Mixing Enhancement in Gas-Stirred Melts by Rotating Magnetic Fields TOBIAS VOGT, ARTUR ANDRUSZKIEWICZ, SVEN ECKERT, KERSTIN ECKERT, STEFAN ODENBACH, and GUNTER GERBETH A model experiment of a submerged gas injection system in a cylindrical vessel under the influence of a rotating magnetic field and its effect on liquid metal mixing is presented. Argon gas is injected through a nozzle into a column of the eutectic alloy GaInSn, which is liquid at room temperature. Without a magnetic field, the bubble plume in the center region of the cylindrical vessel produces a recirculation zone with high fluid velocities near the free surface, while the fluid velocities in the bottom region are rather low. Our measurements revealed the potential of rotating magnetic fields to control both the amplitude of the meridional flow and the bubble distribution and to provide an effective mixing in the whole fluid volume. Various periodic flow patterns were observed in a certain parameter range with respect to variations of the magnetic field strength and the gas flow rate. DOI: 10.1007/s11663-012-9736-1 Ó The Minerals, Metals & Materials Society and ASM International 2012 I. INTRODUCTION BUBBLE-DRIVEN flows are used in many indus- trial facilities. In metallurgical applications, gas bubbles are injected into furnaces, ladles, or similar melt- containing transfer vessels in order to homogenize the melt and their physical and chemical properties. The principle is that a bubble plume accelerates the sur- rounding liquid upward and produces a recirculation zone. This method is used for steelmaking in bottom blown reactors, and the hydrodynamics of such gas- stirred melts were studied in depth by Sahai and Guthrie, [1,2] Johansen et al., [3,4] and Mazumdar et al. [5] The efficiency of gas-stirred systems can be discussed in terms of mixing time, input energy rate, mixing vessel shape, or the type and location of the gas injection. The high relevance of liquid metal stirring makes it worthwhile to search for possible improvements of such a process. A mixing enhancement could yield a better material quality, a reduction of the mixing time, and therefore result in lower mixing gas consumption or lower electric power consumption. With respect to the widespread utilization of liquid metal stirrers, even a slight improve- ment would yield tremendous energy or mixing gas savings. In bottom blown reactors, gas is injected typically from a point source at the bottom into the liquid metal. The density difference between the gas and liquid metal pushes the gas bubbles upward and results in a turbulent bubble plume. Due to their drag, the rising bubbles accelerate the surrounding liquid metal upward. The conservation of mass is responsible for the resulting recirculation flow that takes the upward shifted liquid metal down again. If the gas bubbles are injected at the symmetry axis of a cylindrical vessel, this flow config- uration is attributed to a dead water zone in the lower part of the vessel which is decoupled from the recircu- lation zone in the upper part of the vessel. It is obvious that a long mixing time is needed before a sufficient heat and mass exchange is achieved as soon as dead water zones exist. Variations of the gas flow rate, the number and the design of the gas injection, and the vessel design can be used to influence the mean recirculation velocity, but the basic global flow structure and the existence of dead water zones cannot be avoided. Another way to stir a liquid metal is the application of AC magnetic fields. Some studies have shown that a vertically traveling magnetic field can be used to drive a toroidal flow pattern. [6,7] Likewise, the application of a rotating magnetic field (RMF) is suitable to drive a swirling flow in a liquid metal column. [8–10] But, the mixing in an almost rigidly rotating fluid, which is driven by an RMF, is rather low. This changes when the RMF is used in a pulsed mode. Here, higher mixing rates can be achieved. [11] Only a few number of studies considered a simulta- neous usage of electromagnetic stirring and gas injection. Zhang et al. [12] studied the impact of a vertically traveling magnetic field on the flow in a cylindrical liquid metal bubble plume. They found out that the downward or upward TMF mainly imposes a global co- or counter flow situation compared with the original bubble-driven flow. TOBIAS VOGT, Graduate Student, SVEN ECKERT, Head of Department, and GUNTER GERBETH, Head of Institute, are with the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Institute of Fluid Dynamics, 01314 Dresden, Germany. Contact e-mail: t.vogt@hzdr.de ARTUR ANDRUSZKIEWICZ, Research Scientist, is with Institute of Thermal Engineering and Fluid Mechanics, Wroclaw University of Technology, and also with Technische Universita¨t Dresden, Institute of Fluid Mechanics, 01062 Dresden Germany. KERSTIN ECKERT, Group Leader, and STEFAN ODENBACH, Professor, are with the Technische Universita¨t Dresden, Institute of Fluid Mechanics. Manuscript submitted: May 2, 2012. Article published online October 5, 2012. 1454—VOLUME 43B, DECEMBER 2012 METALLURGICAL AND MATERIALS TRANSACTIONS B