combustion assisted by nanosecond repetitively...

Christophe Laux

Laboratoire EM2CEcole Centrale Paris – CNRS

Grande Voie des Vignes92290 Chatenay-Malabry

Combustion Assisted by Nanosecond Repetitively Pulsed Plasmas

5èmes Journées du Réseau Plasmas Froids, Bonascre, October 4-6, 2006

Contents

• Motivation• Types of discharges• Plasma-assisted ignition• Plasma-assisted stabilization• Conclusions

Motivation• Internal combustion engines• Industrial burners (domestic heating, …)• Gas turbine engines

– Subsonic aircraft– Energy production

• SCRAMJETS (super-stato-réacteurs)

Internal combustion engines

Objectives: • Burn lean or diluted fuel/air mixtures to reduce pollutant emissions• Reduce ignition delay

Gas turbine/aircraft engines

• Objective: burn lean fuel/air mixtures to reduce pollutant emissions

Scramjet engines for hypersonic aircraft

Hyper-X (NASA)• Mach 7-10• Hydrogen-fueled

HyTech (AFOSR)• Mach 4-8• Hydrocarbon-

fueled

• Objective: ignite chamber

Scramjet engines for hypersonic aircraft

∆E3

cavity

flameholder

upstream fueling

plasma igniter

downstream fueling

M = 2

∆E2∆E1

surface electrodes—sequentially pulsed

∆E4kilo

volts

time, msIntegrated Pulsed

Plasma Igniter (IPPI)

combustor mid-plane view

LTE plasma

Elevated Te plasma (non-equilibrium)

∆E3

cavity

flameholder

upstream fueling

plasma igniter

downstream fueling

M = 2

∆E2∆E1

surface electrodes—sequentially pulsed

∆E4kilo

volts

time, msIntegrated Pulsed

Plasma Igniter (IPPI)

combustor mid-plane view

LTE plasma

Elevated Te plasma (non-equilibrium)

Discharges for PAC

• Article Review: S. M. Starikovskaia, “Plasma assisted ignition and combustion”, J. Phys. D: Appl. Phys. 39 (2006) R265–R299

Glow discharges (DC or pulsed)

Sparks (arcs)

Laser discharges (DC or pulsed)

Filamentary discharges (DC or pulsed)

Nanosecond dischargesK=V/Vbreakdown

• Single pulses (widely used for pas 15 years)

– Townsend: K < 1– Streamer/filament: K ≥1– Fast Ionization Wave: K>>1

• Repetitive nanosecond pulses (proposed and demonstrated at Stanford University 1999-2002)– Glow: K < 1– Streamer/filament: K ≥1– (FIW: K>>1)

Repetitively pulsed dischargesVOLTAGE

CURRENT

Pulse duration should be less than the transition to sparkDelay between pulses should match the relevant recombination time

delay betweenpulses

pulseduration

Energy deposition per pulse is very small(3 mJ for 5 kV / 10 ns pulses).

Typical repetitive nanosecond discharge system

Two discharge regimes:• Low voltage: diffuse glow discharge• High voltage: filamentary discharge

Characteristics– Up to 10 kV amplitude– 10 ns long– Fast rise time ~ 2 - 3 ns– Repetition rate: 1 - 30 kHz

Two applications

• Flame ignition in ICE (Internal Combustion Engines)Propose an alternative to conventional spark plug systems, which have several limitations:– Inability to ignite lean or diluted mixtures– Localized ignition– Cathode erosion

• Flame stabilization in LPP (Lean Premixed Prevaporized) reactorsLean flames burn at lower temperatures, thus emit fewer pollutants (NOx, soot), but are unstable

Ignition of PropaneIgnition of Propane--Air Mixtures Air Mixtures by a Sequence of Nanosecond Pulsesby a Sequence of Nanosecond Pulses

S. Pancheshnyi, D.A. Lacoste, A. Bourdon, C.O. Laux

Conventional vs. Repetitive Nanosecond Ignition Systems

SPARK PLUG SYSTEM

1. Siemens HOM 7700 732 263 pulse voltage generator (12V, 5A):

– Amplitude: 25 kV– Duration: 1.5-2.0 ms– Peak pulse power: 20 W– Pulse energy: 60 mJ

2. BOSCH ZR6SPP332 spark plug

REPETITIVE NANOSECOND DISCHARGE SYSTEM

1. FPG 10-30MS pulse generator:– Amplitude: 5 kV – Duration: 10 ns (fixed)– Repetition rate: 1-30 kHz– Peak pulse power: 500 kW– Max pulse energy: 3 mJ

2. Point-to-plane geometry with 1.5-mm interelectrode distance.

– BOSCH Y6DC spark plug– Grounded disk 10 mm in diameter

Ignition and breakdownIgnition requires breakdown of the combustible mixture

Breakdown occurs when the voltage becomes larger than a certain value (the breakdown voltage): 3-4 kV/mm/bar.

If one keeps applying voltage after breakdown, we get a spark ⇒ gas heating, high energy consumption, erosion

For car engine ignition, breakdown requires very high voltages (45-60 kV at 15 bars)⇒ technological issues, EM interferences,…

Alternate approach: use train of pulses below the breakdown voltage

Experimental setup60-cm3 steel chamber with 2 quartz windows

Pressure range: 0.4 – 2.5 bar.

Mixtures used:• Dry air• Propane-air

– Stoichiometric– Lean (φ = 0.7)– Stoichiometric mixture with 30% N2 dilution

Measurements:• Voltage (LeCroy high-voltage probe), current (Pearson monitor)• Pressure trace (AVL quartz pressure transducer)• OH emission (Hamamatsu photosensor module with UV filter)• Spectral emission (Acton spectrometer + ICCD camera)• Ultrafast imaging (Photron intensified camera)

exhaustvacuum pump

compressed airpropane-air premix

FPG

PC

current monitor

high-voltage probe

fiber

pressure transducer

photosensor

fast ICCDcamera

oscilloscopetrigger

generator

pressuregage

flowmeter

spe

ctro

me

ter

Electrical and Optical Diagnostics

Propane-air mixture ignition by trains of 5 kV/10 ns pulses in pressure range

0.5-2.5 bar

Definitions : Ignition delay and combustion rise time

Ignition delay time:Time from spark to a 10% pressure increase

Pressure rise time:Time for the pressure to increase from 10% to 90% of the maximum value

0,00 0,01 0,02 0,03 0,04 0,05

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

1,1

Pres

sure

Incr

ease

, arb

. uni

ts

Time, arb. units

Delay0-10%

Rise Time10-90%

Spar

k

Ignition by repetitive nanosecond discharge

0

1

2

3

4

0 10 20 30 40 50 60 70 80 90 1000,00

0,02

0,04

0,06

0,08

0,10

0,12

0,14

Pres

sure

Incr

ease

, bar

OH

em

issi

on, a

rb. u

nits

Time, ms

13 16 21 94

Stoichiometric propane-air mixture diluted by 30% N2

no ignition30 (1)Ordinaryspark

RiseTime,

ms

DelayTime,

ms

Energy, mJ

(# ofpulses)

Ignitionsource

1-bar propane-air mixture, 1.5 mm gap, train of pulses: 5 kV, 10 ns, 30 kHz

2522160 (94)

252936 (21)

253927 (16)

315622 (13)

Nano-second

repetitivedischarge

Ultrafast imaging of the ignition of a lean C3H8-air mixture: φ = 0.7, 2 bar

Framerate: 30 kHz Exposure time: 33 µs Total time: 35 ms

Conventional ignition system Nanosecond discharge(Ignition requires several sparks)

Influence of the repetition rate

1-bar propane-air mixture φ =1, 1.5 mm gap, train of pulses: 5 kV, 10 ns

• Ignition delay decreases abruptly between 1 and 10 kHz (at 1 bar)• Ignition delay decreases with number of pulses applied

Minimal number of pulses for ignition

1-bar propane-air mixture φ =1, 1.5 mm gap, train of pulses: 5 kV, 10 ns

• Maximum operating pressure: 2.5 bar (for 5-kV pulses at d = 1.5 mm)• To operate at higher pressure, need more than 5 kV or shorter gap

30 kHz

1-bar air plasma produced bya train of 10 pulses

of 5 kV/10 ns at 30 kHz

Discharge in air at 1 bar

1 bar, air, 1.5 mm gap, 10-pulse train: 5 kV, 10 ns, 30 kHz

Ignition occurs only if there is at least Ignition occurs only if there is at least one highone high--current pulse in the traincurrent pulse in the train

Low current High current

Energy deposited


Temperature measurements(from fit of N2 C-B rotational lines)


Summary

1 bar, air, 1.5 mm gap, 10-pulse train: 5 kV, 10 ns, 30 kHzFirst four pulses create favorable First four pulses create favorable conditions for breakdownconditions for breakdown

Observations• A train of repetitive nanosecond pulses below the breakdown

voltage can efficiently ignite lean/diluted propane-air up to some pressure limit (here 2.5 bars with 5 kV pulses across 1.5-mm gap).

• Pressure limit can be increased with higher voltage, shorter gap, or pre-ionization

• The power consumption is 6-20 times lower than spark plug (typically 3-10 mJ required vs. 60 mJ for spark plug)

• Increasing the number of pulses and/or repetition rate reduces the ignition delay time

• Initial pulses create heat, which favors ignition.Question: which active species (radicals, ions, excited molecules or atoms) are formed, how, and in what quantities ?

What we think is going on

• Initial pulses produce radicals via reactions such as:– N2 + e → N2* + e– N2* + O2 → N2 + O + O (« hot » O atoms)

• Hot O atoms transfer energy to gas

• ⇒ T ↑ ⇒ N ↓ ⇒ E/N ↑

• Eventually E/N becomes sufficient to producebreakdown

Stabilization of a Lean Premixed Stabilization of a Lean Premixed AirAir--Propane Turbulent Flame using a Propane Turbulent Flame using a

Nanosecond Repetitively Pulsed PlasmaNanosecond Repetitively Pulsed Plasma

G. Pilla, D. Galley, D. Lacoste, F. Lacas,D. Veynante and C. Laux

Programme INCA “INitiative en Combustion Avancée”SNECMA, CNRS (EM2C, CORIA), ONERA

Flame stabilization strategy

• A single nanosecond high voltage pulse is sufficient to ignite, but not to stabilize a flame

• Flame stabilization requires to excite the gas continuously over extended time (and with reasonable power)

interest of NRPP

Lean Premixed Burner

• Max air flow rate: 25 m3/h• Max propane flow rate: 1 m3/h• Max Reynolds number: 30,000• Max heat release: 25 kW

Lean Premixed Burner

Air/Propane Mixture

Cathode

Cylindrical wire anode

Pulse dischargegenerator

Dischargestabilizing

cone

Bluff-body

Optical diagnostics setup

f = 500 mm

Experimental setup

Mirrors on XYZ translation stage

Burner

ElectrodesFiber

Increasing air flow rate or decreasing Φ

Three flame regimes

V-shaped flame Intermittent flameV-shaped flame Pilot flame(confined to

recirculation zone)

Intermittent flame

Flame stabilization by plasma

PLASMA

OFFPLASMA

ON

Φ = 0.8, Air flow rate = 15 m3/h

OH spontaneous emission

• With the plasma, the pilot flame becomes a V-shaped flame

Parameters:

Air: 14.7 m3/h

Propane: 0.5 m3/h

Pflame=12.5 kW

Pplasma=75 W (streamer mode)

OH* emission without discharge

OH* emission with discharge (P=75 W)

Mask to block theemission of the plasma(discharge behind)

Influence of repetition rate on burner regimes

Flame regimesV-shaped flame to intermittent flame

• Extension of the domain of stability of the flame

2 4 6 8 10 12 14 16 18 20 22 24 260.6

0.7

0.8

0.9

1.0

Air Flow rate in m3/h

Equ

ival

ence

ratio

f = 1 kHz f = 2 kHz f = 5 kHz f = 10 kHz f = 15 kHz f = 20 kHz f = 25 kHz f = 30 kHz

without plasma

Flame regimesIntermittent flame to pilot flame

2 4 6 8 10 12 14 16 18 20 22 24 260.4

0.5

0.6

0.7

0.8

0.9

f = 1 kHz f = 2 kHz f = 5 kHz f = 10 kHz f = 15 kHz f = 20 kHz f = 25 kHz f = 30 kHz


Equ

ival

ence

ratio

Without plasma

Flame regimesPilot flame to extinction

2 4 6 8 10 12 14 16 18 20 22 240

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

30 kHz25 kHz

10 kHz

20 kHz15 kHz

5 kHz

E

quiv

alen

ce ra

tio


Lean flammability limitwithout plasma

2 kHz

• The domain of existence of the pilot flame is considerably extended by the plasma for f > 5 kHz.

Flame regimes

0 5 10 15 20 25 30

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

4 m3/h6 m3/h

12 m3/h

18 m3/h

21 m3/h

Equi

vale

nce

ratio

Frequency in kHz

24 m3/h

Transition from the V-shaped flame to the intermittent flame

• The critical equivalence ratio decreases about linearly with the frequency (by about 10% from 0 to 30 kHz) -> residence time

Optical diagnostics

Time-resolved spectroscopySpectra integrated over the 100 ns following the pulse

• Filamentary discharge in flame produces OH radicals

Stable V-shaped flame Φ= 0.8

Glow in flame

Filamentin flame

Filamentin air

Spectra integrated over the 100 ns following the pulse

• Filamentary discharge produces O atoms in flame, but less than in pure air. No N atoms produced in flame.

Time-resolved spectroscopy

Filamentin flame

Filamentin air



• Filamentary discharge produces H atoms (Hα Balmer line)



Filament in flame

Filament in air


• Filamentary discharge in flame produces H atoms (Hβ Balmer line)



Filamentin air

Filament in flame

Electron number density measurements

485.4 485.6 485.8 486.0 486.2 486.4 486.6 486.80

2

4 Measured spectrum

In

tens

ity a

.u.

Wavelength in nm

Repetition rate of the discharge : 30 kHz, propane/air flame at Φ=0.8

485.4 485.6 485.8 486.0 486.2 486.4 486.6 486.80

2

4 Simulated N2 C-B Measured spectrum

In

tens

ity a

.u.

Wavelength in nm

• Subtraction of a simulated spectrum of N2 C-B

Electron number density measurementsRepetition rate of the discharge: 30 kHz, propane/air flame at Φ=0.8

• Isolated spectrum of Hβ

485.6 485.8 486.0 486.2 486.4 486.6

0.0

0.2

0.4

0.6

0.8

1.0N

orm

aliz

ed in

tens

ity

Wavelength in nm

Measured


485.6 485.8 486.0 486.2 486.4 486.6

0.0

0.2

0.4

0.6

0.8

1.0N

orm

aliz

ed in

tens

ity

Wavelength in nm

ne = 1015 cm-3

ne = 2.14x1015

Measured

Repetition rate of the discharge : 30kHz, propane/air flame at Φ=0.8

• ne = 1015 cm-3 (± 10%)

Theoretical Stark broadened profiles taken from: Gigosos et al., Spectrochim. Acta B, 58 (2003) 1489


• Electron number density constant for frequencies between 10 and 30 kHzne = 1015 cm-3 (± 10%)

10 15 20 25 306E14

8E14

1E15

1.2E15

1.4E15

1.6E15

1.8E15

2E15El

ectr

on n

umbe

r den

sity

in c

m-3

Repetition rate in kHz

propane/air flame at Φ=0.8

371 372 373 374 375 3760.0

0.2

0.4

0.6

0.8

1.0In

tens

ity (a

.u.)

Wavelength in nm

SPECAIR: Tvib = 4000 K, Trot = 2700 K Experimental spectrum

Temperature measurements

N2 C-B (1,3)

• Filamentary regime at 30 kHz in flame: Trot= 2700 K, Tvib= 4000 K.(Adiabatic Flame temperature: 2034 K, propane-air at Φ=0.8)

Repetition rate of the discharge : 30 kHz

Stable V-shaped flame Φ = 0.8

Temperature measurements

0 5 10 15 20 25 30

1600

1800

2000

2200

2400

2600

2800 Rotational temperature of N2 in C state Adiabatic flame temperature

Repetition rate in kHz

Rot

atio

nal t

empe

ratu

re in

Kel

vin

0

20

40

60

80

100

Power input DC

power supply input in W

• temperature increases as the frequency increases• power injected in the discharge increases in similar way

Frequencies from 1 to 30 kHz, propane/air flame at Φ=0.8

What we know

• Power required for stabilization is low: 75 W for 25 kW turbulent flame

• Significant effect of the repetition rate : extension of the domain of stability proportional to the frequency

• Filamentary discharge is much more efficient than glow discharge

• Filamentary discharge produces :• Heat (about 700 K)• Radicals: O, H, OH• Electronically excited species (N2*, OH*,O*,…)• Electrons, and therefore ions: [e] ≅ [NO+] ≅ 1015 cm-3

What we think is important

• Three key characteristic times• T, period between pulses: 33 µs – 1 ms• τ, residence time in plasma region: ~1 ms• tSL time of flame propagation : 2.5 ms

• Location of discharge: must be in hot region (E/N higher) and/or recirculation region (τ higher).

• Need to understand mechanism of radical and ion formation + their effect on time of flame propagation

Conclusions• Repetitive nanosecond discharges are very efficient in reducing ignition delay time and improving the stability of lean/diluted premixed flames

•The power requirements are lower than with conventional spark plug techniques

•There is a clear need for detailed quantitative diagnostics of the species produced by the pulses

• Numerical simulations are in progress with 2-temperature chemistry and flame/plasma/flow coupling

combustion assisted by nanosecond repetitively...

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