Project A5: Liquid metal two-phase flows and magnetic particle separationPI: Gunter Gerbeth (HZDR)
Partners: HZDR, FZJ, TUD, TUBAF
1. Scientific case for the project
1.1 BackgroundMany technical applications in power engineering, metallurgy and casting rely on liquid metal two-phase flows [1-5]. In liquid metal targets, gas bubbles are added to mitigate shock waves upon pulsed operation or sudden changes in load. In metallurgy, lighter particles are introduced into the melt for refinement. As well, gas injection is routinely applied at various stages to promote chemical reactions and to stir the melt for reducing temperature and/or concentration gradients together with promoting an effective removal of impurities. In continuous steel casting, argon bubbles are injected to avoid solidification and to generate desired flow patterns directing impurities to the surface. In aluminium casting, on the other hand, uncontrolled entrapment of gas bubbles may cause serious casting defects. Also in liquid metal cooled fast reactors the flow induced gas entrainment is undesired, as it constitutes an important safety issue. Recycling technologies, eventually, require the separation of varying solid or liquid phases. An example is separation of copper droplets from slag resulting from agglomeration promoted by magnetic stirring. In all the cited applications, functioning, safety and efficiency are below the desired level as they crucially depend on so far incomplete understanding of liquid metal multi-phase flow.
Multi-phase flows with a compressible disperse phase, i.e. bubbles, are a particular challenge and substantially more difficult than corresponding single-phase flows. Different length and time scales coexist even in rather generic flow configurations such as a bubble plume and the stochastic nature of both the carrier-phase turbulence and the dispersed-phase distribution needs to be accounted for. Substantial work, experimental as well as numerical, has been done to improve our understanding of these flows in their great diversity [2-5]. The physical and numerical models, however, still lack sophistication as they need reliable experiments for validation. Compared to the numerous experimental studies on the movement of bubbles in transparent liquids, especially in water, the number of publications dealing with gas bubbles rising in liquid metals is small. Measurements in liquid metals are substantially more difficult but are indispensable because of the distinct differences in material properties compared to water, in particular density and surface tension. Further specific problems concern the influence of surfactants at fluid-gas interfaces. All these conditions cause the gas bubbles to behave very differently in liquid metals compared to water, in particular with respect to bubble formation, dispersion, coalescence and breakup. Substantial work is still to be done in this respect.
1.2 Most important goals of the planned workThe main goal of the project consists in the development and qualification of numerical codes for an appropriate description of dispersed liquid metal two-phase flows. Such a task requires the implementation of suitable model experiments to provide the data base for code validation. To this end, generic studies exploring the magnetic field influence on gas-bubble liquid metal two-phase flows will be conducted. A benchmark experiment will be mounted by the project featuring the generic configuration of a bubble plume in a liquid metal column which can furthermore be exposed to different kinds of magnetic fields. Investigations on the distribution of gas bubbles will be performed applying several measurement techniques in parallel: local resistivity sensors, ultrasonic measurements, and electromagnetic methods.
Additional experiments will rely on X-ray radioscopic diagnostic techniques, for which a unique X-ray lab exists at HZDR. Here, the focus will be on the visualization of bubble coalescence and breakup. Moreover, the phenomenon of the flow-induced gas entrainment will be investigated. Further development and tests of advanced two-phase measurement techniques for liquid metals are an important part of the Alliance works and in this respect the project will establish a close link to the YIG ”Measurement techniques”.
The numerical partners at TUD will perform model development to a level of detail where individual bubbles are resolved and validate against the experimental data for flow field and bubbles in the plumes without and with magnetic field. Furthermore, the challenging modelling of break up and coalescence will be undertaken. This must be done by cooperation between simulation and experiment, since only the latter can provide the relevant data.
Another focus of the project will be on generic studies on electromagnetic phase separation. This is of particular interest for the development of metal slag cleaning processes, which have a great economical impact today in view of modern resource technologies. Numerical studies will be carried out on generic aspects and to optimize magnetic field configurations for an efficient metal separation. The subject of magnetic separation will be investigated in close cooperation with the new Helmholtz Institute of Resource Technology in Freiberg, which is a joint institute of HZDR and TUBAF.
The experimental data of the project will be made available to the entire alliance, in particular project B6 and C3. The numerical capabilities within the project will also be offered to other partners in the alliance, in particular numerical calculation will be performed in cooperation with project C3 with respect to the properties of the two-phase flow in the mould of a steel caster.
2. Existing competencies and infrastructureExperimental activities were carried out at HZDR to investigate the momentum transfer and the turbulent dispersion of gas bubbles in MHD sodium channel flows , to study the behaviour of single bubbles in a longitudinal DC magnetic field  or to assess the impact of both DC and AC magnetic fields on the flow structure inside a rising bubble plume [8,9]. HZDR offers a variety of model experiments to study fundamental aspects of liquid metal two-phase flows. Suitable measuring techniques (local probes, ultrasonic and electromagnetic techniques) have been developed to determine crucial two-phase flow properties such as the void fraction or bubble characteristics (trajectory, velocity, size). A recent study showed the capabilities of X-ray radioscopy to visualize the motion of gas bubbles in a liquid metal .
The Chair of Fluid Mechanics at TUD has long term experience in the simulation of turbulent flows  and MHD flows of weakly conducting fluids . Recently, the code PRIME was developed for efficient parallel simulation of multiphase flows using an immersed boundary method on Cartesian coordinates  which allows to resolve the geometry of individual particles of the disperse phase in their motion through the continuous phase. The code is now routinely applied for laminar and turbulent flows involving heat transfer, buoyancy, and MHD flows laden with particles or bubbles [14,15].
FZJ has long-lasting competences in CFD and uses primarily the commercial codes ANSYS-Fluent and ANSYS-CFX, respectively, which rely on the finite volume method with the capability of simulating multi-phase flows using Euler-Euler or Euler-Lagrange models [16-18]. Previous calculations captured the interplay between multiple fluid phases like gases and liquids, dispersed particles and droplets and even free surfaces. For the Eulerian multiphase model a multiple size group model is available, which allows the simulation of the effect of turbulent breakup and coalescence of different bubble sizes. The Lagrangian particle transport model can be used to simulate disperse phases discretely distributed in a continuous phase, considering phenomena such as particle-wall interaction and particle-particle collision.
3. Resource planning and Budget JustificationWithin the project a PhD student at HZDR will perform the described experiments. Numerical simulations and model development at TUD and FZJ will be conducted by one PhD student each.
Links: The project will be a basis for project B5. Close relations exist also to projects C3, C4 and the YIG.
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