Liquid metal electrochemical cells are electrochemical device with at least one liquid metal electrode. They were adopted in electro-metallurgical processing, heat-to-power conversion, and energy storage applications. The electrode's liquid state ensures high current density, resulting in batteries with high power densities and electro-metallurgical reactors with high conversion yields. Mass transport is critical in all liquid metal electrochemical cells, but especially in concentration cells like liquid metal batteries. These are fully liquid three-layer cells where the two electrodes are separated by a thin molten salt electrolyte. There, the positive electrode alloying and de-alloying processes store energy in the cell. Inhomogeneities in the liquid metal electrode reduce cell efficiency and material use. In this work, we are interested in understanding mass transport within this electrode. From a first study of diffusive heat and mass transport, we established that solutal effects are predominant in liquid metal electrodes. Any thermally driven convection can not significantly affect a compositionally stable stratification. A solutal flow will efficiently mix the electrode regardless of the temperature distribution. Following that, we developed a consistent explanation for the differences in the cell resistance between charge and discharge, as observed multiple times in the literature. The latter was based on quantitative analysis of a new experimental result. The voltage evolution was measured during the cycling of a liquid metal battery. During discharge, light elements are electro-deposited at the positive electrode's top interface, forming a gravitationally stable stratification. As only diffusion transports the light element away from the interface, the concentration difference and the mass transport overvoltage increase with time. During charging, the opposite phenomenon (electro-refining) takes place. The flux at the active interface builds up an unstable, asymmetric, and time-dependent buoyancy distribution in the layer. That leads first to a diffusive transient and then to a convective flow in the layer. This solutal convection was studied numerically with finite-volume and spectral-element-method solvers. The two-dimensional axisymmetric simulations performed covered Schmidt numbers from 1.125 to 288 and five orders of magnitude of flux Rayleigh numbers, starting from 10000. Two regimes were identified as a function of the flux Rayleigh number. At low Rayleigh numbers, diffusion affects the full layer height before the onset of convection. This results in a global flow. Instead, convection originates in the thin concentration boundary layer with characteristic plume structures in the high Rayleigh number regime. In this regime, onset time and concentration difference are independent with respect to the layer height. Thanks to the extensive parametric study, we retrieved robust scaling for velocity and concentration differences as a function of the current density and material properties of the layer. These results can be used in the design and operation of liquid metal electrodes. For example, they allow estimating the mass transport overvoltage during charge. Furthermore, we studied numerically solutal convection in three-dimensional cylindrical electrodes. We showed that the two-dimensional approximation captures quite remarkably the evolution of integral quantities observed in fully three-dimensional simulations. This is not due to the axisymmetric nature of the flow. On the contrary, we observed a rich dynamic, with polygonal-shaped cells forming and evolving in the active interface concentration distribution. Finally, the influence of non-uniform current distribution on mass transport in a liquid metal electrode was investigated. Differences with respect to the homogenous configuration are present in pure diffusion and at the onset of convection. The solutal flow is able to reduce the inhomogeneities in the electrode. / Elektrochemische Flüssigmetallzellen sind Zellen mit mindestens einer Flüssigmetall Elektrode. Sie werden in der Elektrometallurgie, bei der Wandlung von Wärme in Elektrizität, sowie zur Energiespeicherung eingesetzt. Der flüssige Zustand der Elektroden erlaubt hohe Stromdichten, was Batterien mit guter Ratenfähigkeit und elektrometallurgischen Reaktoren mit hoher Raum-Zeit-Ausbeute ermöglicht. Stofftransport ist ein für alle elektrochemischen Flüssigmetallzellen wesentliches Phänomen, insbesonders für Konzentrationszellen wie Flüssigmetallbatterien. Das sind Dreischichtzellen mit komplett flüssigem Inventar, bei denen die beiden Elektroden durch eine Salzschmelze getrennt sind. Die Legierungs- und Entlegierungsprozesse in der positiven Elektroden sind maßgeblich für die Energiespeicherfähigkeit der Zelle. Inhomogenitäten in der Legierungselektrode verringern den Gesamtwirkungsgrad der Zelle und den Nutzungsgrad der Aktivmaterialien. Diese Arbeit widmet sich dem Verständnis des Stofftransports in der positiven Elektrode. Ein Vergleich typischer Kenngrößen für den Wärme-und Stofftransport zeigt auf, dass solutale Effekte die Transportverhältnisse in Flüssigmetallelektroden dominieren. Thermische Konvektion kann eine stabile solutale Schichtung nicht wesentlich beeinflussen. Eine von Konzentrationsgradienten hervorgerufene Strömung durchmischt die Elektrode jedoch unabhängig von der Temperaturverteilung. Darauf aufbauend wird dargelegt, was die Ursache für die wiederholt in der Literatur berichteten Unterschiede im Zellwiderstand zwischen Ladung und Entladung ist. Die Erklärung stützt sich auf eine quantitative Analyse neuer experimenteller Ergebnisse zum Zyklenverhalten von Flüssigmetallbatterien. Während der Entladung werden leichte Elemente am oberen Rand der positiven Elektrode abgeschieden und bilden eine stabile Dichteschichtung. Da das eingelagerte Element geringer Dichte nur durch Diffusion von der Grenzfläche abtransportiert wird, steigen der Konzentrationsgradient und mit ihm die Stofftransportüberspannung mit der Zeit an. Während der Aufladung findet das entgegengesetzte Phänomen (Elektroraffination) statt. Die Extraktion der leichten Komponente an der aktiven Grenzfläche führt zu einer instabilen Dichteschichtung. Dadurch kommt es zunächst zu einer kurzen Phase instationärer Diffusion, die rasch von einer solutal getriebene Konvektion abgelöst wird. Die solutale Konvektion wurde numerisch mit Finite-Volumen- und Spektral-Element-Methoden untersucht. Dazu durchgeführte axialsymmetrische Simulationen umfassten Schmidt-Zahlen von 1,125 bis 288 und fünf Größenordnungen von Rayleigh-Zahlen, beginnend bei 10000. In Abhängigkeit von der Rayleigh-Zahl wurden zwei Regime identifiziert. Bei niedrigen Rayleigh-Zahlen findet Diffusion über die gesamte Schichthöhe statt, bevor es zum Einsetzen der Konvektion kommt. Daraus resultiert eine globale Strömung. Im Bereich hoher Rayleigh-Zahlen entsteht die Konvektion stattdessen bereits in einer dünnen Konzentrationsgrenzschicht an der Phasengrenze und zeigt die charakteristischen Konzentrationsschlieren (plumes). In diesem Regime sind die Eintrittszeit und die zugehörige Konzentrationsdifferenz von der Schichthöhe unabhängig. Die umfassende Parameterstudie bildet die Grundlage zur Herleitung fundierter Skalengesetze für Geschwindigkeit und Konzentration in Abhängigkeit der Stromdichte und der Materialeigenschaften der Fluide. Diese Ergebnisse können zur Auslegung und zum Betrieb von Flüssigmetall-Elektroden verwendet werden. Sie ermöglichen beispielsweise die Bestimmung der Stofftransportüberspannung während des Ladens. Für ausgewählte Konfigurationen wurden dreidimensionalen Simulationen in zylindrischen Elektroden durchgeführt. Die Ergebnisse der zweidimensionalen Simulationen bezüglich der integralen Größen wurden bestätigt. Die dreidimensionalen Simulationen offenbarten eine reichhaltige Dynamik polygonaler Zellen, die sich aufgrund der absinkenden Schlieren in Grenzflächennähe bilden. Abschließend wurde der Einfluss einer ungleichmäßigen Stromverteilung an der Phasengrenze auf den Stofftransport untersucht. Unterschiede zur homogenen Verteilung ergeben sich lediglich bezüglich der rein diffusiven Ausbreitung und des Einsetzens der Konvektion. Die ausgebildete solutale Konvektion durchmischt die Elektrode in beiden Fällen sehr intensiv, so dass in diesem Stadium keine Differenzen feststellbar sind.

Current concentrated solar power plants (CSP) use molten salt at 565°C as a heat transfer and energy storage fluid. Due to thermal energy storage (TES), these solar plants can deliver dispatachable electricity to the grid; however, the levelized cost of electricity (LCOE) for these plants is 12-15 c/kWh, about 2.5 times as high as fossil fuel electricity generation. Molten salt technology limits peak operating temperatures to 565°C and a heat engine efficiency of 40%. Liquid metal (LM), however, can reach >1350°C, and potentially utilize a more efficient (60%) heat engine and realize cost reductions. A 1350 °C LM-CSP plant would require ceramic containment, inert atmosphere containment, additional solar flux concentration, and redesigned internal receiver. It was initially unclear if these changes and additions for LM-CSP were technically feasible and could lower the LCOE compared to LS-CSP. To answer this question, a LM-CSP plant was designed with the same thermal input as a published LS-CSP plant. A graphite internal cavity receiver with secondary concentration heated liquid Sn to 1400°C and transferred heat to a 2-phase Al-Si fluid for 9 hours of thermal energy storage. Input heat to the combined power cycle was 1350°C and had 60% thermal efficiency for a gross output of 168 MW. The cost of this LM-CSP was estimated by applying material cost factors to the designed geometry and scaling construction costs from published LS-CSP estimates. Furthermore, graphite was experimentally tested for reactivity with liquid Sn, successful reaction bonds, and successful mechanical seals. The result is switching to molten metal can reduce CSP costs by 30% and graphite pipes, valves, and seals are possible at least at 400°C.

La France prévoit de construire des réacteurs nucléaires de 4e génération. Ils utiliseraient du sodium liquide comme fluide caloporteur et seraient construits en acier austénitique 316L(N). Afin de garantir un fonctionnement optimal, la tenue de cet acier doit être vérifiée. Pour cela, la corrosion de l’acier 316L(N) par le sodium liquide doit être bien connue.La littérature montre que plusieurs phénomènes de corrosion sont possibles. Pour chacun de ces phénomènes, l’influence de la présence d’oxygène dans le sodium est grande. Nous avons donc étudié la corrosion des aciers austénitiques par le sodium liquide en présence d’oxygène.Les données thermodynamiques permettent de montrer que la formation de chromite de sodium est possible sur les aciers austénitiques immergés dans le sodium contenant de l’ordre de 10 μg.g-1 d’oxygène pour des températures inférieures à 650 °C (conditions réacteurs).L’étude expérimentale montre que la chromite de sodium se forme à 650 °C dans le sodium contenant 200 μg.g-1 d’oxygène. À cette même concentration et à 550 °C, la chromite de sodium est observée avec certitude uniquement pour les longues durées d’immersion (> 5000 h). Les résultats à 450 °C sont moins évidents. Par ailleurs, l’acier est appauvri en chrome dans toutes les conditions étudiées.Ces résultats suggèrent que la chromite de sodium se dissout dans le sodium au fur et à mesure de sa formation. Des modèles de formation de la chromite de sodium, approchéelimitée par la diffusion du chrome dans l’acier (en volume et aux joints de grains) et de dissolution, supposée limitée par le transport dans le métal liquide ont permis de montrer que la formation et la dissolution simultanée de la chromite de sodium est un mécanisme possible pour expliquer nos résultats. / France is planning to construct the 4th generation of nuclear reactors. They will use liquid sodium as heat transfer fluid and will be made of 316L(N) austenitic steel as structural materials. To guarantee optimal operation on the long term, the behavior of this steel must be verified. This is why corrosion phenomena of 316L(N) steel by liquid sodium have to be well-understood.Literature points out that several corrosion phenomena are possible. Dissolved oxygen in sodium definitely influences each of the corrosion phenomenon. Therefore, the austenitic steel corrosion in oxygen-containing sodium is proposed in this study.Thermodynamics data point out that sodium chromite formation on 316L(N) steel is possible in sodium containing roughly 10 μg.g-1 of oxygen for temperature lower than 650 °C (reactor operating conditions).The experimental study shows that sodium chromite is formed at 650 °C in the sodium containing 200 μg.g-1 of oxygen. At the same concentration and at 550 °C, sodium chromite is clearly observed only for long immersion time (> 5000 h). Results at 450 °C are more difficult to interpret. Furthermore, the steel is depleted in chromium in all cases.The results suggest the sodium chromite is dissolved in the sodium at the same time it is formed. Modelling of sodium chromite formation - approached by chromium diffusion in steel (in grain and grain boundaries -, and dissolution - assessed by transport in liquid metal - show that simultaneous formation and dissolution of sodium chromite is a possible mechanism able to explain our results.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1982. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographical references. / by Alfred Walters Reed. / Ph.D.

abstract: Soft polymer composites with improved thermal conductivity are needed for the thermal management of electronics. Interfacial thermal boundary resistance, however, prevents the efficient use of many high thermal conductivity fill materials. Magnetic alignment of ferrous fill material enforces percolation of the high thermal conductivity fill, thereby shifting the governing boundary resistance to the particle- particle interfaces and increasing the directional thermal conductivity of the polymer composite. Magnetic alignment maximizes the thermal conductivity while minimizing composite stiffening at a fill fraction of half the maximum packing factor. The directional thermal conductivity of the composite is improved by more than 2-fold. Particle-particle contact engineering is then introduced to decrease the particle- particle boundary resistance and further improve the thermal conductivity of the composite. The interface between rigid fill particles is a point contact with very little interfacial area connecting them. Silver and gallium-based liquid metal (LM) coatings provide soft interfaces that, under pressure, increase the interfacial area between particles and decrease the particle-particle boundary resistance. These engineered contacts are investigated both in and out of the polymer matrix and with and without magnetic alignment of the fill. Magnetically aligned in the polymer matrix, 350nm- thick silver coatings on nickel particles produce a 1.8-fold increase in composite thermal conductivity over the aligned bare-nickel composites. The LM coatings provide similar enhancements, but require higher volumes of LM to do so. This is due to the rapid formation of gallium oxide, which introduces additional thermal boundaries and decreases the benefit of the LM coatings. The oxide shell of LM droplets (LMDs) can be ruptured using pressure. The pressure needed to rupture LMDs matches closely to thin-walled pressure vessel theory. Furthermore, the addition of tungsten particles stabilizes the mixture for use at higher pressures. Finally, thiols and hydrochloric acid weaken the oxide shell and boost the thermal performance of the beds of LMDs by 50% at pressures much lower than 1 megapascal (MPa) to make them more suitable for use in TIMs. / Dissertation/Thesis / Doctoral Dissertation Mechanical Engineering 2019

Several methods of increasing railgun barrel performance and lifetime are investigated. These include two different barrel-projectile interface coatings: a solid graphite coating and a liquid eutectic indium-gallium alloy coating. These coatings are characterized and their usability in a railgun application is evaluated. A new type of projectile, in which the electrical conductivity varies as a function of position in order to condition current flow, is proposed and simulated with FEA software. The graphite coating was found to measurably reduce the forces of friction inside the bore but was so thin that it did not improve contact. The added contact resistance of the graphite was measured and gauged to not be problematic on larger scale railguns. The liquid metal was found to greatly improve contact and not introduce extra resistance but its hazardous nature and tremendous cost detracted from its usability. The simulated resistivity augmented projectiles were able to mitigate harmful current build-up on the back of a projectile using different conductivity gradients. Within the range of conductivity of aluminum alloys no simulated gradient was able to fully level the current density, however, once the range was expanded to include the lower conductivity of titanium, nearly uniform current density was achieved.

The most common way of producing steel worldwide is with the continuous casting process, where a strand of steel is continuously cast. Further optimization of this process requires data from measurements, measurements which must take place in a hostile high-temperature and corrosive environment such as liquid steel. At Swerim, a Continuous Casting Simulator has been constructed to replicate the flow conditions in the process. A eutectic bismuth-tin alloy with properties similar to steel is used instead to make measurements in liquid metal possible. In this report pressure, velocity and vibration measurements were made using multiple sensors under different flow conditions.

When an electrically conducting fluid moves through a magnetic field, fluid mechanics and electromagnetism are coupled. This interaction is the object of magnetohydrodynamics, a discipline which covers a wide range of applications, from electromagnetic processing to plasma- and astro-physics. 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. 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. 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. 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.

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