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