Combustion Assisted by Nanosecond Repetitively...
Transcript of 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
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
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
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
Temperature measurements(from fit of N2 C-B rotational lines)
1 bar, air, 1.5 mm gap, 10-pulse train: 5 kV, 10 ns, 30 kHz
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
• 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
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
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)
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
Air Flow rate in m3/h
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
Air Flow rate in m3/h
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
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
Stable V-shaped flame Φ= 0.8
Spectra integrated over the 100 ns following the pulse
• Filamentary discharge produces H atoms (Hα Balmer line)
Time-resolved spectroscopy
Stable V-shaped flame Φ= 0.8
Filament in flame
Filament in air
Spectra integrated over the 100 ns following the pulse
• Filamentary discharge in flame produces H atoms (Hβ Balmer line)
Time-resolved spectroscopy
Stable V-shaped flame Φ= 0.8
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
Electron number density measurements
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 measurements
• 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