Magnetic levitation of large liquid volume

It is well known from experiments and industrial applications of cold crucible melting that an intense AC magnetic field can be used to levitate large volumes of liquid metal in the terrestrial conditions. The levitation confinement mechanism for large volumes of fluid is considerably different from the case of a small droplet, where surface tension plays a key role in constraining the liquid outflow at the critical bottom point. The dynamic interaction between the oscillatory motion of the free surface and the effects of turbulent flow is analysed using a unified numerical model, which describes the time dependent behaviour of the liquid metal and the magnetic field. The MHD modified k-? turbulence model is used to describe the mixing and damping properties at smaller scales not resolved by the macro model. The numerical multiphysics simulations suggest that it is possible to levitate a few kilograms of liquid metal in a cold crucible without requiring mechanical support from the container walls. Possible applications to the processing of reactive metals are discussed

TEMHD Effects on Solidification Under Microgravity Conditions

An unexplored potential exists to control microstructure evolution through the use of external DC magnetic fields. Thermoelectric currents form during solidification and interact with this external field to drive microscopic fluid dynamics within the inter-dendritic region. The convective heat and mass transport can lead to profound changes on the dendritic structure. In this paper the effect of high magnetic fields is demonstrated through the use of both 3-dimensional and 2-dimensional numerical models. The results show that the application of a magnetic field causes significant disruption to the dendritic morphology. Investigation into the underlying mechanism gives initial indicators of how external magnetic fields can either lead to unexpected growth behaviour, or alternatively can be used to control the evolution of microstructure in undercooled melts as encountered in levitated droplet solidification

Use of a Static Magnetic Field in Measuring the Thermal Conductivity of a Levitated Molten Droplet

Numerical models are used to analyze the complex behaviour of magnetically levitated droplets in the context of determining their thermophysical properties. We focus on a novel method reported in Tsukada et al. [4] which uses periodic laser heating to determine the thermal conductivity of an electromagnetically levitated droplet in the presence of a static DC field to suppress convection. The results obtained from the spectral-collocation based free surface code SPHINX and the commercial package COMSOL independently confirm and extend previous findings in [4]. By including the effects of turbulence and movement of the free surface SPHINX can predict the behaviour of the droplet in dynamic regimes with and without the DC magnetic field. COMSOL is used to investigate arbitrary amplitude axial translational oscillations when the spherical droplet is displaced off its equilibrium. The results demonstrate that relatively small amplitude oscillations could cause significant variation in Joule heating and redistribution of the temperature. The effect of translational oscillations on the lumped circuit inductance is analysed. When a fixed voltage drive is applied across the terminals of the levitation coil, this effect will cause the coil current to change and a correction is needed to the electromagnetic force acting on the droplet

Vacuum arc remelting time dependent modelling

Vacuum arc remelting (VAR) aims at production of high quality, segregation-free alloys. The quality of the produced ingots depends on the operating conditions which could be monitored and analyzed using numerical modelling. The remelting process uniformity is controlled by critical medium scale time variations of the order 1-100 s, which are physically initiated by the droplet detachment and the large scale arc motion at the top of liquid pool [1,2]. The newly developed numerical modelling tools are addressing the 3-dimensional magnetohydrodynamic and thermal behaviour in the liquid zone and the adjacent ingot, electrode and crucible

Modelling the formation and breakup of particle clusters in metal melt subjected to external fields

Aluminium and magnesium based metal matrix nano-composites (MMNC) with ceramic nano-reinforcements promise low weight with high durability and superior strength, desirable properties in aerospace, automobile and other applications. However, due to the small size of the particles, adhesion force between becomes significant which leads to particle agglomeration. Large clusters of nano-particles are detrimental for the final properties of the MMNC. To prevent agglomeration and to break up clusters, ultrasonic processing is used via an immersed sonotrode, or alternatively via electromagnetic vibration. The collapse of the cavitation bubbles as a result of ultra-sonication is believed to be the main mechanism of breaking up the clusters of nano-particles. The complex interaction of flow and co-joint particles subjected to the shockwave induced by cavitation is addressed in detail using a discrete-element method (DEM) code. Adhesive, elastic and frictional forces between the particles are incorporated and various models of adhesion are compared

A two dimensional unstructured staggered mesh method with special treatment of tangential velocity

A two dimensional staggered unstructured discretisation scheme for the solution of fluid flow problems has been developed. This scheme stores and solves the velocity vector resolutes normal and parallel to each cell face and other scalar variables (pressure, temperature) are stored at cell centres. The coupled momentum; continuity and energy equations are solved, using the well known pressure correction algorithm SIMPLE. The method is tested for accuracy and convergence behaviour against standard cell-centre solutions in a number of benchmark problems: The Lid-Driven Cavity, Natural Convection in a Cavity and the Melting of Gallium in a rectangular domain

Dendritic growth velocities in undercooled melts under static magnetic fields

Dendritic growth in undercooled melts has been an interesting topic for metallurgists, physicists and mathematicians. In recent years, attention has been focused on the effects of melt flow on dendritic growth. Significant thermoelectric currents form in undercooled growth due to the presence of relatively large thermal gradients. Numerical simulations showed that the application of a static magnetic field exerts a complex influence on melt flow due to Lorentz force, damping and thermoelectrically driven convection, affecting growth kinetics in undercooled metallic melts. To verify the simulated results, bulk melts of high purity nickel were undercooled using the glass fluxing technique under static magnetic fields of up to 6 T. A high-speed camera was used for in situ monitoring of the recalescence process of the undercooled samples. The dendritic growth velocities at different melt undercoolings were calculated by simulating the recorded images of the recalescence process. The measured data confirms the predicted effect of heat and mass transport through thermoelectric magnetohydrodynamics flow on dendritic growth

Cold crucible melting with bottom pouring nozzle

Cold crucible melting technique with electromagnetic induction is used to obtain reactive metal castings and produce high quality metal powders for aerospace, automotive and medical applications. An important part of this technology is the nozzle used to pour the molten alloy through the bottom opening. The paper uses mathematical modelling technique, previously validated on multiple similar cases, to investigate a new type of non-consumable nozzle made of copper segments. The design of the nozzle requires to satisfy the narrow balance between the thin solidified protective layer on the wall while avoiding the blockage of the outflow if the nozzle is frozen completely. The sensitivity of the outflow to the nozzle diameter is investigated. The AC electromagnetic force leads to high mixing rates, transitional flow structures and turbulence of the melt, contributing to the melt shape dynamics and the heat loss to walls. The beneficial features of the cold crucible melting to purify the melt from particulate contamination are explained using the particle tracking for the full melting and pouring cycle

Multiple timescale modelling of particle suspensions in metal melts subjected to external forces

Electro-magnetic (EM) fields are widely used in metallurgy in order to stir conducting metals without the risk of contamination or causing an instability or chemical reaction. During the manufacturing of metal matrix composites (MMC), ceramic micro- and nano-particles are added into the metal melt, and ultrasonic (US) processing and EM stirring are used to break the agglomerates and to enhance the dispersion of the particles. EM stirring can also be used to remove the unwanted particles from liquid metal by pushing them towards the walls of the cru-cible where they adhere and can be easily removed. A model has been developed to account for the complex interaction of the particles with each other, with the walls, as well as with the flow of the metal melt. Particles are modelled as elastic spheres with adhesion. Adhesion is incorporated in the model using the Johnson, Kendal, Robert (JKR) and Derjaguin, Muller, Toporov (DMT) theories. The case of the oblique impact of the particles is modelled according to the Thornton and Yin method based on the partial-slip theory developed by Mindlin & Deresievics. The developed particle model is then coupled with the magneto-hydrodynamics (MHD) code PHYSICA in order to demonstrate the effect of the EM stirring and vibration. Multiple time-scales are used which permits modelling the realistic time range of metal-processing and at the same time capture the individual collisions between particles with suffi-cient precision. Several methods of predicting the particle collisions are employed and their ef-ficiency is compared for the case of removing contaminating particles from liquid meta