Mixing Enhancement in Gas-Stirred Melts by Rotating Magnetic Fields
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-DRIVEN flows are used in many industrial facilities. In metallurgical applications, gas bubbles are injected into furnaces, ladles, or similar meltcontaining transfer vessels in order to homogenize the melt and their physical and chemical properties. The principle is that a bubble plume accelerates the surrounding liquid upward and produces a recirculation zone. This method is used for steelmaking in bottom blown reactors, and the hydrodynamics of such gasstirred 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 improvement would yield tremendous energy or mixing gas savings. 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: [email protected] 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
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 configuration is attributed to a dead water zone in the lower part of the vessel which is decoupled from the recirculation 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
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