Upload
others
View
36
Download
0
Embed Size (px)
Citation preview
THERMAL PLASMAS:Properties, Generation,
Diagnostics and Applications
Milan HrabovskyInstitute of Plasma Physics AS CR
Praha, Czech Republic
OUTLINEThermal plasma and non-thermal plasmaComposition, thermodynamic and transport properties of thermal plasmasModeling of thermal plasma flows, arc modelingGeneration of thermal plasmasBasic principles of arc plasma torchesFactors influencing properties of plasma jet in arc plasma torches ( design of the torch, properties of plasma gas)Plasma jet fluctuationsDiagnostics of thermal plasma jetsThermal plasma processing
Electron temperatures and densities in plasmas
102
104
106
108
1012 1016 1020 1024 1028108
Fusion plasma
Thermal plasmaGlow discharges
MHDgeneratorsFlame
Solarcorona
Ionosphere
Electron density [m -3]
Ele
ctro
n te
mpe
ratu
re [K
]
Nonequilibrium plasmas
Thermal (equilibrium) plasmas
plasma is not in thermodynamic equilibriumhigh values of E/p low pressureslow densities of electric current
)TT(TT ieie >≠
ie TT ≅
plasma is in thermodynamic equilibrium - LTElow values of E/p higher pressureshigh densities of electric current
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
Plasma in LTE
Properties of plasma in thermodynamic equilibrium are determined by pressure and temperature
Thermodynamic and transport properties of plasma depend on plasma composition
Composition of plasmaMonoatomic gas – electrons, ions, atoms
Eggert-Saha Equation:
Dalton´s Law:
Quasineutrality:
⎟⎠⎞
⎜⎝⎛ −⎟
⎠⎞
⎜⎝⎛ π
=kTEexp
hkTm2
QQ2
nnn i
2/3
2eiie
( )kTnnnp ie ++=
ie nn =
Thermal plasmas - compositionMonoatomic gases: atoms, ions, electrons
Argon: Ar, Ar+ , Ar++, … , e
Molecular gases: molecules, atoms, molekular ions, atomic ions, electrons
Nitrogen: N2 , N, N2+ , N+ , N++, … , e
Air: N2 , O2 , Ar, NO, N2O, NO2 , N, O, N2+, N+, O2
+, O+, Ar+, N++, O++, Ar++, … , e
Calculation of composition:Equilibrium between dissociation, ionization and recombinationMinimization of Gibbs free energyDepends on chemical potentials of different species present in plasma – can be determined from thermodynamic considerations
Plasma composition – monoatomic gas
Ar+
Ar
Ar++
e
e
Temperature [K]
Plasma composition-molecular gas
N+
e
e
N2+
N2
N
N++
nitrogen
Temperature [kK]
Composition of air plasma
Temperature [kK]
Thermodynamic properties
Mass densityEnthalpyInternal energySpecific heat Entropy
Evaluated for given plasma composition by methods of thermodynamics
The evaluation of thermodynamic properties is based on calculated partition functions for all species:
( )∑ −=s
ss kT/EexpgQ
Plasma density
∑=ρ iimn Density of nitrogen plasma
Decrease of particle concentration
Disociation, ionisation
Increase of number of particles with low mass
Temperature increase
Specific heat capacity
Increase oftemperature
Increase of kinetic energyof thermal motion - CfDissociation Ionization - CrCp = Cf + Cr
Specific heat capacityof various gases
HydrogenHeliumNitrogenArgon
Transport propertiesDiffusion coefficient D
Thermal conductivity k
Electrical conductivity σ
Viscosity µ
Computations of transport coefficients is based on solution of Boltzmann equation.
Knowledge of collision cross sections for all collisionalinteractions between particles is crucial. Transport
coefficients for more complex mixtures are frequently unknown.
ngradD=Γr
Tgradkq =r
Vgradj σ=r
xx vgradf µ=r
Thermal conductivity
Heat transfer
Molecules – kmAtoms - kaIons - kiElectrons - ke
Thermal conductivity of nitrogen
k = km + ka + ki + ke + kD + kI
Dissociation – kDIonization - kI
Copper k = 390 W/mKIron k = 75 W/mKWater k = 0,6 W/mK
Copper σ = 5,8 .107 S/mIron σ = 9,5 .106 S/mGraphite σ = 1,0 .105 S/m
Electric conductivity
Temperature [ 103 K]
Ele
ctri
c co
nduc
tivity
[ S/m
]
Plasma radiationComplex modeling of radiation transfer in plasma with high spatial gradients of temperature is extremely difficult due to
strong dependence of absorption and emission coefficients on temperature. Integration over all wavelengths along the optical path in plasma is necessary. Re-absorption of emitted light is
main problem.
Concept of net emissioncoefficient is frequently used. Net emission coefficientrepresents radiation fromisothermal cylinder of plasma with radius R.
Components of total radiationemission coefficient fo SF6 at 1 atm.
Absorption coefficient of SF6 for temperatures of 300 K and 20 000 K
Components of plasma radiation
Non-equilibrium effectsDeviations from LTE are caused by various mechanisms in
thermal plasma flows.
Flows with high spatial gradients of temperature in the direction of flow
“frozen” plasma composition corresponding to the upstream position
High spatial gradients of temperature – diffusion of species
Higher concentrations of electron in fringes of thermal plasma jets
De-mixing of plasma gases-ambipolar diffusion of ions with different coefficients of diffusion
Rapid changes of plasma temperatureeffect of kinetics of reactions (dissociation, ionization,
recombination) in plasma
Modeling of thermal plasma flowsPlasma can be described as a fluid with thermodynamic and transport properties depending on pressure and temperature
Models of thermal plasma flows are based on solution of equations of balances of mass, momentum and energy.
Equations of fluid dynamics are used with terms representing dissipation of energy by Joule heating,
radiation energy transfer and effect of electromagnetic forces.
Plasma properties are represented by transport and thermodynamic coefficients often calculated separately and given in tables giving dependence on temperature and pressure.
Model of arc column in axial gas flow
continuity equation: ( ) ( )1 vr u 0t r r x
∂ ∂ ∂ρ + ρ + ρ =
∂ ∂ ∂ ,
momentum equations:
( )ru u u p 2 1 uv u j B rvt r x x 3 x r r xθ
⎡ ⎤⎛ ⎞∂ ∂ ∂ ∂ ∂ ∂ ∂ρ + ρ + ρ = − + − η + +⎢ ⎥⎜ ⎟∂ ∂ ∂ ∂ ∂ ∂ ∂⎝ ⎠⎣ ⎦
u 1 u v2 rx x r r r x
⎡ ⎤⎛ ⎞ ⎛ ⎞∂ ∂ ∂ ∂ ∂η + η +⎢ ⎥⎜ ⎟ ⎜ ⎟∂ ∂ ∂ ∂ ∂⎝ ⎠ ⎝ ⎠⎣ ⎦
( )2
xv v v p 2 1 u wv u j B rvt r x r 3 r r r x rθ
⎡ ⎤⎛ ⎞∂ ∂ ∂ ∂ ∂ ∂ ∂ ρρ + ρ + ρ = − − − η + + +⎢ ⎥⎜ ⎟∂ ∂ ∂ ∂ ∂ ∂ ∂⎝ ⎠⎣ ⎦ 2
1 v 2 v u v2 rr r r x r xr
⎡ ⎤⎛ ⎞ ⎛ ⎞∂ ∂ η ∂ ∂ ∂η − + η +⎢ ⎥⎜ ⎟ ⎜ ⎟∂ ∂ ∂ ∂ ∂⎝ ⎠ ⎝ ⎠⎣ ⎦
w w w 1 w wv u rt r x r r r x x
∂ ∂ ∂ ∂ ∂ ∂ ∂⎛ ⎞ ⎛ ⎞ρ + ρ + ρ = η + η −⎜ ⎟ ⎜ ⎟∂ ∂ ∂ ∂ ∂ ∂ ∂⎝ ⎠ ⎝ ⎠ 2vw w wr r rr
ρ η ∂η− −
∂ ,
energy equation: p p p r r x xT T T p p pc vc uc u v j E j Et r x t x r
∂ ∂ ∂ ∂ ∂ ∂ρ + ρ + ρ − = + + + +
∂ ∂ ∂ ∂ ∂ ∂
x r1 T T 5 k T Tr j j Rr r r x x 2 e x r
∂ ∂ ∂ ∂ ∂ ∂⎛ ⎞ ⎛ ⎞ ⎛ ⎞λ + λ + + −⎜ ⎟ ⎜ ⎟ ⎜ ⎟∂ ∂ ∂ ∂ ∂ ∂⎝ ⎠ ⎝ ⎠ ⎝ ⎠&
charge continuity equation:
1 r 0 ,r r r x x
⎛ ⎞ ⎛ ⎞∂ ∂Φ ∂ ∂Φσ + σ =⎜ ⎟ ⎜ ⎟∂ ∂ ∂ ∂⎝ ⎠ ⎝ ⎠
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
Thermal plasma generation
Electric arcs
High current densities– arc column ~ 100 A/cm2
– electrode spots ~> 106 A/cm2
High radiation intensityLow electrode potential drops Vc ~ 10 VDischarge current often limited only by power
supplyArc currents 1 A – 106 AIntensity of electric field 1 V/cm – 103 V/cmArc column length 1 mm – 101 m
liquidvapor
cathode
anode
gas
cathode
anode
Principles of arc stabilization
Gas-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
Gas-stabilized arc plasma torches
Non transferred arcs Transferred arcsHot cathode
Cold cathode
+-
gas plasma
Gas-stabilized plasma torches
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
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
2000
4000
6000
8000
h/S
[m.s
/kg]
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
10000 20000 30000temperature [K]
0.001
0.01
0.1
1
h/ε n
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:
Enthalpy fllow rate G.h0 Enthalpy flow rate G.hR
Operation regimes of gas-stabilizedand water-stabilized dc arc 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
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
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
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)
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 (water) is supplied into the discharge chamber and plasma jet is produced at the exit nozzle
Plasma Jet Instabilities
Gas dynamic instabilities:Fluctuations and turbulences in the arc chamberInteraction of plasma jet with ambient gas outside the
arc chamberArc instabilities:
Fluctuations of arc power due to ripple of rectifieredarc current
Arc column instabilities, changes of arc lengthInstabilities caused by electrode processes
Plasma jet interactionwith ambient gas
Production of vortex structures at the jet boundaryEntrainment of cold gas into plasma flowFormation of plasma flow turbulence
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.
Positions of diodes: zd1 = 2.4 mmzd2 = 4.3 mmzd3 = 6.2 mm
d1 d2 d3
anode
arc
anode jet
nozzle
Development of gas-dynamic instabilityand effect of anode attachment
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
Jet instability caused by anode attachment
Arc anode attachment:-anode jet-movement in the restrike mode
exposure time 3µs
0.05 0.10 0.15t [ms]280
300
Ua[V
]
0.20 0.24 0.28 0.32t [ms]
260
270
280
U a[V]
1 2 3 4 3 4 1 2
Anode restrike and movement of anode attachment
Fluctuations of arc voltage Fluctuations of arc power
Sequence of four images of anode region of plasma jetexit nozzle
anode
Effect of fluctuations of arc current
time
curr
ent
For classical 6-way rectifiers:∆I/I = 0.1 – 0.15Frequency : 300 Hz
Amplitude of fluctuations of arc power depends on relation between τΙ and τα
1
a
1
I
dtdg
g1
dtdI
I1
−
−
⎟⎟⎠
⎞⎜⎜⎝
⎛=τ
⎟⎠⎞
⎜⎝⎛=τTime constant of current
changes
Arc time constant
1. τI << τa Arc has constant conductance: U = U + Rs . ∆IAmplitude of fluctuations of arc power: ∆P/P = 2 ∆I/I + (∆I/I)2
For ∆I/I = 0.1 – 0.15 ………. ∆P/P = 0.21 – 0.32
2. τI >> τa Arc voltage follows static characteristic: Constant arc voltage : ∆P/P = ∆I/IFalling static arc characteristic: ∆P/P < ∆I/I
~ 10-4 – 10-2 s
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
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 10
5
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
e xpe rime ntcalculation
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 characteristicsfrom enthalpy probe measurements
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
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
Stream-wise and counter-stream-wise 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 velocityfrom phase shift of jet fluctuations
Fluctuations of emitted lightat various distances from the torch exit
D - Distance between points∆Φ - Phase shift
Velocity of movementof oscillations
∆Φπ fDv 2=
Determination of flow velocity
smfDv /3002 ==∆Φ
π
Fourier spectrum and dependence of phase shiftof oscillations on frequency – argon, 10 kW
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
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 jets
Ion collecting electric probes are used for study of structure of plasma flow
Determination of probe potential
High biasing resistance – probe potential close to floating potentialLow biasing resistance – probe current close to ion saturation currentNegative biasing voltage increases sensitivity of probe measurement
Lines of the same probe current at the plasma flow in the jet boundary
r
t
Structure of the jet boundary
Thermal plasma processing
Plasma technologies and decisive mechanisms
Heat transferPlasma meltingPlasma cutting
Heat and momentum transferPlasma sprayingSurface modifications
Chemical processes – decomposition and/or synthesis
Waste treatment, gasification, vitrificationPlasma CVD – production of filmsPlasma synthesis
Plasma spraying
Treatedmaterial Syngas
H2 + CO
vitrified lava
Gasification+
vitrification
Plasmatorch
Decomposition of Persistent ChemicalCompounds and Waste Treatment
Plasmareactor
CONCLUSIONSBasic thermal plasma jet properties:
Temperature and velocity profiles, plasma enthalpy, plasma compositionFlow structure and (in)stability
Basic plasma torch design parameters:Arc chamber geometry, length and diameterGas composition, gas flow rate
Basic diagnostics for thermal jetsSpectroscopyEnthalpy probesHigh speed photography, schlieren photographyElectric probes
Thermal plasma applications:Metallurgy, welding, cuttingSurface modifications, coatingsMaterials synthesisDecomposition, waste treatment