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Christophe Laux
Laboratoire EM2C / CNRS
CentraleSupelec
Work supported by:
ANR PLASMAFLAME,
ANR FAMAC,
Chaire d’Excellence
on Optical Diagnostics,
INCA
MUSAF III – Toulouse, September 27-29, 2016
Ignition and Stabilization of Lean Flames by
Nonequilibrium Plasmas
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Contributors
• Da Xu
• Guillaume Pilla
• David Galley
• Diane Rusterholtz
• Séverine Barbosa
• Marien Simeni Simeni
• Sara Lovascio
• Sergey Stepanyan
• Deanna Lacoste
• Gabi Stancu
• Jonas Moeck (TUB)
• Jun Hayashi (Osaka)
2
• Maria Castela
• Benoît Fiorina
• Nasser Darabiha
• Axel Coussement
• Olivier Gicquel
• Denis Veynante
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Motivation
• Improvement of combustion efficiency
• Ignition of lean or diluted flames
• Stabilization of lean flames
• Reduction of pollutant emissions (NOx, Soot)
• Control of thermo-acoustic instabilities
Potential benefits of
plasma-assisted combustion
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Outline
• Nanosecond Repetitively Pulsed (NRP) discharges
• Demonstrations of plasma-assisted combustion:
• Lean flame stabilization
• Control of thermo-acoustic instabilities
• Acceleration of ignition at high pressure
• Fundamental mechanisms in NRP discharges:
• Chemical and thermal effects
• Hydrodynamic effects
• Numerical simulations
4
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Nanosecond Repetitively Pulsed (NRP)
discharges
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1 10 1000
0.2
0.4
0.6
0.8
1.0
O2,N
2 ion.
O2 elect.
N2 elect.
N2 vib.
elastic
O2,N
2 rot.
Fra
ctio
n o
f to
tal p
ow
er
E/N (Td=10-17
Vcm2)
O2 vib.
Motivation
Typical pulses:
10 ns, 5-30 kV
Applied at high frequency, 10-100 kHz
Nanosecond Repetitively Pulsed
(NRP) discharges
0 5 10 15 20 25 30 350
1000
2000
3000
4000
5000
6000
7000
Vo
lta
ge
[V
]
t [ns]
High E/N,
Typically > 100 Td
Aleksandrov et al, High Temp. 19, 1981 and Nighan, Phys. Rev. A 2, 1970
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Stabilization of Lean Premixed Flames
using Nanosecond Repetitively Pulsed
(NRP) discharges
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Mini-PAC burner:
25-kW Lean Premixed Propane-Air Burner
Air/Propane
Mixture
Cathode
Cylindrical wire anode
Pulse
discharge
generator
Bluff-body
0 5 10 15 20 25 30 350
1000
2000
3000
4000
5000
6000
7000
Vo
lta
ge
[V
]t [ns]
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Mini-PAC burner
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10
5 10 15 20
0,65
0,70
0,75
0,80
0,85
0,90
0,95
1,00
limit without plasma
Intermittent flame /
extinction
Fu
el
eq
uiv
ale
nc
e r
ati
o
Air flowrate, m3/h
Stable flame
limit with plasma
• Lean extinction limit decreased by about 10%
Electrical power < 1% of the flame power
Pilla, et al 2006
NRP: 2.3 mJ/pulse, PRF=30 kHz, Plasma power: 70 W
Stability regimes of mini-PAC burner
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Stabilization of
Larger Scale Combustors
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52-kW two-stage swirled gas turbine
injector
C3H8
Air
C3H8
10
0m
m
500 mm
Ignition Spark
Combustion chamber
Air: 105 m3/h
Propane: 2.1 m3/hMax power: 52 kWExit velocity: 40 m/s
Propane/air at 1 bar
Barbosa, Pilla, Lacoste, Scouflaire, Ducruix, Laux,
Veynante, Phil. Trans. Royal Society A, 373, 20140335,
2015
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13
Two-stage swirled gas turbine injectorPremixed propane/air, 52 kW, 1 atm
Combustion chamber
Propane
Swirled air
Swirledair
HV
Swirled air
Swirled air Propane
Propane
Mixingzone
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Lower extinction limit of two-stage burner
Constant air flow rate: 105 m3/hWithout plasma With plasma, 30 kHz
2.1 m3/h Φ=0.47
Lean extinction limit reduced by
a factor 4
2.1 m3/h Φ=0.47
1.95 m3/h Φ=0.44
1.95 m3/h Φ=0.44
Extinction Ф= 0.4
1.8 m3/h Φ=0.4
1.8 m3/h Φ=0.4
1.65 m3/h Φ=0.37
1.35 m3/h Φ=0.3
1.2 m3/h Φ=0.27
Extinction with plasma
Ф=0.11
1.05 m3/h Φ=0.23
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Kerosene/air at 3 bar
• Reduction of the Lean Extinction Limit by a factor 2
• Power consumed by NRP discharge: < 1% of flame power
G. Heid et al, ISABE 2009
Without plasma With plasma, 100 kHz
Extinction: Φ = 0.21Extinction: Φ = 0.44
200 kW Turbulent Aerodynamic Injector
(ONERA/MERCATO)
HT
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Dynamic control of
thermo-acoustic instabilities
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Closed loop control of a turbulent
swirled flame
17Lacoste, Moeck, Durox, Laux, Schuller, J. Eng. Gas Turb. Power, 135, Oct. 2013
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Closed loop control of a turbulent
swirled flame
18
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Ignition of lean combustible
mixtures at 10 bar
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NRP vs Conventional
Ignition
20
C3H8/air (=0.7) mixture - 10 barConventional
NRP 5 ms
32 mm
10 ms 15 ms
5 ms 10 ms 15 ms
55 mJ
22 W
2.7 ms
30 kHz
82 pulses
57 mJ
16 W
3.5 ms
• 20% faster with NRP at high pressure due to more wrinkling
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21
FUNDAMENTAL MECHANISMS
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Chemical and thermal effects
of
NRP discharges
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Experimental approach
4.5 mm
NRP sparl
discharge
(10 kHz)grounded
electrode
Preheated
air at 1000 K
Study NRP discharge in air at 1000 K, 1 atm:
• 10-ns pulse
• 5.7 kV
• Gap: 4.5 mm
• 10 kHz
• 0.670.02 mJ/pulse
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Two-step ultrafast mechanism
for oxygen dissociation and heating
N2 + e → N2* + e (N2* = N2 A, B, C)Thresholds: 6.2, 7.4, 11.0 eV
N2* + O2 → N2 + O + O + DTDT = 1.0, 2.2, 5.9 eV
Measured quantities:
• Electrodynamics: U, I, Energy
• O atoms: TALIF
• N2 A: CRDS
• N2 B and N2 C: OES
• Electrons: Stark broadening
• Temperature: OES (Trot N2 C and Trot N2B)
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Optical diagnostics: TALIF
• O atoms by TALIF • Temperature, N2B,
N2C by OES
25
Pulse generator / delay
ElectrodesPlasma
HV pulser
Digital oscilloscopePD UV
ultrafast
225 nm
Nd:YAG Dye Doubling /
mixing1064 nm /532 nm 570 nm
Power meter lens
BS
LIF collection
optics
Spectrometer ICCD
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HV pulse HV pulse
-20 0 20 40 60 80 100
0.0
2.0x1017
4.0x1017
6.0x1017
8.0x1017
1.0x1018
1.2x1018
1.4x1018
1.6x1018
1.8x1018
O d
ensi
ty [c
m-3
]
Time [s]
1/e lifetime = 25 s
TALIF measurements of O density
during one pulsing cycle (100 s)
• O lifetime in air: 25 s
• About half of the O2 is dissociated by the discharge
Obackground
Stancu, Kaddouri,
Lacoste, Laux,
J. Phys. D., 2010.
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-10 0 10 20 30 40 5010
12
1013
1014
1015
1016
1017
0
1
2
3
4
5
6
0
50
100
150
200
250
300
Time (ns)
N2(A)
N2(C)
Ab
so
lute
de
nsitie
s [
cm
-3]
N2(B)
02.0x10
17
4.0x1017
6.0x1017
8.0x1017
1.0x1018
1.2x1018
O (3P) density
1500
2000
2500
from N2(C-B)
from N2(B-A)
Te
mp
era
ture
[K
]
Temperature
0
10
20
30
40
Cu
rre
nt [A
]
Vo
lta
ge
(V
)
V Iconduction
E/N
[T
d]
27
Synchronized measurements of
V, I, temperature, densities
hheating =21±5%
Rusterholtz et al., J. Phys D., 2013
Electric energy:
670±20 J/pulse
hdiss. = 35±5%
Ultrafast heating:
900 K in 20 ns
50% dissociation
of O2
N2* + O2 → N2 + O + O + DT
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Measured and predicted temporal profiles
of O and Temperature
28
Measurements: present work Simulations: N. Popov, AIAA 2013-1052, Jan. 2013
• Confirmation of the two-step mechanism of ultrafast
heating and oxygen dissociation
• Full reference test case for numerical simulations
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2 m
m
-1.5 μs 50 ns 200 ns 500 ns 900 ns
1.5 μs 3 μs 5 μs 10 μs 16 μs
29
Hydrodynamics
(fast Schlieren imaging)
• Hot channel
• Shock wave
300 K 1m/s
D.A. Xu et al., Appl. Phys. Lett. 99, 121502, 2011
1 kHz
1 mJ/pulse
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Processes involved in flame
stabilization by NRP discharges
e-
N2(A)N2(B)N2(C)
O2N2(X)
N2(X) + 2 O + DE
DT
Thermal and hydrodynamiceffects
O22 O
Chemical effects: RH + O R + OH
Oxidation
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NUMERICAL SIMULATIONS
Castela, Fiorina, Coussement, Gicquel,
Darabiha, Laux Combustion & Flame, 166,
133-147, 2016
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Castela, Stepanyan, Fiorina, Coussement, Gicquel,
Darabiha, Laux., Combustion Symposium, Seoul, 2016.
3D DNS model of plasma-assisted
combustion
• Structured DNS solver – YWC with:
• Compressible reactive flow simulations
• Multicomponent mixture averaged transport properties
• Detailed chemistry
• Detailed combustion kinetics
• Simplified plasma kinetics:
• 20% of electrical energy into fast gas heating (<50 ns)
• 35% of electrical energy into fast O2 dissociation (<50 ns)
• 45% of electrical energy into N2 vibrational excitation
.
Coussement, Gicquel, Caudal, Fiorina, Degrez, J. Comp. Phys. 231, 5571 – 561, J. Comp Phys 2012
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Castela et al., Proceedings of the Combustion Symposium, Seoul, 2016.
3D DNS simulations and experiments
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Ignition of lean propane-air mixture
by a train of 22 pulses, = 0.7, P = 2 bar
34
Interlectrode distance = 0.6 mm, 0.9 mJ/pulse
• Strong hydrodynamic effects at short gaps
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Conclusions
• NRP discharges can efficiently stabilize lean flames and
control instabilities with < 1% of flame power:
• NRP discharges can accelerate lean flame propagation
thanks to wrinkling of flame surface
• Fundamental processes induced by NRP discharges• Production of O and heat: quenching of N2* by O2
• Hydrodynamic effects and shock waves
• Production of NO: quenching of N2* by O ?
• Many challenges remaining:• Real environments with turbulence and high pressures?
• How to reduce NOx?
• Demonstration in larger scale burners
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6th EUCASS Aerospace Thematic Workshop
Fundamentals of Aerodynamic Flow
and Combustion Control by PlasmasApril 9-14, 2017, Kochubei hotel, Pushkin, Saint Petersburg
36
onlinereg.ru/ATW-2017
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NRP vs Conventional
Ignition
39
C3H8/air (=0.7) mixture - 2 bar
Conventional
NRP 5 ms
32 mm
10 ms 15 ms
5 ms 10 ms 15 ms
55 mJ
22 W
2.7 ms
30 kHz
82 pulses
57 mJ
16 W
3.5 ms
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Flame radius development
0 2 4 6 8 10 12 14 16 180
2
4
6
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16
18
0 2 4 6 8 10 12 14 16 180
2
4
6
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0 2 4 6 8 10 12 14 16 180
2
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0 2 4 6 8 10 12 14 16 180
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Ra
diu
s (
mm
)
Time (ms)
5 bar
Ra
diu
s (
mm
)
Time (ms)
3 bar
Audi
NRP-82 pulses
Ra
diu
s (
mm
)
Time (ms)
2 bar
Ra
diu
s (
mm
)
Time (ms)
10 bar
• 20% faster with NRP at high pressure due to more wrinkling
Xu et al, Plasma Chem Plasma Process, 36(1), 309-327, 2016