Chapter 6 Part2 F

8/3/2019 Chapter 6 Part2 F

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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


2/14

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


3/14

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


4/14

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.


6/14

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.


7/14

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.


8/14

Figu

plas

Figu

plas

As the

plasma

minimu

affected

material

particle

e 6.52: Wa

a at variou

e 6.54: Wala at various

F-power di

ets affected

to 15 kW

for air and

is not desir

elocity is r

ll temperatu

RF-power.

l temperatuRF-power.

ssipated in

. More reci

as seen fr

oxygen plas

ed, air and

quired, nitr

re for argon

e for oxyge

ICP torch i

rculation ed

m figure 6

ma.For ma

oxygen is

gen is bette

Figur

plasm

n Figureplasma

s increased,

dies are int

.40 and 6.

terial proce

ore suitabl

r as the vel

6.53: Wall

at various

6.55: Wallgas at vari

the flow p

roduced as

6. But the

sing of po

e as plasma

city can be

temperatur

F-power.

temperatureus RF-pow

attern of ar

the power i

flow patter

ders, wher

medium.

as high as 3

for nitroge

for airr

on and nit

s increased

n marginall

recirculati

henever, h

-35m/sec.

n

ogen

from

y get

on of

igher


9/14

l

Compari

velocity

3) Varia

For vari

and shea

igure 6.56:

pm, Q2 = 3 l

ng figure 6.

also increas

ion of Imp

tion of imp

th gas as 1,

Flow fields

pm and Q3

57 and figur

s. This is b

FigureRF-pow

dance with

dance with

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for differen

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e 6.39, sho

cause of bo

6.57: Axialer

RF- Power

RF power,

spectively.

t gases at 1

s that as th

dy forces (

velocity pro

ICP was si

Figure 6.58

kW RF-po

e power is i

r, Fz) as dis

file at 15 k

ulated wit

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


10/14

of Ltorch

air and n

resistanc

dissipate

Increase

tempera

volume

Figure 6

inductan

Howeve

from fig

As the p

current i

inductan

nitrogentorch ind

and Rplasma

itrogen sho

e depends

d in plasma

in RF-po

re consequ

nd tempera

.58: Plasmae versus R

, in oxygen

re 6.46. So,

wer increa

creases an

e of the p

decreases.uctance (Lto

ith RF-po

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n two facto

increases, p

er results

ntly these r

ure helps pl

resistance a-power for

plasma the

the plasma

es plasma v

hence ther

asma decre

or oxygen,rch) increase

er for argo

F-power in

rs a) plasm

lasma volu

in increase

esults in de

asma resist

nd torchargon plas

effect of te

resistance d

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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

asma resista

as the pow

6.59: Plasnce versus

s more pron

power incre

hich the cr

ween the pl

uctance of

htly decreas.04 micro

air gas res

tance incre

a temperatu

ncrease in p

vity that i

nce. The co

er increases

a resistanceF-power fo

ounced tha

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

.

and torchr nitrogen

volume as

n in figure

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er and henc

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seen

6.58.

uced

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e the


11/14

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


12/14

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


13/14

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


14/14

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

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Chapter 6 Part2 F