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Author: Inchul Choi Publisher: ISBN: Category : Languages : en Pages : 146
Book Description
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.
Author: Inchul Choi Publisher: ISBN: Category : Languages : en Pages : 146
Book Description
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.
Author: Mruthunjaya Uddi Publisher: ISBN: Category : Chemical kinetics Languages : en Pages : 177
Book Description
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.
Author: Paul K. Chu Publisher: CRC Press ISBN: 1466509902 Category : Science Languages : en Pages : 497
Book Description
Written by a team of pioneering scientists from around the world, Low Temperature Plasma Technology: Methods and Applications brings together recent technological advances and research in the rapidly growing field of low temperature plasmas. The book provides a comprehensive overview of related phenomena such as plasma bullets, plasma penetration into biofilms, discharge-mode transition of atmospheric pressure plasmas, and self-organization of microdischarges. It describes relevant technology and diagnostics, including nanosecond pulsed discharge, cavity ringdown spectroscopy, and laser-induced fluorescence measurement, and explores the increasing research on atmospheric pressure nonequilibrium plasma jets. The authors also discuss how low temperature plasmas are used in the synthesis of nanomaterials, environmental applications, the treatment of biomaterials, and plasma medicine. This book provides a balanced and thorough treatment of the core principles, novel technology and diagnostics, and state-of-the-art applications of low temperature plasmas. It is accessible to scientists and graduate students in low-pressure plasma physics, nanotechnology, plasma medicine, and materials science. The book is also suitable as an advanced reference for senior undergraduate students.
Author: Ting Li Publisher: ISBN: Category : Languages : en Pages : 194
Book Description
In this dissertation, the effects of nanosecond, repetitively-pulsed, non-equilibrium plasma discharges on laminar, low-pressure, premixed burner-stabilized hydrogen/O2/N2 and hydrocarbon/O2/N2 flames is investigated using optical and laser-based diagnostics and kinetic modeling. Two different plasma sources, both of which generate uniform, low-temperature, volumetric, non-equilibrium plasma discharges, are used to study changes in temperature and radical species concentrations when non-equilibrium plasmas are directly coupled to conventional hydrogen/hydrocarbon oxidation and combustion chemistry. Emission spectroscopy measurements demonstrate number densities of excited state species such as OH*, CH*, and C2* increase considerably in the presence of the plasma, especially under lean flame conditions. Direct imaging indicates that during plasma discharge, lean hydrocarbon flames "move" upstream towards burner surface as indicated by a shift in the flame chemiluminescence. In addition, the flame chemiluminescence zones broaden. For the same plasma discharge and flame conditions, quantitative results using spatially-resolved OH laser-induced fluorescence (LIF), multi-line, OH LIF-thermometry, and O-atom two-photon laser-induced fluorescence (TALIF) show significant increases in ground-state OH and O concentrations in the preheating zones of the flame. More specifically, for a particular axial position downstream of the burner surface, the OH and O concentrations increase, which can be viewed as an effective "shift" of the OH and O profiles towards the burner surface. Conceivably, the increase in OH and O concentration is due to an enhancement of the lower-temperature kinetics including O-atom, H-atom and OH formation kinetics and temperature increase due to the presence of the low-temperature, non-equilibrium plasma. High-fidelity kinetic modeling demonstrates that the electric discharge generates significant amounts of O and possibly H atoms via direct electron impact, as well as quenching of excited species rather than pure thermal effect which is caused by Joule heating within the plasma. These processes accelerate chain-initiation and chain-branching reactions at low temperatures (i.e. in the preheat region upstream of the primary reaction zone in the present burner-stabilized flames) yielding increased levels of O, H, and OH. The effects of the plasma become more pronounced as the equivalence ratio is reduced which strongly suggest that the observed effect is due to plasma chemical processes (i.e. enhanced radical production) rather than Joule heating supports the kinetic modeling.
Author: Moon Soo Bak Publisher: ISBN: Category : Languages : en Pages :
Book Description
In this study, the use of non-equilibrium plasmas is examined as possible methods of active control of combustion. The plasmas investigated here include nanosecond-pulsed repetitive discharges as well as nanosecond-pulsed laser-produced breakdowns. These sources are used to stabilize both premixed and jet-diffusion flames of various fuel types. The use of nanosecond-pulsed repetitive discharges to stabilize lean premixed fuel-air mixtures is found to extend the equivalence ratio for complete combustion to lower values, in some cases, below the so-called lean flammability limits. This extension depends strongly on the pulse repetition frequency or average discharge power. Simulations reveal that a significant production of radicals associated with gas heating is responsible for flame stabilization and this is attributed mainly to a dissociative quenching of electronically excited species by molecular oxygen. In jet diffusion flames, anchoring of the flame-base is best when the discharge plasma is positioned where the local equivalence ratio is between 0.8 and 1.9. Lastly, the discharge plasma source is replaced by laser-induced breakdowns. Two successive laser pulses with a variable time delay are employed to mimic repetitive breakdowns expected from a future high frequency laser source of sufficient power. From studies first carried out in pure air, it is found that the first laser breakdown causes a temporal region virtually transparent to the subsequent laser pulse during the interval from 100 ns to 60 μs. This is attributed to heating by the plasma, reducing the density below threshold levels needed for absorption of a laser pulse. In premixed fuel-air mixtures, the first breakdown induces a second region of transparency during the interval from 100 μs to 2 ms after the pulse due to the heat released by combustion. These findings limit the laser repetition rate to a maximum of 500 Hz when the equivalence ratio is 1. Time-resolved imaging of CH* chemiluminescence reveals flame front merging confirming that flame stabilization can be achieved at these moderate laser repetition rates.
Author: Ainan Bao Publisher: ISBN: Category : Hydrocarbons Languages : en Pages : 188
Book Description
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.
Author: Yohan Seepersad Publisher: ISBN: Category : Dissertation, Academic Languages : en Pages : 354
Book Description
The state of plasma is widely known as a gas-phase phenomenon, but plasma in liquids have also received significant attention over the last century. Generating plasma in liquids however is theoretically challenging, and this problem is often overcome via liquid-gas phase transition preceding the actual plasma formation. In this sense, plasma forms in gas bubbles in the liquid. Recent work at the Drexel Plasma Institute has shown that nanosecond pulsed electric fields can initiate plasma in liquids without any initial cavitation phase, at voltages below theoretical direct-ionization thresholds. This unique regime is poorly understood and does not fit into any current descriptive mechanisms. As with all new phenomena, a complete fundamental description is paramount to understanding its usefulness to practical applications. The primary goals of this research were to qualitatively and quantitatively understand the phenomenon of nanosecond pulsed discharge in liquids as a means to characterizing properties that may open up niche application possibilities. Analysis of the plasma was based on experimental results from non-invasive, sub-nanosecond time-resolved optical diagnostics, including direct imaging, transmission imaging (Schlieren and shadow), and optical emission spectroscopy. The physical characteristics of the plasma were studied as a function of variations in the electric field amplitude and polarity, liquid permittivity, and pulse duration. It was found that the plasma size and emission intensity was dependent on the permittivity of the liquid, as well as the voltage polarity, and the structure and dynamics were explained by a 'cold-lightning' mechanism. The under-breakdown dynamics at the liquid-electrode interface were investigated by transmission imaging to provide evidence for a novel mechanism for initiation based on the electrostriction. This mechanism was proposed by collaborators on the project and developed alongside the experimental work in this research. Finally, analysis of emission spectra obtained from the OH(A-X) band at 308 nm by the excited hydroxyl radical was performed to quantify the temperature parameters of the plasma. Boltzmann analysis was performed to quantify the rotational temperature of OH which correlates well to the liquid temperature, and Stark broadening of the ionic lines belonging to hydrogen and oxygen was analysed to estimate electron temperature. It was found that the liquid temperature remained close to bulk temperature with T_(n,i)
Author: Caroline Winters Publisher: ISBN: Category : Aerospace engineering Languages : en Pages : 228
Book Description
This work has produced extensive sets of new data on low-temperature plasma-assisted fuel oxidation in hydrogen-oxygen-argon and hydrocarbon-oxygen-argon mixtures. The measurements have been made in two different plasma flow reactors, at an initial temperature of 500 K and pressures ranging from 300 Torr to 700 Torr. In both reactors, the plasma is generated by a high peak voltage, ns pulse discharge, operated at high pulse repetition rates (up to 20 kHz). Metastable Ar atom number density distributions in the discharge afterglow are measured by Tunable Diode Laser Absorption Spectroscopy (TDLAS), and used to characterize plasma uniformity. Temperature in the discharge-excited reacting flow is measured by Rayleigh scattering. Two-photon Absorption Laser Induced Fluorescence (TALIF) is used to measured absolute H and O atom number densities. The results are compared with predictions of a kinetic model analyzing reaction kinetics of excited species and radicals generated by the plasma at low temperatures and high pressures. The modeling predictions show good agreement with the data, with the exception of fuel-limited mixtures, when nearly all fuel available in the mixture of reactants is oxidized in the discharge. Kinetic modeling analysis identified dominant processes of generation and decay of atomic and radical species in the discharge and in the afterglow. At the present low-temperature conditions, the effect of chain branching reactions on plasma-assisted fuel oxidation kinetics is insignificant.
Author: Inchul Choi
Author: Inchul Choi
Author: Mruthunjaya Uddi
Author: Paul K. Chu
Author: Ting Li
Author: Moon Soo Bak
Author: Ainan Bao
Author: Yohan Seepersad
Author: Caroline Winters
Author: 
Author: Katharina Grosse