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 Spectroscopy The 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) NO x 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 -Creates a pool of radicals and excited electronic species (O, H, N 2 (A, B, C)) -Translational/rotational temperature is ~300K -Allows us to study low temperature kinetics Discharge Test Section -Electrode dimensions 65 mm(l) x 14 mm(w) -Quartz cell dimensions 220 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 pulses 40 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 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/ C 2 H 4 -  =0.5 O atom creation O 2 + N 2 (A,B,C) → O + O + N 2 (X) O atom decay C 2 H 4 / air is more rapid than in air by a factor ~ 100 O + C 2 H 4 → products, k=4.9∙ cm 3 /s vs O + O 2 + M → O 3 + 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 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 H 2 -Air Plasma at  =1, P=40 Torr At 40 Torr plasma is uniform Faint emission between the pulses is suggestive of ignition *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 I CARS ~ I w1a *I w1b *I w2 *N 2 Signal has resonant and NR contributions ω 1a ω2ω2 ω 1b ω CARS ω 1a ω 1b ω 2 ω CARS Virtual states  E rotation

The Ohio State University Nonequilibrium Thermodynamics Laboratory Pure Rotational CARS Apparatus f=500mm Beam dump Periscope of 532nm mirrors 780nm mirror 780nm 50/50 splitter Dichroic mirror 532nm Polarizer 780nm waveplate 780nm horizontal 780nm vertical 532nm horizontal Horizontal beams Vertical beams f=100mm Short pass filter Signal is now horizontal Nd:YAG Broadband Titanium:Sapphire Spectrometercamera f=400mm f=-200mm

The Ohio State University Nonequilibrium Thermodynamics Laboratory Ti:Sapphire Laser Output Coupler High Reflector 532 nm mirrorlens Output coupler or High reflector Nd:YAG

The Ohio State University Nonequilibrium Thermodynamics Laboratory Ti:Sapphire Spectral Output and Intensity Squared Correction Factor  1a  1b

The Ohio State University Nonequilibrium Thermodynamics Laboratory Air vs. H 2 -Air  1) CARS Spectra After 400 Pulses, P=40Torr 5 accumulations of 600 laser shots

The Ohio State University Nonequilibrium Thermodynamics Laboratory Experimental and Synthetic CARS Spectra

The Ohio State University Nonequilibrium Thermodynamics Laboratory Hydrogen-Air Plasma Chemistry Model N 2 + e - → N 2 (A 3  3  3  a’ 1  e - H 2 + e- → H + H + e- O 2 + e - → O( 3 P) + O( 3 P, 1 D) + e- N 2 (a’ 1  2 → N 2 + H + H N 2 (C 3  2 → N 2 (B 3  2 N 2 (B 3  2 → N 2 (A 3  2 N 2 (a’ 1  2 → N 2 (B 3  2 N 2 (A 3  2 → N 2 + H + H N 2 (B 3  2 → N 2 (A 3  2 O( 1 D) + H 2 → H + OH N 2 (A 3  2 → N 2 + 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, N 2, O, O 2, O 3, NO, NO 2, N 2,O) excited species (N 2 (A 3 Σ), N 2 (B 3 Π), N 2 (C 3 Π), N 2 (a‘ 1 Σ), O( 1 D)) electrons in the plasma (Kossyi, 1992) –H 2 -air chemistry: 22 reactions of H, O, OH, HO 2, H 2 O, H 2 O 2 (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 H 2 -Air at P=40 Torr,  =40 kHz -Air only -H 2 -Air  =0.05 -H 2 -Air  =0.5 -H 2 -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 H 2 -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 Time, ms

The Ohio State University Nonequilibrium Thermodynamics Laboratory H 2 -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 + O 2 + M ↔ HO 2 + M OH + H 2 ↔ H + H 2 O O + H 2 ↔ H + OH H + O 2 ↔ O + OH H 2 + O 2 ↔ OH + OH OH + O 2 ↔ O + HO 2 OH + HO 2 ↔ H 2 O + O 2 H + HO 2 ↔ H 2 O + O OH + OH ↔ H 2 O + O H + OH + M ↔ H 2 O + M H + H + M ↔ H 2 + M H + HO 2 ↔ OH + OH O + O + M ↔ O 2 + M OH + M ↔ H + O + M H 2 + O 2 ↔ H + HO 2 HO 2 + H 2 ↔ OH + H 2 O HO 2 + HO 2 ↔ H 2 O 2 + O 2 OH + OH + M ↔ H 2 O 2 + M OH + H 2 O 2 ↔ HO 2 + H 2 O H + H 2 O 2 ↔ HO 2 + H 2 H + H 2 O 2 ↔ OH + H 2 O O + H 2 O 2 ↔ OH + HO 2

The Ohio State University Nonequilibrium Thermodynamics Laboratory Burst Mode Comparison for Full and Reduced Reaction Sets (H 2 -air  =1, P=40 Torr) Temperature during burst mode ns pulse discharge (40 kHz pulse rep rate) predicted by plasma chemical kinetic model for full and reduced reaction sets Full Reaction Set Reduced Reaction Set

The Ohio State University Nonequilibrium Thermodynamics Laboratory Effect of Discharge O and H Generation (H 2 -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

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: –H 2 -air temperature measurements at different locations within the discharge and at different pressures. –H 2 vibrational temperature measurements using ps CARS. –Measurement of additional species such as H atom (TALIF), OH (LIF), and HO 2 (CRDS) to study the influence of plasmas on the following chain branching and termination processes: H + O 2 → OH + H H + O 2 + M → HO 2 + 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