Numerical Simulation And Mechanism Study Of Metal Behavior Under Electromagnetic Suspension Conditions

Electromagnetic levitation(EML)is a novel contact-less process which avoid the contaminant from the crucible at high temperature.EML is also praised for providing great kinetics condition during the refining process of high performance metal and alloys.Thus,the behavior of the lifted metallic droplet by EML has received extensive attention in the field of novel material production and metallurgical fundamental research.In this thesis,we focused on describing the droplet behaviors,including levitation behavior,oscillation behavior and transfer behavior,during EML process.The purpose is to provide the suggestions for droplet behavior control,which will benefit experimental precision.Multi-physical coupled models were developed to predict the electromagnetic induction,levitation process as well as reactions on the droplet surface.These models were verified by experimental works which measured the levitation force exerted on copper droplets,temperature increasing of a stable levitated copper droplet and decarburization rate of lifted Fe-Cr-C droplets.The models were then worked with theoretical tools like dimensionless analysis to investigate the levitation and oscillation process of droplets in a considerable material range(copper,palladium,silver,aluminum and gold).Based on the results,the balancing position of a Fe-Cr-C droplet was determined,and the transfer phenomena inside and outside the droplet were discussed.The dimensionless number Sh was calculated to describe the mass transfer rate during the decarburization process.The main conclusions of the work are listed as follows:(1)The effect of levitation environmental parameters to the droplet levitation process was investigated.Numerical model was utilized to investigate the parameters which may have influence on levitation force.It was found out that the current I,current frequency w,droplet size D,relative permeability ?,electrical resistivity ?E all have influence on the levitation force.Through the dimensionless analysis,the relationship between the above parameters to the levitation force was found out:(?)Coupled with numerical modeling,the equation can be used to predict the parameters,which are necessary for droplet levitation,and explain droplet behaviors during levitation.(2)In order to describe the oscillation during the levitated droplet melting,a mathematical model modified based on Hooke's law(spring)was developed.With the mathematical model,the oscillation frequency of different droplets,namely,copper,palladium,silver,aluminum and gold during induced melting were calculated.The result shows that the oscillation frequency of the droplets was around 10 to 30Hz when the current was around 300-450A.The prediction is expected to provide plans for oscillation control.With the subtilized control of molten droplets oscillation,EML will have the potential for more applications.(3)The transfer phenomena inside the EML droplet were investigated by numericalmodeling.Electromagnetic stirring induced by EML was proved to be useful for homogenization of heat transfer inside the Fe-Cr-C droplet,as only a minor temperature gradient was found(less than 4K).With the prediction presented by the model,a huge eddy was found in the fluid field.The maximum velocity of the fluid field inside copper droplet and Fe-Cr-C droplet was 0.39m/s and 0.272m/s,respectively.With the increasing of the deformation ratio,the eddy kept growing with the strengthen of electromagnetic stirring.The deformation ratio will also lead to the increasing of Joule heat.(4)With the transfer phenomena discovered by numerical modeling,a fundamental study was carried out to investigate the mechanism of decarburization on the surface of a lifted Fe-Cr-C droplet.The result shows that:The restrictive link of the decarburization process in the current study is gas transfer.With the effect of temperature gradient on the surface of the droplet,it was suggested to utilize hydraulic diameter as characteristic length for mass transfer coefficient calculation.In addition,instead of gas velocity near inlet,fluid velocity near droplet surface was more suitable to be chosen as characteristic velocity.With these improvements,the mass transfer coefficient(Sherwood number)during decarburization of levitated Fe-Cr-C alloy by Ar-CO2 gas mixture was calculated in the present research range,where the sphere to column diameter ratio(dp/dc)was around 0.58 to 0.8.The Sherwood number was in the range of 26.86-82.14,when the Reynold number(Reh)was in the range of 4.19 to 132.16.