In this course the physical and technical basics of induction melting processes and technologies will be explained. During the introduction the author will demonstrate along typical features of induction melting, why today induction melting is used in many industrial processes. In the first part of this course the physical basics will be discussed by explaining the fundamental equations. The most important features of induction melting like, Joule heat effect, induced current and power density distribution in the melt, electromagnet forces, free melt surface deformation, turbulent melt flow and stirring will be explained.In the following the physical principle of the industrial most important induction melting furnaces, the induction crucible furnace and the induction channel furnace, will be shown. In the second part of this course the author will explain, how the heat and mass transfer processes in the melt of induction furnaces are caused and influenced by the turbulent melt flow. Along industrial oriented examples it will be shown how numerical simulation based on sophisticated turbulent models can be used today for improved process understanding and design of the melting processes and devices. The recapitulation of the most important features of induction melting processes and technologies will conclude this course.

1. E. Baake: 12-05-2016
EPM Academy
Webinar May 12, 2016
1
Physical and technical basics of
induction melting processes
Egbert Baake
Institute of Electrotechnology
Leibniz University of Hannover
Hannover / Germany

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Outline
 Introduction
 Physical principles of induction melting
 Electromagnetic shaping and stirring
 Process simulation
 Power balance and efficiency
 Classical applications
 New developments
 Conclusions

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Induction Melting:
Multifunctional Applications

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Industrial process requirements for
melting in induction furnaces
 Mixing and homogenisation
of the entire melt
 Homogenisation of the
temperature, avoiding of
local overheating, but realizing of
sufficient superheating of the
entire melt
 Intensive stirring at the melt
surface (melting of small-sized
scrap)
 Avoiding of erosion and clogging
of the ceramic lining
 Avoiding of melt instabilities,
splashing or pinching
 Intensive stirring for cleaning or
degassing of the melt

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Why induction technologies for
melting processing solutions?
Heat can be generated within the material
High energy density and fast processing if required
High temperature
if required
Homogeneous temperature distribution in the melt
High efficiency
Low specific energy consumption
High reliability
Low thermal inertia Less environmental impact
Clean melting in any atmosphere
Electromagnetic stirring
High level of automation
Reproducible process
conditions
Good integration in
the production line
Electromagnetic processing
High reliability
Electromagnetic shaping

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electro-
magnetic field
velocity field temperature field
free surface shape
liquid-solid
interface
Physical correlations in induction
melting processes processes
alloy composition
non-linear
material properties
6

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Principle of induction heating Example: Induction heating of a tube
Source: RWE-Information
Induktive Erwärmung
Principle of induction heating
Eddy currents

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J: Stromdichte
J
8
Current density distribution in a
cylindrical workpiece (approximation)
Electromagnetic penetration depth
Source: RWE-Information Induktive Erwärmung
Current density distribution and skin effect
With: ρ = spec. electrical resistance
μ = permeability
f = frequency
J: current density

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melt
steel-
construction
concrete-ring
meniscus
melt flow
crucible
induction
coil
magnetic
yoke
9
Induction furnaces for melting
Induction crucible furnace Induction channel furnace
 Used mainly for melting
 Medium high efficiency
 Operating frequency: 50 ... 1000 Hz
 Used mainly for holding and pouring
 High efficiency
 Operating frequency: 50 Hz, 60 Hz
Source: RWE-Information Prozesstechnik
thermal
isolation
magnetic
yoke
inductor
channel
furnace
vessel
refractory
melt
induction
coil

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Scheme of an induction crucible furnace
melt
steel-
Construction
meniscus
melt flow
crucible
induction coil
magnetic yoke

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Example of medium frequency induction crucible
furnace: 12 t/9,3 MW/250 Hz
source: ABB Industrietechnik AG

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Meniscus shape and melt flow of the
crucible induction furnace

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Meniscus shape and melt flow of the
crucible induction furnace
Inductor current: J1

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Meniscus shape and melt flow of the
crucible induction furnace
Magnetic field: B
Inductor current: J1

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Meniscus shape and melt flow of the
crucible induction furnace
Magnetic field: B
Inductor current: J1
Induced current
density in the melt:J2

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Meniscus shape and melt flow of the
crucible induction furnace
Magnetic field: B
Inductor current: J1
Induced current
density in the melt:J2
Electromagnetic
force density:
F = J2 x B

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Meniscus shape and melt flow of the
crucible induction furnace
Magnetic field: B
Inductor current: J1
Induced current
density in the melt:J2
Electromagnetic
force density:
F = J2 x B
Melt flow
pattern

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Spec. kinetic energy of turbulence:
k = ½ (v´x1
2 + v´x2
2 + v´x3
2)
local melt flow velocity in
dependence on time
Shared in:
1. Time averaged flow velocity
 convective heat and mass
transfer
2. Instationary fluctuations
and oscillations
 turbulent heat and mass
transfer
Characteristics of turbulent flow in
induction furnaces
Vmax ≈ 20 cm/s

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Influence of power and frequency on the melt
movement
melt
steel-
construction
concrete-ring
meniscus
melt flow
crucible
induction coil
magnetic
yoke
Velocity v of the melt flow is
proportional to the inductor
current I:
v ~ I for const. frequency f
and therefore proportional to
the square root of the spec.
power in the melt PS:
for const. frequency fSPv~
The height of the meniscus:
fPKh SÜ ⋅=
Melt flow velocity v is
proportional to:
0,4
S f1Pv ⋅~

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Meniscus shape and melt flow in dependence on the filling level
filling level 80% filling level 100% filling level 120%
filling level: height of the melt when power is switched off relating to the upper end of active coil

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Numerical simulation
approach
r << λEM
Harmonic
EM field
Alloy shape
Alloy HD
movement
Secondary
EM field
Steady part of EM force is
calculated for fixed free surface
shape
Volume of Fluid
numerical technique
for calculation of
two-phase fluid
hydrodynamics.
VF = 0 – phase #1 (air);
VF = 1 – phase #2 (melt);
0 < VF < 1 – free surface
intersects mesh element.
Unsteady Reynolds
Averaged Navier Stokes
equation for
incompressible fluid
with k – ω SST isotropic
turbulence description and
0
( )
0
ef ef
ROT
B B
f
µ
⋅∇ ⋅
=
 

2
2
HD
v
p ρ=
0 0Rem r vσµ=
0
2
EMδ
µ σω
=
2
0
04
EM
B
p
µ
=

qvol = 0
FEM –
Ansys Classic
FVM –
FLUENT and CFXReciprocal interaction
Rem << ω
˄
21

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RANS (k-ε model)
• Whole energy spectrum is modelled
• Relatively low mesh resolution
requirements
• Steady-state simulations
Direct Numerical Simulation (DNS)
• All scales are resolved directly
• Very high requirements for
computational resources
• Simulations of industrial installations
are impossible
Large Eddy Simulation (LES)
• Large scales are resolved directly while only small
scales are modelled
• Relatively high mesh resolution requirements
• Transient 3D simulations
CFD problem
Re ≥ 104
Numerical simulation of CFD problems
Mesh
22

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Time-averaged flow pattern [m/s] (3D-transient LES)
P = 4 540 KW
Hind = 1.33 m
Rcr = 0.49 m
3D hydrodynamic model of an industrial
induction crucible furnace melting cast iron

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Cross-section of the transient
melt flow [m/s]
3D transient simulation of the melt flow of an industrial
induction crucible furnace using LES model
Length-section of the transient
melt flow [m/s]

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Inductor efficiency of a cylindrical arrangement of inductor and workpiece
as function of workpiece diameter to penetration depth ratio
Source: RWE-Information
Induktive Erwärmung
Electrical efficiency of induction crucible
furnace
steel 20°C, μr=100, ρ=0.13
steel 400°C, μr=30, ρ=0.45
graphite 200…1000°C, ρ=10
steel 1000°C, μr=1, ρ=1.2
stainless steel 900°C, μr=1, ρ=1.2
stainless steel 20°C, μr=1, ρ=0.8
aluminium 100°C, ρ=0.038
copper 100°C, ρ=0.022
workpiece diameter / penetration depth

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75%
4700 kW
390 kWh/t
1%
60 kW
15%
940 kW
1,5%
95 kW
3%
190 kW
1,5%
95 kW
100%
6270 kW
520 kWh/t
3%
190 kW
Trans-
former
Converter
Capacitor
bus bars
Induction
coil
Furnace
Construction
Heat
Losses
Energy balance for a medium frequency (MF) induction
crucible furnace for melting of grey cast iron (example)

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Induction channel furnace: main features
 Joule heat and Lorentz forces
are generated in the melt of
the inductor channel
 heat transport from the inductor
channel to the furnace vessel is
important
thermal
isolation
magnetic
yoke
inductor
channel
furnace
vessel
refractory
melt
induction
coil
Primary and secondary current

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3D electromagnetic and fluid-dynamic model for ICF
(examples)
~ 0.8 m
throat
Iron yoke
induction
coil
channel
cooling
shield
electromagnetic model
fluiddynamic model
channel
vessel inductor
throat

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Distribution of the electromagnetic force density and melt flow in
the cross section of the channel
29
EM-force density Time averaged melt flow distribution

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Kraftdichteverteilung Melt flow (Simulation)Melt flow (Measurement)
Distribution of the electromagnetic force density and melt
flow in the cross section of the channel

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3D-simulation of the melt flow velocity in the
induction channel furnace

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Channel of a double loop induction channel furnace for
melting of copper after solidification of the melt
32
„Solidified“
flow eddies structure
Complete channel
Part of the channel

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3D transient result of the temperature distribution
33
Original design with symmetrical
channel: power: 240 kW Overheating ΔT: 50 K

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melt with
meniscus
shape
crucible
segment
inductor
bottom
slit
skull
current
melt flow
EM-forces
heat conduction
radiation
(water cooled)
(water cooled)
(water cooled)
Features of the Induction Furnace
with Cold Crucible
 slitted crucible to realize
efficient electromagnetic
transparency
 free melt surface and
intensive melt stirring,
based on electromagnetic
forces
 water cooled bottom and
crucible segments leads to
solid layer (skull)
 heat losses by radiation
and conduction depending
on the meniscus shape

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Melting in the Induction Furnace with Cold Crucible (IFCC)
 High reactive and high
purity materials, e.g. TiAl
 Melting, alloying, over-
heating and casting
in one process
 Good homogenization
of the melt caused by
electromagnetic stirring
 Overheating temperature
is the key-parameter of the
process

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Principle of induction skull melting technology
Intensive cooling of inductor
and bottom
 formation of a skull
 skull protects against impurities
layer of
initial material
water-cooled inductor
skull
melt
water-cooled bottom
magnetic field
Advantages of induction
skull melting
High process temperatures
High power density
High purity of the melt
and the final product
High efficiency
Process can be realized
in different gas atmospheres
or in vacuum
Compact melting equipment

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Electromagnetic levitation melting (ELM)
Features:
• contactless melting process
• no reaction with crucible
• high purity of the molten material
• high temperature of the melt
• high efficiency
• mixing and homogenization of the melt
• defined fast solidification due to maximal
under cooling of the melt is possible
Applications:
• precision casting of special alloys
• medicine and dental applications, e.g. implantats
• aeronautics, e.g. precision casting components
• investigation of new material compositions
• determination of material properties
• investigations of under-cooled melts

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New setup for EM-levitation melting
Ief = 1.1 kA, f1 = 30 kHz, f2 = 40 kHz
Yoke: μr = 5000, Bef < 250 mT
Horizontal magnetic fields generated by
two pairs of induction coils with different
frequencies
Magnetic flux guiding system
500 g
38

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Time-averaged flow
v, m/s
Supporting Lorentz force forms a single
torroidal vortex with an upward directed
flow on the z-axis

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EM levitation melting of aluminum sample
m = 500 g
40

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Induction melting processes – Summary
 melt homogenisation due to melt stirring
 small material losses because no local overheating
 exact adjustment of the alloy composition
 exact temperature control
 easy automatic process control
 high quality of the melt
 high throughput due to fast heating up speed
 high furnaces efficiency
 relatively simple handling
 good energy control
 friendly working conditions
 environmental friendly (small emission of dust, no exhaustion gas)
 new future oriented technologies, like skull melting and levitation melting

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Thank you very much
for your attention!