Efficient Melt Stirring Using Pulse Sequences of a Rotating Magnetic Field: Part I. Flow Field in a Liquid Metal Column
- PDF / 8,009,711 Bytes
- 12 Pages / 593.972 x 792 pts Page_size
- 71 Downloads / 230 Views
CTION
ALTERNATING current (AC) magnetic fields are commonly used in industrial practice for melt stirring.[1– 6] The requirements arising from the particular metallurgical or casting operation are manifold. For example, the electromagnetic stirring should provide an efficient mixing of the melt or counterbalance buoyancy-driven flows. In principle, different magnetic fields (rotating, traveling, pulsating, and combinations of these three) are available, but each field type gives rise to a more or less symptomatic flow pattern. For the optimization of melt-stirring operations, a suitable magnetic field design has to be chosen. Furthermore, the magnetic-field parameters (amplitude, frequency, and spatial and temporal structure) have to be adjusted according to the requirements. In the present article, we consider the standard case of electromagnetic stirring by means of a rotating magnetic field (RMF). The rotary stirring is thought to be employed in steelmaking not only for reasons of ladle mixing or flow control at the mold region in continuous casting,[2–4] but also during the solidification of metal alloys, where it has been proven a powerful method for achieving a purposeful alteration of the microstructure of casting ingots. The RMF-driven convection promotes a transition from a columnar to an equiaxed dendritic growth (CET) and provokes a distinct grain-refining effect.[7,8] However, it was also observed that the RMF stirring of a solidifying metal alloy causes macrosegregations.[9–12] S. ECKERT, Senior Research Scientist, D. RA¨BIGER, Graduate Student, and G. GERBETH, Group Leader, are with the MHD Department, Forschungszentrum Dresden-Rossendorf (FZD), D-01314, Dresden, Germany. Contact e-mail: [email protected] P.A. NIKRITYUK, Postdoctoral Fellow, and K. ECKERT, Group Leader, are with the Institute for Aerospace Engineering, Dresden University of Technology, D-01062, Dresden, Germany. Manuscript submitted May 3, 2007. Article published online November 6, 2007.
METALLURGICAL AND MATERIALS TRANSACTIONS B
In the majority of cases, the RMF stirring is applied as continuous stirring. For a metal alloy in a cylindrical column, the imposition of an RMF leads basically to a swirling motion of the liquid, usually called a primary flow. A secondary recirculating flow in the r-z plane results from the Ekman pumping at the horizontal walls. Although the amplitude of the primary motion is several times higher than the secondary flow, its contribution to an efficient bulk mixing is marginal, because it consists mainly of an almost rigidly rotating core. The convective transport in the vertical and radial directions is mainly achieved by the secondary flow, which consists of a double-vortex structure for the laminar case. Therefore, an amplification of the secondary flow causes an intensification of the mixing rate, which can be realized in the simplest way by increasing the magnetic field. At sufficiently high field intensities, so-called Taylor–Go¨rtler vortices occur, moving along the sidewalls of the cylinder and dissipating inside the Bo¨dewadt
Data Loading...
