19 Jun 2018 4 minute read

CFD simulation of Electromagnetic Stirring of Liquid Metals in ANSYS FLUENT

Project Description

Current density contours and magnetic field vector in ANSYS FLUENT

Course: BS Thesis Research

Duration: January 2014 - June 2014

Advisor: Prof. Pramod Kumar & Jayasankar Variyar

Institute: Indian Institute of Science, Bangalore, India

Motivation

Traditional die casting has quality issues due to porosity, shrinkage, gas entrainment because of turbulent nature of die filling. Semisolid metals processing (SSM) uses a semisolid slurry (ketchup like) for casting. When the ketchup bottle is shaken, the ketchup flows due to a reduction in viscosity. It thickens when it is allowed to stand again (known as thixotropy). The slurry is maintained between the liquidus and solidus temperatures. The thixotropic nature of the slurry is utilized during die casting where the material is forced to fill a die. The figure below shows a semisolid Aluminum billet which can be sliced by a knife 1.

Semisolid metal processing is a near net shape manufacturing process with the following advantages over die casting:

  1. Eliminates porosity and entrainment defects because of laminar flow

  2. Less solidification shrinkage because the material does not cool from liquid state

  3. Increased die life because of lower temperatures

To utilize the thixotropic behavior, the microstructure of the semisolid metal alloy should be spheroidal and not dendritic 2. One way to achieve this by stirring liquid metal, which breaks the dendrites.

Semisolid Aluminum billet Spheroidal and dendritic microstructure Dendrite fragmentation due to stirring

Electromagnetic stirring is a widely used stirring method because it is non-contact. It consists of circular coils placed around a vessel containing electrically conducting metal (Eg: continuous caster or a ladle containing molten Steel) as shown below3. Electromagnetic fields generated because of current flow through helical coils results induces currents (eddy currents) in the liquid metal. The combination of eddy currents and the magnetic fields produces stirring effect because of Lorentz force

Electromagnetic stirrer schematic

The flowchart below explains the stirring effect due to the interaction between electromagnetic field and fluid flow, which is the basis of an electromagnetic stirrer.

Electromagnetic stirrer

In continuous casting, $v_{\mathrm{molten\,metal}}\approx 2\, \mathrm{m}/\mathrm{min}$. Usually, the velocity of molten metal for computing the Lorentz force is neglected. My contribution comprised of including the effect of velocity in computing the Lorentz force. This was added as a Source Term using User Defined Function (UDF) in ANSYS FLUENT. Including the velocity couples or links the fluid flow (Navier-Stokes) and Maxwell’s electromagnetic equations.

Highlights

  • Solved Maxwell’s electromagnetic equations in ANSYS FLUENT using User Defined Functions, programmed in C

  • Validated model with analytical solutions

  • Enhanced model functionality to include custom inputs (waveforms) of AC current

  • Developed CFD simulations by coupling electromagnetic and fluid flow equations

Conversion of 3D geometry to 2D axisymmetric model geometry for validation with analytical solutions Magnetic field due to current carrying straight conductor Magnetic field due to current carrying circular coil

Publications

  1. BS Thesis: Modeling of Electromagnetic Stirring of Liquid Metals

  2. ANSYS FLUENT UDF code for solving Maxwell’s equations: Code

Skills

  • Customizing CFD software

  • Enhancing CAE model functionality

  • Benchmarking models with reference solution (test cases)

  • Writing User defined functions (UDF)

  1. Image from Semi-Solid Metal Casting: Reducing the Cost of Copper Alloy Parts, OTA Technology Fact Sheet 

  2. Image from National Facility for Semisolid Forming laboratory, IISc 

  3. Image courtesy Prof Pramod Kumar, IISc 


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