ignition and stabilization of lean flames by nonequilibrium...
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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
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
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
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
5
Nanosecond Repetitively Pulsed (NRP)
discharges
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
7
Stabilization of Lean Premixed Flames
using Nanosecond Repetitively Pulsed
(NRP) discharges
8
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]
9
Mini-PAC burner
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
11
Stabilization of
Larger Scale Combustors
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
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
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
15
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
16
Dynamic control of
thermo-acoustic instabilities
Closed loop control of a turbulent
swirled flame
17Lacoste, Moeck, Durox, Laux, Schuller, J. Eng. Gas Turb. Power, 135, Oct. 2013
Closed loop control of a turbulent
swirled flame
18
19
Ignition of lean combustible
mixtures at 10 bar
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
21
FUNDAMENTAL MECHANISMS
22
Chemical and thermal effects
of
NRP discharges
23
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
24
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)
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
26
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.
-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
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
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
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
NUMERICAL SIMULATIONS
Castela, Fiorina, Coussement, Gicquel,
Darabiha, Laux Combustion & Flame, 166,
133-147, 2016
32
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
33
Castela et al., Proceedings of the Combustion Symposium, Seoul, 2016.
3D DNS simulations and experiments
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
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
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
37
38
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
Flame radius development
0 2 4 6 8 10 12 14 16 180
2
4
6
8
10
12
14
16
18
0 2 4 6 8 10 12 14 16 180
2
4
6
8
10
12
14
16
18
0 2 4 6 8 10 12 14 16 180
2
4
6
8
10
12
14
16
18
0 2 4 6 8 10 12 14 16 180
2
4
6
8
10
12
14
16
18
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
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