A Double-Phase High-Frequency Traveling Magnetic Field Developed for Contactless Stirring of Low-Conducting Liquid Materials

The use of low electrically conducting liquids is more and more widespread.This is the case for molten glass,salt or slag processing,ionic liquids used in biotechnology,batteries in energy storage and metallurgy.The present paper deals with the design of a new electromagnetic induction device that can heat and stir low electricallyconducting liquids.It consists of a resistance-capacity-inductance circuit coupled with a low-conducting liquid load.The device is supplied by a unique electric power source delivering a single-phase high frequency electric current.The main working principle of the circuit is based on a double oscillating circuit inductor connected to the solid-state transistor generator.This technique,which yields a set of coupled oscillating circuits,consists of coupling a forced phase and an induced phase,neglecting the influence of the electric parameters of the loading part(i.e.,the low-conductivity liquid).It is shown that such an inductor is capable to provide a two-phase AC traveling magnetic field at high frequency.To better understand the working principle,the present work improves a previous existing simplified theory by taking into account a complex electrical equivalent diagram due to the different mutual couplings between the two inductors and the two corresponding induced current sets.A more detailed theoretical model is provided,and the key and sensitive elements are elaborated.Based on this theory,equipment is designed to provide a stirring effect on sodium chloride-salted water at 40 S/m.It is shown that such a device fed by several hundred kiloHertz electric currents is able to mimic a linear motor.A set of optimized operating parameters are proposed to guide the experiment.A pure electromagnetic numerical model is presented.Numerical modelling of the load is performed in order to assess the efficiency of the stirrer with a salt water load.Such a device can generate a significant liquid motion with both controlled flow patterns and adjustable amplitude.Based on the magnetohydrodynamic theory,numerical modeling of the salt water flow generated by the stirrer confirms its feasibility.

Parametric instability of a liquid metal sessile drop under the action of low-frequency alternating magnetic fields

A 2-D mathematical model is developed in order to simulate a parametric electromagnetic instability oscillation process of a liquid metal droplet under the action of low frequency magnetic field. The Arbitrary Lagrangian-Eulerian （ALE） method and weak form constraint boundary condition are introduced in this model for implementation of the surface tension and electromagnetic force on liquid droplet free surface. The results of the numerical calculations indicate the appearance of various regimes of oscillation. It is found that according to the magnetic field frequency various types of oscillation modes may be found. The oscillation is originated from an instability phenomenon. The stability diagram of liquid metal droplet in the parameter space of magnetic frequency and magnetic flux density is determined numerically. The diagram is very similar to that found in the so-called parametric instability.

In-Situ Analyzing the Influence of Thermoelectromagnetic Convection on the Nucleation Ahead of the Advancing Interface During Directional Solidification

Sn-3wt%Pb alloy was directionally solidified without and with a 0.08T transverse magnetic field(TMF),and real-time recorded by in-situ synchrotron X-ray imaging.Results indicate that TMF shortened the distance from the location of nucleation to the advancing interface,and accelerated the growth rate of the equiaxed crystal,which caused the columnar-to-equiaxed transition(CET)finally.The thermoelectromagnetic convection(TEMC)in front of the interface and around the crystal’s dendritic branch should respond to changes of the distance and the growth rate.

Thermo-Electric Motions and Structures Generated by Static Magnetic Fields During the Solidification of Metallic Alloys

Thermo-electric currents in the presence of static magnetic fields generate significant electromagnetic forces(TEM forces).The thermo-electric currents are due to the Seebeck effect when temperature gradients exist in the material. Those forces may produce various phenomena like pumping,stirring in liquid metals as well as solid motions,stresses in the solid metal.Those effects may be encountered especially during the solidification of metallic materials because of the existence of significant temperature gradients.In liquid metals the application of a static magnetic field enhances TEM convection at moderate intensity but also damps it when it is strong enough.This means that there exists a maximum of the convection which occurs when the Hartmann layers are comparable to the considered length scale.However,the smaller the length scales are(for example when primary or secondary dendrite arm spacings are considered),the higher the magnetic field strength which is needed to damp the TEM convection.So far,many solidification experiments on various types of alloys(e.g.,Sn-Pb,Al-Cu,Al-Si,Al-Ni etc.)have been carried out.The experiments have shown that TEM convection occurs both in the liquid bulk but also in the deep mushy zone.TEM convection may strongly influence the meso-macrosegregation patterns,the solidification structures and the grain boundaries leading to the striking grain boundary structure of the mushy zone.The flow pattern and accordingly the segregations may be controlled by changing the orientation of the applied magnetic field,i.e.,axial or transverse.We have shown that very high magnetic field strengths,up to 16 T,are needed to damp the TEM convection.Heuristic numerical and analytical investigations show that the TEM force density is very important in the liquid,but can be even more important in the solid.Indeed,for high Hartmann number,the electric currents are confined in a small skin layer near the solid boundaries leading to a decay of the flow.However,the situation is different in the solid,since the force is still important and is not confined.This is confirmed experimentally,since the solidified structures are subject to a much greater rate of dislocations and twinning effects.Furthermore,the TEM forces on the solid promote the appearance of direct Columnar-to-Equiaxed Transitions thanks to the possible enhanced fragmentation of the dendrites.Such mechanisms have been recently partly observed by X-ray in situ observations,where channel formation effects as well as detachment of grains along with horizontal motions were observed.

Parametric Instability of a Liquid Metal Sessile Drop Under a High Frequency Amplitude-Modulate Magnetic Field

The parametric instability behavior of a liquid mercury sessile drop under high frequency Amplitude-Modulate Magnetic Field(AMMF),i.e.a high frequency magnetic field(carder wave)modulated by a low frequency sine wave(modulate wave),is investigated experimentally.The free surface contour of the mercury drop is observed by a CCD camera while varying the frequency and amplitude of the high frequency AMMF.At a given frequency and amplitude,the edge deformations with an azimuthal wave numbers(modes n=3,4,5,6)were excited.