1. Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory Absolute OH Number Density Measurements in Lean Fuel-Air Mixtures Excited by a Repetitively Pulsed Nanosecond Discharge Zhiyao Yin, Aaron Montello, Walter R. Lempert, and Igor V. Adamovich International Symposium on Molecular Spectroscopy 68 th meeting Michael A. Chaszeyka Nonequilibrium Thermodynamics Laboratories, Department of Mechanical and Aerospace Engineering, The Ohio State University, Columbus, OH 43210

2. Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory Plasma Assisted Ignition High-voltage, nanosecond pulse discharge assisted ignition ■ T electron ~1-10 eV ■ T gas ~ initial temperature ■ Non-thermal ignition: radical species generation Energy branching in an applied field RotationalVibrationalElectronic Electron energy (eV)~0.030.2-23-10 E/N (Td)<14-110120-1000 Electron impact excitation and dissociation Spark ignition ■ T electron ~5000-10000 K ■ T gas ~1000-10000 K ■ Thermal ignition Heating

3. Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory Nonequilibrium Plasma Assisted Ignition Above self-ignition threshold Shock-preheated: T>1000 K, P~0.5 bar, single discharge pulse Kosarev et al, Combust. Flame, 156:221-233, 2009 Kinetic modeling (plasma chemistry + combustion chemistry) reproduces experiments well Radicals produced by the discharge trigger a more rapid fuel decomposition and chain reactions Ignition at lower initial T Without discharge With discharge Drastic reduction in ignition delay

4. Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory Nonequilibrium Plasma Assisted Oxidation Below self-ignition threshold Uddi et al, J. Phys. D: Appl. Phys., 42:075205, 2009 Lean CH 4 -air, P=60 torr, T=300 K, a single discharge pulse Baseline model (plasma chemistry + combustion chemistry) cannot reproduce experiments New reaction channels with N 2 (v) and O 2 (b 1 Σ) have been proposed; but rates are not well-known NO concentration Well-known combustion mechanisms: ■ GRI 3.0 (for CH 4 ): 1000-2500 K ■ Konnov (for H 2 ): 950-2700 K ■ Konnov (for C1-C4): >910 K Applicability to low-moderate T?

5. Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory Objectives Kinetic studies of plasma assisted fuel oxidation at low–moderate temperatures ■ Temperature and Hydroxyl radical (OH) concentration measurements ■ Assessment of different conventional combustion mechanisms by kinetic modeling

6. Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory Plasma flow reactor (T 0 =500 K, P=100 torr) Experimental Setup Peak voltage ~30 kV, pulse duration ~10 ns, pulse repetition rate at 10 kHz Measured coupled pulse energy ~1.5-2 mJ Photograph Laser beam

7. Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory LIF Experimental Setup Excitation transitions in OH A 2 Σ + ← X 2 Π (v ’ =1, v ’’ =0) and (v ’ =0, v ’’ =0) bands Calibration with Rayleigh scattering at 308 nm for inferring absolute OH number density

8. Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory Experimental Setup N 2 CARS Pump/Probe: 532 nm; Stokes: centered at near 604 nm, FWHM~5-6 nm Spectral resolution: ~0.4 cm -1, partially resolve the rotational structure in the Q-branch of N 2

9. Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory Plasma Uniformity Exclude thermal heating effect from hot filaments 0-D kinetic model Single-shot ICCD imaging of broad band plasma emission (T 0 =500 K) Imaging 1 st pulse 10 th pulse 100 th pulse Air P=200 torr H 2 -air ϕ =0.3 P=100 torr C 2 H 4 -air ϕ =0.3 P=100 torr Through the Brewster window 50 nsec camera gate electrodes Imaging Through the right angle prism 50 nsec camera gate, ϕ =0.3 electrodes

10. Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory Characterization of the Discharge Afterglow Decouple conventional fuel chemistry from reactions involving plasma generated excited species Discharge operation 10 kHz, 50 pulses Afterglow Electronically excited species, mainly N 2 (C 3 Π), T 0 =500 K Through the right angle prism 490-ps camera gate Broadband plasma emission during a single discharge pulse Vibrational non-equilibrium in the afterglow, T 0 =500 K In air only, T v (N 2 ) ~850 K after 50 pulses In fuel-air, T v (N 2 ) ~600- 700 K Measured gas temperature after 50 pulses, T g ~570 K End of discharge burst

11. Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory Temperature after the discharge burst OH LIF thermometry N 2 CARS Comparison (T 0 =500 K, P=100 torr, 10 kHz, 50 pulses) End of discharge burst 2µs after the end of discharge burst

12. Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory Calibration using Rayleigh scattering lΩβ: Optical collection constant Rayleigh scattering signal: The absolute OH number density is: D Rayleigh Spectrally-integrated LIF signal N2N2 Absolute OH number density after the discharge burst H 2 -air (T 0 =500 K, P=100 torr, 10 kHz, 50 pulses) Symbols: Expt.; Lines: Model Popov H 2 -O 2 mechanism Konnov H 2 -O 2 mechanism Modeling

13. Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory Absolute OH number density after the discharge burst CH 4 -air (T 0 =500 K, P=100 torr, 10 kHz, 50 pulses) Symbols: Expt.; Lines: Model USC/Wang mechanism Better agreement with the Konnov mechanism Konnov mechanism

14. Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory C 2 H 4 -air (T 0 =500 K, P=100 torr, 10 kHz, 50 pulses) Symbols: Expt.; Lines: Model Absolute OH number density after the discharge burst USC/Wang mechanism Better agreement with the Konnov mechanism Konnov mechanism

15. Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory Absolute OH number density after the discharge burst C 3 H 8 -air (T 0 =500 K, P=100 torr, 10 kHz, 50 pulses) Symbols: Expt.; Lines: Model USC/Wang mechanismKonnov mechanism Neither mechanism reproduces the experiments

16. Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory Summary Discharge uniformity is verified by ICCD imaging of plasma emission Time-resolved [OH] is measured in lean H 2 -air, CH 4 -air, C 2 H 4 -air, and C 3 H 8 -air at T 0 =500 K and P=100 torr, after 50-pulse discharge burst Konnov’s mechs show better overall agreement for H 2 -, CH 4 -, and C 2 H 4 -air, compared to Popov’s and USC mechs For C 3 H 8 -air, neither of the mechs reproduces the experiments Needed: an accurate, predictive low-T plasma chemistry / fuel chemistry kinetic model applicable to high-C fuels (C3 or higher)

17. Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory Acknowledgement This work is supported by The U.S. Air Force Office of Scientific Research MURI “Fundamental Aspects of Plasma Assisted Combustion” Chiping Li – Technical Monitor Thanks for your attention Questions?

18. Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory Appendix I: Plasma Uniformity Exclude thermal heating effect from hot filaments 0-D kinetic model Averaged PLIF imaging 50-shot on-CCD accumulation, 100-ns camera gate C 2 H 4 -air, ϕ =0.1 T 0 =500 K, P=100 torr Imaging electrodes PlasmaRelative OH concentrationTemperature T 0 =300 K, P=60 torr Non-uniform plasma Two-line thermometry, with P 1 (1.5) and Q 1 (4.5) transitions in OH A 2 Σ + ← X 2 Π (v ’ =1, v ’’ =0)

19. Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory Appendix II: LIF Excitation Scans Data Processing: LIF excitation scan spectrum across 14 major transitions in the R-branch in the OH A-X (0,0) Step I Laser was scanned across R 1 (4.5) and Q 21 (4.5) transitions in OH A-X (0,0), at y=0 mm Correct LIF spectrum with laser absorption ~f(v L ) R 1 (4.5) Step II R 1 (4.5) Step III The dashed lines are individual transitions extracted from the fit Integrate each fitted transition individually over a large spectral range

20. Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratory Appendix III: Kinetic Modeling Plasma Chemistry Air Hydrogen Hydrocarbons A1N 2 + e - = N 2 (A 3 Σ, B 3 Π, C 3 Π, a' 1 Σ) + e - A2N 2 + e - = N( 4 S) + N( 4 S) + e - A3O 2 + e - = O( 3 P) + O( 3 P, 1 D) + e - A4N 2 (C 3 Π) + O 2 = N 2 (B 3 Π ) + O 2 A5N 2 (a' 1 Σ) + O 2 = N 2 (B 3 Π) + O 2 A6N 2 (B 3 Π) + O 2 = N 2 (A 3 Σ) + O 2 A7N 2 (A 3 Σ) + O 2 = N 2 + O + O H1H 2 + e - = H + H + e - H2N 2 (a' 1 Σ) + H 2 = N 2 + H + H H2N 2 (B 3 Π) + H 2 = N 2 (A 3 Σ) + H 2 H4N 2 (A 3 Σ) + H 2 = N 2 + H + H H5O( 1 D) + H 2 = H + OH M1CH 4 + e - = CH 3 + H + e - M2N 2 (A 3 Σ) + CH 4 = N 2 + CH 3 + H M3N 2 (B 3 Π) + CH 4 = N 2 + CH 3 + H M4N 2 (C 3 Π) + CH 4 = N 2 + CH 3 + H M5N 2 (a' 1 Σ) + CH 4 = N 2 + CH 3 + H E1C 2 H 4 + e- = products 3 + e- E2N 2 (A 3 Σ) + C 2 H 4 = N 2 + C 2 H 3 + H E3N 2 (B 3 Π) + C 2 H 4 = N 2 + C 2 H 3 + H E4N 2 (C 3 Π) + C 2 H 4 = N 2 + C 2 H 3 + H E5N 2 (a' 1 Σ) + C 2 H 4 = N 2 + C 2 H 3 + H P1N 2 (A 3 Σ) + C 3 H 8 = N 2 + C 3 H 6 + H 2 P2N 2 (B 3 Π) + C 3 H 8 = N 2 + C 3 H 6 + H 2 P3N 2 (C 3 Π) + C 3 H 8 = N 2 + C 3 H 6 + H 2 P4N 2 (a' 1 Σ) + C 3 H 8 = N 2 + C 3 H 6 + H 2 H 2 -air ■ Popov’s Mechanism (Popov, 2008) ■ Konnov’s Mechanism (Konnov, 2008) *only dominant processes shown here Combustion Chemistry Hydrocarbon (HC)-air ■ GRI-Mech 3.0 ■ USC Mech II ■ Konnov 0.5