Bulletin of the American Physical Society
60th Gaseous Electronics Conference
Volume 52, Number 9
Tuesday–Friday, October 2–5, 2007; Arlington, Virginia
Session BT1: Plasma Combustion and Chemistry |
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Chair: Leanne Pitchford, LAPLACE Room: Doubletree Crystal City Crystal Ballroom A |
Tuesday, October 2, 2007 8:00AM - 8:30AM |
BT1.00001: Plasma-Assisted Flame Ignition and Stabilization using Nanosecond Repetitively Pulsed Discharges Invited Speaker: Ever more stringent environmental regulations are providing impetus for reducing pollutant emissions, in particular nitric oxides and soot, in internal combustion and aircraft engines. Lean or diluted combustible mixtures are of particular interest because they burn at lower flame temperatures than stoichiometric mixtures and thus produce lesser amounts of thermal nitric oxides. Over the past decade, high voltage nanosecond pulsed discharges have been demonstrated as energy efficient way to ignite such mixtures. However, the practical application of these discharges for ignition purposes is limited by the very high electric fields required, especially in high pressure combustion chambers. Moreover, stabilization requires a steady-state addition of energy that cannot be achieved with single or low repetition frequency pulses. In the present work, we investigate the applicability and effectiveness of high voltage nanosecond discharges with high pulse repetition frequencies, typically up to 100 kHz. The high repetition frequencies are chosen to exceed the recombination rate of chemically active species. In this way, the concentration of active species can build up between consecutive pulses, thus yielding significantly higher concentrations than with low frequency pulses. These discharges are investigated for two applications, the ignition of diluted air/propane mixtures at pressures up to several bars in a constant volume chamber, and the stabilization of atmospheric pressure lean premixed air/propane flames. Time-resolved electric and spectroscopic measurements are presented to analyze the discharge regimes, the energy deposition, the gas temperature evolution, the electron number density, and the production of excited species. The results show that nanosecond repetitive pulses produce ultrafast gas heating and atomic oxygen generation, both on nanosecond time scales, via excitation of molecular nitrogen followed by dissociative quenching of molecular oxygen. These effects result in a significant reduction of the lower flammability limit and in the subsequent extension of the domain of flame stability, for a power consumption typically less than 1{\%} of the heat released by the flame. [Preview Abstract] |
Tuesday, October 2, 2007 8:30AM - 8:45AM |
BT1.00002: Spatially and Temporally Resolved Atomic Oxygen Measurements in Short Pulse Discharges by Two Photon Laser Induced Fluorescence Walter Lempert, Mruthunjaya Uddi, Eugene Mintusov, Naibo Jiang, Igor Adamovich Two Photon Laser Induced Fluorescence (TALIF) is used to measure time-dependent absolute oxygen atom concentrations in O$_{2}$/He, O$_{2}$/N$_{2}$, and CH$_{4}$/air plasmas produced with a 20 nanosecond duration, 20 kV pulsed discharge at 10 Hz repetition rate. Xenon calibrated spectra show that a single discharge pulse creates initial oxygen dissociation fraction of $\sim $0.0005 for air like mixtures at 40-60 torr total pressure. Peak O atom concentration is a factor of approximately two lower in fuel lean ($\phi $=0.5) methane/air mixtures. In helium buffer, the initially formed atomic oxygen decays monotonically, with decay time consistent with formation of ozone. In all nitrogen containing mixtures, atomic oxygen concentrations are found to initially increase, for time scales on the order of 10-100 microseconds, due presumably to additional O$_{2}$ dissociation caused by collisions with electronically excited nitrogen. Further evidence of the role of metastable N$_{2}$ is demonstrated from time-dependent N$_{2}$ 2$^{nd}$ Positive and NO Gamma band emission spectroscopy. Comparisons with modeling predictions show qualitative, but not quantitative, agreement with the experimental data. [Preview Abstract] |
Tuesday, October 2, 2007 8:45AM - 9:00AM |
BT1.00003: Mechanisms of iodine atoms production by pulse discharge Anatoly Napartovich, Igor Kochetov, Nikolay Vagin, Nikolay Yuryshev Pulsed electric discharge is most effective to turn COIL operation into pulse mode by instant production of iodine atoms. Numerical model is developed for simulations of an electric discharge in a mixture of gas flow outgoing from the singlet oxygen generator (SOG) with CF$_{3}$I. Electron scattering cross sections from CF$_{3}$I molecules are analyzed to reproduce recently published swarm data for CF$_{3}$I and N$_{2}$ mixtures. The model comprises a system of kinetic equations for neutral and charged species, electric circuit equation, gas thermal balance equation, and the photon balance equation. Reaction rate coefficients for processes involving electrons are found by solving the electron Boltzmann equation, which is re-calculated in a course of computations when plasma parameters changed. The processes accounted for in the Boltzmann equation include excitation, dissociation and ionization of atoms and molecules, electron-ion recombination, electron-electron collisions, second-kind collisions, and stepwise excitation of molecules. The last processes are particularly important because of a high singlet oxygen concentration in gas flow from the SOG. Results of numerical simulations for conditions of the experiments are compared with results of measurements. [Preview Abstract] |
Tuesday, October 2, 2007 9:00AM - 9:15AM |
BT1.00004: Pulsed Nanosecond Discharge Development and Production of Active Particles Evgeny Mintoussov, Ainan Bao, Walter R. Lempert, Igor V. Adamovich Pulsed nanosecond discharges are being actively used for different engineering applications such as plasma-assisted ignition, plasma flow control, and gas dynamics lasers. The main advantages of using of this type of discharge are (i) efficient production of active particles, and (ii) sustaining uniform, volume filling plasmas at high pressures and power loadings. In the present work, development of a nanosecond pulse discharge (pulse amplitude up to 40 kV, pulse repetition rate up to 100 kHz, pulse duration of 4 ns) was studied at different pressures. Discharge parameters, such as fast ionization wave amplitude and velocity have been measured. Energy input into the flow was also determined. Active particle production in high-speed combustible flows (up to 100 m/s) was estimated by comparing heating of air-fuel flow and air flow in the discharge. The results suggest that additional heat release in air-fuel flows is due to plasma chemical fuel oxidation reactions, which at certain conditions leads to ignition. Kinetic model describing production of active particles in the discharge, subsequent plasma chemical reactions, and ignition process is developed. [Preview Abstract] |
Tuesday, October 2, 2007 9:15AM - 9:30AM |
BT1.00005: An improved description of the vibrational energy transfers in nitrogen discharges Vasco Guerra, M. Lino da Silva, S. Goci\'c, J. Loureiro The vibrational levels of ground-state N$_2(X)$ molecules are often the main energy reservoirs in nitrogen discharges and their post-discharges. As a consequence, they have a direct and crucial importance in the understanding of several fundamental phenomena occurring in nitrogen, such as dissociation, ionization, gas heating and the nitrogen pink afterglow. In recent years, nitrogen discharges have been modeled assuming the vibrational levels to be described by a Morse oscillator. Accordingly, the resulting number of bound vibrational states is 45. In this work we investigate how the vibrational energy distribution function of N$_2(X)$ molecules and the relaxation of vibrational energy are modified when a more realistic intra-molecular potential is used. To this purpose, the ground-state potential curve has been reconstructed with the RKR method and a total of 59 vibrational bound levels were obtained. The discharge and the afterglow were modeled by solving the electron Boltzmann equation, coupled with a system of rate-balance equations for the creation of the most important heavy- particles. The relevant rate coefficients for vibrational exchanges were obtained using the Forced Harmonic Oscillator theory. [Preview Abstract] |
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