The Ohio State University Nonequilibrium Thermodynamics Laboratory Pure Rotational CARS Thermometry in Nanosecond Pulse Burst Air and Hydrogen-Air Plasmas

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


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


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


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)


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.


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


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)


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


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


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


Ti:Sapphire Laser

OutputCoupler

HighReflector

532 nm mirror lens Output coupler or High reflector

Nd:YAG


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


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


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)


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


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.


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


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


Sensitivity Analysis: O and H Reactions in Full and Reduced Kinetic Model


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


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


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


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


Acknowledgements

• Air Force Office of Scientific Research– Julian Tishkoff – Technical Monitor

• National Science Foundation– Phil Westmoreland – Technical Monitor

Documents

The Ohio State University Nonequilibrium Thermodynamics Laboratory Pure Rotational CARS Thermometry in Nanosecond Pulse Burst Air and Hydrogen-Air Plasmas