1. Nonequilibrium Thermodynamics Laboratories The Ohio State University OH Laser-Induced Fluorescence Measurements in Nanosecond Pulse Discharge Plasmas Inchul Choi, Igor V. Adamovich and Walter R. Lempert Michael A. Chaszeyka Non-Equilibrium Thermodynamics Laboratories Department of Mechanical Engineering The Ohio State University, Columbus, OH 43210 65 th INTERNATIONAL SYMPOSIUM ON MOLECULAR SPECTROSCOPY Columbus, OH, 21-25 June, 2010

2. Nonequilibrium Thermodynamics Laboratories The Ohio State University Background: Nonequilibrium Plasma Assisted Combustion (PAC) Electron energy (1~10 eV) of Nonequilibrium plasma → Favorable chemistry (e.g. active radical generation) Combustor for high speed flow Supersonic airflow Must occur at few msec efficiently KIMM Arvinmeritor Fuel reforming NOx reduction Combustion chemistry NASA

3. Nonequilibrium Thermodynamics Laboratories The Ohio State University Path to Understanding Nonequilibrium Plasma Ignition Kinetics Predictive modeling of energy release rate and ignition delay time in low- temperature, repetitive nanosecond pulse fuel-air plasmas requires: Knowledge of reduced electric field (E/N) in the plasma, individual pulse energy coupled to the plasma, and their scaling with pressure, temperature, pulse waveform, and mixture composition Air plasma and fuel-air plasma chemistry: reactions among ground state species, excited species and radicals generated in the plasma, and their effect on energy release rate Approach: Predict plasma E/N and coupled pulse energy using nsec pulse plasma / sheath model Incorporate results into fuel-air plasma chemistry model Compare model predictions with experiments (CARS, TALIF, LIF)

4. Nonequilibrium Thermodynamics Laboratories The Ohio State University Repetitive Nanosecond Pulse Discharge (plane copper electrodes mounted outside a quartz channel) Flow / discharge geometry: Quartz channel 220 mm long, 22 x 10 mm cross section / 14 x 65 mm electrode plates Flow velocity ~1 m/sec, residence time in discharge ~50 msec (w/o plasma) Pulse burst: 20 kV, 25 ns pulses at 40 kHz (25 µsec between pulses) Pre-ionization: UV lamp (to achieve breakdown on the first pulse) Application: studies of C x H y /air and H 2 /air ignition kinetics by a low-temperature plasma 310 nm filter HV Pulser

5. Nonequilibrium Thermodynamics Laboratories The Ohio State University Typical Voltage Pulse Profile HV Pulser Specification - 24 kV peak voltage of negative polarity - Frequency: 10-50 kHz - Pulse Duration: ~25 nsec - Pulse Energy: ~1 mJ/pulse Typical single pulse voltage waveform in air and in H 2 -air at P=40 torr.

6. Nonequilibrium Thermodynamics Laboratories The Ohio State University Burst Mode Operation 10 Hz, 100 ms Time between pulses 40 kHz, 25 ms 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 ms spacing. Burst is repeated at 10 Hz to match laser repetition rate. Fresh sample of gas with every burst.

7. Nonequilibrium Thermodynamics Laboratories The Ohio State University Experimental setup for OH LIF Excitation: X 2 Π→A 2 Σ + (1, 0) Detection: A 2 Σ + → X 2 Π (1, 1) Linear Regime, <10μJ

8. Nonequilibrium Thermodynamics Laboratories The Ohio State University Absolute OH Number Density Measurements using Adiabatic Hencken Flat-Flame Burner C2H4C2H4 N2N2 Nitrogen flow Measurement Location – 10mm Above Surface (Lucht et al. 1997) Absolute concentration data highly desirable for validating kinetic model Equilibrium in [OH]

9. Nonequilibrium Thermodynamics Laboratories The Ohio State University Adiabatic Hencken Calibration Burner: Boltzmann Plot for Temperature NASA-Glenn Chemical Equilibrium Code prediction: T=2337 K, OH mole fractions = 0.00464 Q(1) Q(2) Q(3) Q(4) Q(5) C 2 H 4 -Air, Ф=0.95 Wavelength, nm Relative Intensity

10. Nonequilibrium Thermodynamics Laboratories The Ohio State University Typical LIF time dependence: Burst Mode Discharge (40 kHz, 600 pulses) H 2 -Air, Ф=1.0 P=40 Torr

11. Nonequilibrium Thermodynamics Laboratories The Ohio State University OH LIF Measurements: Single Pulse Discharge (H 2 -Air, P=40 Torr, Ф=1.0) H + O 2 + M → HO 2 + M(1) O + HO 2 → OH + O 2 (2) OH + H 2 → H + H 2 O(3) Temporal evolution of absolute OH concentration for single pulse discharge [OH] peaks at approximately at 1.8x10 13 cm -3

12. Nonequilibrium Thermodynamics Laboratories The Ohio State University OH LIF Measurements: Burst Mode Discharge (40 kHz, P=40 Torr) Temporal evolution of absolute OH concentration for burst mode discharge [OH] rising rapidly during ~10 pulses, steady-state after 20 pulses, and rising again after 300 pulses (after ~20 msec, ignition)

13. Nonequilibrium Thermodynamics Laboratories The Ohio State University Summary OH LIF measurements were performed in hydrogen-air nanosecond pulsed discharges at single pulse and burst mode conditions: ~24 kV peak voltage, ~25 nsec FWHM, ~1.28 mJ coupled to the load. Absolute OH concentration obtained by near-adiabatic non-premixed Hencken flat flame burner. For time-resolved OH evolution, Hydrogen-air plasma chemistry model (with nanosecond pulse energy model prediction) was compared with experimental results with good agreements.

14. Nonequilibrium Thermodynamics Laboratories The Ohio State University Acknowledgments U.S. Air Force Office of Scientific Research (Julian Tishkoff, technical monitor) NASA Glenn Research Center (Aaron Auslender, Isaiah Blankson, technical monitor) National Science Foundation

15. Nonequilibrium Thermodynamics Laboratories The Ohio State University Quenching Rate Coefficients: Burst Mode Discharge (40 kHz, P=40 Torr)