This work presents the results of number density measurements of metastable Ar atoms and ground state H atoms in diluted mixtures of H 2 and O 2 with Ar, as well as ground state O atoms in diluted H 2-O 2-Ar, CH 4-O 2-Ar, C 3 H 8-O 2-Ar, and C 2 H 4-O 2-Ar mixtures excited by a repetitive nanosecond pulse discharge. The measurements have been made in a nanosecond pulse, double dielectric barrier discharge plasma sustained in a flow reactor between two plane electrodes encapsulated within dielectric material, at an initial temperature of 500 K and pressures ranging from 300 Torr to 700 Torr. Metastable Ar atom number density distribution in the afterglow is measured by tunable diode laser absorption spectroscopy, and used to characterize plasma uniformity. Temperature rise in the reacting flow is measured by Rayleigh scattering. H atom and O atom number densities are measured by two-photon absorption laser induced fluorescence. The results are compared with kinetic model predictions, showing good agreement, with the exception of extremely lean mixtures. O atoms and H atoms in the plasma are produced mainly during quenching of electronically excited Ar atoms generated by electron impact. In H 2-Ar and O 2-Ar mixtures, the atoms decay by three-body recombination. In H 2-O 2-Ar, CH 4-O 2-Ar, and C 3 H 8-O 2-Ar mixtures, O atoms decay in a reaction with OH, generated during H atom reaction with HO 2 , with the latter produced by three-body H atom recombination with O 2. The net process of O atom decay is O + H → OH, such that the decay rate is controlled by the amount of H atoms produced in the discharge. In extra lean mixtures of propane and ethylene with O 2-Ar the model underpredicts the O atom decay rate. At these conditions, when fuel is completely oxidized by the end of the discharge burst, the net process of O atom decay, O + O → O 2 , becomes nearly independent of H atom number density. Lack of agreement with the data at these conditions is likely due to diffusion of H atoms from the partially oxidized regions near the side walls of the reactor into the plasma. Although significant fractions of hydrogen and hydrocarbon fuels are oxidized by O atoms produced in the plasma, chain branching remains a minor effect at these relatively low temperature conditions.
Plasma-assisted oxidation of atmospheric pressure hydrogen-and hydrocarbon-oxygen mixtures diluted in argon is analyzed by kinetic modeling, over a wide range of temperatures. In the experiments, preheated reactant mixtures are excited by a repetitively pulsed, double dielectric barrier ns discharge in plane-to-plane geometry, at near-isothermal conditions. Plasma images and temperature distributions in the discharge indicate that the reactant flow is nearly uniform, justifying the use of quasi-0D approximation in kinetic modeling. The kinetic model is based on a plasma chemistry mechanism combined with a conventional combustion reaction mechanism. The model does not contain adjustable parameters such as reduced electric field in the plasma, used in a number of previous 0D modeling studies. Comparison of the modeling predictions with the experimental data for hydrogen and methane oxidation exhibits good agreement between measured and predicted fuel concentrations over a wide range of temperatures, showing that the yield of primary radicals generated in the plasma is predicted accurately. Concentrations of intermediate hydrocarbon species predicted by the model are also in good agreement with the experiments, with the exception of acetylene below the hot ignition point. For ethylene and propane oxidation, the model overpredicts fuel consumption at low temperatures, also overpredicting concentration of CO, the dominant oxidation product, and underpredicting acetaldehyde concentration. This indicates that low-temperature pathways of formaldehyde formation, a major precursor for CO, as well as low-temperature reactions of several radicals which are precursors for formaldehyde and acetaldehyde, are not represented accurately in both conventional reaction mechanisms used. Although concentrations of intermediate hydrocarbon species in ethylene and propane are predicted relatively well, kinetics of formation and decay of acetylene remains not understood. This may be due to inaccurate branching ratio for dissociative quenching of metastable argon by heavy hydrocarbon species, as well as deficiencies of the conventional reaction mechanisms.
This work presents results of time-resolved, absolute measurements of OH number density, nitrogen vibrational temperature, and translational-rotational temperature in air and lean hydrogen-air mixtures excited by a diffuse filament nanosecond pulse discharge, at a pressure of 100 Torr and high specific energy loading. The main objective of these measurements is to study kinetics of OH radicals at the conditions of strong vibrational excitation of nitrogen, below autoignition temperature. N2 vibrational temperature and gas temperature in the discharge and the afterglow are measured by ns broadband coherent anti-Stokes Raman scattering. Hydroxyl radical number density is measured by laser induced fluorescence, calibrated by Rayleigh scattering. The results show that the discharge generates strong vibrational nonequilibrium in air and H2-air mixtures for delay times after the discharge pulse of up to ~1 ms, with a peak vibrational temperature of Tv ≈ 1900 K at T ≈ 500 K. Nitrogen vibrational temperature peaks at 100–200 µs after the discharge pulse, before decreasing due to vibrational-translational relaxation by O atoms (on the time scale of several hundred µs) and diffusion (on ms time scale). OH number density increases gradually after the discharge pulse, peaking at t ~ 100–300 µs and decaying on a longer time scale, until t ~ 1 ms. Both OH rise time and decay time decrease as H2 fraction in the mixture is increased from 1% to 5%. Comparison of the experimental data with kinetic modeling predictions shows that OH kinetics is controlled primarily by reactions of H2 and O2 with O and H atoms generated during the discharge. At the present conditions, OH number density is not affected by N2 vibrational excitation directly, i.e. via vibrational energy transfer to HO2. The effect of a reaction between vibrationally excited H2 and O atoms on OH kinetics is also shown to be insignificant. As the discharge pulse coupled energy is increased, the model predicts transient OH number density overshoot due to the temperature rise caused by N2 vibrational relaxation by O atoms, which may well be a dominant effect in discharges with specific energy loading.
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