Liquid metal multiphase flows
Multiphase flows with a compressible disperse phase, i.e. gas bubbles, are a major challenge for research and technology development as they tend to form very complex flow patterns. Measurements in liquid metals are challenging, but, indispensable because of distinct differences in material properties compared to water.
Liquid metal two-phase flows are of particular importance for many processes in metallurgy and metal casting. For example, the secondary-metallurgical treatment of liquid steel relies on the injection of purge gas for improving the steel cleanliness. Objectives are the enhancement of mixing and homogenization and the separation of inclusions by flotation.
In case of the flotation process the efficiency is determined not only by the properties of the inclusions in the melt, but also by the size and the specific surface area of the dispersed gas phase. Fundamental analysis indicated that using smaller bubbles in flotation is the most effective approach because it increases the probability of collision between bubbles and particles and reduces the probability of particle detachment at the liquid-gas interface. Bubble interaction which leads to coalescence, breakup or separation plays an important role with regard to the resulting bubble size distribution and the interfacial area within the melt.
Systematic studies devoted to the behavior of the interaction between gas bubbles in a liquid metal are carried out at HZDR. These experiments employ GaInSn, a ternary alloy that is liquid at room temperature and whose material properties are very similar to those of liquid steel. The dynamics of the bubble interactions were visualized by means of X-ray radiography using high-speed video imaging.
The figure below presents exemplary snapshots of bubbles rising in GaInSn for different Argon gas flow rates. Both the bubble size and the release frequency increase with increasing gas flow rate. The rising bubbles form a bubble chain at flow rates below 800 cm3/min (see figure a-d) while for higher gas flow rates the bubbles are ejected as clusters.
|Schematic drawing of the experimental setup.|
|Single frames illustrating bubbles rising behavior in GaInSn for different Argon gas flow rates: a) 50 cm3/min, b) 200 cm3/min, c) 600 cm3/min, d) 800 cm3/min, e) 1200 cm3/min, f) 2400 cm3/min.|
Download video/mp4 - 21,9 MB / 854x480 px
|Bubble detachment from a nozzle at a gas flow rate of 700 cm3/min|
Download video/mp4 - 29,9 MB / 854x480 px
|Bubble separation and gas entrainment at a liquid metal free surface|
Krull, B.; Strumpf, E.; Keplinger, O.; Shevchenko, N.; Fröhlich, J.; Eckert, S.; Gerbeth, G.
Combined experimental and numerical analysis of a bubbly liquid metal flow
IOP Conference Series: Materials Science and Engineering 228(2017), 012006
Keplinger, O.; Shevchenko, N.; Eckert, S.
Validation of X-ray radiography for characterization of gas bubbles in liquid metals
IOP Conference Series: Materials Science and Engineering 228(2017), 012009
Vogt, T.; Boden, S.; Andruszkiewicz, A.; Eckert, K.; Eckert, S.; Gerbeth, G.
Detection of gas entrainment into liquid metals
Nuclear Engineering and Design (2015)294, 16-23
Timmel, K.; Shevchenko, N.; Röder, M.; Anderhuber, M.; Gardin, P.; Eckert, S.; Gerbeth, G.
Visualization of liquid metal two-phase flows in a physical model of the continuous casting process of steel
Metallurgical and Materials Transactions B 46(2015)2, 700-710