Transient Numerical Analysis of Induction Heating of Graphite Cruciable at Different Frequency

International Journal of Electromagnetics ( IJEL ), Vol 1, No 1, August 2016
35
TRANSIENT NUMERICAL ANALYSIS OF INDUCTION
HEATING OF GRAPHITE CRUCIBLE AT DIFFERENT
FREQUENCY
B. Patidar, M.M.Hussain, A. Sharma, A.P. Tiwari
Bhabha Atomic Research Centre, Mumbai
Abstract
Mathematical modeling of Induction heating process is done by using 2D axisymmetric geometry.
Induction heating is coupled field problem that includes electromagnetism and heat transfer. Mathematical
modeling of electromagnetism and heat transfer is done by using maxwell equations and classical heat
transfer equation respectively. Temperature dependent material properties are used for this analysis. This
analysis includes coil voltage distribution, crucible electromagnetic power, and coil equivalent impedance
at different frequency. Induction coil geometry effect on supply voltage is also analyzed. This analysis is
useful for designing of induction coil for melting of nonferrous metal such as gold, silver, uranium etc.
Keywords:
Induction heating, FEM, Coil design, Graphite
1.INTRODUCTION
Graphite has been widely utilized in different industries applications, because of it physical
properties like good thermal stability, corrosion resistance, high electrical conductivity, thermal
shock resistance, high melting temperature, high purity, refractoriness, machinability etc [1].
Graphite electrical, mechanical and thermal properties makes, it suitable for induction melting of
non ferrous materials such as gold, silver, uranium etc.
In induction heating, graphite crucible is coupled with pulsating magnetic field produced by
induction coil, which generates the electro motive force and eddy current in graphite crucible and
that will heat it by joules effect. This heat is transferred to the charge (Material that is supposed to
melt) through conduction, convection and radiation [2] [3].
Induction heating is multiphysics phenomena i.e combination of electromagnetism and heat
transfer [4]. These physics are nonlinearly coupled with each other due to temperature dependent
material properties. Mathematical modeling of electromagnetism and heat transfer is done by well
known maxwell equations and classical heat transfer equation respectively [2][4] [4].Field
equations are solved by using finite element method.
This paper presents mathematical modeling of induction heating of graphite crucible.
Electromagnetic power induced in different wall thickness of graphite crucibles, and voltage
distributions in induction coil are analyzed at different frequency. This model helps to design and
optimized the induction coil and graphite crucible for heating application.

36
This paper is organized as follows, section II gives brief description of induction heating system.
Mathematical modeling and numerical solution procedure are explained in section III. In section
IV, numerical results and analysis are present. Finally conclusion is given in section V.
2.SYSTEM DESCRIPTION
Induction melting set up comprises of power source, water cooled copper induction coil, graphite
crucible and charge. Power source supplies high frequency current to the induction coil to
generate varying magnetic field and heat the crucible.
C o p p e r
I n d u c t i o n
c o i l
C h a r g e
G r a p h i t e
c r u c i b l e
I n d u c t i o n
m e l t i n g
p o w e r
s o u r c e
3 - P h a s e
m a i n s
p o w e r
s u p p l y
Figure 1:- Schematic of Induction heating system
3.MATHEMATICAL MODEL
Mathematical modeling of electromagnetism and heat transfer is done separately.
Electromagnetic model is governed by maxwell equations as shown below [5] [6] [7] [8] [9],
∇. = 0 (1)
∇. = (2)
∇ × = − (3)
∇ × = + (4)
Here, H: - Magnetic field strength (A/m)
E: - Electric field strength (V/m)
σ: - Electrical conductivity (S/m)
J= Current density (A/m2
)

37
D= Electric flux density(C/m2
)
ρc=Electric charge density(C/m3
)
B= Magnetic flux density (Wb/m2
)
Constitutional equations for linear isotropic medium,
= ( ) (5)
= (6)
= (7)
Here,
µ0= Free space magnetic permeability (H/m)
µr= Relative magnetic permeability
ε0= Free space electric permittivity (F/m)
εr= Relative electric permittivity
Magnetic field problems are generally solved by using magnetic vector potential formulation and
which is derived by using maxwell equations. Magnetic vector potential (A) is defined as,
= ∇ × (8)
From eq (3), (4) and (8), Magnetic vector potential equation in frequency domain can be written
as,
( )
∇!
+ " − #$ ( ) = 0 (9)
Here,
Js= source current density (A/m2)
ω=Angular frequency (rad/sec)
For solving eq (9) in axisymmetric geometry shown in figure 2, following assumption are
considered,
1. The system is rotationally symmetric about Z-Axis.

38
2. All the materials are isotropic.
3. Displacement current is neglected.
4. Electromagnetic field quantities contents only single frequency component.
For different domain of figure (2), eq (9) can be written as,
(%)
∇!
= 0 in Ω1 (10.1)
(%)
∇!
+ " − #$ ( ) = 0 in Ω2 (10.2)
(%)
∇!
− #$ ( ) = 0 in Ω3 (10.3)
Figure 2:- 2-D Axisymmetric geometry of induction heating system
Eddy current and induced electromagnetics power in graphite crucible are calculated by using
magnetic vector potential as shown below,
'( = ( )(#$ ) (11.1)
) =
*+
,
-( )
= ( )(#$ )!
(11.2)
Here,
Je= induce eddy current density (A/m2
)

39
Electromagnetism is coupled to heat transfer by power induced (Q) in graphite crucible and that
is used as forcing function in heat transfer equation. Temperature in the graphite crucible is
governed by the classical heat transfer equation [2] [5] [9] [10],
.( ). (∇! ) + ) = /0( ) (12)
Here,
T= Temperature (DegK)
ρ= Density (Kg/m3)
cp= Specific heat ((J/(Kg.K))
K=Thermal conductivity (W/(m.K))
Qconv= convection heat loss (W/m2
)
Qrad= radiation heat loss (W/m2
)
t =Time (Sec)
For electromagnetism, Dirichlet (A=0) and Neumann boundary conditions and for heat transfer,
convection and radiation boundary conditions are used.
Convection heat loss can be represent as,
) 123 = ℎ. ( − 567) W/m2
(13)
Radiation heat loss can be represent as,
) 58 = 9 7. : ;
− 567
;
< W/m2
(14)
Here,
h=convection coefficient (W/m2
K)
ϵ=Emissivity
σb= Boltzmann constant(5.67X10-8 W/m2
K4
)
Tamp= Ambient temperature (DegK)
Induction heating is non linear coupled field problem that makes difficult to solve by analytical
method. Hence, Numerical method is used to solve the field equations. Finite element method
(FEM) is simple and most preferred numerical method for solving field equation, therefore, FEM
is chosen for this analysis. FEM converts continuous equations i.e eq (10) into discrete equation
by discretizing geometry of solution domain [2].Discretization can be done by using trigular,
rectangular or hexagonal elements. Discretized equations are solved by using segregated or
coupled field solver.

40
4.SIMULATION
Simulation is done in three steps i.e. preprocessing, processing and post processing using FEM
based multiphysics software [7].
Preprocessing
Processing
No
Yes
Post processing
Figure 3. Computation steps
Preprocessing steps includes creating geometry, assigning materials and their properties to
different sub domains, define boundary conditions, initial conditions, forcing function and
domain discretization. Processing steps includes the computation of parameters as shown in
figure 3. Post processing step includes computation of parameters such as coil impedance,
convection and radiation losses etc for further analysis.
Magnetic vector
potential (A)
calculation
Electromagnetic
power (Q)
calculation
Temperature field
(T) calculation
t > t set
Output
Update
Material
Properties
σ (T), K (T),
cp (T)
Geometry, material
and their properties,
initial & boundary
conditions, forcing
function, domain
discretization
Calculate parameters such as
electromagnetic power, coil
impedance, temperature
gradient etc

41
5.NUMERICAL RESULTS
Geometry shown in figure 4 (a) is used for simulation. Solution domain is discretized using
trigular elements. Meshing density is more on Surface of graphite crucible and induction coil due
to skin effect as shown in the figure 4(b). Graphite temperature dependent material properties are
given in figure 4.
4(a) 4(b)
Figure 4(a). 2-D axisymmetric geometry, 4(b) Domain discretization (Meshing)
Dimension details of induction coil is given in table-I. Graphite temperature dependent material
properties such as electrical conductivity, thermal conductivity, specific heat, are shown in figure
5.
0 400 800 1200 1600 2000
0
140
150
160
170
ElectricalconductivityinkS/m
Temperature in DegK
Graphite Electrical Conductivity
0 500 1000 1500 2000
0
40
60
80
100
120
140
160
180
200
220
ThermalcoductivityinW/mK
Temperature in DegK
Graphite Thermal Conductivity
5(a) 5 (b)

42
0 400 800 1200 1600 2000
0
800
1200
1600
2000
Graphite Specific Heat
SpecificheatinJ/Kg.K
Temperature in DegK
5 (c)
Figure 5:- Temperature dependent material properties of Graphite [2]
Table-I Induction coil dimensions and properties
Induction coil Description
Material Copper
Inside diameter 215
Outside diameter 247
Height 200
Coil tube diameter 16
Coil tube thickness 5
No. of turn 9
Initial conditions, boundary conditions and forcing function for electromagnetism and heat
transfer are given in table-II and table-III respectively. Electromagnetism equations are solved in
complete solution domain (induction coil, graphite crucible, air). Heat transfer analysis is done
only in graphite crucible, because induction coil is water cooled, and always at room temperature.
Table-II Boundary condition and forcing function for electromagnetism
Boundary
condition
Description
Outer boundary A=0
Asymmetry axis ∂A
∂n
= 0
Induction coil
current
707.21A

43
Table-III Initial and boundary condition for Heat transfer
Boundary condition Description
Initial temperature 303 DegK
Convection coefficient(h) 10 (W/m2
K)
Emissivity( Graphite
surface)
0.7
Emissivity(Refractory
surface)
0.01
As induction coil carries single frequency current (because of series resonance configuration),
Hence, electromagnetic analysis is done in frequency domain. Heat transfer equation is solved in
transient domain. Magnetic field produced by induction coil at 1 kHz, 5 kHz and 9 kHz is shown
in the figure 7.
Figure 7. Magnetic field at 1 kHz, 5 kHz, and 9 kHz
6.RESULT AND ANALYSIS
From figure 7, it is observed that, magnetic field is high on graphite crucible surface and it
reduces toward centre of the crucible. At 1 kHz, magnetic field penetrates more compared to 5
kHz and 9 kHz. Figure 8 shows the crucible temperature at different frequency, from figure, it is
observe that as frequency increases crucible heat up at faster rate and that is because of increase
in coupling between crucible and induction coil.

44
0 200 400 600 800 1000
0
200
400
600
800
1000
1200
1 kHz
5 kHz
9 kHz
TemperatureinDegC
Time in Sec
Figure 8. Graphite crucible temperature
Figure 9 shows, voltage distribution in different turn of the induction coil. From figure it is
observed that from centre turn to outer turn of induction coil, impedance of coil turns are reduced
and that is due to maximum proximity effect experience by centre turn compared to other turns.
That makes higher voltage drop and power loss in centre turn of the induction coil. This analysis
helps the designer for designing of efficient cooling system for induction coil.
1 2 3 4 5 6 7 8 9
5
10
15
20
25
30
35
40
45
50
55
1 kHz
5 kHz
9 kHz
VoltageinV
Turn
1 2 3 4 5 6 7 8 9
0.00
0.01
0.02
0.03
0.04
0.05
1 kHz
5 kHz
9 kHz
Impedenceinohm
Turn
9(a) 9(b)
Figure 9. (a) Turn voltage, 9(b) Turn impedance
Figure 10 shows that electromagnetic power induced in graphite crucible at different frequencies.
From figure, it is observed that during initial period, power induced in graphite crucible is first
reduced and then increase and the reason for this is uniqueness of graphite electrical conductivity,
which has both positive temperature coefficient and negative temperature coefficient as shown in
figure 5(a).

45
0 200 400 600 800 1000
0
10000
20000
30000
40000
50000
60000
1 kHz
9 kHz9 kHz
5 kHz
ElectromagneticpowerinW/m
Time in Sec
Figure 10. Electromagnetic power in crucible at different frequency
Figure 11 shows the variation of electromagnetic power in crucible at different thickness. From
figure it is observed that as crucible thickness increase induce power is increases upto a level and
after that it reduces. This is due to the skin effect, means 84%~ power is induced in one skin
depth and 98 % ~ in second skin depth.
0 20 40 60 80
30000
40000
50000
60000
70000
EMpowerincrucibleinW/m
Crucible thickness in mm
Figure 11. Electromagnetic power in crucible at different thickness
7. CONCLUSION
Transient numerical analysis of Induction Heating of Graphite Crucible was carried out
successfully at different frequency. Electromagnetic power induced in graphite crucible is
analyzed at different temperature that gives better understanding of heat transfer in graphite
crucible. Voltage at each coil turns are analyzed, that helps in designing of cooling system for
induction coil. Effect of crucible wall thickness on induced power is analyzed, that helps for
selection of optimum crucible thickness.

46
REFERENCES
[1] A.W.Moore, “The induction heating of pyrolytic graphite”, Pergamon press ltd. Printed in Great
Britain, carbon 1967, Vol.5, pp 159-165.
[2] Valery Rudnev, Don loveless, Raymond Cook, Micah Black, “Handbook of Induction heating”,
INDUCTOHEAT,Inc., Madison Heights,Michigan,U.S.A.
[3] E.J Davies and P.G. Simpson, Induction Heating Handbook. McGraw Hill, 1979.
[4] C Chabodez, S Clain, R.Glardon,D, D Mari, J.Rappaz, M. Swierkosz, “Numerical modeling in
induction heating for axisymmetric geometries”, IEEE transactions on Magnetics.Vol33, No.1
January 1997, P 739-745.
[5] Jiin-Yuh Jang, Yu-Wei Chiu, Numerical and experimental thermal analysis for a metallic hollow
cylinder subjected to step-wise electro-magnetic induction heating, Applied thermal engineering
2007, 1883-1894.
[6] Andrzej Krawczyk, John A. Tegopoulos, “Numerical modeling of eddy current,” Oxford science
publications, P-17.
[7] Ion Carstea, Daniela Carstea, Alexandru Adrian Carstea, “A domain decomposition approach for
coupled field in induction heating device,” 6th WEEAS international conference on system science
and simulation in engineering, Venice, Italy, November 21-23, 2007, P63-70
[8] B. patidar, M. T. Saify, M. M. Hussain, S. K. Jha and A. P. Tiwari, “Analytical, Numerical and
Experimental Validation of Coil Voltage in Induction Melting Process", International Journal of
Electromagnetics(IJEL), Vol 1, No. 1, 2015, pp19-31 , [2015]
[9]Bo Yun Jang, Joon Soo Kim, Young Soo Ahn, “ Induction heating process using segmented
graphite crucible for silicon heating”, Solar Energy Materials & Solar Cell 95 (2011),pp 101-106.
[9] Y.Favennec, V.Labbe, F.Bay, “Induction heating processes optimization a general optimal control
approach”, Journal of Computational Physics 183 (2003), pp68-94.

Transient Numerical Analysis of Induction Heating of Graphite Cruciable at Different Frequency

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