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6.2.3: User-defined function in FLUENT:
After validation of the User Defined Function (UDF) in FLUENT developed for standard
geometry with argon gas, numerical simulation was done using different gases. For practical
applications of ICP torches like material processing, the role of different plasma gases has beenstudied in details. The minimum sustainable power required for operation of stable plasma in
gases like argon, nitrogen, oxygen and air has been estimated. In determining the useful range of
operating parameters of the ICP torch for material processing, the following requirements for the
ICP torch are taken in to account:
(a) Hot enough temperature in the coil region, so that electron density (more than 10 21 /m3)
should be high enough to satisfy LTE assumption
(b) Large volume of the hot plasma for industrial application like material processing, the
requisites are high temperature at the coil plasma region
(c) No recirculating flow inside the plasma especially near the inlet
(d) Sufficient cooling of the tube wall by the sheath gas, so that wall temperature should be
below the melting point of quartz
The objective of present study is to provide a comparative study of influential operating
parameters on the flow field, temperature contours, axial velocity, impedance and heat
distribution. The gases used for study are argon, nitrogen, oxygen and air working at
atmospheric pressure with oscillator frequency of 3 MHz. The investigated parameters can be
used to design a RF-ICP system depending on the industrial application.
1) Minimum sustaining Power :The minimum sustaining power for various gases, that is the power below which it is not
possible to sustain RF-ICP plasma using that particular gas. The total gas flow rate for this
simulation study is considered as 25 lpm, the central gas flow rate, plasma gas flow rate and
sheath gas flow rate is 1, 3 and 21 lpm respectively. Although, ionization intiates at temperatures
between 5300-5500 K for these gases, the maximum core temperature should be such that LTE
assumption hold valid. The criteria for the power to be termed, as minimum sustainable is that a
minimum RF-power is required for the convergence of numerical solution and the maximum
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core tem
minimu
1 kW.
standard
The mini
respectiv
oxygen.
nitrogen
perature sh
sustainabl
ccording t
geometry
mum sustai
ely and ma
Even thoug
is different.
Figure 6.3
Figur
uld be arou
power for
o our simul
ith argon g
nable powe
ximum cor
h air consti
Because th
9: Tempera
6.40: Flow
nd 6500 K
an ICP torc
ated data,
s is 0.9 kW
for nitroge
temperatu
tutes 80 %
specific he
ure contour
fields for di
in the coil r
h working
the minimu
that gives
n, oxygen a
re for nitro
of nitroge
at of nitroge
of different
fferent gase
egion. As r
t atmosphe
m sustaina
maximum
d air are 1
en is 9000
, the maxi
n is more a
gases at th
s at minimu
ported in li
ic pressure
le power
core tempe
.4 kW, 12.
K and 65
um tempe
compared t
ir minimum
m RF-powe
terature [38
is approxim
or ICP tor
ature of 96
kW and 13
0 K for ai
ature of ai
o air.
power
r
], the
ately
h of
0 K.
.7kW
and
and
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Figure 6.39 and 6.40 shows the temperature and flow field for argon, nitrogen, oxygen and air
obtained at minimum power condition. Argon gas has maximum core temperature at minimum
power as compared to other gases. Plasma volume of oxygen is least as compared to air and
nitrogen. Therefore, argon gas is the best for generating hot plasma at less power.
As seen in figure 6.40, there are less recirculation eddies in oxygen and air plasma. This is
because radial and axial electromagnetic forces (as seen in figure 6.41 and 6.42) are more
towards the wall. The position of electromagnetic field is offaxis for argon and nitrogen plasma,
which results in recirculation eddies.
Figure 6.41: Radial electromagnetic force contours at minimum power
Figure 6.42: Axial electromagnetic force contours at minimum power
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Due to large eddy, the magnitude of axial velocity of nitrogen is more as compared to other
plasma as seen in figure 6.43. The axial velocities of oxygen and air plasma are between 1.8 6
m/s. One can thus choose the suitable plasma medium in consideration of residence time for
material processing applications.
2) Effect of variation of RF-power on different plasma gas:In this case the inductively coupled torch was operated at different dissipated RF-power with
constant central, plasma and sheath gas flow as 1, 3, 21 lpm respectively. Figure 6.44 shows the
temperature contour for argon gas operating at RF-power of 8.8 kW and 15 kW. The maximum
temperature for 8.8 kW is 10200 K and for 15 kW is 10600 K. Figure 6.45, 6.46 and 6.47 shows
temperature contours of nitrogen, oxygen and air respectively at different RF-power. The
maximum core temperature for nitrogen at 15 kW and 25 kW are 9100 K and 9700 K
respectively.
Figure 6.43: Axial velocity for minimum power
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Figure 6.44: Temperature contour at different
power for argon plasmaFigure 6.45: Temperature contour at different
power for nitrogen plasma
Figure 6.46: Temperature contour at differentpower for oxygen plasma
Figure 6.47: Temperature contour at differentpower for air plasma
For oxygen plasma, the maximum temperature for 15 and 20 kW are 9000 K and 10000 K
respectively. Similarly, for air plasma maximum temperature is 6600 K for 15 kW and 7200 K
for 29.5 kW. These figures show that as the RF- power is increased, the plasma expands radially
and axially. This is due to increase in electrical conductivity of plasma, which is due to increase
in ionization. Therefore temperature of the plasma increases. Consequently, skin depth reduces
and plasma core volume expands.
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For 15 kW RF-power, argon plasma has more plasma volume and oxygen plasma has least
plasma volume in comparison with other gas plasma. If high temperature and maximum plasma
volume is desired then argon is best.However if less plasma volume with high temperature is
desired then oxygen plasma is the best.
The RF-power dissipated to an ICP discharge is dissipersed through radiation (Qrad), thermal
conduction through wall (Qwall) and convection in form of exhausted flame enthalpy (Qout). To
study, how the RF power (heat) is distributed in the ICP torch using different plasma gases, the
flow rates of central, plasma and sheath gas were kept at 1, 3 and 21 lpm respectively. As the
power increases, loss due to radiation (Qrad) increases for all plasma gas considered. For argon
plasma as RF-power increases, losses at the exit (Qout) rapidly decreases and convective losses
(Qwall) at the wall increases and then reaches a saturation value after 10 kW as shown in figure
6.48.
Figure 6.48: Heat distribution profile as a
function of RF-power for argon plasmaFigure 6.49: Heat distribution profile as a
function of RF-power for nitrogen plasma
For nitrogen plasma, convective heat transfer to the wall (Qwall) decreases slowly as seen from
figure 6.49, however Qout decreases very slowly as the power increases and after 15 kW it
almost remains constant. Similar heat distribution trend is seen for oxygen plasma as shown in
figure 6.50. In air plasma as shown in figure 6.51, the Qwall percentage reduces as power
increases and Qout decreases very slowly as the power increases and after 20kW it almost
remains constant.
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Figure 6.50: Heat distribution profile as afunction of RF-power for oxygen plasma
Figure 6.51: Heat distribution profile as afunction of RF-power for air plasma
The wall temperature profile with respect to wall of the quartz tube for various RF-powers for
argon plasma is shown in figure 6.52. As power increases the wall temperature also increases.
Therefore, for designing an ICP with RF-power beyond 15kW, the wall temperature should be
monitored (as melting point of quartz tube is 1683 K) or water can be used for cooling the wall.
For nitrogen plasma as observed earlier in figure 6.49, that Qwall loss is around 60-70%.
However, the wall temperature of quartz tube doesnot go beyond 900 K as RF-power is
increased from 10.4 kW to 25 kW as seen in figure 6.53. Same situation is for oxygen plasma,
even though the losses at the wall is 70-80%, the wall temperature are well below melting point
of quartz tube as seen in figure 6.54. Qwall losses are more for oxygen than nitrogen but still the
maximum wall temperature at 15 kW is less for oxygen plasma than nitrogen. This is because the
axial plasma core volume of nitrogen is more compared to oxygen plasma. The wall temperature
is around 700 K even at 30 kW for air plasma as shown in figure 6.55.
Observation of wall temperature profile for all the gases show three peaks in the profile. These
peaks are at the coil turn position of the ICP torch. This is due to the electric field intensity in
azimuthal direction.
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Figu
plas
Figu
plas
As the
plasma
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material
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e 6.52: Wa
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ets affected
to 15 kW
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is not desir
elocity is r
ll temperatu
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ssipated in
. More reci
as seen fr
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e for oxyge
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ma.For ma
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.40 and 6.
terial proce
ore suitabl
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at various
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the flow p
roduced as
6. But the
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city can be
temperatur
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temperatureus RF-pow
attern of ar
the power i
flow patter
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as high as 3
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on and nit
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n marginall
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henever, h
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y get
on of
igher
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l
Compari
velocity
3) Varia
For vari
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igure 6.56:
pm, Q2 = 3 l
ng figure 6.
also increas
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Flow fields
pm and Q3
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s. This is b
FigureRF-pow
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for differen
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e 6.39, sho
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6.57: Axialer
RF- Power
RF power,
spectively.
t gases at 1
s that as th
dy forces (
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ICP was si
Figure 6.58
kW RF-po
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r, Fz) as dis
file at 15 k
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6.59, 6.60
wer with Q1
ncreased th
cussed earli
flow rate
and 6.61 sh
= 1
maximum
er.
f central, pl
ws the vari
axial
asma
ation
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of Ltorch
air and n
resistanc
dissipate
Increase
tempera
volume
Figure 6
inductan
Howeve
from fig
As the p
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.58: Plasmae versus R
, in oxygen
re 6.46. So,
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ntly these r
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the plasma
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hence ther
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or oxygen,rch) increase
er for argo
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rs a) plasm
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esults in de
asma resist
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effect of te
resistance d
olume incre
is less flux
ases and th
the plasmas from 6.02
n, nitrogen,
reases the
volume a
e increases
in electric
crease in pl
nce increas
aFigure
inducta
plasma
mperature i
ecreases as
ases due to
leakage bet
erefore ind
volume sligmicro H to
oxygen and
plasma resi
d b) plasm
leading to i
al conducti
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as the pow
6.59: Plasnce versus
s more pron
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hich the cr
ween the pl
uctance of
htly decreas.04 micro
air gas res
tance incre
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ncrease in p
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nce. The co
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a resistanceF-power fo
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ases as sho
oss-section
asma and th
he torch f
es with po.
ectively. A
ses. The pl
re. As the p
lasma resist
turn incr
mbined eff
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and torchr nitrogen
volume as
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e coil. Ther
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rgon,
asma
ower
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eases
ct of
seen
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uced
fore,
and
e the
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Figure 6
inductan
plasma
4) Var
In this c
flow of
plasma g
Comput
temperat
indicates
high flo
marginal
rate, the
6.65.
.60: Plasma
e versus R
ation in flo
se, for co
entral gas
as flow rate
tional simu
ure contour
the plasma
rate of t
ly increase
radial plas
resistance a
-power for
rate:
paritive stu
is varied fr
constant at
ation show
and the flo
volume shri
e gas. Dec
f radial pla
a volume d
nd torch
oxygen
dy, the diss
m 1 to 5 l
3 lpm. Tota
that the var
w patterns.
nks radially
rease in sh
ma volume
creases for
Figure 6.
inductanc
ipated RF-p
pm and she
l gas flow r
iation of sh
Increase in
at the axis.
eath gas fl
towards the
all the gase
61: Plasma
e versus RF
ower is kep
ath gas fro
ate for all t
eath and th
central inj
This is due
ow rate fro
wall. Ther
s as shown i
resistance a
-power for
t at 15 kW
21 to 17
e gases is c
central ga
ction flow
to the conv
m 21 to 1
fore, due to
n figure 6.6
d torch
ir plasma
for all gase
lpm keepin
onstant (25
flow affec
from 1 to 5
ctive cooli
lpm, resul
variation in
2, 6.63, 6.6
s and
g the
lpm).
s the
lpm
g by
ts in
flow
and
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Figure 6.64: Temperature contour at
different flow rates for oxygen plasmaFigure 6.65: Temperature contour at
different flow rates for air plasma
Increase in central gas flow from 1-5 lpm and decrease in the sheath gas flow from 21-17 lpm,
the axial temperature profile indicates that the plasma core region shifts to the wall as mentioned
earlier. For argon gas and nitrogen plasma, the maximum axial temperature does not change
much with variation in sheath or central gas flow rate as shown in figure 6.66 and 6.67.
Figure 6.62: Temperature contour atdifferent flow rates for argon plasma
Figure 6.63: Temperature contour atdifferent flow rates for nitrogen plasma
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Howeve
in figure
Figur
plasm
Figu
plas
For argo
temperat
there is
oxygen
respectiv
, for air and
6.68 and 6.
6.66: Axia
at differen
e 6.68: Axi
a at differe
n and nitrog
ure profile t
onsiderabl
and air pl
ely. Profile
oxygen pla
9.
l temperatur
flow rates
al temperatu
t flow rates
en plasma it
aken at Z =
fall in radi
sma, the r
indicate th
sma the flo
e for argon
re for oxyg
is seen that
0.042 m is n
al temperat
dial tempe
t if central
rate affect
Figure
plasma
n Figureplasma
at 1 and 3
ot affected.
re at the i
rature prof
gas injectio
the maxim
6.67: Axial
at different
6.69: Axial
at different
pm central
But for 5 lp
let as seen
ile is show
flow is inc
um axial te
temperature
low rates
temperatur
flow rates
injection flo
m central in
in figure 6.
n in figur
reased from
perature as
for nitroge
e for oxyge
w rate, the
ection flow
70 and 6.71
6.72 and
1 to 3 lpm
seen
adial
rate,
. For
6.73
there
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is major
similar t
region.
Fig
arg
Fi
ox
drop in tem
that of 3 l
ure 6.70: R
on plasma a
ure 6.72:
ygen plasm
perature at
pm sheath
adial tempe
t different fl
adial tempe
at different
he injectio
as flow rat
ature for
ow rates
rature for
flow rates
region. At
is no maj
Figure
nitroge
Figure
plasma
5 lpm cent
r change in
6.71: Radi
n plasma at
6.73: Radial
at different
al gas flow
temperatur
l temperatu
different fl
temperatur
low rates
rate, the tre
e at the inje
re for
w rates
e for air
nd is
ction