Intermittent Behavior Caused by Surface Oxidation in a Liquid Metal Flow Driven by a Rotating Magnetic Field
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TING magnetic fields (RMFs) are used widely in metallurgical engineering for melt stirring during metal refinement, in continuous casting for improving the cleanliness of steel slabs, or for controlling melt flow during solidification of metal alloys or single crystal growth.[1–8] Predictions with respect to the resulting heat and mass transfer in such electromagnetically stirred liquid metals are of key interest for an optimization of the respective industrial technologies. RMF-driven liquid metal flows inside infinitely long as well as finite cylindrical containers have been investigated frequently by numerical simulations.[9–17] The cylinder axis is aligned vertically, whereas the magnetic field rotates in a horizontal plane. In case of a finite cylinder, both situations of a free surface of the melt or a solid boundary at the top of the fluid vessel have been considered. The finite size of the fluid domain is responsible for the existence of an axial gradient of the angular velocity that drives the secondary motion in the radial-meridional plane in the form of a double vortex structure.[18] Recent developments in the field of measuring techniques for liquid metal flows[19] enabled CHAOJIE ZHANG, formerly PhD Student, MHD Department, Helmholtz-Zentrum Dresden-Rossendorf, D-01314 Dresden, Germany, is now Process Engineer, TGE Gas Engineering GmbH, Bonn, Germany. VICTOR SHATROV, Senior Researcher, SVEN ECKERT, Group Leader, and GUNTER GERBETH, Department Head, are with MHD Department, Helmholtz-Zentrum DresdenRossendorf. Contact e-mail: [email protected] JANIS PRIEDE, Lecturer, is with the Applied Mathematics Research Centre, University of Coventry, Coventry CV1SFB, U.K. Manuscript submitted February 17, 2011. Article published online June 22, 2011. 1188—VOLUME 42B, DECEMBER 2011
experimental studies, which provided quantitative measurement data with respect to the velocity field within the metal melts being exposed to continuously applied or modulated magnetic fields.[20–23] Numerical simulations usually consider idealized assumptions of either an enclosed fluid vessel or a free surface as velocity boundary conditions. The surface of the metal melts in almost all technical applications, where melt stirring is considered as a tool for flow control, it is covered certainly by nonrigid layers of slag or oxide particles. We are not aware of numerical or experimental studies in the published literature so far that consider a potential influence of an oxide film at the free surface on the flow structure within the fluid. The oxidation of the free surface is the key issue with respect to the occurrence of the flow phenomena considered in this article. The investigations reported in this study are focused on the flow of a eutectic GaInSn alloy inside a cylindrical column driven by a steady RMF. The fluid vessel has a circular cross section and electrically insulating walls. The free surface of the liquid is covered by a closed oxide film. Such a layer behaves similar to a membrane floating on the liquid. The oxide layer is in contact with the inner
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