Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Liquid metal two-phase flows are widely used in metallurgical processes. For example Argon gas bubbles are injected into a bulk liquid to enhance mixing and homogenization of the melt. Also the Argon gas bubbles remove undesired inclusions by transporting them towards the slag layer at the free surface improving the melt cleanliness. This process is highly dependent both on the properties of the inclusions and on the size and surface characteristics of the dispersed gas phase. The bubble size distribution and interfacial area inside the melt are strongly influenced by the bubble coalescence and breakup which are controlled by the turbulent flow that develops inside the melt. In order to improve the final product quality an external magnetic field is applied to control the fluid motion and bubble behavior. Despite an increasing number of numerical and experimental studies on bubble rise in liquid metals only few experimental data on bubble rise in the presence of a magnetic field exists. These works are mainly focused on investigations of single bubbles in the absence of turbulence. Since bubble rising dynamics in a bubble chain or cluster is often affected by bubble-wake and bubble-bubble interactions direct investigation of bubble chains and clusters rising in liquid metals under the influence of magnetic field becomes crucial.
Bubble chain ascending in non-transparent liquid metal under the influence of magnetic field was examined by X-ray radiography through high-speed video imaging. The Argon gas bubbles were injected through a single bevel-shaped nozzle positioned in the middle at the bottom of a flat Plexiglas vessel. The vessel was filled with eutectic GaInSn alloy at isothermal conditions. We present experimental results accompanied by statistical analysis of the bubble size distribution, shape deformation, velocities, etc. for Argon gas flow rates lying in the range 150-1200 cm³ /min. In general, the increase of the gas flow rate leads to increase in bubble size and velocity. In turn, the velocity shows periodic oscillations related to the zig-zag motion of the bubbles. Both the velocity and oscillation amplitude decrease with increasing the magnetic field strength. Bubble pairing regime appears at higher gas flow rates for bubbles moving in the magnetic field: at 400 cm³ /min against 300 cm³ /min for bubbles moving without magnetic field. Therefore, the appearance of bubble coalescence and breakup is also shifted to higher gas flow rates. The integral gas distribution for bubbles moving without magnetic field is symmetrical due to the bubble chain oscillation in the observation plane. In contrast, the bubbles move almost along the same bubble path in a magnetic field leading to the asymmetry of gas distribution. Further image processing reveals that the major axis of the ellipses fitted to the bubbles at moderate gas flow rates (≤400 cm³ /min) is aligned almost parallel to the bottom of the vessel in the presence of the highest magnetic field used in our experiments (for B ~270 mT). Also the bubble shape oscillations are damped with increasing magnetic field at moderate gas flow rates (≤400 cm³ /min) when the turbulence is strongly suppressed.