Numerical Investigation of the Position and Asymmetric Deformation of a Molten Droplet in the Electromagnetic Levitation
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TRODUCTION
A novel experimental technique based on electromagnetic heat flow theory, is electromagnetic levitation (EML). This technique provides a contactless environment for metal purification, material synthesis, and property measurements, and has been widely used in many fields, such as metallurgical research, materials processing and microgravity environment simulation.[1,2] Compared with traditional melting processes, EML can prevent contamination from crucible materials and also reduce segregation through electromagnetic stirring.[3,4] Due to the contactless aspect of EML, the effects of wide temperature ranges can be evaluated, and materials with a melting point above three thousand degrees can be melted in an EML facility.[5] Thus EML is an ideal means for the investigation of refractory metals, high purity metals, reactive metals, and high temperature metallurgical chemistry. Half-Heusler alloy (Zr1-xTixNiSn0.975Sb0.025) with the excellent thermoelectric performance was synthesized by Jiang et al. using EML equipment.[6] Zhang et al.[7] measured the surface tension of Ni85Sn15 alloy using an EML melting approach. In addition, when the EML facility was used for the simulation of space microgravity, Szekely et al.[8] found that the buoyancy convection effect caused by gravity could be avoided, and heterogeneous nucleation generated by contact with a refractory wall was avoided.
PENG YAN and GUIFANG ZHANG are with the Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, 650093, China. Contact e-mail: [email protected] YINDONG YANG and ALEXANDER MCLEAN are with the Department of Materials Science and Engineering, University of Toronto, Toronto, ON, M5S 3E4, Canada. Manuscript submitted June 4, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Operational conditions for EML are very complex and it is not an easy task to achieve stable levitated droplets. The process can be described as follows. An alternating magnetic field is created by passing a high-frequency current through a water-cooled copper coil, which is specifically designed to provide droplet levitation at predictable temperature. When a conductor, in this case a metallic specimen, experiences change in the magnetic field, eddy current is induced on the surface of the specimen. In addition to heating the specimen, the induced circulating current also provides a repulsive effect, or lifting force within the alternating magnetic field. However, some instability phenomena are associated with the EML process as indicated by the following aspects: (1) Nonlinear oscillation in the vertical direction was investigated by Asakuma et al.[9] during the melting of silicon droplets. (2) Wang et al.[10] found that wavering and lateral migration of the droplet could cause it to stick to the quartz tube wall. (3) The surface fluctuations and deformation of a levitated droplet were investigated by Li et al.[11] Some unstable phenomena captured in the current research on EML are exhibited in Figure 1. The downward migration of
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