Search Results (1 - 10 of 10 Results)

Sort By  
Sort Dir
 
Results per page  

Zhu, Yonry RApplications and Modeling of Non-Thermal Plasmas
Bachelor of Science (BS), Ohio University, 2018, Engineering Physics
This thesis focuses on validation of a 0D plasma kinetic model and its subsequent use as an explanatory tool to support the results of hot-fire tests of a plasma assisted rotating detonation combustor. The plasma model predictions showed good agreement with experimentally measured values of various ground state species number densities, vibrationally excited N2 number densities, plasma temperatures, and ignition delay times. Once validated, the plasma model was combined with a ZND detonation model and semi-empirical correlation to determine the effects of a non-thermal plasma on the reduction of the detonation cell size for an H2 - air mixture. The modeling results showed that non-thermal plasma significantly reduces the detonation cell size. This effect is most pronounced at lean conditions, where the model predicted a reduction in cell size by a factor of more than 100. For stoichiometric and rich conditions, the cell size reduction was around a factor of 5. An investigation was conducted to determine the viability of using a non-thermal plasma to expand the operating regime of a rotating detonation combustor. The plasma was produced with a nanosecond pulse generator connected to a ceramic and metal centerbody electrode. Hot-fire testing results showed that the plasma causes detonation onset in conditions that would otherwise not support detonation. This effect was most prominent at near-stoichiometric conditions, with a reduced effect for richer or leaner mixtures.

Committee:

David Burnette (Advisor)

Subjects:

Aerospace Engineering; Mechanical Engineering; Plasma Physics

Keywords:

non-thermal plasma; plasma assisted combustion; nanosecond pulsed plasma; plasma modeling; detonation combustion; rotating detonation combustor; detonation;

Heinrichs, Joseph AloysiusPlasma Assisted Combustion and Flameholding in High Speed Cavity Flows
Master of Science, The Ohio State University, 2012, Mechanical Engineering
This thesis presents an experimental study of non-equilibrium, low temperature, large volume plasma assisted ignition and flameholding in high-speed, non-premixed fuel-air flows. The plasma is produced between two electrodes powered by a high-voltage, nanosecond pulse generator operated at a high pulse repetition rate. Ignition in this type of plasma occurs due to production of highly reactive radicals by electron impact excitation and dissociation, as opposed to more common thermal ignition. Previously, it has been shown that this type of plasma can reduce ignition delay time and ignition temperature. The experiments performed in this thesis focus on application of these plasmas to ignition, and flameholding in high-speed cavity flows. The experiments discussed in this thesis continue previous work using a high-speed combustion test section with a larger cavity, and the previous results are compared to the present work. Several modifications have been made to the test section and electrodes compared to the design used in previous work in order to reduce the cavity effect on the main flow and maintain diffuse plasma between the electrodes in the cavity. The electrodes used in these experiments are placed in a cavity recess, used to create a recirculation flow region with long residence time, where ignition and flameholding can occur. In order to analyze the nanosecond pulse plasma and the flame, various diagnostics were used, including current and voltage measurements, UV emission measurements, ICCD camera imaging, static pressure measurements, and time-averaged emission spectroscopy. The experiments in this thesis were performed at relatively low pressures (P=150-200 torr) using hydrogen and ethylene fuels injected into the cavity. Current and voltage measurements showed that ~1-2 mJ was coupled to the plasma by each pulse. ICCD imaging and UV emission data revealed that the plasma sustained in quiescent air was diffuse. When ethylene was injected into the cavity to ignite the flow, ICCD imaging and UV emission data showed arcing to bare metal surfaces in the test section occurred shortly after ignition, which prompted switching to hydrogen fuel. Using hydrogen, ICCD imaging and UV emission showed that the plasma remained diffuse and confined to the area between electrodes. Time-average emission spectroscopy measurements revealed that the air-flow temperature remained low until fuel was injected and ignition occurred. Pressure and UV emission measurements were used to find velocity limits within which the flow ignited. It was found that the upper limit of velocity depends strongly on the static pressure in the test section. The highest flow velocity at which combustion was achieved in H2-air flows was 270 m/s at 180 torr. This represents considerable improvement compared to previous work using nanosecond pulse discharge for ignition in cavities. Preliminary results show that plasma generation and ignition are possible using a smaller diameter electrode such that the cavity size can be further reduced, and that a supersonic flow can be produced in the present test section using a Mach 2 nozzle placed upstream of the cavity. The appendix details a study on the production of oxygen atoms using a pulsed excimer laser.

Committee:

Igor Adamovich, PhD (Advisor); Walter Lempert, PhD (Committee Member)

Subjects:

Mechanical Engineering

Keywords:

plasma assisted combustion; plasma assisted ignition; nonequilibrium plasma; temperature plasma; cavity ignition; cavity flameholding

Yin, ZhiyaoFuel Oxidation and Ignition by Nanosecond Pulse Discharges at Elevated Temperatures
Doctor of Philosophy, The Ohio State University, 2013, Mechanical Engineering
Kinetic studies of plasma assisted oxidation and ignition have been performed in fuel-air mixtures excited by nanosecond dielectric-barrier discharges at elevated temperatures and low pressures. The following topics have been extensively investigated: (i) accuracy of Rayleigh scattering calibration of OH Laser-Induced Fluorescence (LIF), (ii) characterization of nanosecond pulse discharges, (iii) non-thermal plasma e ffect on ignition delay, temperature, and OH concentration.

Committee:

Igor Adamovich (Advisor); Walter Lempert (Committee Member); Bill Rich (Committee Member); Jeffrey Sutton (Committee Member)

Subjects:

Mechanical Engineering

Keywords:

Plasma Assisted Combustion, Nanosecond Pulse Discharge, Low Temperature Plasmas, Laser Induced Fluorescence, Rayleigh Scattering, Kinetic Modeling

Choi, InchulOH LIF Studies of Low Temperature Plasma Assisted Oxidation and Ignition in Nanosecond Pulsed Discharge
Doctor of Philosophy, The Ohio State University, 2011, Mechanical Engineering
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.

Committee:

Walter R. Lempert, PhD (Advisor); Igor V. Adamovich, PhD (Committee Member); Joseph W. Rich, PhD (Committee Member); Jeffrey A. Sutton, PhD (Committee Member)

Subjects:

Mechanical Engineering

Keywords:

Laser-Induced Fluorescence; Hydroxyl; Plasma Assisted Combustion; Kinetics; Optical Diagnostics

Uddi, MruthunjayaNon-Equilibrium Kinetic Studies Of Repetitively Pulsed Nanosecond Discharge Plasma Assisted Combustion
Doctor of Philosophy, The Ohio State University, 2008, Mechanical Engineering

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 Φ=0.5. Burst measurements in ethylene-air at Φ=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 ~ 225μs, 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.

Committee:

Walter Lempert, PhD (Advisor); Igor Adamovich, PhD (Committee Member); Vishwanath Subramaniam, PhD (Committee Member); William Rich, PhD (Committee Member); Mohammed Samimy, PhD (Committee Member); Sampath Parthasarathy, PhD (Committee Member)

Subjects:

Chemistry

Keywords:

Plasma assisted combustion; non equilibrium kinetics; laser diagnostics; TALIF

Dutta, AshimCavity Ignition and Flameholding of High Speed Fuel-Air Flows by a Repetitively Pulsed Nanosecond Discharge
Doctor of Philosophy, The Ohio State University, 2011, Mechanical Engineering

The dissertation presents an experimental study of ignition and flameholding of high speed, room temperature fuel-air flows using a diffuse, large volume, low temperature plasma produced by a repetitive nanosecond pulse discharge sustained in a cavity. Experiments are performed in premixed, partially premixed, and non-premixed ethylene-air and hydrogen-air flows in a pressure range of P = 0.2 – 0.3 atm. The dissertation also incorporates kinetic modeling of plasma assisted ignition of ethylene-air and hydrogen-air mixtures, to study the effect of radical generation in the plasma on ignition delay. The experimental results demonstrate that repetitive nanosecond pulse plasma assisted ignition occurs via formation of multiple arc filaments in the fuel-air plasma, although air plasma remains diffuse and low-temperature until the fuel is added. Comparison of ignition and flameholding achieved in premixed ethylene-air flows using a repetitive nanosecond pulse discharge and a DC arc discharge of approximately the same power (100 W) demonstrated that DC discharge resulted in sporadic ignition and flame blow-off, much lower burned fuel fraction, and significantly lower velocity (35 m/sec) at which ignition is achieved.

For premixed and partially premixed near-stoichiometric ethylene-air flows, ignition and stable flameholding have been achieved up to a flow velocity of 100 m/sec at P=0.2 atm. During these experiments, nearly complete combustion is achieved. For partially premixed hydrogen-air flows, stable ignition and flameholding at P=0.2 atm has been achieved at flow velocities of up to 100 m/sec and equivalence ratios of φ=0.44-0.96. Time averaged plasma temperature measurements using nitrogen emission spectroscopy showed that the air plasma temperature is within 70° C to 200° C, while plasma temperature in presence of a stable flame is 700-1000° C. During non-premixed combustion experiments in ethylene-air at P=0.2 atm, ignition and stable flameholding is observed up to a flow velocity of 90 m/s at global equivalence ratio of φ=0.1. The highest flow velocity at which stable flameholding is observed in non-premixed hydrogen-air flows at P=0.26 atm is 190 m/sec, at φ≈0.04. Flow choking in the combustor is observed for average flow velocities above 200 m/sec. High frame rate NO Planar Laser Induced Fluorescence (PLIF) imaging and schlieren imaging have been performed to observe the dynamics of fuel-air mixing in the cavity during fuel injection.

Kinetic modeling is used to study the mechanism of low-temperature nanosecond pulse plasma assisted ignition. The reduced kinetic mechanism of plasma assisted ignition of hydrogen has been identified and compared with the full mechanism in a wide range of temperatures and pressures, showing good agreement. Kinetic modeling calculations performed to study the effect of non-thermal radical generation in nanosecond pulse discharge plasma on oxidation/ignition of hydrogen-air mixtures demonstrated that removal of plasma chemical radical generation processes inhibits low-temperature exothermic chemical reactions, thus blocking ignition. It is also observed that presence of radicals produced by the plasma accelerates ignition process significantly and reduces ignition temperature. Finally, the kinetic model has been used to interpret the results of flameholding experiments in premixed ethylene-air and hydrogen-air flows.

Committee:

Igor Adamovich, PhD (Advisor); Walter Lempert, PhD (Committee Member); J. William Rich, PhD (Committee Member); Mohammad Samimy, PhD (Committee Member)

Subjects:

Aerospace Engineering; Mechanical Engineering

Keywords:

Plasma assisted combustion; Flameholding; Cavity flow; Nanosecond pulse discharge; Plasma chemical reactions; Kinetic modeling

Bowman, Sherrie S.Atomic and Molecular Oxygen Kinetics Involved in Low Temperature Repetitively Pulsed Nonequilibrium Plasmas
Doctor of Philosophy, The Ohio State University, 2013, Chemistry
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 (<100 Torr) pulsed plasma in a plane-to-plane dielectric barrier discharge was studied using a high repetition rate (40 kHz) high voltage pulsed discharge. Second, a higher temperature (~1000 K) and low pressure (<100 Torr) pulsed plasma in a bare metal, spherical electrode geometry was studied using a 60 Hz repetition rate high voltage pulsed discharge. Third, a high temperature (~1200 K) and high pressure (~760 Torr) pulsed plasma in a pin-to-plane geometry was studied using a 10 Hz repetition rate high voltage pulsed discharge. Additionally, a study of the role of electronically excited molecular oxygen, SDO, on the kinetics of a low temperature (~300 K) and low pressure ( <100 Torr) nonequilibrium plasma in a plane-to-plane dielectric barrier discharge was conducted. Kinetic modeling results were compared to all the experimental results. UV ICCD camera imaging was used to confirm the stable and diffuse nature of the plasma under all of the conditions that were studied. Current and voltage traces were measured using commercially available probes to determine the energy coupled to the plasma. All of these results were used for modeling of experimental results. Two photon Absorption Laser Induced Fluorescence (TALIF) measurements were used for determining atomic oxygen concentration.. Calibration by comparison with xenon gas gave absolute O atom concentration in a variety of gas mixtures and discharge geometries. IR emission spectroscopy was used for electronically excited molecular oxygen, SDO, measurements. Calibration by comparison with a blackbody source was used for absolute scale results. The effect of SDO on ignition delay time was measured spontaneous OH A¿X(0,0) emission spectroscopy was used. Ignition delay was defined as the onset of continuous OH emission between discharge pulses. It was found that while, in general, the mechanism for atomic oxygen formation and decay in each of the plasmas studied can be compared there are significant differences in quantitative values in each case. Initial conditions, such as the coupled energy and number density of electrons, play a strong role in determining how the chemistry propagates in time. The role of SDO was found to be complicated by concurrent NOx chemistry happening in the discharge and significantly higher concentrations would be needed to differentiate these effects.

Committee:

Walter Lempert (Advisor); Heather Allen (Committee Member); Anne McCoy (Committee Member); Frank DeLucia (Committee Member)

Subjects:

Atoms and Subatomic Particles; Chemistry; Experiments; Gases; Molecular Chemistry; Molecular Physics; Optics; Plasma Physics

Keywords:

Lasers; Laser Diagnostics; Plasmas; Plasma Assisted Combustion; Kinetics; Nonequilibrum Thermodynamics

Sheehe, Suzanne Marie LanierHeat Release Studies by pure Rotational Coherent Anti-Stokes Raman Scattering Spectroscopy in Plasma Assisted Combustion Systems excited by nanosecond Discharges
Doctor of Philosophy, The Ohio State University, 2014, Chemistry
Heat release studies of plasma assisted combustion have been performed in fuel-air mixtures excited by nanosecond dielectric barrier discharges initially at room temperature and maintained at low pressure (~40 – 50 torr). The following topics have been extensively investigated: (i) the applicability of pure O2 broadband Rotational Coherent Anti-Stokes Raman Scattering spectroscopy at very low O2 pressures of ~8 torr or less to obtain rotational temperature, (ii) validation of a proposed low temperature fuel-oxidation kinetics mechanism fully decoupled from NOx chemistry, (iii) characterization of nanosecond pulse discharges in a dielectric barrier discharge cell and a pin-to-pin discharge geometry, and (iv) effect of fuel addition on heat release in a pin-to-pin discharge geometry at low pressure. For the first topic, the applicability of pure O2 broadband Rotational Coherent Anti-Stokes Raman Scattering (RCARS) Spectroscopy at very low O2 partial pressure of ~ 8 torr or less to obtain rotational temperature has been demonstrated. Very good experimental precisions of ~ ± 1 to 2 K has been demonstrated for diffuse and volumetric plasmas excited by a repetitively pulsed nanosecond discharge. It is shown that the electron-multiplication feature of an EMCCD camera increases the signal to noise ratio significantly. For the second topic, the pure O2 RCARS system was applied to the dielectric barrier discharge cell to obtain time-resolved temperature measurements in nanosecond pulse discharges in 20% O2-Ar, H2-O2-Ar and C2-H2-O2-Ar mixtures, initially at room temperature, operated at a high pulse repetition rate of 40 kHz, in plane-to-plane double dielectric barrier geometry at a pressure of 40 Torr. Nitrogen was deliberately excluded from the system so as to decouple NOx chemistry from the plasma fuel-oxidation processes. It was found that a 0-D model predictions for temperature are in very good agreement in the baseline mixture without fuel and the hydrogen containing mixtures. However, the model predicts that the heat release in hydrogen containing mixtures is only weakly dependent on equivalence ratio, which is inconsistent with experimental results. Furthermore, In C2H2 containing mixtures, the model consistently under-predicts the temperature, further delineating the need for more accurate low-temperature plasma/combustion chemistry decoupled from NOx processes for both hydrogen and ethylene fuels. For the third topic, plasma characterization has been carried out for the mixtures in the aforementioned dielectric barrier discharge cell in addition to air-fuel mixtures in a pin-to-pin discharge geometry. The pin-to-pin discharge was excited by a nanosecond discharge at 60 Hz. Broadband plasma emission images were obtained for both types of discharges using an (ICCD) camera. In the dielectric barrier discharge, the 20% O2-Ar and O2-Ar-H2 mixtures were both shown to be diffuse and volumetric. The O2-Ar-C2H4 mixtures, on the other hand, showed significant striation and plasma constriction. In the pin-to-pin discharge, the plasma filament in both air and air-hydrogen was fairly homogeneous along the discharge gap, but radially decreases outward from the filament centerline. The energy coupling to the plasma for both types of discharges was determined using current-voltage waveforms. The dielectric barrier discharge couples a very small amount of the energy, ~0.1 mJ/pulse, that is stored in the capacitive load formed during breakdown to the plasma. Good agreement between these energy coupling results and a prediction from a 0-D analytical model was found. On the other hand, the pin-to-pin discharge has a higher energy loading of ~3 mJ/pulse and a model is currently in development. On the fourth topic, in the pin-to-pin discharge geometry it is demonstrated that a fast heating and a slow heating regime exist in air and air-fuel mixtures and are clearly distinct from each other after the onset of the discharge pulse. It is indicated that air-ethylene mixtures do not exhibit a clear distinction between slow and fast heating. In all cases, with increasing fuel addition, the rate of the heat release increases. Radial temperature profiles were taken for air at three different time points relative to the onset of the pulse. The radius was found to be the same in all three cases, strongly indicating that there is no contraction or expansion of the plasma filament. Preliminary results with a 1-D model still in development show very good agreement, which is promising. It is expected that the model will attribute fast heating primarily to collisional quenching of N2 excited states in air and air-hydrogen containing mixtures. In ethylene mixtures, ethylene oxidation processes are expected to have a larger contribution as experimental results indicate a strong dependence on equivalence ratio. Slow heating is expected to be dominated by V-T transfer from vibrationally excited N2 by collisional quenching of O-atoms, with additional release by fuel-oxidation.

Committee:

Walter Lempert, PhD (Advisor); Anne McCoy, PhD (Committee Member); Terry Gustafson, PhD (Committee Member)

Subjects:

Chemistry; Engineering; Physical Chemistry

Keywords:

CARS; pure Rotational Coherent Anti-Stokes Raman Scattering Discharge; dielectric barrier discharge; plasma instability; Plasma Assisted Combustion; Gas heating; third-order non-linear susceptibility; pin-to-pin discharge; nanosecond discharge

Li, TingExperimental Study of the Effects of Nanosecond-Pulsed Non-equilibrium Plasmas on Low-Pressure, Laminar, Premixed Flames
Doctor of Philosophy, The Ohio State University, 2014, Aero/Astro Engineering
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.

Committee:

Jeffrey Sutton (Advisor); Igor Adamovich (Advisor); Walter Lempert (Committee Member); Joseph Rich (Committee Member)

Subjects:

Aerospace Engineering

Keywords:

Plasma-Assisted Combustion, Combustion Chemical Kinetics, Laser Diagnostics

Bowman, Sherrie S.Kinetics of Low-Temperature Fuel Oxidation and Ignition by Repetitively Pulsed Nonequilibrium Plasmas
Master of Science, The Ohio State University, 2010, Chemistry

This thesis presents the results of a non-equilibrium kinetic study of oxidation and ignition in large volume hydrogen/air and ethylene/air mixtures by repetitively pulsed non-equilibrium plasmas. These measurements were conducted at low temperatures (~300K) and pressures (40 to 80 Torr), and a 40 kHz pulse repetition rate. Most measurements were also conducted at a 10 Hz discharge repetition rate. UV ICCD imaging and OH emission spectroscopy are used to ensure plasma uniformity and ignition delay time, respectively. Atomic oxygen Two Photon Absorption Laser Induced Fluorescence (TALIF) measurements, with Xenon calibration, give the absolute O atom concentration in a variety of mixtures. Finally, the sensitivity of a plasma chemistry kinetic model has been studied and gives insight into the chemical processes that are occurring.

UV ICCD camera imaging shows that the plasma remains stable and diffuse at the conditions explored in the atomic oxygen TALIF measurements (40 Torr pressure and 40 kHz discharge repetition rate). While plasma uniformity can be confirmed at these conditions, weak emission left ignition questionable. Images taken at higher pressures and equivalence ratios show emission between the pulses more clearly, indicating ignition. Also, a series of images taken at longer delay times after the burst, and with a larger camera gate, show the duration of the ignition event.

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.

TALIF is used to measure the absolute atomic oxygen concentrations in air, hydrogen/air, and ethylene/air non-equilibrium plasmas as a function of burst size (15 to 1000 pulses, or 0.375 to 25 milliseconds). In air, there is a very rapid increase in O atom concentration (~2 milliseconds) that levels off into a near steady-state value. In the presence of very small amounts of hydrogen (f=0.05 or ~1.5%), there is significantly less atomic oxygen formed, by approximately an order of magnitude. The kinetic model predicts a very rapid rise in O atom concentration after 12 or 15 milliseconds in both the f=0.5 and 1.0 cases, however the experimental results only show this increase (more gradually) in the f=0.5 case.

Finally, sensitivity analysis of a plasma chemistry kinetic model shows that in a single pulse discharge, at low initial gas temperature (T=300K), the kinetics can be described by a reduced model that is independent of chain branching that rapidly increases the rate fuel oxidation. At higher temperatures (T=500 to 600K), this reduced model must include chain branching that becomes dominant and there is an increase in net energy release. In burst mode, a reduced model comprised of reactions from both the single pulse cases are found to be important in describing the kinetics of the system.

Committee:

Walter Lempert, Dr. (Advisor); Heather Allen, Dr. (Committee Member)

Subjects:

Chemistry

Keywords:

TALIF; plasma assisted combustion; lasers