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Electrolysis stands as a pivotal method for environmentally sustainable hydrogen production. However, the formation of gas bubbles during the electrolysis process poses significant challenges by impeding reactions, diminishing cell efficiency, and dramatically increasing energy consumption. Furthermore, the inherent difficulty in detecting these bubbles arises from the non-transparency of the wall of electrolysis cells. Fortunately, these gas bubbles induce alterations in the cell’s conductivity, leading to corresponding fluctuations in the surrounding magnetic flux density. In this context, we can leverage external magnetic sensors to measure the magnetic flux density fluctuations induced by gas bubbles. Next, by solving the inverse problem of the Biot-Savart Law, we can estimate the conductivity, bubble size, and location within the cell. Nevertheless, reconstructing a high-resolution conductivity map from limited induced magnetic flux density measurements poses a formidable challenge as an ill-posed inverse problem. To overcome this challenge, we employ Invertible Neural Networks (INNs) to reconstruct the conductivity field. The inherent property of INNs, characterized by a bijective mapping between the input and output space, makes them exceptionally well-suited for resolving ill-posed inverse problems. We conducted extensive qualitative and quantitative evaluations to compare the performance of INNs with traditional approaches such as Tikhonov regularization. Our experiments demonstrate that, particularly in the presence of noise in the magnetic sensor data, our INN-based approach outperforms Tikhonov regularization in accurately reconstructing bubble distributions and conductivity fields. We hope that, given the efficacy of INNs shown in this work, they will become an indispensable deep-learning based approach for addressing inverse problems not only in Process Tomography but across various other domains.

Aktuelle Studien illustrieren die Entwicklung von Strömungsmustern der Magnetokonvektion bis hin zu chaotischen Strömungen.

The formation and behavior of gas bubbles is experimentally investigated in a liquid metal downward pipe flow, a configuration that largely corresponds to the situation in a submerged entry nozzle (SEN) in the continuous casting process in steel making. The experimental mockup is operated at room temperature using the ternary alloy GaInSn as model fluid. Argon gas is injected through an orifice located in the SEN wall. The gas distribution in the pipe is visualized by means of the X-ray radiography. The set-up is completed by an electromagnetic stirrer, which is used to create a swirling flow in the tube. Depending on the volume flow rates of the gas and the liquid metal, as well as the intensity of the swirl flow generated by the stirrer, 4 flow regimes are observed: (1) the formation of an almost stationary gas pocket in the region below the injection point without any electromagnetic stirring, (2) a twisted void zone along the side wall, (3) a straight void zone in the center of the pipe, and (4) a bubble chain in the core of the pipe flow. The experiments reveal that the wetting conditions at the inner SEN wall have a decisive influence on the resulting flow regime.

We present an evaluation study on the characterization of bubbles rising in liquid sodium by applying two-plane ultrafast X-ray computed tomography (UFXCT). It includes a new method for determining the three-dimensional shape and velocity vector of each individual bubble. In the experimental part, argon gas was injected through a single nozzle located slightly above the bottom of a cylindrical vessel filled with liquid sodium. The gas flow rate was varied between 10 and 635 cm3/min to obtain a chain of individual bubbles. In this parameter range, collisions of bubbles, coalescence or breakup are not expected. Measurements were carried out in a wide spatial range starting near the nozzle up to a height of about 200 mm above it. It was convincingly demonstrated that two-plane UFXCT imaging, in combination with the data processing presented here, allows a reliable characterization of the size, shape and velocity of bubbles with a size of a few millimeters in a sodium column of 54 mm diameter. Observed experimental results include a reproducible fluctuation of shape, position and velocity in the lower part of the column as well as lower terminal rise velocities compared to bubble chains in water.

Understanding the behavior of rising bubbles in a liquid metal under the influence of a magnetic field (MF) is crucial for optimizing continuous casting processes. The study experimentally investigated the effects of a hor- izontal MF on the behavior of bubble chains in a gallium alloy. High-speed ultrasonic computed tomography was used to measure the instantaneous bubble crossing positions in a cylindrical column with an inner diameter of 50 mm. With an increase in the MF strength, the oscillations of the bubbles were suppressed, resulting in the crossing position being concentrated in a certain area of the cross-section. The fluctuations in the time intervals of the chain bubbles decreased. These effects were more pronounced when the magnetic interaction parameter (or Stuart number) was greater than 1. The distribution of bubbles in the direction perpendicular to the MF was widespread slightly compared to that in the direction parallel to the MF; this was noticeable at higher flow rates. The suppression of the wake turbulence induced by the Lorentz force was larger in the direction parallel to the MF than that in the direction perpendicular to the MF. Our results have the potential to be used for the direct verification of numerical models.

This paper presents the analysis of particle-laden liquid metal flow around a cylindrical obstacle at different obstacle Reynolds numbers. Particles in liquid metal are imaged using dynamic neutron radiography. We present the results of particle tracking velocimetry of the obstacle wake flow, and demonstrate the capabilities to assess both temporal and spatial characteristics of turbulent liquid metal flow, and validating the precision and accuracy of our methods against theoretical expectations, numerical simulations and experiments reported in literature. We obtain the expected linear vortex shedding frequency scaling with the obstacle Reynolds number and correctly identify the universal algebraic growth laws predicted and observed for trajectory curvature in isotropic homogeneous two-dimensional turbulence. To our knowledge, this is the first such result for liquid metals. Particle residence times within the obstacle wake and velocity statistics are also derived and found to be physically sound. Finally, we outline potential improvements to our methodology and plans for further research using neutron imaging of particle-laden flow.

Non-metallic inclusions in metallic materials are a key challenge in metallurgical processing such as steelmaking. Aiming to control the population of inclusions and to remove them from the metal in its molten state, gas bubbles are commonly used for melt stirring, homogenisation and purification during ladle treatment. However, the effects of bubble–inclusion interactions in molten metals are not yet well researched, as experimental investigations at high processing temperatures are challenging. To circumvent these harsh conditions, model experiments are performed at room temperature, employing low-melting alloys based on gallium. In such laboratory-scale experiments, the interactions between gas and solid phases in the liquid metal are observable by means of transmission imaging with X-rays or neutron radiation. Starting from the essentials of the measurement principle, this contribution presents two exam-ples of dynamic X-ray and neutron radiography studies in liquid metals, thus showcasing the unique capabilities as well as limitations of imaging measurements at high spatial and temporal resolution. X-ray radiography is able to image both, gas bubbles and solid particles in the liquid metal, at high contrast-to-noise ratio, but only if these particles are rather coarse and heavy [1]. Using neutron radiography, the focus is on a configuration motivated by a single bubble: the particle-laden liquid metal flow around a cylindrical obstacle, measured at 100 Hz imaging frame rate [2]. Combining particle image velocimetry and particle tracking algorithms, we detected particle trajectories in the cylinder wake flow [3], derived particle residence times and velocity statistics [4]. Such radiography studies provide valuable insights into multi-phase liquid metal flows, and the experimental findings may improve the understanding of the inclusion behaviour in bubble-stirred metallurgical reactors. References [1] Lappan T., Sarma M., Heitkam S. et al. Materials Processing F

In water electrolysis, the porous transport layer (PTL) is an essential component of both proton (PEM) as well as anion exchange membrane (AEM) electrolysers. Besides establishing an electrical contact, the PTL enables the electrolyte to be transported to the anode. In the opposite direction, the oxygen (O2) formed at the anode must be transported away, resulting in a complex counterflow of liquid and gas through the PTL, thus limiting the mass transport and, consequently, the conversion of electrical energy. The further development of electrolysers faces so far unexplored operating conditions, in particular by increasing the electric current density. This, in turn, affects the formation and transport of gas bubbles in the PTL, which is not yet sufficiently understood. As the gas-liquid two-phase flow in the PTL is inaccessible for flow measurement by optical methods, we employed time-resolved X-ray and neutron radiography. Using the model experiment sketched in Fig. 1, we aimed for imaging measurements of the gas transport through open-porous foam by mapping the gas fraction distribution over time. In previous experimental studies, we have used X-ray and neutron radiography for flow visualisation in optically opaque fluids such as liquid metal [1] and aqueous foam [2]. Similar to the approach of radiographic measurements of the liquid fraction in aqueous foam [3], this conference contribution showcases the detection and tracking of bubbles based on their gas fraction in X-ray or neutron images. As exemplarily illustrated in Fig. 2, we observed preferred paths of the bubbles moving upwards through the open-porous foam samples. Moreover, we found that bubbles smaller than the pore size are significantly slowed down, even in the case of a hydrophilic surface character of the foam. In summary, the measurement results and conclusions from our experimental parameter study are available for comparison with computational fluid dynamics.

n this paper, we present an experimental investigation that centers on exploring the fluid dynamics within a precessing cylinder. Our research is part of the DRESDYN project at Helmholtz-Zentrum Dresden-Rossendorf, specifically focusing on the precession dynamo experiment. The primary objective of our study is to examine how different rotation configurations influence the dominant flow modes inside the precessing cylinder, specifically considering the prograde and retrograde rotations. Our main focus lies on two significant flow modes: the directly forced mode (m1, k1) and the non-geostrophic axisymmetric mode (m0, k2). These modes hold substantial potential for precession-driven dynamo action. By analyzing the outcomes between the prograde and retrograde configurations, we gain valuable insights into the prevailing flow patterns within the precessing cylinder.

This study aims to investigate how pulsed electromagnetic fields (PEMF) can affect grain refinement and microstructure during the solidification of a model Ga-In alloy. The magnetic system used generates field intensities of 8 - 11 mT and frequencies in the domain between 10 and 300 Hz, a duty cycle of 50%. We record the dendritic structures at the end of solidification experiments after switching OFF the electromagnetic field via X-ray radiographic imaging. Preliminary lab-scale results show that the solidification under frequencies above 100 Hz leads to dendrite fragmentation and solute redistribution in the mushy zone. No evidence of a CET is observed despite numerous fragmentation events. The fragments that detached from the dendritic network were unable to grow as equiaxed dendrites in the liquid as they became trapped within the dendritic network.

Foam processes are essential in many industrial applications e.g., in froth flotation for material separation. A detailed understanding of foam flows is vital for improvements in process efficiency. X- Ray and Neutron imaging can measure flow fields in foam, but require a complex setup and cannot be performed in-situ. Magnetic particle tracking (MPT) is an alternative approach, that measures the trajectory of a small magnetic tracer particle inside the foam as a representation of its movement. Different magnetic field sensors can be applied to detect the magnetic tracer particle. We chose thin film sensors based on the planar Hall effect (PHE) due to their small size, high sensitivity, high signal-to- noise ratio and low cost. Our sensors have a size of only 2 mm x 2 mm and are capable of measuring magnetic fields as low as 10 nT at a sampling frequency of 1 Hz. A sensitivity of 20 V/T at a driving current of 1 mA was achieved by means of sensor bridging. Our PHE-sensors are almost as precise as currently used Fluxgate probes, but offer several advantages due to their reduced size. This includes being installed closer to the area to be measured, enabling finer grids of sensors and decreasing the detection volume, which increases the precision of the MPT as well as other tomographic methods.

Static magnetic fields have been shown to have a significant effect on channel formation in the GaIn freckle defect forming alloy. Inter-dendritic convective solute transport driven by the Thermoelectric Magnetoydrodynamics (TEMHD) phenomena leads to repositioning of the channel, preferential growth of secondary arms, plume migration and complex grain boundary interactions. This paper focuses on a secondary TEMHD mechanism that is generated by larger scale thermoelectric currents that circulate between the liquid and the entire mushy zone. This secondary mechanism is strongly dependent on the thermal profile and this leads to further modification of the bulk flow and ultimately plume migration. This mechanism has been observed by Xray synchrotron experiments and predicted by TESA (ThermoElectric Solidification Algorithm), a parallel Cellular Automata Lattice Boltzmann based numerical model, providing new insights into the intimate coupling between thermal solidification conditions and the effect of the magnetic field.

The flow behaviour of liquid foam is of central importance in froth flotation for mineral processing. Flotation separates valuable mineral particles from gangue material based on the surface wettability. To this end, the solids are finely ground and suspended in an aqueous solution with flotation reagents. In aerated flotations cells, gas bubbles selectively attach to the hydrophobic mineral particles, rise to the surface, and form a froth. To recover the valuables, they are transported out of the flotation cell with the froth. In flotation plants, the recovery of solid and liquid is monitored by optical observation of the overflowing froth. However, this monitoring is limited to the free surface of the particle-laden froth. Aiming for detailed insights into the flow behaviour underneath the surface-near foam bubbles, the laboratory-scale experiment in this work investigates the velocity field of an overflowing foam in combined optical and X-ray measurements. For this purpose, foam was generated continuously, moved similar to a plug-flow in a vertical channel with rectangular cross-section, and flowed off over a one-sided horizontal weir into the open surrounding. The imaging measurements focused on the foam flow in the region of interest around the weir. Simultaneously, the liquid fraction of the foam was monitored by measuring its electric conductivity between electrode pairs mounted near the weir. We used aqueous foams of two different surfactant concentrations but similar bubble size range and superficial gas velocity, yielding around 10 % liquid fraction. The optical measurements carried out through the transparent side wall of the flow channel as well as at the free surface of the overflowing foam. They captured light reflections on the foam bubbles were analysed by an adapted particle image velocimetry algorithm. While the opacity of the foam limits optical measurements to the wall- or surface-near foam bubbles, our approach of X-ray particle tracking velocim

Contactless Inductive Flow Tomography (CIFT) is a flow measurement technique allowing for visualizing the global flow in electrically conducting fluids. The method is based on the precise measurement of very weak induced magnetic fields arising from the fluid motion under the influence of one or several primary excitation magnetic field(s). The simultaneous use of more than one excitation magnetic field is necessary to fully reconstruct three-dimensional liquid metal flows, yet is not trivial as the scalar values of induced magnetic field at the sensors need to be disentangled for each contribution of the excitation fields. Another approach is to multiplex the excitation fields. Here the temporal resolution of the measurement needs to be kept as high as possible. We apply two trapezoidal-shaped excitation magnetic fields with perpendicular direction to each other to a mechanically driven liquid metal flow. The consecutive application by multiplexing enables to determine the flow structure in the liquid with a temporal resolution down to 3 s with the existing equipment.

Magnetic fields have been shown to have a significant effect during solidification in a wide range of conditions from the slow growth of traditional casting to the more rapid growth of Additive Manufacturing. An underlying phenomenon is Thermoelectric Magnetohydrodynamics (TEMHD), which, due to inherent thermal gradients, generate thermoelectric currents and ultimately a Lorentz force through interaction with the magnetic field. In casting this leads to inter-dendritic convective solute transport. This can be used to control freckle defect formation in the GaIn system, where the magnetic field can be used to reposition channel formation, introduce preferential growth of secondary arms, plume migration and complex grain boundary interactions. These mechanisms have been observed by X-ray synchrotron experiments and predicted by TESA (ThermoElectric Solidification Algorithm), a parallel Cellular Automata Lattice Boltzmann based numerical model. In laser AM, melt pools are subject to large thermal gradients and consequently form relatively large thermoelectric currents. The system is highly dependent on the orientation and strength of the magnetic field with competition between Marangoni flow and TEMHD resulting in control of the depth, width and potential deflections of the melt pool. This leads to significant changes in the microstructure including modification to the melt pool boundary layer and epitaxial growth. The numerical predictions also compare favourably to X-ray synchrotron experiments.

X-ray radiographic imaging is an efficient tool for investigating flow phenomena and solidification processes in optically opaque metallic alloys. This contribution is an overview of the latest advances in in-situ radiographic experiments made by the authors, as well as recent applications, including magnetohydrodynamic systems. We investigated a range of phenomena, such as bubble flow in liquid metal under an applied magnetic field, collective bubble dynamics, particle flow in liquid metal channels, and mesoscale solidification of alloys. Radiography measurements in liquid/solidified metal experiments are inevitably performed under adverse conditions of low signal-to-noise ratio, low image contrast, scattering, etc. To extract meaningful information from experimental data we combine both well-known methodology of data processing and our original codes. Examples of image analysis and results of in-situ experiments performed with low melting point alloys are presented and discussed in this contribution. A focus of these experiments is exploring scaled-down representative systems of industrial processes in metallic alloys.

In situ X-ray observations are scarce for ternary and multi-component alloys. A Ga-In-Bi alloy is solidified in a Hele-Shaw cell under buoyancy-driven convection. A complex and strongly disoriented dendrite-type solid phase is formed that differs from a regular dendrite network. It is shown that primary arms of dendrites in a ternary system adapt their velocity to the local concentration ahead of their tips and change continuously or abruptly the growth direction. Some grains exhibit a morphology that is rather similar to the "seaweed" pattern. The appearance of seaweed grains is usually related to a solid/liquid interfacial energy. Further, we focus on the role of melt flow in transition from dendritic arrays to seaweed structures. In particular, it is shown that the splitting of a dendrite tip is preceded by the oscillation of the local intensity of the X-ray pattern which is related to the local concentration of the components.

The cost-efficient production of green hydrogen using renewable energies requires next-generation proton exchange membrane (PEM) electrolysers to be operated at higher current density. Under this new operating condition, the elevated temperature of the ultra-pure water and its supersaturation with oxygen on the anode side have strong effects on the formation and transport of gas bubbles. The resulting gas-liquid two-phase flow through the porous transport layer at the membrane electrode assembly is characterised by up to 50 % gas fraction, which is exceptionally high. Such a foam-like flow within the porous medium is not accessible by optical measurements. Instead, we performed imaging flow measurements by means of time-resolved radiography using polychromatic X-rays as well as thermal neutrons at 100 frames per second imaging frame rate. On a laboratory scale, we aimed to study the bubble transport by mapping the local gas fraction distribution over time. This conference contribution presents two model experiments with open-porous metal and polymer foams, namely made of nickel and polyurethane, showcasing the advantages but also limitations of X-ray and neutron radiography for investigating bubble transport phenomena within such foam structures. In both experiments, foam samples of approximately 70 mm x 70 mm in width and height were sandwiched between the X-ray- or neutron-transparent front and back windows of a vessel filled with deionised water. As neutrons are strongly attenuated in water, the thickness of the water-filled vessel and the foam sample were set to 5 mm along the beam direction in all measurements. Bubbles were generated continuously by injecting compressed air at different but constant volumetric flow rates through a single hollow needle releasing the bubbles directly into the water-soaked foam. Based on calibration radiographs acquired both in the absence and presence of water, quantitative image analysis yielded a pixelwise mapping of the gas fr

Electrolysis is crucial for eco-friendly hydrogen production, but gas bubbles generated during the process hinder reactions, reduce cell efficiency, and increase energy consumption. Additionally, these gas bubbles cause changes in the conductivity inside the cell, resulting in corresponding variations in the induced magnetic field around the cell. Therefore, measuring these gas bubble-induced magnetic field fluctuations using external magnetic sensors and solving the inverse problem of Biot-Savart’s Law allows for estimating the conductivity in the cell and, thus, bubble size and location. However, determining high-resolution conductivity maps from only a few induced magnetic field measurements is an ill-posed inverse problem. To overcome this, we exploit Invertible Neural Networks (INNs) to reconstruct the conductivity field. Our qualitative results and quantitative evaluation using random error diffusion show that INN achieves far superior performance compared to Tikhonov regularization.

A novel concept of a measurement technology for the localization and determination of the size of gas bubbles is presented, which is intended to contribute to a further understanding of the dynamics of efficiency-reducing gas bubbles in electrolyzers. A simplified proof-of-concept (POC) model is used to numerically simulate the electric current flow through materials with significant differences in electrical conductivity. Through an automated approach, an extensive data set of electric current density and conductivity distributions is generated, complemented with determined magnetic flux densities in the surroundings of the POC cell at virtual sensor positions. The generated data set serves as testing data for various reconstruction approaches. Based on the measurable magnetic flux density, solving Biot-Savart’s law inversely is demonstrated and discussed with a model-based solution of an optimization problem, of which the gas bubble locations are derived.

In mineral processing, froth flotation is based on recovering valuable mineral particles by means of the overflowing froth. Industrial-scale froth flotations cells are typically equipped with optical measurement systems, which monitor bubble sizes and flow velocities at the froth surface. However, the velocity profile of the overflowing froth underneath its free surface is not accessible by optical observation. The present study combines X-ray radiography and particle tracking velocimetry in a laboratory-scale experiment, aiming to measure local flow velocities within an opticially opaque foam at a horizontal overflow. For this purpose, we prepared custom-tailored tracer particles: light- weight tetrahedra with an edge length of 4 mm were 3D-printed from a polymer material, and metal beads of 0.5 mm in diameter glued at each corner of a tetrahedron served as radiopaque features. In parallel to the velocity measurements by means of X-ray particle tracking, we determined the liquid fraction of the overflowing foam by electric conductivity measurements using electrode pairs. The experiment was performed with aqeuous foams of two different surfactant concentrations, but similar bubble size range and superficial gas velocity, yielding around 10 % liquid fraction near the overflow. Employing the particles as tools for flow tracing in X-ray image sequences, we identified an unexpected velocity maximum underneath the free surface of the overflowing foam. In a sequel, we will compare the X-ray radiographic measurements with optical measurements of the foam flow velocity through a transparent wall and at the free surface.

In this study we investigate the capabilities of Fiber Bragg Gratings (FBG) sensors to measure temperature variations in the bulk of liquids. In particular, we want to show to what extent the utilisation of optical glass fibers without encapsulation can fulfill the requirements with regard to robustness and measurement accuracy. We built an experimental test setup as a benchmark, in which 23 FBG measuring positions are instrumented in parallel with thermocouples for validation. We present a special assembly procedure in which the fiber is placed under a defined tension to improve its stiffness and immobility for given flow conditions. We propose the application of a single FBG sensor as a strain reference measuring the strain effect in real time and allowing for accurate relative temperature measurements at the other FBG sensor points. The further addition of a well-calibrated, strain-independent thermometer at the reference FBG provides satisfactory absolute temperature data. We apply this method on two test cases: (i) a stable gradient in the liquid metal GaInSn and (ii) a local source of heat in water. We succeeded in accurately tracking the temperature variation both spatially and temporally.

Mit der kontaktlosen induktiven Strömungstomografie (CIFT) lassen sich Geschwindigkeitsfelder in elektrisch leitfähigen Flüssigkeiten global bestimmen. Kenntnisse über den Strömungszustand in Metallschmelzen sind für industrielle Prozesse, wie das Stranggießen von Stahl, von immenser Bedeutung und können auch in der Grundlagenforschung nutzbringend angewendet werden, z.B. zur Analyse von konvektiven Flüssigmetallströmungen als Modellsysteme des Wärmetransportes. Das Verfahren beruht auf der präzisen Messung kleinster Magnetfeldänderungen durch geeignete Sonden und der nachfolgenden Rekonstruktion der Strömungsstruktur durch die Lösung eines linearen inversen Problems. In dieser Veröffentlichung geben wir einen Überblick über die Entwicklungen der letzten Dekade und diskutierten je einen Anwendungsfall für CIFT aus der grundlegenden und der angewandten Fluiddynamik. >>Contactless Inductive Flow Tomography (CIFT) is a flow measurement technique that can reconstruct the global 3D flow in electrically conducting fluids. Knowledge of the flow conditions in liquid metals is of outmost importance for industrial processes as continuous casting of steel. CIFT can also be applied to fundamental research, e.g. to measure convective liquid metal flows. The technique is based on the precise measurement of very small magnetic field changes and the subsequent reconstruction of the flow field by solving the linear inverse problem. This publication illustrates one use-case from fundamental and applied fluid mechanics each.

Solute channel formation introduces compositional and microstructural variations in a range of processes, from metallic alloy solidification to salt fingers in ocean and water reservoir flows. Applying an external magnetic field interacts with thermoelectric currents at solid/liquid interfaces generating additional flow fields. This thermoelectric (TE) magnetohydrodynamic (TEMHD) effect can impact on solute channel formation ,via a mechanism recently drawing increasing attention. To investigate this phenomenon, we combined in situ synchrotron X-ray imaging and Parallel Cellular Automata Lattice Boltzmann method-based numerical simulations to study the characteristics of flow and solute transport under TEMHD. Observations suggest the macroscopic TEMHD flow appearing ahead of the solidification front, coupled with the microscopic TEMHD flow arising within the mushy zone are the primary mechanisms controlling plume migration and channel bias. Two TE regimes were revealed, each with distinctive mechanisms that dominate the flow. Further, we show that grain orientation modifies solute flow through anisotropic permeability. These insights led to a proposed strategy for producing solute channel-free solidification using a time-modulated magnetic field.

We present measurement results for the flow rate of liquid sodium at temperatures up to 700 °C that were obtained with a high temperature prototype of an immersed Eddy Current Flow Meter. The experimental campaign was conducted at the SOLTEC-2 sodium loop at KIT. The main objective of the experiments is the high temperature qualification of the Eddy Current Flow Meter as part of the safety instrumentation of generation IV liquid metal cooled fast reactors. There it is intended to be used for monitoring the flow rate of the coolant and to detect possible blockages of sub assemblies. Due to the large liquid metal volume, the sensor has to be located close to the sub assemblies, therefore measurements from outside of the vessel are not possible and an immersed sensor is required. We demonstrate the successful application of the immersed Eddy Current Flow Meter at such high temperatures and identify the relevant effects with impact on the sensor performance.

In recent years there is an attempt to control the gas stirring intensity in metal-making ladles with the aid of vibration measurements. Understanding better the induced vibrations in two-phase flows can substantially improve the existing models for gas stirring control. In this work, highly sensitive accelerometers were used for the vibration measurements in a liquid metal alloy; Sn–40 wt pctBi alloy at 200 °C and water at 20 °C. The examination of the liquids was conducted in the ladle mockup integrated into the Liquid Metal Model for Steel Casting facility at Helmholtz-Zentrum Dresden Rossendorf. Single bubbles were generated in the respective liquids by controlled argon injection at low flow rates in the range of 0.01 to 0.15 NL/min through a single nozzle installed at the bottom of the ladle. Obtained results demonstrate differences between the induced vibrations in the examined liquids in terms of the magnitude of the root mean square values of vibration amplitude and the shape of the resulting curves with increasing flow rate. Furthermore, continuous wavelet transform reveals variations in the duration and vibrational frequency of the evolved bubble phenomena. The findings suggest that differences in the physical properties of the examined liquids result in variations in the vibrations induced during bubble evolution.

The interplay between solidification and convection, which are usually strongly coupled, occurs via many different mechanisms resulting in very complex dynamics. Melt convection changes the solutal field near the solidification front leading to different microstructures or formation of freckle defects. Quantitative of dendritic structure evolution and melt flow during in situ solidification experiments is rather challenging and requires new/improved approaches to image processing. We present an image processing algorithm designed for quantitative analysis of meso-scale solidification of metal alloys in thin cells via X-ray imaging. Our methodology enables one to identify the bulk liquid volume, liquid channels and cavities, and separate them from the solidified structures. It also enables morphological analysis within the solid domain, including automatic decomposition into dominant grains by orientation and connectivity. Furthermore, convective plumes within the bulk liquid can also be studied. The applied image filters enable the developed code (will be made open-source) to reliably operate even for single images with low signal- and contrast-to-noise ratio at low image resolution. This is demonstrated by applying the code to several in situ dynamic X-ray imaging experiments involving a solidifying gallium-indium alloy in a thin cell. We show that primary spacings, grain (and global) dendrite orientation statistics, convective plume parametrization, etc. can be obtained. The limitations of the presented approach are also explained.

Raw data from - optical fiber FBG, acquired with FiSens software - thermocouple data, qcauired with custom Labview

Biological invasions may result from multiple introductions, which might compensate for reduced gene pools caused by bottleneck events, but could also dilute adaptive processes. A previous common-garden experiment showed heritable latitudinal clines in fitness-related traits in the invasive goldenrod Solidago canadensis in Central Europe. These latitudinal clines remained stable even in plants chemically treated with zebularine to reduce epigenetic variation. However, despite the heritability of traits investigated, genetic isolation-by-distance was non-significant. Utilizing the same specimens, we applied a molecular analysis of (epi)genetic differentiation with standard and methylation-sensitive (MSAP) AFLPs. We tested whether this variation was spatially structured among populations and whether zebularine had altered epigenetic variation. Additionally, we used genome scans to mine for putative outlier loci susceptible to selection processes in the invaded range.Despite the absence of isolation-by-distance, we found spatial genetic neighborhoods among populations and two AFLP clusters differentiating northern and southern Solidago populations. Genetic and epigenetic diversity were significantly correlated, but not linked to phenotypic variation. Hence, no spatial epigenetic patterns were detected along the latitudinal gradient sampled. Applying genome-scan approaches (BAYESCAN, BAYESCENV, RDA, and LFMM), we found 51 genetic and epigenetic loci putatively responding to selection. One of these genetic loci was significantly more frequent in populations at the northern range. Also, one epigenetic locus was more frequent in populations in the southern range, but this pattern was lost under zebularine treatment. Our results point to some genetic, but not epigenetic adaptation processes along a large-scale latitudinal gradient of S. canadensis in its invasive range.   &nbsp

Contactless inductive flow tomography (CIFT) can reconstruct the global 3D flow field in liquid metals. The technique is based on measuring very small magnetic fields induced by currents in the conducting liquid arising from the fluid motion under the influence of two primary excitation fields and solving the according linear inverse problem [1]. We present experimental results of CIFT measurements on a large cylindrical vessel with a height of 640 mm and a diameter of 320 mm, i.e. aspect ratio 0.5, filled with the eutectic alloy GaInSn. The liquid metal is heated from the bottom and cooled from the top, i.e. a so-called Rayleigh-Bénard (RB) convection cell. The temperature gradient drives a complex flow, that varies spatially and temporally [2-5]. The experimental set-up includes a number of Fluxgate sensors for highly sensitive measurements of the induced magnetic field in the order of 10 nT [6] as well as Ultrasound-Doppler velocimetry probes, that directly measure the velocity of the flow along their line-of-sight. Our preliminary measurements used only the excitation field in vertical direction, yet show a very high agreement of the time-dependent velocity profiles measured by UDV and the according CIFT data, projected onto the line-of-sight of the UDV sensors. Acknowledgement This work was supported by the German Research Foundation (DFG) under project no. 374994652. T.V. and F.S. also thank the DFG for support under the grant VO 2331/1. REFERENCES [1] Stefani F., Gundrum T., Gerbeth G., Physical Review E 70: 056306. 2004. [2] Akashi M. et al., J. Fluid Mech. 932 (2022). [3] Zürner T. et al., J. Fluid Mech. 876 (2019). [4] Vogt T. et al., PNAS 115 (2018). [5] Mitra R. et al., Magnetohydrodynamics, vol. 58 (2022) [accepted for publication]. [6] Sieger M. et al., Magnetohydrodynamics, vol. 58 (2022) [accepted for publication].

Contactless inductive flow tomography (CIFT) is a flow measurement technique developed at Helmholtz-Zentrum Dresden-Rossendorf that can reconstruct the global 3D flow field in electrically conducting fluids such as liquid metals. The velocity field of the moving fluid can be reconstructed by solving the underlying inverse problem using appropriate regularization methods. This publication introduces the key concept and mathematical foundation of the method and illustrates the measurement capability of CIFT on two examples: continuous casting of steel in applied fluid dynamics and Rayleigh-Bénard convection as a paradigmatic system in fundamental fluid dynamics.

Froth flow is of central importance for mineral processing by froth flotation. In flotation plants, the recovery of solid mineral particles and liquid from the overflowing froth is monitored by optical observation and, therefore, limited to the froth’s free surface. The laboratory-scale experiment in this work investigates the flow behaviour of an aqueous foam at a horizontal overflow (Fig. 1) in combined optical and X-ray radiographic measurements. Simultaneously, the foam’s liquid fraction was determined by measuring the electrical conductivity between electrode pairs. The optical measurements, performed both through a transparent wall and at the free surface of the overflowing foam, captured light reflexions on the foam bubbles, which were analysed by adapting particle image velocimetry algorithms. While the opacity of the foam limits optical measurements to the surface-near bubbles, our approach of X-ray particle tracking velocimetry (X-PTV) sheds light on the three-dimensional foam flow. The customised tracer particles used in this work consisted of a 3D-printed tetrahedral polymer structure with a total of four small metal beads at its corners (Fig. 1). Owing to their shape and the light-weight material composite, the tracers adhered to the bubble-scale foam structure and were carried by the foam. X-ray radiography visualises the motion paths of each tracer’s metal beads, representing the local streamlines of the foam flow. Further, the X-ray radiographs map the foam’s liquid fraction distribution, thus extending the local measurement of the liquid fraction by means of the electrode pairs. X-PTV reveals comparatively high flow velocities of the three-dimensional foam flow, in particular near the overflow, whereas the optical measurements are sub-jected to wall or surface effects, yielding lower flow velocities. However, X-PTV with customised foam flow tracers comes to its limit in unstable foams at high liquid fraction and high flow velocity.

The Eddy Current Flow Meter is an inductive flow rate sensor which can be used in many liquid metal applications and is well suited for operation in liquid metal cooled fast reactors. There it can be used as part of the safety instrumentation in order to detect a loss of flow in the reactor core. To further qualify the Eddy Current Flow Meter for use in liquid metal cooled reactors, measurements with a high temperature prototype of the sensor have been performed at the sodium loop SUPERFENNEC for sodium temperatures between 200 °C and 300 °C and flow velocities of up to 2.5 m/s. By measuring the magnitude or phase shift of the output voltage of the sensor, the flow rate or velocity of the liquid sodium in a certain volume around the sensor can be determined. Depending on the frequency of the excitation current, the sensitivity of the sensor is changing. Therefore, measurement results for different frequencies and temperatures are presented as well as the results of a frequency sweep for determining the optimal excitation frequency of the sensor.

In situ real-time observations, which are important in studies of binary alloy systems for verification of theoretical hypotheses, are scarce for ternary and multi-component alloys. This work is devoted to in situ visualization of solidification patterns observed during bottom-up solidification of a Ga-In-Bi alloy in a Hele-Shaw cell under buoyancy-driven convection. The investigations are based on a combination of micro-focus X-ray radiography and advanced image processing techniques. A model ternary system is selected from the family of low-temperature metallic alloys with working temperatures below 100 °C. The phase diagram of the ternary Ga-In-Bi system indicates the appearance of an indium-based primary solid phase and only a minor amount of intermetallic phase for concentrations chosen within this study. Our observations show complex and strongly disoriented solidification patterns with curved primary and secondary branches. By studying the solidification dynamics we establish that some grains exhibit a morphology that is rather similar to the "seaweed" pattern. The appearance of seaweed grains is usually related to the low anisotropy of the surface energy of metal crystals that can be relatively easy modified by the presence of another element. We focus on the role of melt flow in transition from dendritic arrays to seaweed structures in this ternary system. Morphological analysis and comparison of dendritic and seaweed patterns are performed using a new X-ray image processing approach.

The Eddy Current Flow Meter is an inductive velocity sensor which can be used in liquid metal applications, such as liquid metal cooled fast reactors. There it can be used as part of the safety instrumentation in order to monitor the coolant flow through subassemblies under normal operating conditions or to detect and locate blockages in case of a local freezing of the coolant. Typically the Eddy Current Flow Meter is used in pipe flows where the flow is mostly parallel to the sensor axis, whereas the flow angle may change significantly above subassemblies in a liquid metal cooled reactor. In the first part, the paper therefore deals with investigating the influence of varying flow angles on the performance of the Eddy Current Flow Meter. By performing measurements in a model experiment, the effect of different flow angles on the measured velocities will be demonstrated. In the second part of the paper, multiple Eddy Current Flow Meters in an array are used to detect and locate blockages in an array of seven subassemblies in the same model experiment. All experiments are carried out at room temperature with a liquid alloy of gallium, indium and tin.

Schaum und dessen Strömungsverhalten sind von zentraler Bedeutung bei der Schaumflotation zur Mineralaufbereitung. Die zu gewinnenden Mineralpartikel lagern sich an aufsteigenden Gasblasen an und werden mit dem Schaum aus der Flotationszelle heraustransportiert. In industriellen Flotationsanlagen wird die Rückgewinnung von Feststoff- und Flüssigphase aus der überlaufenden Schaumströmung mittels optischer Messtechnik überwacht und ist daher auf die freie Oberfläche des partikelbeladenen Schaums beschränkt. Die vorliegende Arbeit untersucht das Strömungsverhalten eines wässrigen Schaums an einem Überlauf vergleichend mittels optischer und Röntgen-Messungen. Der Schaum wurde kontinuierlich erzeugt, strömte in vertikaler Richtung durch einen Kanal mit quadratischem Querschnitt und floss durch einen einseitigen horizontalen Überlauf in die freie Umgebung. Die bildgebenden Strömungsmessungen fokussierten sich auf den Schaum im Bereich des Überlaufs. Gleichzeitig wurde hier der Flüssigkeitsanteil des Schaums mittels Messung der elektrischen Leitfähigkeit zwischen jeweils zwei Elektroden bestimmt. Die optischen Messungen erfolgten einerseits durch die transparente Kanalwand und andererseits an der freien Oberfläche des überlaufenden Schaums. Die dortigen Strömungsgeschwindigkeiten wurden mittels angepasster PIV-Algorithmen ausgewertet, welche Lichtreflexionen auf den Schaumblasen als Messinformation nutzten. Aufgrund der Undurchsichtigkeit des Schaums erfassen diese optischen Messungen nur die oberflächennahen Schaumblasen. Unsere Variante der Röntgen-Radiographie mit maßgeschneiderten Tracer-Partikeln (X-PTV) gibt Aufschluss über die drei-dimensionale Schaumströmung. Die in dieser Arbeit verwendeten Tracer bestanden aus einer 3D-gedruckten polymeren Trägerstruktur in Form eines Tetraeders mit insgesamt vier kleinen Metallkügelchen an dessen Ecken. Dank der Tetraeder-Form und des geringen Gewichts des Materialverbunds hafteten die Tracer in der Schaumstruktur und wurden in

Experimental observations point to structural mechanics as a factor that can significantly alter the development of a cast metal alloy’s microstructure. Forces such as gravity or drag due to solute flow can induce dendrites to deform and/or change orientation. Such changes in microstructural development can lead to defects including stray grains and slivers that degrade macroscopic material properties. However, the interaction of microstructure evolution with structural mechanics is often neglected as a factor in numerical models, potentially rendering them incapable of capturing key defect formation mechanisms. A numerical method coupling a Finite Volume Structural Mechanics solver to a Cellular Automata microstructure solidification solver has been developed, where the growth behaviour of solidifying dendrites is altered by changes to the crystallographic orientation obtained from the calculated displacements. Scenarios where small deformations lead to large orientation changes to accumulate were examined, finding the behaviour to be analogous to that observed in experiments.

The development of closed control loops for electromagnetic actuators in continuous casting based on the current flow conditions of the liquid steel in the mold is challenging due to the opaqueness and the high temperature of the melt. In this work we will investigate the applicability of Contactless Inductive Flow Tomography (CIFT) as a real-time measurement technique for controlling the strength of a ruler type Electromagnetic Brake (EMBr) in a model of a continuous caster. Because CIFT relies on the measurement of the small flow induced perturbation of an applied magnetic field, the measurement system is very sensitive to changes of the strength the EMBr. We will shortly delineate the developed compensation method that is able to cope with the non-linearity of the ferromagnetic yoke of the brake. In combination with the real-time reconstruction algorithm for solving the linear inverse problem, CIFT is able to visualize the flow structure in the mold in real-time with a time resolution of 1 Hz. As a proof of concept of a closed control loop, we implemented a simple controller which switches the EMBr off, when the impingement positions of the jets at the narrow faces are below a critical threshold. The implemented controller and the experiment will be described in detail.