Non-equilibrium Kinetic Studies of Repetitively Pulsed Nanosecond Discharge Plasma Assisted Combustion PDF Download

Author: Mruthunjaya Uddi
Publisher:
ISBN:
Category : Chemical kinetics
Languages : en
Pages : 177

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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: Nicholas Tsolas
Publisher:
ISBN:
Category :
Languages : en
Pages :

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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.

Author: Moon Soo Bak
Publisher:
ISBN:
Category :
Languages : en
Pages :

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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: Colin A. Pavan
Publisher:
ISBN:
Category :
Languages : en
Pages : 0

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Book Description
Plasma assisted combustion (PAC) is a promising technology for extending combustion operating envelopes with a low energy cost relative to flame power. It has been investigated for use in various situations, particularly those where combustion is being performed near flammability limits imposed by equivalence ratio, residence time, etc. While the fundamental processes allowing plasma to modify combustion dynamics have been well studied, there are still many unresolved questions in determining the relative contribution of different actuation pathways in different situations (thermal enhancement, kinetic enhancement or transport-induced effects) and how the plasma will evolve and interact with the flame in a dynamic combustion environment. The plasmas being used for PAC are typically non-equilibrium and are often produced by the nanosecond repetitively pulsed discharge (NRPD) strategy. The development of these discharges is highly dependent both on applied voltage and also on the gas environment (composition, temperature, flow field, etc.). As the plasma affects the combustion, so too does the combustion affect the plasma structure and energy deposition pathways. This two-way coupling means that the plasma's ability to modify the combustion, and the mechanisms by which it achieves these effects, will vary as the environment changes due to combustion dynamics. This impact of the combustion on the plasma has received considerably less attention than the other direction of interaction, especially in environments with transient or propagating flames. The first main objective of this thesis is to explore the development of NRPDs in dynamic combustion environments and in particular how the plasma develops on the timescales of transient combustion (many accumulated pulses). This is performed first in a laminar, mesoscale platform to probe the interaction in detail, and the important insights are later shown to be relevant to high power systems of practical interest. While the impact of the plasma on the flame has been considerably better studied and the fundamental processes are well understood, there are still hurdles that must be overcome before PAC systems can begin to be designed and implemented for use outside of the laboratory. The development of versatile and flexible engineering models of the impact of the plasma will be necessary to allow system designers to make predictions about combustor operation when plasma is applied. The second main objective of this thesis is to develop such an engineering model and demonstrate its predictive capabilities across a variety of configurations. The model is developed for a laminar mesoscale platform and is shown to correctly predict the impact of the plasma in several different configurations, indicating a path forward towards physics[1]informed design of PAC systems. The model also provides important physical insight of the impact of plasma on flame, such as the role of pressure waves in disturbing the flame dynamics, even when considering uniform DBD discharges.

Author: L.M. Biberman
Publisher: Springer
ISBN: 9781468416657
Category : Science
Languages : en
Pages : 0

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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.

Author: Inchul Choi
Publisher:
ISBN:
Category :
Languages : en
Pages : 146

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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: Sherrie S. Bowman
Publisher:
ISBN:
Category :
Languages : en
Pages : 178

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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 (

Author: Ting Li
Publisher:
ISBN:
Category :
Languages : en
Pages : 194

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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:
Publisher:
ISBN:
Category :
Languages : en
Pages : 1

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The paper represents experimental and numerical investigations of nonequilibrium plasma formation by high voltage pulsed nanosecond discharge.

Author: Kuninori Togai
Publisher:
ISBN:
Category :
Languages : en
Pages :

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Plasma-assisted combustion (PAC) is a promising combustion enhancement technique that shows great potential for applications to a number of different practical combustion systems. In this dissertation, the chemical kinetics associated with PAC are investigated numerically with a newly developed model that describes the chemical processes induced by plasma. To support the model development, experiments were performed using a plasma flow reactor in which the fuel oxidation proceeds with the aid of plasma discharges below and above the self-ignition thermal limit of the reactive mixtures. The mixtures used were heavily diluted with Ar in order to study the reactions with temperature-controlled environments by suppressing the temperature changes due to chemical reactions. The temperature of the reactor was varied from 420 K to 1250 K and the pressure was fixed at 1 atm. Simulations were performed for the conditions corresponding to the experiments and the results are compared against each other. Important reaction paths were identified through path flux and sensitivity analyses. Reaction systems studied in this work are oxidation of hydrogen, ethylene, and methane, as well as the kinetics of NOx in plasma.In the fuel oxidation studies, reaction schemes that control the fuel oxidation are analyzed and discussed. With all the fuels studied, the oxidation reactions were extended to lower temperatures with plasma discharges compared to the cases without plasma. The analyses showed that radicals produced by dissociation of the reactants in plasma plays an important role of initiating the reaction sequence. At low temperatures where the system exhibits a chain-terminating nature, reactions of HO2 were found to play important roles on overall fuel oxidation. The effectiveness of HO2 as a chain terminator was weakened in the ethylene oxidation system, because the reactions of C2H4 + O that have low activation energies deflects the flux of O atoms away from HO2. For the ethylene and methane oxidation systems, the reaction pathways important for the formation of intermediate species are discussed. The reactions of CH3 and C2H5 were found to influence the production channels of minor species.In the studies on the kinetics of NOx in plasma, several mechanistic insights were obtained, including the identification of formation and consumption steps of N2O and the extensive review of three NO formation schemes found in the current reaction mechanism. Efforts to address the known inaccuracies of the current model are also reported.