Dans les travaux de recherche développés durant cette thèse, une première approche desécoulements magnétohydrodynamiques (MHD) multiphasiques est proposée. Cette approche seconcentre sur les phénomènes liés à l’interaction entre une dynamique interfaciale et un écoulementde coeur MHD. Le couplage induit entre rhéologie de surface etMHDde sous-phase est particulièrementillustré par le développement d’un viscosimètre annulaire surfacique, dédié à l’étude desmétaux liquides progressivement oxydés. En premier lieu sont introduits les éléments théoriquespropres à laMHDet à la rhéologie de surface. La modélisation de leur couplage fait intervenir deuxparamètres interfaciaux : les viscosités surfaciques dilatationnelle et de cisaillement. L’influence respectivede ces deux paramètres sur l’écoulement MHD de sous-phase est étudiée analytiquementet numériquement dans le cas d’un écoulement (stratifié) annulaire MHD permanent. Dans la configurationretenue, un champ magnétique uniforme vertical est imposé, perpendiculairement à lasurface liquide graduellement oxydée. Le rôle décisif des contraintes visqueuses interfaciales concernantl’(in)activation des couches de Hartmann est démontré, conduisant à des topologies atypiquesd’écoulement MHD. Le viscosimètre annulaire MHD est ensuite proposé en tant que méthodeexpérimentale originale, permettant la mesure sélective des viscosités surfaciques de fluides électroconducteurs.Les premières campagnes expérimentales aboutissent à une estimation de la viscositéde cisaillement interfacial d’un alliage métallique (GaInSn) liquide à température ambiante. Finalement,une ouverture sur l’écoulement MHD à proximité d’une inclusion gazeuse sphérique rigideest discutée en annexe de ce projet, en lien avec des conditions mécaniques variables à l’interfaceliquide/gaz pilotées par la rhéologie de surface, constituant une première approche vers la descriptiondes écoulements MHD dispersés. / In this thesis work, a first approach towards the description of magnetohydrodynamic(MHD) multiphase flows is proposed, based on the investigation of the role of surface rheologyin permanent regime. The study of the coupling between bulk MHD and surface rheology is particularlymotivated by the development of an annular surface viscometer devoted to liquid metalstopped with an oxidation layer. First, theoretical foundations of MHD and of surface rheologyare separately introduced. The modelling brings out a strong coupling between bulk and surfacevelocities, the latter being dependent on two interfacial parameters: the surface shear and dilatationalviscosities. Their respective influence is analytically and numerically investigated in the caseof a permanent annular (stratified) MHD end-driven flow. In the considered geometry, a vertical(uniform) magnetic field is imposed, perpendicular to the gradually oxidising liquid surface. Thecontribution of planar surface viscous stresses to the possible electrical activation of Hartmannlayers is demonstrated, leading to a variety of atypical MHD flow patterns. The annular MHDviscometer is then developed as a first device able to perform selective measurement of the surfaceviscosities of electroconductive fluids. First experiments lead to an estimated value of the surfaceshear viscosity for a liquid alloy (GaInSn) at room temperature. Some hints are finally given to investigatethe study case of aMHDflow past a rigid fluid sphere, with varying interfacial conditionsgoverned by surface rheology, as an additionalwork constituting a first step towards the descriptionof dispersed MHD flows.

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When an electrically conducting fluid moves through a magnetic field, fluid mechanics and electromagnetism are coupled.<p>This interaction is the object of magnetohydrodynamics, a discipline which covers a wide range of applications, from electromagnetic processing to plasma- and astro-physics.<p><p>In this dissertation, the attention is restricted to turbulent liquid metal flows, typically encountered in steel and aluminium industries. Velocity measurements in such flows are extremely challenging because liquid metals are opaque, hot and often corrosive. Therefore, non-intrusive measurement devices are essential. One of them is the Lorentz force flowmeter. Its working principle is based on the generation of a force acting on a charge, which moves in a magnetic field. Recent studies have demonstrated that this technique can measure efficiently the mean velocity of a liquid metal. In the existing devices, however, the measurement depends on the electrical conductivity of the fluid. <p><p>In this work, a novel version of this technique is developed in order to obtain measurements that are independent of the electrical conductivity. This is particularly appealing for metallurgical applications, where the conductivity often fluctuates in time and space. The study is entirely numerical and uses a flexible computational method, suitable for industrial flows. In this framework, the cost of numerical simulations increases drastically with the level of turbulence and the geometry complexity. Therefore, the simulations are commonly unresolved. Large eddy simulations are then very promising, since they introduce a subgrid model to mimic the dynamics of the unresolved turbulent eddies. <p><p>The first part of this dissertation focuses on the quality and reliability of unresolved numerical simulations. The attention is drawn on the ambiguity that may arise when interpretating the results. Owing to coarse resolutions, numerical errors affect the performances of the discrete model, which in turn looses its physical meaning. In this work, a novel implementation of the turbulent strain rate appearing in the models is proposed. As opposed to its usual discretisation, the present strain rate is in accordance with the discrete equations of motion. Two types of flow are considered: decaying turbulence located far from boundaries, and turbulent flows between two parallel and infinite walls. Particular attention is given to the balance of resolved kinetic energy, in order to assess the role of the model.<p><p>The second part of this dissertation deals with a novel version of Lorentz force flowmeters, consisting in one or two coils placed around a circular pipe. The forces acting on each coil are recorded in time as the liquid metal flows through the pipe. It is highlighted that the auto- or cross-correlation of these forces can be used to determine the flowrate. The reliability of the flowmeter is first investigated with a synthetic velocity profile associated to a single vortex ring, which is convected at a constant speed. This configuration is similar to the movement of a solid rod and enables a simple analysis of the flowmeter. Then, the flowmeter is applied to a realistic three-dimensional turbulent flow. In both cases, the influence of the geometrical parameters of the coils is systematically assessed. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

Investigating and mapping fundamental processes in nature is a driving force for breakthroughs in research and technology. Doing so, requires knowledge of the smallest scales of the world. One way of performing measurements on these scales is through intense x-ray sources, which have improved greatly over the last decades. By combing these sources with state of the art optics, even higher flux densities can be reached, allowing for faster measurements and ground-breaking discoveries.  This study aims to explore the performance of polycapillary optics, when aligned to one of the most intense x-ray micro sources in the world, the liquid-metal-jet D2+. Knife edge scans were performed together with a photon-counting medipix x-ray camera to quantify focus properties such as, flux, flux density, transmission, gain and beam width. Measurements were conducted with a 20 μm source spot that was compared to a simulated 200 μm source spot, both at 260 W electron beam power. The data from vertical and horizontal scans were combined to reconstruct the 2D functionality of the polycapillary optic. The flux density were almost four times higher with the 20 μm spot compared to the simulated 200 μm spot. This result correlated with the condition for total external reflection and the local divergence. The conclusion is that the small source spot of the liquid-metal-jet source improves the efficiency of the polycapillary optic.  The efficiency could still be improved, if the deviation in the pointing accuracy could be minimized. Furthermore, the combination of liquid-metal-jet x-ray source and the polycapillary optic, achieved extremely high flux densities. This was specially compared to an x-ray source used for confocal micro XRF, where the flux was almost nine times higher with the liquid-metal-jet x-ray source. This allows for faster measurements within confocal micro XRF and other techniques demanding very high flux densities, but with low demands on beam divergence and spectral purity.

This diploma thesis deals with the optimization of gating systems of shape-complex aluminum castings produced by investment casting process. Two castings whose massive gating systems make problem with the use of liquid metal (so called yield) were chosen for optimization. Each casting is described in its own chapter. The individual subchapters deal with description of the original and newly designed gating system. Simulation, test castings, X-ray inspection and metallographic examination were performed for each variant. At the end of each chapter there is an evaluation of the results of the optimization of gating system.

Liquid metal batteries are discussed today as an economic grid-scale energy storage, as required for the deployment of fluctuating renewable energies. These batteries consist of three stably stratified liquid layers: two liquid metal electrodes are separated by a thin molten salt electrolyte, this way forming an electrochemical concentration cell. The completely liquid interior, which is on the one hand very beneficial for the energy efficiency, also poses some major challenges on the other hand. Strong cell currents in combination with electromagnetic fields make liquid metal batteries highly susceptible to various kinds of magnetohydrodynamic instabilities. In particular, the so-called metal pad roll instability, which can drive uncontrollable wave motions in both interfaces, was identified as a key limiting factor for the operational safety. The metal pad roll instability is well known from conceptually similar aluminum reduction cells, but still poorly understood in the framework of liquid metal batteries. Mainly by developing analytical wave models, but also by employing numerical simulations and by setting up a newly designed wave experiment, the present thesis pursues the goal of providing a better understanding of interfacial wave dynamics and the manifestation of the metal pad roll instability in liquid metal batteries. As a main result, a three-layer formulation of standing gravity-capillary waves reveals that the pressure coupling between the two interfaces plays a crucial role in the cell stability. Three different coupling regimes, which partially involve novel types of interfacial wave instabilities, are identified and classified by two dimensionless parameters. Building on this theoretical work, the wave experiment is exploited to further investigate different metal pad roll-related wave properties. The crucial importance of the contact line dynamics is emphasized and viscous damping, which is important for the estimation of instability onsets, is discussed as a function of the layer heights. Finally, a hybrid interfacial sloshing model is formulated and equipped with recently derived two-layer damping rates to account for viscous dissipation. The model allows to study and interpret the forced wave mechanics in the wave experiment as a function of eight dimensionless parameters and can, as an additional application, be exploited to optimize mixing in orbitally shaken bioreactors. As a further key result, the sloshing model reveals the formation of novel spiral wave patterns under the effect of strong damping.

Stahl wird heutzutage fast ausschließlich im kontinuierlichen Stranggussprozess zu Halbzeugen verschiedener Geometrien verarbeitet. Flüssiger Stahl wird dabei über ein Tauchrohr in eine wassergekühlte, nach unten offene Kokille gefüllt und erstarrt an deren Wänden. Der innen noch flüssige Strang wird kontinuierlich nach unten aus der Kokille abgezogen und ist erst nach mehreren Metern komplett erstarrt. Die Qualität des Halbzeugs wird vorwiegend durch die Strömung in der Kokille beeinflusst. In der Industrie werden seit langer Zeit verschiedene elektromagnetische Aktuatoren eingesetzt, um die Strömungsstruktur in der Kokille mithilfe von Lorentzkräften zu beeinflussen. Bedingt durch die hohe Temperatur des flüssigen Stahls und dessen Undurchsichtigkeit lässt sich die Strömungsstruktur im realen Prozess jedoch kaum untersuchen. Zur Optimierung sowie zum besseren Verständnis der Vorgänge in der Kokille sind daher numerische Simulationen und Modellexperimente notwendig. In dieser Arbeit werden die Ergebnisse experimenteller Untersuchungen zum kontinuierlichen Stranggießen von Stahl unter dem Einfluss verschiedener elektromagnetischer Aktuatoren präsentiert. Die Experimente wurden an der Mini-LIMMCAST-Anlage des Helmholtz-Zentrum Dresden-Rossendorf (HZDR) durchgeführt. Mini-LIMMCAST ist ein Modell einer Strangguss-Anlage, welches isotherm mit der bei Raumtemperatur flüssigen Metalllegierung GaInSn betrieben wird. Durch die Verwendung von flüssigem Metall als Modellfluid können die Auswirkungen elektromagnetischer Aktuatoren untersucht werden, jedoch kann aufgrund der Undurchsichtigkeit des Modellfluids die Geschwindigkeitsverteilung in der Kokille nicht mittels optischer Verfahren erfasst werden. Daher wird zur Messung der Geschwindigkeitsverteilung in der Kokille die Ultraschall-Doppler-Velocimetrie eingesetzt. Die Ergebnisse von Messungen an zwei verschiedenen Kokillen-Geometrien werden vorgestellt: zum Einen solche an einer zylindrischen Kokille, bei welcher auf verschiedene Arten eine Drehströmung erzeugt wurde. Dies erfolgte durch elektromagnetisches Rühren in der Kokille sowie durch ein Tauchrohr mit spiralförmigen Auslässen. Zum Anderen wird eine Kokille mit rechteckigem Querschnitt, wie sie in Brammen-Anlagen verwendet wird, betrachtet. In dieser wird die Strömung sowohl durch ein statisches Magnetfeld, einer sogenannten elektromagnetischen Bremse, als auch durch elektromagnetische Wechselfelder beeinflusst. Die Ergebnisse dieser Arbeit zeigen eine Reihe zuvor nicht beschriebener Phänomene, wie das zeitliche Verhalten des Jets in der Rundkokille mit elektromagnetischem Rührer oder den Mechanismus der in der Brammen-Geometrie mit elektromagnetischer Bremse für eine Veränderung der Geschwindigkeit an der freien Oberfläche der Kokille sorgt. Umfassende experimentelle Parameterstudien erlauben detaillierte Einblicke in die beim Stranggießen auftretenden Strömungsstrukturen, wie sie mit numerischen Simulationsmodellen nur unter einem immensen Einsatz von Rechenleistung zu erzielen wären. / Today, steel is almost exclusively processed into semi-finished products of various geometries by the continuous casting process. Liquid steel is filled through a submerged entry nozzle into a water-cooled, downwardly open mould and solidifies on its walls. The strand, which is still liquid on the inside, is continuously drawn downwards out of the mould and is then completely solidified after several metres. The quality of the semi-finished product is mainly influenced by the flow in the mould. For many years, various electromagnetic actuators have been used in industry to influence the flow structure in the mould by the application of Lorentz forces. Due to the high temperature of the liquid steel and its opacity, however, the flow structure can hardly be investigated in the real process. Therefore, numerical simulations and model experiments are necessary to optimise and better understand the processes in the mould. In this dissertation the results of experimental investigations on continuous casting of steel under the influence of different electromagnetic actuators are presented. The experiments were performed at the Mini-LIMMCAST facility of the Helmholtz-Zentrum Dresden-Rossendorf (HZDR). Mini-LIMMCAST is a model of a continuous casting machine which is operated isothermally with the metal alloy GaInSn, which is liquid at room temperature. By using liquid metal as model fluid, the effects of electromagnetic actuators can be investigated, but due to the opacity of the model fluid, the velocity distribution in the mould cannot be determined by optical methods. Therefore, ultrasonic Doppler velocimetry is used to measure the velocity distribution in the mould. The results of measurements using two different mould geometries are presented. One is a cylindrical mould in which a rotational flow was generated in different ways. This was achieved by electromagnetic stirring in the mould and by an submerged entry nozzle with spiral-shaped outlets. On the other hand, a mould with a rectangular cross-section, as used for slab casting, is considered. Here the flow is influenced by a static magnetic field, a so-called electromagnetic brake, as well as by alternating electromagnetic fields. The results of this work show a number of previously undescribed phenomena, such as the temporal behaviour of the jet in the round mould with an electromagnetic stirrer or the mechanism that causes a change in the speed at the free surface of the mould in the slab geometry with an electromagnetic brake. Comprehensive experimental parameter studies allow detailed insights into the flow structures occurring during continuous casting, which could only be achieved with numerical simulation models with an immense input of computing power.

Diese Arbeit behandelt numerisch die Fluiddynamik in Flüssigmetallbatterien. Insbesonders die Tayler-Instabilität und Elektrowirbelströmungen werden ausführlich betrachtet. Die Motivation der Untersuchungen besteht zum einen in einer Steigerung von Leistung und Sicherheit und zum anderen in der Senkung von Produktions- und Betriebskosten von Flüssigmetallbatterien. Es wird ein Lösungsverfahren für zeitabhängige magnetohydrodynamische Strömungen entwickelt und in OpenFOAM implementiert. Die Basisversion des Lösers erlaubt die Analyse einer flüssige Elektrode. Eine Erweiterung dient der Untersuchung des Einflusses von Stromsammler und Zuleitung der Batterie. Simulationen werden vorwiegend für zylindrische, aber auch für quaderförmige Elektrodengeometrien durchgeführt. Der Hauptteil der Arbeit widmet sich der stromgetriebenen Tayler-Instabilität, die in großen Batterien bei Strömen von einigen Kiloampere auftritt und dort zu einer Strömung in Form von Konvektionszellen führt. Das Auftreten, Wachstum und die Geschwindigkeiten dieser Instabilität werden analysiert und deren Bedeutung für die Batterie diskutiert. Zur Dämpfung bzw. Unterdrückung der Strömung werden eine Reihe von Gegenmaßnahmen vorgestellt und deren praktischer Nutzen bewertet. Der zweite, kürzere Teil der Arbeit befasst sich mit Elektrowirbelströmungen, deren Charakterisierung und ihren Wechselwirkungen mit der Tayler-Instabilität. Die besondere Bedeutung von Elektrowirbelströmungen für die Integrität der Elektrolytschicht sowie ihre Anwendbarkeit für die Verbesserung des Stofftransports in Flüssigmetallbatterien werden vervorgehoben.

Multi-phase flows of small solid particles and gas bubbles in optically opaque fluids play a key role in both mineral and metallurgical processing, which use the principle of froth flotation and bubble flotation, respectively. To gain visual insight into such particle-laden multi-phase flows, this dissertation investigates the application of radiographic techniques, employing both X-rays and neutron radiation. Lab-scale experiments are performed with model particles in liquid foams and liquid metals, focussing on the time-resolved measurement of the particles’ motion in the multi-phase flows, aiming for a sufficient contrast-to-noise ratio in the X-ray or neutron image sequences. The model experiments in this dissertation demonstrate the capabilities of X-ray and neutron radiography to image multi-phase flows in particle-laden and optically opaque fluids, especially to measure the motions of small particles with high spatial and temporal resolution. X-ray radiography enables to track custom-tailored tracer particles acting as tools for experimental investigations of flow phenomena in three-dimensional liquid foams. Both radiographic techniques supplement each other for imaging measurements of multi-phase flows with gas bubbles and solid particles in liquid metals. However, to visualise smallest model particles in liquid metal flows, neutron radiography proves to be the more promising technique compared to X-ray radiography. All in all, this dissertation contributes to paving the way for systematic radiographic measurements and further studies of particle-laden flows in optically opaque fluids.