The Impact of a Vertically Travelling Magnetic Field on the Flow in a Cylindrical Liquid Metal Bubble Plume
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etal two-phase flows are widespread in metallurgical engineering. For instance, gas injection is applied routinely at various stages of melt refining in steelmaking.[1] The gas is injected into the metal melt to promote chemical reactions or mixing. The bubble-induced flow reduces gradients of temperature and/or concentration and supports an effective removal of deoxidation products and other impurities. The control of mixing in these refining operations becomes more and more important. The mixing is a combination of both convection and eddy diffusion processes. In practice, the mixing time has been often used as a quantitative measure for mixing.[2,3] It represents the time within which the liquid content inside a vessel reaches homogeneous distributions of temperature or concentration. The mixing time can also be interpreted in terms of the turnover rate of a vessel’s content. Such an approach emphasizes the importance of convection with respect to mixing. Many studies have been devoted to the mixing in bottom blown reactors for steelmaking.[2–10] The mixing efficiency was discussed to depend primarily on the input energy rate, the specific shape of the vessel, or the mode of gas injection. A bottom gas agitation into a stagnant liquid pool often implicates an insufficient convective exchange of heat and species between separated flow regions in the upper and lower parts of the fluid vessel. The gas injection from a point source generates a turbulent C. ZHANG, Postgraduate, S. ECKERT, Group Leader, and G. GERBETH, Department Head, are with the MHD Department, Forschungszentrum Dresden-Rossendorf (FZD), 01314, Dresden, Germany. Contact e-mail: [email protected] Manuscript submitted November 5, 2008. Article published online August 21, 2009. 700—VOLUME 40B, OCTOBER 2009
plume of rising bubbles, which entrains the surrounding liquid. Although the gas bubbles escape at the free surface, the transported liquid spreads out laterally and produces a global liquid circulation inside the fluid container. Obviously, the liquid motion becomes considerably weaker the lower the position inside the fluid vessel. Dead flow zones may occur in the bottom part laterally with respect to the gas injector. These regions are decoupled effectively from the main circulation, which implies long mixing times for a complete homogenization of the melt. Variations of the gas flow rate and the vessel geometry, as well as the number, location, or the type of the gas injector usually influence the amplitudes of the mean velocity and respective turbulent fluctuations, the intensity and extension of recirculation zones. However, the global flow pattern cannot be substantially modified. Electromagnetic fields can be used to control, to some extent, both the flow pattern and the void distribution and, hence, the efficiency of the considered metallurgical operation. The electromagnetic methods allow for a completely contactless agitation of the liquid metal. Thereby, the flow intensity can be controlled directly and easily by tuning electrical control parameters like field amplitu
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