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The Ohio State University Nonequilibrium Thermodynamics Laboratory
Pure Rotational CARS Thermometry in Nanosecond Pulse Burst Air and
Hydrogen-Air Plasmas
Yvette Zuzeek, Sherrie Bowman, Inchul Choi, Igor V. Adamovich and Walter R. Lempert
th International Symposium on Molecular SpectroscopyThe Ohio State University – June 21-25, 2009
The Ohio State University Nonequilibrium Thermodynamics Laboratory
Some Examples of Why We Study Plasma Assisted Combustion
• High speed propulsion systems (ignition and flame holding)
• NOx reduction• Interest in using the ability of a nonequilibrium
plasma to create radical species (O, H, OH, NO, etc.) at low temperature to accelerate combustion
• Improve understanding of low temperature combustion chemistry
The Ohio State University Nonequilibrium Thermodynamics Laboratory
Goals of This Study
• Perform detailed kinetic measurements of heat release and ignition
• Validate predictions of plasma kinetic model developed at OSU
• Develop insight into key PAC kinetic processes
The Ohio State University Nonequilibrium Thermodynamics Laboratory
Nanosecond Pulse Discharge Test Section and Voltage Profile
0 5 0 1 0 0 1 5 0 2 0 0 2 5 0
-2 0
-1 0
0
1 0
2 0
T im e , n sec
V o ltag e , k V
A ir, P = 4 0 to rr
g ro u n d e d
flo a tin g
-Creates a pool of radicals and excited electronic species (O, H, N2(A, B, C)) -Translational/rotational temperature is ~300K-Allows us to study low temperature kinetics
Discharge Test Section-Electrode dimensions65 mm(l) x 14 mm(w)-Quartz cell dimensions220 mm(l) x 22 mm(w) x 10 mm(h)
The Ohio State University Nonequilibrium Thermodynamics Laboratory
Burst Mode Operation
10 Hz, 100 ms
Time between pulses40 kHz, 25 s with Chemical Physics Technologies Power Supply
Laser delay time after last pulse
time
Burst of pulses
Laser pulse
• Pulser produces a rapid “burst” of 2-1000 pulses with 25 s spacing.• Burst is repeated at 10 Hz to match laser repetition rate.• Fresh sample of gas with every burst.
The Ohio State University Nonequilibrium Thermodynamics Laboratory
Previous Work: Atomic Oxygen TALIF Measurements in Air and Air/ C2H4 - =0.5
1 .0 E -7 1 .0 E -6 1 .0 E -5 1 .0 E -4 1 .0 E -3 1 .0 E -2
0 .0 E + 0
1 .0 E -5
2 .0 E -5
3 .0 E -5
4 .0 E -5
5 .0 E -5
T im e , seco n d s
O a to m m o le frac tio n
A ir
A ir-e th y len e , = 0 .5 O atom creation
O2 + N2 (A,B,C) → O + O + N2 (X)
O atom decay
C2H4/ air is more rapid than in air by a factor ~ 100
O + C2H4 → products, k=4.9∙10-13 cm3/svs
O + O2 + M → O3 + M
Coupled Pulse Energy 0.76mJ/pulse
The Ohio State University Nonequilibrium Thermodynamics Laboratory
Previous Work: CARS Temperature Measurements in Air and Ethylene-Air Plasmas at 40Torr
0 5 1 0 1 5
0
1 0 0
2 0 0
3 0 0
4 0 0
5 0 0
6 0 0
7 0 0
8 0 0
T im e , m se c
T , 0C
P = 4 0 to rr , = 4 0 k H z
A ir
A ir, m o d e l
F u e l-a ir , = 0 .1
F u e l-a ir , = 0 .1 , m o d e l
F u e l-a ir , = 1 .0
F u e l-a ir , = 1 .0 , m o d e l
• Heat release in fuel mixture is greater than in air.• Initial heating rates in =0.1 and 1.0 are the same.• Heating rate diverges after about 5 ms.• For =1.0 model predicts achieving steady state temperature but data continues to rise. (Otherwise agreement is quantitative)
The Ohio State University Nonequilibrium Thermodynamics Laboratory
Stability of H2-Air Plasma at =1, P=40 Torr
• At 40 Torr plasma is uniform• Faint emission between the pulses is suggestive of ignition
Time, microseconds
20 30 40 50 60 70 80
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
pulses (40 kHz)2 microsecond plasma gate 18 microsecond flame gate
*Images taken by Sherrie Bowman and Inchul Choi
The Ohio State University Nonequilibrium Thermodynamics Laboratory
Pure Rotational Coherent Anti-Stokes Raman Spectroscopy (CARS)
• .~20 mJ/pulse per beam (centered at ~780 nm)
• .mJ/pulse (532 nm)• Collection
– ½ meter spectrometer– 1800 grooves/mm grating– ICCD camera
• Burst and laser repetition rate is 10 Hz
• ICARS ~ Iw1a*Iw1b*Iw2*N2
• Signal has resonant and NR contributions
ω1a
ω2
ω1b
ωCARS
ω1a ω1b ω2 ωCARS
Virtual states
Erotation
The Ohio State University Nonequilibrium Thermodynamics Laboratory
Pure Rotational CARS Apparatus
f=500mm f=500mm
Beam dump Periscopeof 532nmmirrors
780nm mirror
780nm 50/50 splitter
Dichroic mirror
532nmPolarizer
780nm waveplate
780nm horizontal
780nm horizontal
780nm vertical
532nm horizontal
Horizontal beams
Vertical beams
f=100mm
Short pass filter
Signal is nowhorizontal
Nd:YAG
Nd:YAGBroadbandTitanium:Sapphire
Spectrometercamera
f=400mm
f=-200mm
The Ohio State University Nonequilibrium Thermodynamics Laboratory
Ti:Sapphire Laser
OutputCoupler
HighReflector
532 nm mirror lens Output coupler or High reflector
Nd:YAG
The Ohio State University Nonequilibrium Thermodynamics Laboratory
Ti:Sapphire Spectral Output and Intensity Squared Correction Factor
100 Shot Average
Wavenumbers
12400 12600 12800 13000 13200
No
rma
lize
d I
nte
ns
ity
0.0
0.2
0.4
0.6
0.8
1.0
1.2Beam Contribution
Raman Shift (wavenumbers)
0 200 400 600 800
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Single Shot
Wavenumbers
12200 12400 12600 12800 13000
No
rmali
ze
d I
nte
ns
ity
0.0
0.2
0.4
0.6
0.8
1.0
1.210 Shot Average
Wavenumbers
12600 12800 13000 13200
No
rmalized
In
ten
sit
y
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1a
1b
The Ohio State University Nonequilibrium Thermodynamics Laboratory
Air vs. H2-Air1) CARS Spectra After 400 Pulses, P=40Torr
Raman Shift (cm-1)
50 100 150 200 250
Nor
mal
ized
Int
ensi
ty
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Hydrogen-AirAir
NitrogenJ=13 J=15 J=17
5 accumulations of 600 laser shots
The Ohio State University Nonequilibrium Thermodynamics Laboratory
Experimental and Synthetic CARS Spectra
Raman Shift (cm-1)
60 80 100 120 140 160 180 200
Inte
nsity
(au
)
5
10
15
20
25 DataTheory (T=740K)
The Ohio State University Nonequilibrium Thermodynamics Laboratory
Hydrogen-Air Plasma Chemistry Model
N2 + e- → N2(A 333a’1e- H2 + e- → H + H + e-
O2 + e- → O(3P) + O(3P, 1D) + e- N2(a’ 12 → N2 + H + H
N2(C 32 → N2(B 32 N2(B 32 → N2(A 32
N2(a’ 12 → N2(B 32 N2(A 32 → N2 + H + H
N2(B 32 → N2(A 32 O(1D) + H2 → H + OH
N2(A 32 → N2 + O + O
• First step →model chemistry during 25 ns discharge – two-term expansion Boltzmann equation for plasma electrons and electron
impact cross sections• Second step → model subsequent chemistry
– Air reactions used• ground state neutral species (N, N2, O, O2, O3, NO, NO2, N2,O)• excited species (N2(A 3Σ), N2(B 3Π), N2(C 3Π), N2(a‘ 1Σ), O(1D))• electrons in the plasma (Kossyi, 1992)
– H2-air chemistry: 22 reactions of H, O, OH, HO2, H2O, H2O2 (Popov, 2008) • Quasi-1-D conduction heat transfer to the walls• Dominant radical species generation in the plasma
The Ohio State University Nonequilibrium Thermodynamics Laboratory
Summary of Temperature Measurements in Air and H2-Air at P=40 Torr, =40 kHz
0 5 1 0 1 5 2 0 2 5 3 0
3 0 0
6 0 0
9 0 0
1 2 0 0
0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0
T im e , m sec
N u m b e r o f p u lse s
T , K
P = 4 0 to rr
= 0 (a ir)
= 0 .0 5
0 5 1 0 1 5 2 0 2 5 3 0
3 0 0
6 0 0
9 0 0
1 2 0 0
0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0
T im e , m sec
N u m b er o f p u lse s
T , K
P = 4 0 to rr
= 0 .5
= 1
-Air only-H2-Air =0.05
-H2-Air =0.5-H2-Air =1
Unlike eth-air, initial heating rate is somewhat dependent on equivalence ratio.0.05 is close to air.Heating rate is higher with more fuel (and Model predictions and experimental results show a maximum in temperature.
The Ohio State University Nonequilibrium Thermodynamics Laboratory
Oxygen and Hydrogen Mole Fractions
• H2-air =1, P=40Torr after 400 pulses
• Oxygen mole fraction – inferred from CARS
synthetic spectra (Sandia CARS Code)
– predicted by plasma kinetic model
• Hydrogen mole fraction– predicted by plasma
kinetic model
0 5 1 0 1 5 2 0 2 5 3 0
0 .0
0 .1
0 .2
0 .3
T im e , sec
M o le frac tio n s
P = 4 0 to rr , = 1
O 2 , C A R S sp e c tra
O 2 , m o d e l
H 2 , m o d e l
Time, ms
The Ohio State University Nonequilibrium Thermodynamics Laboratory
H2-Air CARS Conclusions
• CARS data and model predictions are in good agreement.
• Indications of Ignition– Maximum temperature in experiment and
code predictions at =0.5 and 1.0– Rapid reduction in oxygen mole fraction
near peak
The Ohio State University Nonequilibrium Thermodynamics Laboratory
Sensitivity Analysis: O and H Reactions in Full and Reduced Kinetic Model
The Ohio State University Nonequilibrium Thermodynamics Laboratory
H + O2 + M ↔ HO2 + MOH + H2 ↔ H + H2OO + H2 ↔ H + OH H + O2 ↔ O + OH
H2 + O2 ↔ OH + OH OH + O2 ↔ O + HO2
OH + HO2 ↔ H2O + O2
H + HO2 ↔ H2O + OOH + OH ↔ H2O + O
H + OH + M ↔ H2O + MH + H + M ↔ H2 + M
H + HO2 ↔ OH + OH O + O + M ↔ O2 + MOH + M ↔ H + O + M
H2 + O2 ↔ H + HO2
HO2 + H2 ↔ OH + H2OHO2 + HO2 ↔ H2O2 + O2
OH + OH + M ↔ H2O2 + MOH + H2O2 ↔ HO2 + H2O
H + H2O2 ↔ HO2 + H2
H + H2O2 ↔ OH + H2OO + H2O2 ↔ OH + HO2
The Ohio State University Nonequilibrium Thermodynamics Laboratory
Burst Mode Comparison for Full and Reduced Reaction Sets (H2-air =1, P=40 Torr)
0 5 1 0 1 5 2 0 2 5 3 0
3 0 0
6 0 0
9 0 0
1 2 0 0
0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0
T im e , m sec
N u m b e r o f p u lse s
T , K
H 2 -a ir , P = 4 0 to rr
= 1
m o d e l, fu ll se t
m o d e l, re d u c e d se t
Temperature during burst mode ns pulse discharge (40 kHz pulse rep rate) predicted by plasma chemical kinetic model for full and reduced reaction sets
0 5 1 0 1 5 2 0 2 5 3 0
1 .0 E -6
1 .0 E -5
1 .0 E -4
1 .0 E -3
1 .0 E -2
1 .0 E -1
1 .0 E + 0
T im e , sec
S p ec ie s m o le frac tio n s
H 2
O
H 2O
O H
H
H O 2
0 5 1 0 1 5 2 0 2 5 3 0
1 .0 E -6
1 .0 E -5
1 .0 E -4
1 .0 E -3
1 .0 E -2
1 .0 E -1
1 .0 E + 0
T im e , sec
S p ec ie s m o le frac tio n s
H 2
O
H 2 O
O H
H
H O 2
Full Reaction Set Reduced Reaction Set
The Ohio State University Nonequilibrium Thermodynamics Laboratory
Effect of Discharge O and H Generation (H2-air =1, P=40 Torr)
• Reduced reaction set with and without plasma chemical processes of O and H atom generation
• Ignition is NOT predicted without
0 5 1 0 1 5 2 0 2 5 3 0 3 5
3 0 0
6 0 0
9 0 0
1 2 0 0
0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0
T im e , m sec
N u m b er o f p u lses
T , K
= 1
fu ll se t
n o O an d H g e n e ra tio nb y p la sm a
The Ohio State University Nonequilibrium Thermodynamics Laboratory
Conclusions/Future Work
• Conclusions:– Diffuse volumetric ignition in uniform low temperature plasma.– Good agreement between pure rotational CARS temperature
measurements and the model with no adjustable parameters.– Reduced low temperature chemistry model identified.
• Future Work:– H2-air temperature measurements at different locations within the
discharge and at different pressures.– H2 vibrational temperature measurements using ps CARS.– Measurement of additional species such as H atom (TALIF), OH
(LIF), and HO2 (CRDS) to study the influence of plasmas on the following chain branching and termination processes:
• H + O2 → OH + H• H + O2 + M → HO2 + M
The Ohio State University Nonequilibrium Thermodynamics Laboratory
Acknowledgements
• Air Force Office of Scientific Research– Julian Tishkoff – Technical Monitor
• National Science Foundation– Phil Westmoreland – Technical Monitor