Control of gas bubble injection into liquid metals by electromagnetic forces

Due to significant differences in material properties such as density or surface tension the behaviour of gas bubbles reveals some peculiarities in liquid metal applications as compared with ordinary water flows. Moreover, questions about the wetting of solid surfaces or the role of impurities which have a minor importance in water systems, however, show a more dominant influence in metallic melts.
In view of the production of metallic foams the control of the properties of liquid metal two-phase flows by electromagnetic forces in a contactless way would be very attractive. Some examples of magnetic field applications will be presented with respect to the bubble generation process, the dispersion of gas bubbles or the momentum and heat transfer properties of liquid metal bubbly flows.
If gas bubbles are injected into a liquid metal characterised by a large surface tension one should take care to get a good wetting between the fluid and the surface of the gas injector. Otherwise, the gas would try to spread out along this interface to form gas layers. A control of the bubble size and formation rate becomes difficult. The comparison between experiment and theoretical models describing bubble formation processes requires an ideally wetted gas injector. The bubble formation in mercury and the eutectic alloy InGaSn has been studied by means of several methods of gas injection, for instance through single orifices or injectors made from sintered metals with a mean porosity of a few microns. X-ray measurements have been used to directly observe the resulting gas bubbles rising in the liquid metal. In the case of a single orifice the influence of electromagnetic forces on the bubble frequency has been demonstrated.
The transport properties of small argon bubbles have been studied in turbulent upwards channel flows of sodium and mercury. The bubbles were injected by a single orifice located in the centre of the channel cross section. After a distinct distance the local void fraction and the bubble velocity have been measured by means of electrical resistivity probes. The flow has been exposed to external magnetic fields directed transverse or longitudinal to the mean flow direction. Experimental results showing the effect of the magnetic field on the horizontal gas distribution and the ratio between gas and liquid velocity will be presented.
The variety of standard measuring techniques to characterise liquid metal flows is limited due to the nature of metallic melts. However, the availability of diagnostic methods to get information about the structure of liquid metal two-phase flows is a crucial point. A selection of measuring techniques which have been developed in our group to determine flow parameters such as the phase velocities, the void fraction or the bubble size will be discussed with respect to their capabilities and restrictions in various applications.