Coupling Analysis on the Multi-Physics Field and Optimization of Process Parameters in an Aluminum Melting Furnace with Electromagnetic Stirring

Abstract: Electromagnetic stirring is an advanced stirring method in aluminum alloy melting processing. With the application of Electromagnetic stirring technology, the molten aluminum is stirred with alternating magnetic field produced by electromagnetic stirrer, and uniform temperature and composition in the melt may be obtained. Thus the product quality can be improved and the fuel consumed can be saved. It is practical in studying the distribution of physical field in molten aluminum with electromagnetic stirrer systematically, which is helpful to evaluate the stirring effect and optimize the process parameters.Based on the project "Simulation and optimization of 50t aluminum melting furnace used for casting aluminum ingots" supported by a factory, three dimensional finite element model for a 50t aluminum melting furnace and the bottom electromagnetic stirrer was built by ANSYS software. According to the theory of the electromagnetic and fluid flow and heat transfer, the numerical simulation of magnetic field and flow field and temperature field in molten aluminum were studied with sequential coupling method, then the process parameters were optimized. The main conclusions are as following:The electromagnetic stirrer produces alternating magnetic field in the surrounding, which changes with the period of the low frequency AC current loaded on the model. The magnetic field travels right like waves with the speed of 1.44m/s along the long axis of the aluminum melting furnace when the current frequency is 0.4Hz in the reference condition, and the electromagnetic force field shows a similar feature and its speed is twice the former. The magnetic induction intensity and the electromagnetic force increase from the edge to the center of the furnace, and point to the aluminum-in surface, while the sum electromagnetic force points at the aluminum-in surface by the aluminum-out surface in the horizontal plane. Both the electromagnetic force and the magnetic induction intensity point to the top surface in a vertical cross section of aluminum melting furnace. The electromagnetic force decays faster than the magnetic induction intensity and penetrates about 0.4m along the height of molten aluminum. The vertical-projected area of electromagnetic stirrer in the bottom of molten aluminum is the core area where magnetic induction density and electromagnetic force are large.After the electromagnetic stirrer working for 50 seconds, the flow velocity of aluminum liquid is getting stable, the fluid in the central region moves to the aluminum-in surface by the aluminum-out surface because of the electromagnetic force, thus forms two counter-clockwise vortexes in both the aluminum-out surface by the burner side and the furnace door surface by the aluminum-in side, which is consistent with field observation. After stirrer working for one minute, the maximum flow velocity of liquid aluminum is 0.470m/s, while the value in most region of molten aluminum is between 0.1m/s and 0.3m/s. The flow velocity is larger in the areas near aluminum-out surface and furnace door, while weaker in both the center region of vortex and near aluminum-in surface. The temperature distribution of molten aluminum is getting uniform and temperature difference reduced from the beginning of 56.8K to 7.0K at the moment, which shows a significant effect of electromagnetic stirring.Reasonable current intensity and current frequency can increase the stirring intensity and make the stirrer operate more efficiently. The conclusions show that, it is reasonable to select frequency range from 1.2Hz to 1.8Hz and current intensity in a range from 700A to 750A for a 50t aluminum melting furnace. In practice, current frequency should be adjusted at first, then changing the current intensity may be suitable. In order to improve the stirring efficiency, stirring period can be shorten and the amount of aluminum in the furnace may be increased slightly.
Key words: aluminum melting furnace; electromagnetic stirring; magnetic field; flow field; temperature field; numerical simulation;

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