N2 Vibrational Temperature, Gas Temperature, 72nd International Symposium on Molecular Spectroscopy June 19-23, 2017 - Champaign-Urbana, Illinois Nonequilibrium Thermodynamics Laboratory N2 Vibrational Temperature, Gas Temperature, and OH Number Density Measurements in ns Pulse H2-air Plasmas Introduce people Caroline Winters, Yichen Hung, Elijah Jans, Kraig Frederickson, and Igor V. Adamovich

Motivation and Objective: Chemical Reactions in Fuel-Air Plasmas Radical species such as H, O, and OH are critical in low-temperature plasma-assisted combustion In fuel-air plasmas, a large fraction of discharge input energy goes to N2 vibrational excitation Question Is there coupling between N2 vibrational excitation and radical reaction kinetics? Hypothesis (Starikovskiy and Aleksandrov, 2013): N2(v=1) + HO2 → N2(v=0) + HO2(v2+v3) → N2 + H + O2 → N2 + OH + O Energy transfer from N2 breaks up HO2, extends H and OH lifetime. Dissociation and ionization dominate in (a), vibrational excitation in (b) Objectives: Measure [OH], T, and Tv (N2) in plasmas with strong N2 vibrational excitation Obtain insight into radical reaction kinetics at these conditions using kinetic modeling predictions

“Diffuse filament” discharge between hollow spherical electrodes Discharge Test Cell and Plasma Images Collinear CARS, axial optical access through hollow electrodes T0 =300 K, P=100 Torr Pulse repetition rate = 10-60 Hz Peak voltage = 10-11 kV Coupled Energy = 5-10 mJ/pulse Pulse duration = ~100 ns

Coherent Anti-Stokes Raman Scattering (CARS) Experimental Schematic Probe Length ~ 4 mm Nd: YAG pump/probe beam, 7 mJ/pulse, 0.015 cm-1 with injection seeding, 0.4 cm-1 without seeding Broadband dye laser (Stokes beam), 3.2 mJ/pulse, ~ 100 cm-1 FWHM Access to N2(v=0-3) levels Spectra taken 800 ns to 14 ms after discharge pulse

Typical N2 CARS Spectra ωprobe ωpump ωCARS ωStokes ωvib T=557±50K Trot-trans=292±12K Coherent Anti-Stokes Raman Spectroscopy T and TV(N2) Collinear phase matching CARS Probe length ~4 mm Plasma filament length ~8 mm ωp= ωprobe= 532 nm ωStokes= 607 nm ωCARS= 473 nm Sandia National Laboratory, CARSFIT

CARS T and Tv(N2) Results in Discharge Afterglow T0 =300 K, P=100 Torr, E~8 mJ/pulse Tv T Tv T Tv is much higher than T, indicating strong vibrational non-equilibirum Tv rise at t= 1-200 μs due to “downward” N2-N2 V-V exchange, decay at t=200 μs – 1 ms due to V-T relaxation of N2 by O atoms T increases from 400 K to 500 K at t = 400-800 μs After t > 1 ms, both T and Tv drop due to radial diffusion from the filament Modeling predictions are in fairly good agreement with experimental data Results in air and H2-air mixtures are similar

OH Laser Induced Fluorescence (LIF) Experimental Schematic 308 nm OH A→X (0,0) R2(3) 532 nm output of Nd:YAG laser pumps a dye laser, produces 615 nm output Frequency doubling by a BBO crystal, to 308 nm LIF quenching rate is measured directly Fluorescence is collected at 900, calibrated by Rayleigh scattering

[OH] LIF line images and signal distribution OH LIF Images and Signal Distribution 3% H2- Air: T0 =300 K, P=100 Torr, E~8 mJ/pulse t < 50 µs: OH formed near grounded electrode t = 50-300 µs: OH formed near both electrodes Effect more prominent at higher coupled energy t > 300 µs: nearly uniform distribution [OH] Centerline OH signal intensity distribution [H] Ground HV

Time resolved [OH] in plasma afterglow Absolute [OH] in Discharge Afterglow T0 =300 K, P=100 Torr, E= 8.0 mJ/pulse [OH] and Tv(N2) peaks coincide in time approximately Model predictions are in satisfactory agreement with experimental data However, [OH] peak is not caused by N2(v=1) + HO2 → N2(v=0) + HO2(v2+v3) reaction (adding it reduces [OH]) [OH] peak is not caused by temperature rise induced by N2 vibrational relaxation In the afterglow, modeling prediction shows [OH] follows [H] and [O]; peak may be caused by diffusion of H atoms from plasma regions with higher density. [OH] Tv T [H]

Summary Diffuse filament, ns pulse discharge sustains strong vibrational non- equilibrium in air and H2-air mixtures (up to Tv~2000 K, T~ 500 K) on time scales of up to ~1 ms CARS measurements of Tv(N2), T show that N2 vibrational excitation air and H2-air mixtures is similar [OH] in the afterglow follows [H] and [O]; Reaction N2(v=1) + HO2 → N2(v=0) + HO2(v2+v3), temperature rise do not affect [OH] kinetics Transient rise of [OH] may be caused by H atom transport from other regions in the plasma Laser H probe region

Thank you Any Questions?

Chemical kinetics of vibrationally excited N2 in air Time resolved “first” level N2 vibrational temperature and rotational-translational temperature in air “Rapid” heating: N2* quenching by O2 N2(A,B,C)+ O2  N2(X) + O + O + e [OH] Downward V-V energy transfer: N2(w) + N2(v=0)  N2(w-1) + N2(v=1) V-T relaxation by O atoms, “slow” heating: N2(v=1) + O  N2(v=0) + O Electron impact, electronic excited N2 Downward V-V energy transfer from 1-200 us, such that population of v=1 increases and Tv increases. V-T relaxation of v=1 N2 molecules. This drop of Tv at 200-800 us after the discharge pulse is not observed in pure N2 case. The results are consistent with modeling calculation. [H] Radial diffusion Air: T=300 K, P=100 Torr

Modeling Prediction of Radical Species in 3% H2-air Plasma Coupled Energy = 8 mJ/pulse