Thermal plasma jets generated in dc arc

plasma torches - generation, properties, diagnostics

Milan HrabovskyInstitute of Plasma Physics AS CR

Praha, Czech Republic

OUTLINE

Generation of thermal plasma jetsBasic principles of arc plasma torchesFactors influencing properties of plasma jet in arc plasma torches ( design of the torch, properties of plasma gas)Water and hybrid gas/liquid plasma torchesSpecial diagnostic tools for thermal plasma jets

Thermal plasmas

Basic plasma propertiesLocal thermodynamic equilibriumTemperatures Te = Th ~ 8 – 50.103 KPressures 10 kPa - 1 MpaE/p ~ 1 – 100 V/m.kPa

Common sources of thermal plasmasInductively coupled discharges in gasesElectric arcs – stabilized by gas flow

– stabilized by vortex of liquid (Gerdien arcs)

1.0E-004 1.0E-001 1.0E+002pressure [kPa

100

1000

10000

100000

tem

pera

ture

[K] Te

Th

Thermal Plasma Jets

0 10 20 30 40 50z [mm]-3

-2-10123

r [m

m]

T = 13 kK

161820

14In most plasma generators gas is

supplied into the discharge chamberand plasma jet is produced at the exit

nozzle

Inductively coupled plasma torchesInductively coupled discharge is maintained in an open tube in the presence of streaming gas. Low velocity plasma jet is formed at the exit.

Frequencies: 100 kHz – 100 MHzPower: 1 kW – 1 MWPressure: 104 – 106 PaPlasma temperature: 6 000 – 10 000 KPlasma velocity: 10 – 102 m/s

gas flow

plasma

RF

Arc plasma torchesNon transferred arcs Transferred arcs

Hot cathode

Cold cathode

+-

gas plasma

liquidvapor

cathode

anode

gas

cathode

anode

Principles of arc stabilizationGas-stabilized arc Liquid-stabilized (Gerdien) arc

• Gas flows along the arc in the nozzle• Usually gas flow has vortex component

for better stabilization and for anodespot movement

• Anode created by exit nozzle or transferred arcs

• Power level: 1 kW – 10 MW• Plasma temperatures: 6 000 – 20 000 K

• Liquid vortex is created in cylindrical chamber with tangential injection• Arc is stabilized by its interaction with the vortex• Anode is outside of arc chamber • Power level: 10 – 200 kW• Plasma temperatures: 8 000 – 50 000 K

Basic factors determining properties of plasma jetin arc torches

Principal design factors: arc chamber geometry, arc currentplasma gas, gas flow rate

Dominant mechanisms of energy balance of unit length of arc columnin axial flow:Energy dissipationby Joule heating

Increase of axialenthalpy flux

Power loss byradial conduction

Power lossby radiation

Following simple relations can be derived from energy balance equation for basic torch parameters:

Torch Power:

Torch Efficiency:

= + +

21

2G

F hRG.L.IU.IP

σπηθ

θθ== σ

1

F

Gn2

F

G2

hS

GL2

hGLR41

−

++=

θπθε

θθπη

design factors plasma gas properties

Properties of plasma gases

8000 12000 16000 20000temperature [K]

0

10000

20000

30000

40000

50000

h/σ

[J.m

.V/k

g.A

10000 20000 30000temperature [K]

0.001

0.01

0.1

1

h/ε n

10000 20000 30000temperature [K]

0

2000

4000

6000

8000h/

S [m

.s/k

g]

steam argon/hydrogen 3:1 argon nitrogen/hydrogen 2:1

Torch operation parameters are determined by physical properties of plasma gas.Decisive gas properties:Power:ratio between enthalpy and electrical conductivity

Efficiency:Ratio between enthalpy and heat conductivityRatio between enthalpy and radiation emissivity

0 200 400 600 800 1000arc current [A]

0E+000

1E+008

2E+008

3E+008

plas

ma

enth

alpy

[MJ/

kg]

G/L=0.01

G/L=0.001

Plasma enthalpy in dependence on arc current

steamN2

Ar/H2 (33/10)

G/L=0.1

6 200240.105.0200500N2/H2 (235/94)

10 80015.30.151.9344500Ar/H2 (65/3)

17 5003200.0060.33176600water

15 8002520.0040.284300water

12 10013.50.080.9825750Ar/H2(33/10)

2 5002.96.432115300N2

3 0003.68.040180700N2

Tbulk

[K]Hbulk

[MJ/kg]

G/L[kg/s.m

]

G[g/s]

Power[kW]

Current[A]

Plasma gas

Typical parameters of dc arcplasma spraying torches

Efficiency of utilizing plasma enthalpy for processing

Plasma inPlasma and reaction products out

Reaction space

Temperature T0Enthalpy h0

Temperature T TREnthalpy h hR

Reaction TemperatureTR

≥≥

0

R

0

R0R h

h1h.G

)hh.(G−=

−=η

Only enthalpy ∆H = G.h0 – G.hR can be used for the treatment of material

Treated material

Process efficiency:

Plasma enthalpy flux G.h0 Plasma enthalpy flux G.hR

0 10 20 30heat flux potential S [kW/m]

0

20

40

60

80

100

plas

ma

enth

alpy

h [M

J/kg

]

nitrogen/hydrogen (2:1)argon/hydrogen (3/1)water

Relation between enthalpy and heat flux potential for common plasma gases

K2000TdT)T(kS 0

T

T0

== ∫

Thermal efficiency of water-stabilizedand gas-stabilized torches

η = (Pjet – G.hT )/ Pjet

= (1 – hT . G/Pjet )

hT=h(T) … T …S(T)

0.001 0.01 0.1 1 10 100heat flux potential S [kW/m]

0

0.2

0.4

0.6

0.8

1

effi

cien

cy η

waterargon/hydrogen (3/1)nitrogen/hydrogen (2:1)

Typical spray ratesfor dc arc plasma torches

plasma medium arc power[kW]

spray rate [kg/h] ceramics metals

water 160 25-45 80-100

gas - N2/H2 200 5-8 25-45

gas-Ar/H2/He 50 2-4 6-8

Spray rates in kg of powder per hour for water torch WSP-500and for gas spraying torches Plasmatechnik F4 and PlasJet.

Water Stabilized Plasma Torch

arc current: 300 - 600 Aarc power: 80 - 176 kWarc voltage: 267 - 293 V

exit centerline temperature: 19 000 - 28 000 K

exit centerline velocity: 2500 - 7000 m/s

centerline plasma density:0.9 - 2.0 g/m-3

Operation regimes of gas-stabilizedand water-stabilized torches

Mean plasma enthalpy:

Gas torches:10 – 40 MJ/kg

Water torches:100 – 300 MJ/kg

0 2 4 6mass flow rate [g/s]

0

50

100

150

200

pow

er [k

W]

Gas torches

Water torches

Operation regimes are expressed by the relation betweenarc power and mass flow rate

Joule heating of arc plasmaconvection convection

heating of liquid – power loss

absorption in vapor

cond

uctio

n

radi

atio

n

evaporation

heating of vapor

dissociationionization

increase of pressure

accelerationof plasma

Basic Processes in Liquid-Stabilized Arc

absorption in boiling layer

arc core

vapor

water

Components of radial heat transfer

Qexp., Qmodel.-total radial heat fluxQcond.- heat transferred by conductionQrad.- heat transferred by radiation

ε

0

0.2

0.4

0.6

0.8

1

α, β

, γ, ε

300 400 500 600arc current [A]

γ

αexp.

β

αmodel

.elmodn22

elmod E.I/)S2R4( π+επ=αm.km=ε

)E.I./(Q.m .ev α=β

E.I.Pw

α=γ

300 400 500 600arc current [A]

0

0.2

0.4

0.6

0.8

1

Q/I

E

Qcond.= 2πS/(I.E)Qrad.= (4π2 R2 εn)/(I.E)Qmodel./(I.E)Qexp./(I.E)

- low part of radially transferred energy is spent for water evaporation- high part of radially transferred energy is spent for vapor heating

plasma with high enthalpy and temperature and low density is generated

The balance of energy transfer and thus arc characteristics and plasma propertiescan be changed by changing boiling temperature

or latent heat of evaporation of liquid.

Effect of balance of radial transfer of energyon arc plasma properties.

For high arc plasma temperatures radiation energy transfer is dominantMost of radiation in UV waveband is absorbed in vapor sheathdue to photodissociation and photoionisation.Low fraction of radiation is absorbed in boiling water layer.

0 2 4 6mass flow rate [g/s]

0

50

100

150

200

pow

er [k

W]

Gas torches

Water torches

Principle of hybrid gas/water plasma torch

Operation regimes of dc arc plasma torches

Hybridtorches

Hybrid gas/water dc arc plasma torch

Ar

Ar

Water

Water

Plasma Jet

RotatingAnode

Arc

Ar

Ar

Water

Water

Plasma Jet

RotatingAnode

Arc

Profiles of plasma temperature and velocity at the exit nozzle

Profiles of plasma temperaturefor arc current 150 A and

three argon flow rates, z = 2 mm

Profiles of plasma velocityfor arc current 150 A and

three argon flow rates, z = 2 mm

Effect of argon flow rate and arc current onplasma velocity and shape of the jet

Centerline plasma velocity in the anoderegion as a function of arc current for

various argon flow rates.

12.5 slm

22.5 slm

Arc current – 200 A

12.5 slm

22.5 slm

Arc current – 200 A

Argon flow rate Spread angle 12.5 6.45° 17.5 8.35° 22.5 9.46°

Images of luminous jet core recorded by short shutter camera and spread angles of the jet for various flow rates of argon

150 200 250 300 350 400arc current [A]

1000

2000

3000

4000

5000

6000

7000

plas

ma

velo

city

[m/s

]

Argon flow rate [slm]32 26 22 15

Diagnostics of thermal plasma jets

Fluctuations:Spatial distributionFrequency spectraPhase velocity of oscillations

Shape of the jetJet structure

Plasma jet core:High gradients of temperatureand velocityLaminar flow

Turbulent jet:Smooth T and v profilesHeterogeneous mixtureof plasma and cold gas

Time averaged characteristics:Temperature profilersVelocity profilesPlasma compositon

Evaluation of mass flux at the exit nozzle

Temperature profilefrom optical spectroscopy

Profiles of plasma enthalpy h, density ρand sound velocity c

Equilibrium calculations of transportand thermodynamic coefficients of plasma

Velocity profile

v (r) = M . c(r)

Mach number at the exit nozzle

∫ ∫−+−+

−=

E ER

0

R

00BwAr drcr2)]TT(C[)f1(drchr2

PWMρπλρπ

Power balance of arc insidethe arc chamber

Mass flow rate at the exit nozzle

drcr2MGER

0N ρπ∫=

Basic plasma characteristics evaluatedfrom measurements:

Electron number densityTemperatureMolar concentrations of components

Basic methods:Absolute intensities of spectral linesLine intensity ratiosStark Broadening

725 730 735 740 745 750 755 760 765 7700

1

2

3

4

5

6

7

8x 105

ArII ArII

ArI

OI

ArI ArI

ArI ArII OII

TS ArII/ArI: 420/745, 745/745 nm

470 480 490 5000

2

4

6

8

10

x 106

experimentcalculation

Problems:Non LTE conditionsHigh gradients of temperatureelectron diffusionde- mixing of plasma gases

Non symmetrical cross section of the jet

Emission spectroscopy

1. Tare – no plasma flowQ – Heat flow rate P – Pressure

2. Sampling – plasma flows into the probeQ – Heat flow rateG – plasma flow rate

3. Mass spectrum

Measurements:Isokinetic coefficientppiso vR/GK ρπ 2=

Enthalpy probe

Plasma composition

Evaluation of plasma jet characteristics

Local plasma velocity

Plasma enthalpy:

Plasma temperatureThermodynamic and transport coefficients

Measurement of flow velocity

Enthalpy probe-Pitot tubeTime of flight of fluctuations - emitted light

- electric probe currentPropagation of waves in plasma

amplifier30 Hz – 50 MHz

measurement oftorch parameters

synchronizationunit

short shuttercamera

plasma torch

array of photodiodes

objectiveDiagnostics of jet fluctuationsand instabilities

I = 400 A, argon flow rate FAr = 8.5 slm

exposure time t = 700 ns.

Main sources of plasma flow instabilities

anode

arc

jet heating

production of jetinstability

anode jet

plasma jet

top view

side view

exit nozzle

anode anode jet

laminar jet turbulent jet

anode

Arc phenomena

Arc anode attachment:-movenet in the restrikeš mode-anode jet

exposure time 3µs

0 10000 20000 30000t [20 ns]

-100

0

100

200

300

Ua[ V

]

-3

-2

-1

0

1

2

3

Ud[ V

]

Ua

Ud3

Ud2

Ud1

Arc voltage and light fluctuations at various axial positions.

Distances from the nozzle:zd1 = 2.4 mmzd2 = 4.3 mmzd3 = 6.2 mm

d1 d2 d3

anode

arc

anode jet

nozzle

Main sources of plasma flow instabilitiesGas-dynamic instability

0 50 100 150 200 250t [20 ns]

-1.5

-1

-0.5

0

0.5

1

Ud

[V]

240

260

280

300

Ua

[V]

Ud1Ud2

Ud3

Ud4Ud5Ua

Ua - Arc voltage

Signals of photodiodes:d1 = 1 mm (z/D = 0.167)d2 = 4.3 mm (z/D = 0.717) d3 = 7.7 mm (z/D = 1.28)d4 = 11 mm (z/D = 1.83)d5 = 14.3 mm (z/D = 2.38)

Determination of flow velocity from propagation of disturbance caused by an anode restrike

Anode restrikein the position d3

300 350 400 450 500arc current [A]

02468

1012141618202224

velo

city

[km

/s] streamwise

counter-streamwise

Streamwise and counter-streamwise velocities of perturbationscaused by a formation of new anode spot

and flow velocity evaluated as their difference

280 320 360 400 440 480 520I [A]

0

2000

4000

6000

8000

10000

12000

v [m

/s]

present measurementsmeasurements [4]

Determination of flow velocity

0 100 200 300 400 500t [0.1µs]

-1.5

-1

-0.5

0

0.5

1

1.5

phot

odio

des v

olta

ge [V

]

d6 d5

d4d3

d2

d1

Determination of flow velocity

Fluctuations of emitted lightat various distances from the torch exit

D - Distance between points∆Φ - Phase shift

Velocity of movementof oscillations

∆Φπ fDv 2=

0.5 1 1.5 2 2.5 3

x 105

0

5

10

15

D1

0.5 1 1.5 2 2.5 3

x 105

0

2

4

6

D4

0.5 1 1.5 2 2.5 3

x 105

0

5

10

15

f [Hz]

D6

Fourier spectra of oscillations at variousaxial positions along the jet

z=17.5 mmz/D=2.92

z=26.1 mmz/D=4.35

z=31.7 mmz/D=5.28

d6

d5

d4

d3

d2

d1

0 400 800 1200t [100 ns]

0

4

8

12

Ud

[rel

. u.]

Determination of flow velocity

smfDv /3002 ==∆Φ

π

Fourier spectrum and dependence of phase shiftof oscillations on frequency – argon, 10 kW

Ii [A]

U [V]Upl

I = U/Ra

I = U/Rb I = U/Ra - UB

y

z

r

U

-110 V

to voltage dividers

R = 10 kΩ

cathode

waterswirl

anode

vprobe

y

z

r

U

-110 V

to voltage dividers

R = 10 kΩ

cathode

waterswirl

anode

vprobe

anode

vprobe

Electric probes in thermal plasma jet

Ion collecting electric probes are used for study of structure of plasma flow

Determination of probe potential

Lines of the same probe current at the plasma flow in the jet boundary

r

t

Structure of the jet boundary

Development of shape of structuresat the jet boundary

r

t

D = 5 mm D = 1 mm

laser

plasma

lens screenlens increasingn(ρ)

Schlieren photography

plasma jet produced in water stabilized arc

screendiameter D

0

t [µs]:

10

20

30

5

15

25

35

High speed photography

Interaction of plasma jet with arc anode attachment