waste containment system in a high precipitation environment, and to optimize combinations of local soil layers for barrier construction. A multi-phase flow model is proposed considering the movements of the gas and water phases simultaneously. The governing partial differential equations are solved in COMSOL Multiphysics software. Several combinations of lean clay with sand (CL), clayey sand with gravel (SC), silty sand with gravel (SM), sandy silt (ML), and well-graded gravel with silt (GW-GM) are examined. The rates of percolation water and gas emission are used as indicators to compare the performance of different combinations. A fine-grained surface soil layer reduces both water infiltration and gas emission due to its low desaturation rate and high water-retention capacity. The coarse middle layer plays a critical role, promoting capillary effects and hindering water infiltration during rainfall as well as draining any infiltrated water or percolated gas.

We present simulations and experiments of drainage processes in a micro-model. A direct numerical simulation is introduced which is capable of describing wetting phenomena on the pore scale. A numerical smoothed particle hydrodynamics model was developed and used to simulate the two-phase flow of immiscible fluids. The experiments were performed in a micro-model which allows the visualization of interface propagation in detail. We compare the experiments and simulations of a quasistatic drainage process and pure dynamic drainage processes. For both, simulation and experiment, the interfacial area and the pressure at the inflow and outflow are tracked. The capillary pressure during the dynamic drainage process was determined by image analysis.

This paper provides an overview of the aims, objectives and the main findings of the CO(2)QUEST FP7 collaborative project, funded by the European Commission and designed to address the fundamentally important and urgent issues regarding the impact of the typical impurities in CO2 streams captured from fossil fuel power plants and other CO2 intensive industries on their safe and economic pipeline transportation and storage. The main features and results recorded from some of the unique test facilities constructed as part of the project are presented. These include an extensively instrumented realistic-scale test pipeline for conducting pipeline rupture and dispersion tests in China, an injection test facility in France to study the mobility of trace metallic elements contained in a CO2 stream following injection near a shallow-water qualifier and fluid/rock interactions and well integrity experiments conducted using a fully instrumented deep-well CO2/impurities injection test facility in Israel. The above, along with the various unique mathematical models developed, provide the fundamentally important tools needed to define impurity tolerance levels, mixing protocols and control measures for pipeline networks and storage infrastructure, thus contributing to the development of relevant standards for the safe design and economic operation of CCS.

Numerical Analysis

Investigation of two- and multiphase flow phenomena inside technical apparatuses and hydraulic components are of highest interest for equipment designers and operators since the knowledge helps to assess and understand complex flows, e.g. in chemical and process engineering. This in turn expedites the development of safer and more efficiently operated industrial facilities. Furthermore, measured data are used to validate new models developed for multiphase computational fluid dynamics (CFD). For non-intrusive two-phase flow investigations, two radiation-based computed tomography (CT) scanners were developed and are operated at the Helmholtz-Zentrum Dresden - Rossendorf (HZDR), Institute of Fluid Dynamics: a high-resolution gamma-ray computed tomography scanner (HireCT) first presented by Hampel et al. (2007) and an ultrafast electron beam X-ray CT scanner (ROFEX) first presented by Fischer et al. (2010). Both CT scanners are able to recover cross-sectional phase distribution images of a scanned plane or volume section within the flow as time averaged images or time resolved image sequences. The capabilities of both CT systems are introduced exemplarily with reference to experiments on an industrial scale bubble column, a fluidized bed and a static mixer.

Carbon Capture and Storage (CCS) is widely seen as an effective technique to reduce what are perceived to be excessive concentrations of Carbon Dioxide (CO2) in the atmosphere. In the CCS chain, transportation of CO2 through high-pressure pipelines constitutes an important link. Although CO2 pipelines are generally very safe, an unplanned release of CO2 from a pipeline presents a potential risk to human and animal populations as well as the environment. Therefore, to facilitate the risk assessment, it is necessary to gain a better understanding of CO2 releases from high-pressure pipelines, including the prediction of depressurisation of the pipe flow, the near-field atmospheric expansion and the far-field atmospheric dispersion.

Investigations on two- and multiphase flow phenomena inside technical apparatuses or feedings are of highest interest for designers and operators since the knowledge helps to understand the fundamental physics behind processes, e.g. in chemical and process engineering. It expedites the development of safer and more efficiently operated industrial facilities. Furthermore, measured data are used to validate new models developed for multiphase flow simulation, e.g. CFD. For non-intrusive two-phase flow investigations, two radiation-based computed tomography (CT) scanners are operated at the Helmholtz-Zentrum Dresden - Rossendorf (HZDR) at the department of Fluid Dynamics: a high-resolution gamma-ray computed tomography scanner (HireCT) and an ultrafast electron beam X-ray CT scanner (ROFEX). They are able to recover non-superimposed cross-sectional material distributions of the scanned plane or volume section within the flow as time averaged images or time resolved image sequences. The capabilities of both CT systems are demonstrated exemplarily at experiments on an industrial scale bubble column, a fluidized bed and a static mixer.

For many multi-phase flow and transport processes in porous media, miscibility of the constituent components (i.e., the chemical compounds of the fluids) is a non-negligible part of the governing physics. This thesis covers several theoretical and numerical aspects of such flows. First, the continuum-scale equations are motivated starting from the molecular scale. Then, techniques to discretize these equations are presented while keeping their parallel implementation by computer programs in mind. After the theoretical, numerical, and implementation-related aspects of such flows are covered, this thesis examines some sample applications to illustrate various computational and physical properties of such flows. Finally, this thesis is concluded and a list of related fields which it does not cover is given. The main contribution of this thesis to the scientific state of the art is the proposal of non-linear complementarity problem (NCP) based model assumptions to handle phase transitions in compositional simulations of flow in porous media as well as an extensive evaluation of its properties relative to established approaches - such as the primary variable switching (PVS) and the black-oil models., Für viele Strömungsprozesse in porösen Medien ist Mischbarkeit der chemischen Komponenten aus denen sich die Fluidphasen zusammensetzen ein nicht vernachlässigbarer Teil der zu betrachtetenden physikalischen Vorgänge. Diese Dissertation bearbeitet verschiedene theoretische und numerische Aspekte solcher Strömungen. Zuerst werden die kontinuumsskaligen Gleichungen mithilfe der molekularen Skala motiviert. Anschließend werden Diskretisierungstechniken für diese Gleichungen besprochen, wobei ihre Implementierung in parallelen Computerprogrammen mitberücksichtigt wird. Nachdem die theoretischen, numerischen und Implementierungsabhängigen aspekte dieser Strömungen behandelt wurden, betrachtet diese Dissertation einige Beispielanwendungen um die verschiedenen numerischen und physischen eigenschaften dieser zu untersuchen. Zuletzt wird ein kurzer Ausblick auf weiterführende Themen gegeben, die nicht im Rahmen dieser Arbeit besprochen werden konnten. Der Hauptbeitrag dieser Arbeit zum Stand der Forschung ist die Behandlung von Phasenübergängen als nichtlineare Komplementaritätsprobleme (NCP), sowie die ausführliche Untersuchung der Eigenschaften dieser Herangehensweise im Vergleich zu existierenden Modellen wie beispielsweise dem Primärvariablentauschmodell (PVS) und dem Black-Oil Modell.

Recently, gamma-ray computed tomography (GCT) has been established at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) for the investigation of two- or multiphase flows in technical apparatuses. The GCT systems at HZDR are operated with collimated 137Cs isotopic sources emitting gamma photons with an energy of 662 keV. This gamma radiation is able to penetrate dense housings and delivers sufficient phase contrast between, e.g., liquid and gas. The radiation detector arcs consist of in-house developed gamma-ray detector modules based on scintillation detector technology. Special effort has been spent on their thermal design. With a thermal stabilisation of better than 1 K for each detector element, a high repeat measuring accuracy could be achieved for varying environmental temperatures within a temperature range of 20 K. A current project is engaged in gas phase distribution determination within industrial centrifugal pumps at various operating states. Because the application of those pumps is limited to single liquid phase flow, gas entrainment reduces its delivery performance as well as the process efficiency. For detailed studies, a thermo hydraulic test facility was assembled at HZDR replicating authentic operating conditions for industrial centrifugal pumps. Defined gas volume fractions can be injectect in form of two different flow regimes, disperse and tubular. Thus, effects onto delivery perfomance and corresponding gas fraction distribution and gas holdup within the impeller region could be determined.

In the last decade, applications of Computational Fluid Dynamic (CFD) methods for nuclear applications received more and more attention, as they proved to be a valuable complementary tool for design and safety. The main interest towards CFD consists in fact in the possibility of obtaining detailed 3D complete flow-field information on relevant physical phenomena at lower cost than experiments. Typically free surfaces manifest as stratified and wavy flows in horizontal flow domain where gas and liquid are separated by gravity. Stratified two-phase flows are relevant in many nuclear applications, e.g. pipelines, main coolant lines, horizontal heat exchangers and storage tanks. Special flow characteristics as flow rate, pressure drop and flow regimes have always been of engineering interest. Wallis (1973) analyzed the onset of slugging in horizontal and near horizontal gas-liquid flows. Flow maps which predict transitions between horizontal flow regimes in pipes were introduced e.g. by Taitel and Dukler (1976) and Mandhane et al. (1974). The most important flow regimes are smooth stratified flow, wavy flow, slug flow and elongated bubble flow. Taitel and Dukler (1976) explained the formation of slug flow by the Kelvin-Helmholtz instability. They also proposed a model for the frequency of slug initiation (Taitel and Dukler, 1977). CFD simulations for free surface flows require the modeling of the non-resolved scales. For modeling of interfacial transfers it is necessary to select the adequate interfacial transfer models and to determine the interfacial area. The numerical solution can resolve the statistically averaged motion of the free surface (including waves) which may not be too small relatively to the channel height and to the characteristic length of the spatial discretization. However, the detailed structure of interacting boundary layers of the separated continuous phases and surface ripples cannot be resolved. Instead, its influence on the average flow must be

A two-dimensional multi-phase numerical model for incompressible, immiscible and variable density fluids has been developed based on Navier-Stokes equations. The governing equations were discretized in a collocated grid system and solved with a finite volume method. The present model was parallelized in a shared memory system using openMp which reduced the computational time. The model was verified against Boussinesq and non-Boussinesq lock exchange problems. In addition, the model was applied to simulate salinity intrusion into Ohashi River that connects Nakaumi and Shinji brackish coastal lakes. The developed model successfully predicted the density current propagation in the experimental cases while it still needs refinement for field study cases.

This thesis presents the study of laminar to turbulent transition of particle laden flows. When a flow becomes turbulent, the drag increases one order of magnitude compared to a laminar flow, therefore, much research is devoted to understand and influence the transition. Previous research at the Linne Flow Centre at KTH has concentrated on the understanding of the bypass transition process of single-phase fluids. Though there are still questions, the principles of this process are now, more or less, known. However, little is known of the influence of particles on transition. While experiments in the 1960s already showed that particles can reduce the friction in turbulent channel flows significantly. The question explored in this thesis is whether this can be attributed to their influence on transition. The initial onset of transition has been investigated with both modal and non-modal linear stability analysis in a Poiseuille flow between two parallel plates. Particles are introduced as a second fluid and they are considered to be solid, spherical and homogeneously distributed. When the fluid density is much smaller than the particle density, ξ (≡ ρf/ρp) << 1, an increase of the critical Reynolds number is observed. However, transient growth of streamwise vortices resulting in streaks is not affected by inclusion of particles. Particles with ξ ∼ 1 hardly seem to have an effect on stability. Although linear analysis shows that particles hardly influence the transient growth of disturbances, they might affect other (non-linear) stages of transition. To investigate such effects, the full Navier-Stokes equations for 3D Poiseuille flow between two parallel plates are numerically solved and particles are introduced as points with two-way coupling. For particles in a channel flow with ξ<<1, results show that the transition to turbulence is delayed for mass fractions ƒ (=mp N / ρf) larger than 0.1. For a mass fraction of ƒ=0.4 the initial disturbance energy need, QC 20131030

This study deals with fibre suspension flows through cylindrical pipes. Thepresent work aims at measurements of opaque flows, which are common inindustries. Nuclear magnetic resonance imaging (NMRI) and ultrasound velocimetryprofiling (UVP) were employed as non-invasive and optic-independenttools to measure the velocity profiles. As a first experiment, a paper-pulp suspensionflow through a sudden contraction and expansion was investigated.The results show the NMRI technique can be used to measure the stronglyunsteady flow such as separated regions though the MR signal is attenuateddue to the turbulence in the flow. The flow loop had however an insufficientinlet length which caused asymmetric profiles at the test section. As a secondexperiment, a flow loop which provided fully developed flows at the test sectionwas designed. After that, the velocity profiles of rayon-fibre and micro-spheresuspension flows were measured by the NMRI and the UVP independently.In principle, these two techniques measure the different velocities of the fibresuspensionflows, i.e. the velocity of the water and the fibre. In dilute suspensionflows, where the velocities of the two phases were assumed to be thesame, the velocity profiles were in good agreement. This shows the validityof the two measurement techniques. However, it should be pointed out thatthere is a limitation of the current UVP method for highly concentrated flows.The velocity profiles obtained by the UVP at high concentrations seems notto represent physics while the NMRI is not affected by the concentrations. Itis argued that the advances of the NMRI for the measurement of the highlyconcentrated flows.

Detailed hydrodynamic characterization of transient multiphase flows is significant in the field of process engineering. The ultrafast X-ray tomography system ROFEX was used to analyze the complex transient multiphase flow and its hydrodynamic characteristics in monolithic structures. Introducing the applied tomography system and the particular experimental set-up, selected experiments and investigations will be explained. Furthermore several methods for the computation and visualization of gas and liquid phase distribution, the calculation of liquid holdup and the characterization of the occurring flow structures will be presented. As a conclusion a selection of achieved results and the derivable information will be summarized.

Am HZDR werden in enger Zusammenarbeit mit ANSYS neue Ansätze wie das Algebraic Interfacial Area Density (AIAD)-Modell zur numerischen Beschreibung horizontaler, stratifizierter Strömungen entwickelt. Der Inhalt dieser Diplomarbeit ist die Weiterentwicklung des AIAD-Modells hinsichtlich des Impulsaustausches und der Turbulenzbehandlung an der freien Oberfläche. Die Diplomarbeit ist sehr anspruchsvoll. Das Dokument gliedert sich in Einleitung und Stand der Technik, Grundlagen der Berechnung von Mehrphasenströmungen, Modellierung des Widerstandsbeiwertes, Turbulenzmodellierung und Validierung der Modelle an einem Experiment. Eigene Lösungsansätze wurden selbständig entwickelt und in den Rechencode eingearbeitet. Andere Lösungsansätze werden diskutiert, konnten aber in der Laufzeit der Arbeit nicht umgesetzt werden. Die Arbeiten wurden sehr sorgfältig durchgeführt und die Interpretation der Ergebnisse ist fundiert. Insgesamt ist es eine sehr gute wissenschaftliche Arbeit.

Fluid flow through curved ducts is influenced by the centrifugal action arising from duct curvature and has behaviour uniquely different to fluid flow through straight ducts. In such flows, centrifugal forces induce secondary flow vortices and produce spiralling fluid motion within curved ducts. Secondary flow promotes fluid mixing with intrinsic potential for thermal enhancement and, exhibits possibility of fluid instability and additional secondary vortices under certain flow conditions. Reviewing published numerical and experimental work, this thesis discusses the current knowledge-base on secondary flow in curved ducts and, identifies the deficiencies in analyses and fundamental understanding. It then presents an extensive research study capturing advanced aspects of secondary flow behaviour in single and two-phase fluid flow through curved channels of several practical geometries and the associated wall heat transfer processes.As a key contribution to the field and overcoming current limitations, this research study develops a new three-dimensional numerical model for single-phase fluid flow in curved ducts incorporating vortex structure (helicity) approach and a curvilinear mesh system. The model is validated against the published data to ascertain modelling accuracy. Considering rectangular, elliptical and circular ducts, the flow patterns and thermal characteristics are obtained for a range of duct aspect ratios, flow rates and wall heat fluxes. Results are analysed for parametric influences and construed for clearer physical understanding of the flow mechanics involved. The study formulates two analytical techniques whereby secondary vortex detection is integrated into the computational process with unprecedented accuracy and reliability. The vortex inception at flow instability is carefully examined with respect to the duct aspect ratio, duct geometry and flow rate. An entropy-based thermal optimisation technique is developed for fluid flow through curved

This thesis deals with the stability of particle laden flows. Both modal and non-modal linear analyses have been performed on two-way coupled particleladen flows, where particles are considered spherical, solid and either heavy or light. When heavy particles are considered, only Stokes drag is used as interaction term. Light particles cannot be modeled with Stokes drag alone, therefore added mass and fluid acceleration are used as additional interaction forces. The modal analysis investigates the asymptotic behavior of disturbances on a base flow, in this thesis a pressure-driven plane channel flow. A critical Reynolds number is found for particle laden flows: heavy particles increase the critical Reynolds number compared to a clean fluid, when particles are not too small or too large. Neutrally buoyant particles, on the other hand, have no influence on the critical Reynolds number. Non-modal analysis investigates the transient growth of disturbances, before the subsequent exponential behavior takes over. We investigate the kinetic energy growth of a disturbance, which can grow two to three orders of magnitude for clean fluid channel flows. This transient growth is usually the phenomenon that causes transition to turbulence: the energy can grow such that secondary instabilities and turbulence occurs. The total kinetic energy of a flow increases when particles are added to the flow as a function of the particle mass fraction. But instead of only investigating the total energy growth, the non-modal analysis is expanded such that we can differentiate between fluid and particle energy growth. When only the fluid is considered in a particle-laden flow, the transient growth is equal to the transient growth of a clean fluid. Besides thes Stokes drag, added mass and fluid acceleration, this thesis also discusses the influence of the Basset history term. This term is often neglected in stability analyses due to its arguably weak effect, but also due to difficulties in implem, QC 20111013

Stirring tanks are very important equipments used for mixing, separating, chemical reaction, etc. A typical stirring tank is a cylindrical vessel with an agitator driving the fluid and generating turbulence to promote mixing. Flotation cells are widely used stirring tanks in phase separation where multiphase flow is involved. Flotation refers to the process in which air bubbles selectively pick up hydrophobic particles and separate them from hydrophilic solids. This technology is used throughout the mining industry as well as the chemical and petroleum industries. In this research, efforts were made to investigate the multi-phase flow hydrodynamic problems of some flotation cells at different geometrical scales. Pitot-static and five-hope probes were employed to lab- pilot- and commercial-scale tanks for velocity measurements. It was found that the tanks with different scales have similar flow patterns over a range of Reynolds numbers. Based on the velocity measurement results, flotation tanks' performance was evaluated by checking the active volume in the bulk. A fast-response five-hole probe was designed and fabricated to study the turbulence characteristics in flotation cells under single- and multi-phase flow conditions. The jet stream in the rotor-stator domain has much higher turbulence intensity compared with other locations. The turbulent dissipation rate (TDR) in the rotor-stator domain is around 20 times higher than that near tank's wall. The TDR could be used to calculate the bubble and particle slip velocities. An isokinetic sampling probe system was developed to obtain true samples inthe multi-phase flow and then measure the local void fraction. It was found that the air bubbles are carried out by the stream and dispersed to the whole bulk. However, some of the bubbles accumulate in the inactive regions, where higher void fractions were detected. The isokinetic sampling probe was then extended to be an isokinetic borescope system, which was used to dete

The entrainment process in two-phase buoyant jets differs significantly from their singlephase counterparts, and is not well understood. Entrainment models developed for singlephase flow are often used in two-phase jetting simulations, albeit with limited success. In this work, Particle Image Velocimetry (PIV) and shadowgraph flow visualization experiments have been conducted on submerged round gas jets of varying speeds and nozzle diameters with the goal of improving our understanding of the entrainment process in a two-phase (gas-liquid) jet. The total entrainment estimated using the PIV measurements is higher than the respective values suggested by a common empirical model developed for singlephase buoyant jets. A two-phase theoretical entrainment model used for comparison shows an overestimation of entrainment, but predicts the increase in the rate of entrainment with axial distance from the jet nozzle seen in the PIV results. This thesis also presents advances in PIV processing methodology that were developed concurrently with the entrainment research. The novel Spectral Phase Correlation (SPC) allows for particle displacement to be determined directly from phase information in the Fourier domain. Some of the potential benefits of the SPC explored here include (1) avoidance of errors introduced by spatial peak-finding routines; (2) use of a modal analysis that can be used to provide information such as correlation quality; and (3) introduction of a means of incorporating information from multiple image windows. At low image noise levels, the method performs as well as an advanced CC-based method. However, difficulties unwrapping the aliased phase information cause the SPC's performance to degrade at high noise levels.

Gas-liquid flows are central to oil and gas production and because of the very flexible nature of the interface between the fluids, the flows can be very complex exhibiting a wide variety of three dimensional structures. In the field the steel pipes in which these two-phase mixture flow make it difficult to observe them. Even in the laboratory where pipes can be made of transparent plastic, we can still normally only see what is occurring near the pipe wall as bubbles clustering near the wall, or wavy films of liquid flowing along the walls, obscure what is occurring at the middle of the pipe. One way in which these visual limitations can be overcome is to employ electrical tomography methods. In research at the University of Nottingham we are working with Forschungzentrum Rossendorf-Dresden from Germany and Tomoflow Limited to use electrical tomography to 'see' into the pipe.

The lecture introduces computerized tomography as applied in process and flow diagnostics. In many indutrial fields multiphase flows play a key role in efficiency and safety aspects of indiustrial plants and processes. In the lecture principles of tomographic imaging modalities, such as electrical tomography, gamma and X-ray tomography, electron beam tomography, PET and PEPT are discussed along with exemplary applications in oil industry, chemical industry, nuclear engineering and other industrial and scientific fields.

In the present study, motions of two small solid particles, which are lifted by the shock-induced flow behind a propagating shock wave, are numerically simulated to investigate the dynamic characteristics from the viewpoint of direct and indirect interactions between the two particles. The particle Reynolds number is 6 000, and the time period considered ranges from the time when the shock passes the particles to the time when the boundary layer thickness grows to the same order as the particle size. Three cases are considered for their initial arrangement on the wall. In the first case, they are placed tandem, aligned with the direction of shock propagation. Results show that they are lifted up, keeping contact each other. In the second case, they are seperated by three times as large as the particle size. As a result, the rear particle is lifted higher because the flow before the rear particle has been changed by the front particle. In the third case, they are placed in the spanwise direction. They are also lifted by the shock-induced flow, increasing the separation distance between them due to repulsive fluid force. In these three cases, the average height of two particles becomes larger than that in the case of one particle alone, which was studied in the previous work.

We developed a fast read-out electronics which is used in conjunction with a radiation detector for gamma ray computed tomography. The detector comprises 320 single scintillation detector elements and is operated in pulse counting mode. Digital pulses corresponding to gamma or x-ray absorption events in the scintillation crystals are counted by a counter bank implemented fully parallel in FPGA electronics. The counter data may be read-out at a maximum rate of 5 MHz via micro controller and USB 2.0 interface. This enables to take full projection data within intervals of down to 23 µs which in turn gives the opportunity to produce sharp tomographic slice images of objects rotating with more than 1000 rotations per minute. As an example we demonstrate the reconstruction of the gas distribution in the turbine region of a stirred tank reactor.