The Influence Of Static Magnetic Field On The Stability Of The Electromagnetic Levitated Molten Droplet

For electromagnetically levitated(EML) molten droplet, some unstable factors usually exist in the droplet, such as internal fluid convection, quick spin and horizontal displacement and so on. As a result, stabilizing the droplet is very important for EML technology. In this thesis, the influence of the induction coil geometry and the static magnetic field on the stability of the droplet was investigated by experiments and numerical simulations.The temperature of molten copper droplet under different coils arrangements was measured experimentally. In the condition of the same radius of bottom turn, the lower temperature appears in the molten droplet with Cone coil compared to the Column one. Temperature decreases with the reduction of the turn number in upper coils, while the melting time becomes longer. Increasing of the distance between the upper and lower coils leads to a lower temperature in the droplet, whereas, it also raises the chance of horizontal drift. Therefore, the Cone coil 3-2 with a small distance(S=6mm) was used in the follow-up experiments.A horizontal static magnetic field was imposed on an EML Cu droplet through a U-shaped static magnetic component. The shape oscillation of a Cu droplet was recorded continuously under different magnetic field intensity using a high speed camera. The effects of static magnetic field on the oscillation frequency, amplitude and spin angle of the droplet were analyzed from the recorded data of droplet shape. The result shows that when the strength of the static magnetic field exceeds 0.3T the solid Cu is levitated statically without any spin and horizontal movement. For molten Cu droplet, its amplitudes of the R-, A and Dmax were reduced by 25%, 76% and 60% respectively when a static magnetic field with 0.15 T is imposed. With the increase of the magnetic field strength the amplitude, frequency of m=±1 and m=±2 oscillation decease continuously. However, the horizontal static magnetic field has little effect on the frequency of m=0 oscillation. Finally, the result exhibits that the horizontal static magnetic field can inhibit the spin of the levitated droplets. For instance, when the strength of the magnetic field is 0.53 T the droplet spins within a very narrow angle of 10°, which is quite smaller than that of the case without static magnetic field. These results exhibit that the imposed horizontal static magnetic field can validly improve the stability of electromagnetic levitated droplet. According to the experimental data, the surface tension coefficient of molten copper with the temperature 1963 K is calculated to be 693.4 m Nm-1 under the condition of the static magnetic field 0.53 T in the air environment.Through simulation results of the internal flow and temperature distribution in the levitated drop, we analyze the influence of the coil geometry and static magnetic field on the stability of electromagnetic levitation copper droplets. In the condition of the bottom turns with the same size, we found that alternating magnetic field in the levitation regions of Column coil is greater than that of in Cone coil. And it is enhanced with the increase of upper coil turns. As the alternating magnetic field increases the temperature of molten droplets is raised and the flow in the molten droplets is enhanced significantly. Both static magnetic fields imposed in the radial and horizontal directions can effectively suppress the flow in the molten droplets. Under the same intensity, static magnetic field imposed in radial direction gives more significant inhibition. Numerical results exhibits the internal flow clearly. The internal fluid flow is driven by the electromagnetic force and the flow pattern is characterized by two counter-rotating recirculating loops. The one locates near the equator, and the other lies in the bottom left corner. The temperature distribution is characterized by higher values in the bottom region of the drop and lower in the upper. The highest temperature occurs near the bottom left corner, and the temperature difference in the droplet is about 3.74 K under a static magnetic field of 0T. With the static magnetic field increase the temperature difference becomes larger due to the flow is suppressed. At static magnetic field exceeding 0.4T, the temperature difference in the droplet reaches 3.95 K.