OH A to X(0,0) spontaneous emission spectroscopy is used to confirm ignition. The presence of a "footprint" in the emission trace (indicating emission, and thus ignition, between the pulses) is observed in pressures from 50 to 100 Torr and in equivalence ratios from f=0.3 to 1.0. Ignition delay time, as defined as the onset of continuous OH emission between pulses, is found to be a strong function of pressure, but only a weak function of equivalence ratio.
Abstract: This dissertation presents novel results in the study of nanosecond pulsed, non-equilibrium plasmas. Specifically, an in-depth experimental study of the role of atomic oxygen on the kinetic mechanisms involved in three distinct discharge geometries was conducted. First, a low temperature (~300 K) and low pressure (
Abstract: In recent years, plasma assisted ignition and flame-holding in high speed flows has attracted considerable attention due to potential applications for turbojet engines and afterburners operating at high altitudes, as well as scramjet engines. Conventional methods of igniting a flow in the combustor using a spark or an arc discharge are known to be ineffective at low pressures and high flow velocities, since the ignition kernel is limited by a small volume of the spark or arc filament. Single photon LIF spectroscopy is used to study hydroxyl radical formation and loss kinetics in low temperature hydrogen-air repetitively pulsed nanosecond plasmas. Nanosecond pulsed plasmas are created in a rectangular cross section quartz channel / plasma flow reactor. Flow rates of hydrogen-air mixtures are controlled by mass flow controllers at a total pressure of 40-100 torr, initial temperature T0=300-500 K and a flow velocity of approximately u=0.1-0.8 m/sec. Two rectangular copper plate electrodes, rounded at the corners to reduce the electric field non-uniformity, are attached to the outside of the quartz channel. Repetitively pulsed plasmas are generated using a Chemical Physics Technologies (CPT) power supply which produces ~25 nanosecond pulses with ~20 kV peak voltage. Absolute hydroxyl radical mole fraction is determined as both a function of time after application of a single 25 nsec pulse, and 60 microseconds after the final pulse of a variable length "burst" of pulses. Relative LIF signal levels are put on an absolute mole fraction scale by means of calibration with a standard near-adiabatic Hencken flat flame burner at atmospheric pressure. By obtaining OH LIF data in both the plasma and the flame, and correcting for differences in the collisional quenching and Vibrational Energy Transfer (VET) rates, absolute OH mole fraction can be determined. For a single discharge pulse at 27 °C and 100 °C, the absolute OH temporal profile is found to rise rapidly during the initial ~0.1 msec after discharge initiation and decay relatively slowly, with a characteristic time scale of ~1 msec. In repetitive burst mode the absolute OH number density is observed to rise rapidly during the first approximately 10 pulses (0.25 msec), and then level off to a near steady-state plateau. In all cases a large secondary rise in OH number density is also observed, clearly indicative of ignition, with ignition delay equal to approximately 15, 10, and 5 msec, respectively, for initial temperatures of 27 °C, 100 °C, and 200 °C. Plasma kinetic modeling predictions capture this trend quantitatively.
The first research on plasma was done in connection with the study of electrical discharges in gases. The focus of attention for physicists was the partially ionized plasma, the kinetics of which is governed by various collisional and radiative processes. The choice of this area of research was motivated largely by the practical problems of that time the creation of gas-discharge light sources, rectifiers, and inverters. Since the early 1950s interest in plasma physics has risen sharply, particularly in the study of the completely ionized plasma with its various collective phenomena, insta bilities, and the interesting and sometimes unexpected effects attending the propagation of electromagnetic waves in such a plasma and the action on it of external electric and magnetic fields. Interest in hot plasmas has been stimulated not only by the diverse and novel physical phenomena, but also by the problems arising in connection with controlled nuclear fusion. The advent, in the early 1960s, of new technical fields such as gas-discharge lasers, magnetohydrodynamic generators, thermoemission converters, plasma chemistry, plasma propul sion devices, various methods in plasma technology, etc. , has led to increased interest in weakly ionized low-tempera ture plasmas. This is particularly true of nonequilibrium plasmas, which are characterized by an extraordinary diver sity of states and properties.
Abstract: The dissertation presents non-equilibrium chemical kinetic studies of large volume lean gaseous hydrocarbon/ air mixture combustion at temperatures (~300K) much below self ignition temperatures and low pressures (40-80torr), in ~25 nanosecond duration repetitive high voltage (~18kV) electric discharges running at 10 Hz. Xenon calibrated Two Photon Absorption Laser Induced Fluorescence (TALIF) is used to measure absolute atomic oxygen concentrations in air, methane-air, and ethylene-air non-equilibrium plasmas, as a function of time after initiation of a single 25 nsec discharge pulse at 10Hz. Oxygen atom densities are also measured after a burst of nanosecond discharges at a variety of delay times, the burst being run at 10Hz. Each burst contains sequences of 2 to 100 nanosecond discharge pulses at 100 kHz. Burst mode measurements show very significant (up to ~0.2%) build-up of atomic oxygen density in air, and some build-up (by a factor of approximately three) in methane-air at [phi]=0.5. Burst measurements in ethylene-air at [phi]=0.5 show essentially no build-up, due to rapid O atom reactions with ethylene in the time interval between the pulses. Nitric oxide density is also measured using single photon Laser Induced Fluorescence (LIF), in a manner similar to oxygen atoms, and compared with kinetic modeling. Fluorescence from a NO (4.18ppm) +N2 calibration gas is used to calibrate the NO densities. Peak density in air is found to be ~ 3.5ppm at ~ 225us, increasing from almost initial levels of ~ 0 ppm directly after the pulse. Kinetic modeling using only the Zeldovich mechanism predicts a slow increase in NO formation, in ~ 2 ms, which points towards the active participation of excited N2 and O2 molecules and N atoms in forming NO molecules. Ignition delay at a variety of fuel/ air conditions is studied using OH emission measurements at ~ 308nm as ignition foot prints. The ignition delay is found to be in the range of 6-20ms for ethylene/ air mixtures. No ignition was observed in the case of methane/ air mixtures. All these measurements agree well with kinetic modeling developed involving plasma reactions and electron energy distribution function calculations.
The volume includes selected and reviewed papers from the 3rd Conference on Ignition Systems for Gasoline Engines in Berlin in November 2016. Experts from industry and universities discuss in their papers the challenges to ignition systems in providing reliable, precise ignition in the light of a wide spread in mixture quality, high exhaust gas recirculation rates and high cylinder pressures. Classic spark plug ignition as well as alternative ignition systems are assessed, the ignition system being one of the key technologies to further optimizing the gasoline engine.
Abstract: The dissertation presents experimental and kinetic modeling studies of ignition of hydrocarbon-air flows by a high voltage, repetitively pulsed, nanosecond pulse duration plasma. A high reduced electric field during the pulse results in efficient electronic excitation and molecular dissociation, and extremely low duty cycle of the repetitively pulsed nanosecond discharge improves the plasma stability and helps sustain a diffuse and uniform nonequilibrium plasma.
This book gathers outstanding papers presented at the 17th Annual Conference of China Electrotechnical Society, organized by China Electrotechnical Society (CES), held in Beijing, China, from September 17 to 18, 2022. It covers topics such as electrical technology, power systems, electromagnetic emission technology, and electrical equipment. It introduces the innovative solutions that combine ideas from multiple disciplines. The book is very much helpful and useful for the researchers, engineers, practitioners, research students, and interested readers.
A new experimental facility was developed to study the reactive chemical kinetics associated with plasma-assisted combustion (PAC). Experiments were performed in a nearly isothermal plasma flow reactor (PFR), using reactant mixtures highly diluted in an inert gas (e.g., Ar, He, or N2) to minimize temperature changes from chemical reactions. At the end of the isothermal reaction zone, the gas temperature was rapidly lowered to terminate any continuation in reaction. Product composition as a result of any observed reaction was then determined using ex situ techniques, including non-dispersive infrared (NDIR), and by sample extraction and storage into a multi-position valve for subsequent analysis by gas chromatography (GC). Hydroxyl radical concentrations were measured in situ, using the laser induced fluorescence (LIF) technique. Reactivity maps for a given fuel system were achieved by fixing the flow rate or residence time of the reactant mixture through the PFR and varying the isothermal temperature. Fuels studied were hydrogen, ethylene and C1 to C7 alkane hydrocarbons, to examine pyrolysis and oxidation kinetics with and without the effects of a high-voltage nanosecond pulse duration plasma discharge, at atmospheric pressure from 420 K to 1250 K. In select instances, experimental studies were complimented with detailed chemical kinetic modeling analysis to determine the dominant and rate-controlling mechanisms, while elucidating the influence of the plasma chemistry on the thermal (neutral) chemistry.In the hydrogen oxidation system, no thermal reaction was observed until 860 K, consistent with the second explosion limit at atmospheric pressure, at which point all the hydrogen was rapidly consumed within the residence time of the reactor. With the plasma discharge, oxidation occurred at all temperatures examined, exhibiting a steady increase in the rate of oxidation starting from 470 K, and eventually consuming all the initial hydrogen by 840 K. For ethylene, kinetic results with the discharge indicated that pyrolysis type reactions were nearly as important as oxidative reactions in consuming ethylene below 750 K. Above 750 K, the thermal reactions coupled to the plasma reactions to further enhance the high temperature fuel consuming chemistry. Modeling analysis of plasma-assisted pyrolysis revealed that ethylene dissociation by collisional quenching with electronically-excited argon atoms formed in the presence of the plasma, resulted in the direct formation of acetylene and larger hydrocarbons by way of the ethyl radical. Similarly, during plasma-assisted oxidation, excited argon was able to directly dissociate the initial oxidizer to further enhance fuel consumption, but also facilitate low temperature oxidative chemistry due to the effective production of oxygenated species controlled by R+O2 chemistry. At the highest temperatures, the radical production by neutral thermal reactions became competitive and the effectiveness associated with the plasma coupled chemistry decreased. Under the effects of the plasma, alkane fuels exhibited extended limits of oxidation over the entire temperature range considered, compared to that of the thermal reactions alone. At atmospheric pressure, propane and butane exhibited cool flame chemistry between 420 K to 700 K, which normally occurs at higher pressures (P > 1 atm) for thermally constrained systems. This chemistry is characterized by the alkylperoxy radical formation, isomerization to the hydroperoxyalkyl radical, followed by dissociation to form aldehydes and ketones. Whereas, intermediate temperature chemistry between 700 K to 950 K, is characterized by beta-scission of the initial alkyl radical to form alkenes and smaller alkanes. The culmination of these studies demonstrate new insight into the kinetics governing PAC and provides a new experimental database to facilitate the development and validation of PAC-specific kinetic mechanisms.
Pure rotational CARS thermometry is used to study low-temperature plasma assisted fuel oxidation kinetics in a repetitive nanosecond pulse discharge in ethene-air at stoichiometric and fuel lean conditions at 40 Torr pressure. Air and fuel-air mixtures are excited by a burst of high-voltage nanosecond pulses (peak voltage 20 kV, pulse duration ~25 nanosecond) at a 40 kHz pulse repetition rate and burst repetition rate of 10 Hz. The number of pulses in the burst is varied from a few pulses to a few hundred pulses. The results are compared to the previously developed hydrocarbon-air plasma chemistry model, modified to incorporate non-empirical scaling of the nanosecond discharge pulse energy coupled to the plasma with the number density, as well as one-dimensional conduction heat transfer. Experimental time-resolved temperature, determined as a function of number of pulses in the burst, is found to agree well with the model predictions. The results demonstrate that the heating rate in fuel-air plasmas is much faster compared to air plasmas, primarily due to energy release in exothermic reactions of fuel with O atoms generated by the plasma. It is found that the initial heating rate in fuel-air plasmas is controlled by the rate of radical (primarily O atoms) generation and is nearly independent of the equivalence ratio. At long burst durations, heating rate in lean fuel air-mixtures is significantly reduced when all fuel is oxidized.
