Generation and Microwave Scattering Diagnostics of Small Volume Plasmas

This dissertation focuses on the development of novel generation and microwave scattering diagnostic techniques for small volume plasmas. The small volume plasmas presented in this work fall under the two generalized categories: 1) laser-induced plasmas and 2) non-equilibrium microdischarges. Chapter I presents the application of microwave scattering theory to laser-induced breakdown in air. The MIE solution to Maxwell’s equations is employed to reveal three distinct phases of the evolution of the laser-induced breakdown in air. Chapter II presents a novel method of quantifying thresholds for laser-induced breakdown. These thresholds are established via total electron number measurement from dielectric calibration of microwave scattering. Chapter III presents high-repetition-rate (HRR) nanosecond laser pulse train scheme for laser ignition. Demonstration of the ignition of combustible gaseous mixtures is shown to have an order-of-magnitude reduction in per-pulse energy using the HRR LI method over traditional laser ignition methods. Chapter IV presents ion-kinetic measurements of a laser induced plasma in sodium-argon and sodium-air gaseous mixtures. Coherent microwave Rayleigh scattering (Radar) from Resonance Enhanced Multi-Photon Ionization (REMPI) is utilized for the measurement of sodium ion neutral stabilized and cluster dissociative recombination rates. Chapter V presents rotational temperature measurements in a DC microdischarge produced in air. Radar REMPI measurements of O2 rotational temperature is performed at eight axial locations between pin-to-pin electrodes. Chapter VI presents relative concentration measurements of atomic oxygen in DC and pulsed Discharges. Relative atomic oxygen concentrations were obtained via Radar REMPI. The effects of pressures, gas composition, and discharge voltage were explored for the DC and pulsed discharges. Comparisons between two-photon absorption laser induced fluorescence (TALIF) and Radar REMPI techniques were made for atomic oxygen concentration measurements in a pulsed discharge. Chapter VII presents a method of reducing the breakdown voltage of a DC microdischarge via metal nanoparticle seeding. Reductions in the breakdown voltage were seen to be as high as 25% for a PD scaling of 40 Torr-cm from the seeding of iron and aluminum nanoparticles into the discharge gap

Application of Resonant and Non-Resonant Laser-Induced Plasmas for Quantitative Fuel-to-Air Ratio and Gas-Phase Temperature Measurements

In this work, two laser-induced plasma techniques are used for gas-phase chemical and temperature measurements. The first technique, laser-induced breakdown spectroscopy (LIBS) is applied for fuel-to-air ratio (FAR) measurements in a well calibrated Hencken flame. In Chapter I, relevant technical and background information for each technique is provided. In Chapter II, measurements are first performed for high-pressure (1-11 Bar) methane-air flames, for which calibration curves are generated using the emission ratio of hydrogen at 656 nm and ionic nitrogen at 568 nm. The effect of pressure on the sensitivity and precision of the resulting calibrated curves is evaluated. Results indicate a degradation of measurement precision as environmental pressure increases, with data indicating that fluctuations of the plasma play a major part in this behavior. Expanding upon this work with LIBS, a comparison of FAR calibration curve results for atmospheric methane-air Hencken flame using three different laser pulse widths, femto-, pico-, and nanosecond regimes, is done in Chapter III. The results are discussed in the context of potential advantages for high-pressure LIBS-based FAR measurements. Results indicate that while nanosecond duration pulses provide better precision at 1 Bar conditions, femtosecond duration pulses might be better suited for high-pressure measurements.In Chapter IV, the radar REMPI technique, which uses microwave scattering from a plasma created by selective multiphoton ionization of molecular oxygen, is used for gas-phase temperature measurements through the wall of ceramic-enclosed environments. Specifically, measurements are done through the wall of a heated laboratory flow reactor and through the wall of a ceramic well-stirred reactor. Results show good agreement with thermocouple and/or computational modeling and the effectiveness of radar REMPI for through-the-wall measurements.In Chapter V, a new technique is discussed, namely acoustic REMPI, which utilizes the pressure wave generated from the creation of the REMPI plasma for diagnostics. The acoustic emission from the plasma is characterized and used for gas-phase temperature measurements. Comparison, with radar REMPI shows a high-level of agreement.Finally, in Chapter VI, a summary of the work in this dissertation is provided along with a discussion of potential for work in the future

Optical and Laser Spectroscopic Study of Microwave Plasma-Assisted Combustion

Nonthermal plasma-assisted combustion (PAC) has been demonstrated to be a promising potential method to enhance combustion performance and reduce the pollutant emissions. To better understand the mechanism in PAC, we have conducted a series of studies on the combustion enhancement by plasma using a home-developed PAC platform which employs a nonthermal microwave argon plasma and a suit of optical diagnostic tools including optical imaging, optical emission spectroscopy, and cavity ringdown spectroscopy. A new PAC system in which a continuous atmospheric argon microwave plasma jet is employed to enhance combustion of methane/air mixtures was reported. Reactive species in PAC were characterized in a state-resolved manner including the simultaneously measurements of OH(A) and OH(X) radicals in the PAC flames. Roles of the state-resolved OH(A) and OH(X) radicals in microwave PAC of premixed methane/air mixture were explored. It was concluded that if both OH(A) and OH(X) radicals assisted the ignition and flame stabilization processes, then we may hypothesize that the role of OH(A) was more dominant in the ignition enhancement but the role of OH(X) was more dominant in the flame stabilization. The effect of fuel injection configurations was investigated in the comparative study between PAC of the premixed and nonpremixed methane/air mixtures. It was found that emissions from the CH (A-X) and C2 Swan systems only exist in the nonpremixed PAC which suggest that the reaction pathways are different between premixed and nonpremixed PAC. The PAC of premixed methane/oxygen/argon mixtures was investigated. A U-shaped dual-layer curve of fuel ignition/flame stabilization limit showing the effects of the plasma power on the fuel ignition and flame stabilization was observed and reported. A parametric study of the microwave PAC of the premixed ethylene/air mixtures was conducted. Behavior of the OH, CH, and C2 radicals and their dependence on plasma power, argon flow rate, and total ethylene/air mixture flow rate were also studied

Optical and Laser Spectroscopic Study of Microwave Plasma-Assisted Combustion

Nonthermal plasma-assisted combustion (PAC) has been demonstrated to be a promising potential method to enhance combustion performance and reduce the pollutant emissions. To better understand the mechanism in PAC, we have conducted a series of studies on the combustion enhancement by plasma using a home-developed PAC platform which employs a nonthermal microwave argon plasma and a suit of optical diagnostic tools including optical imaging, optical emission spectroscopy, and cavity ringdown spectroscopy. A new PAC system in which a continuous atmospheric argon microwave plasma jet is employed to enhance combustion of methane/air mixtures was reported. Reactive species in PAC were characterized in a state-resolved manner including the simultaneously measurements of OH(A) and OH(X) radicals in the PAC flames. Roles of the state-resolved OH(A) and OH(X) radicals in microwave PAC of premixed methane/air mixture were explored. It was concluded that if both OH(A) and OH(X) radicals assisted the ignition and flame stabilization processes, then we may hypothesize that the role of OH(A) was more dominant in the ignition enhancement but the role of OH(X) was more dominant in the flame stabilization. The effect of fuel injection configurations was investigated in the comparative study between PAC of the premixed and nonpremixed methane/air mixtures. It was found that emissions from the CH (A-X) and C2 Swan systems only exist in the nonpremixed PAC which suggest that the reaction pathways are different between premixed and nonpremixed PAC. The PAC of premixed methane/oxygen/argon mixtures was investigated. A U-shaped dual-layer curve of fuel ignition/flame stabilization limit showing the effects of the plasma power on the fuel ignition and flame stabilization was observed and reported. A parametric study of the microwave PAC of the premixed ethylene/air mixtures was conducted. Behavior of the OH, CH, and C2 radicals and their dependence on plasma power, argon flow rate, and total ethylene/air mixture flow rate were also studied

Publications of the Jet Propulsion Laboratory, July 1961 through June 1962

Jpl bibliography on space science, 1961-196

A study of the application of microwave techniques to the measurement of solid propellant burning rates Progress report, 1 Sep. 1966 - 31 Aug. 1967

Microwave techniques for measurement of solid rocket propellant burning rate

EXTENDING THE LEAN LIMIT OF METHANE-AIR MIXTURES BY OXYGEN, NITROGEN AND CARBON DIOXIDE INJECTION IN THE GAP OF THE SPARK PLUG’S ELECTRODES

An experimental investigation is carried out to investigate the feasibility of extending the lean limit for methane–air mixtures by injecting oxygen and other gases such as nitrogen and carbon dioxide in the area between spark plug electrodes. Additionally, the effect of oxygen injection on exhaust gas recirculation (EGR) is investigated. The pressure in the chamber and the equivalence ratio of the mixture are varied from 0.35 MPa to 1.03 MPa and 0.46 to 1.0, respectively. Delco 12620540 spark plug is used as the ignition source and the gas injection into spark plug gap is accomplished through a 0.159-cm OD stainless steel injection nozzle. With oxygen injection the lean limit is extended from an equivalence ratio of 0.54 to 0.46 at all pressures investigated in the present study. Similar trend is observed with nitrogen and carbon dioxide injection in the area between the spark plug’s electrodes. This indicates that the extension of the lean limit of methane-air mixtures is mainly due to induced charge motion. From high-speed video camera the flame kernel between the electrodes in the case of oxygen, nitrogen and carbon dioxide injections is observed to be much larger than the flame kernel in the case of without injection. Thus, it appears that one of the effects of injection is to enlarge the initial flame kernel to a radius exceeding the critical radius. As a result the flame front grows and propagates, resulting in more energy transferred to the unburned mixture, enhancing the burning and reaction rates. The effect of the oxygen injection on percent EGR represented by carbon dioxide dilution in a stoichiometric methane-air mixture is also investigated. Without oxygen injection combustion of a stoichiometric methane-air mixture is impossible to obtain with 18 % EGR, however with oxygen injection combustion is achieved even at 22 % EGR. As a result of oxygen injection the peak pressure at 22 % EGR is the same as at 15 % EGR. In contrast it is impossible to ignite the methane-air mixture with nitrogen and carbon dioxide injection, even at 10 % EGR

Characterization of Corona Discharge for Ignition Improvement

Advanced spark ignition (SI) engines may better operate under lean or diluted conditions for fuel efficiency improvement. Under lean or diluted conditions, the ignition and complete combustion of the mixture is a challenge with conventional spark plugs. The small spark gap with limited spark energy delivery and the associated heat loss to the ground electrode, remain to be an unsolved problem. This research explores the corona discharge as an alternative ignition technology to counter the challenges of lean or diluted combustion, including the intensive gas flow condition. Without an adjacent ground electrode, the corona discharge tends to generate a larger ignition volume and multiple flame kernels. The study is based on an in-house designed alternating-current (AC) corona system with adjustable electrical parameters. By controlling stable corona discharge, the plasma length is characterized, including the effects of discharge voltage, discharge duration, discharge frequency, and background conditions on corona discharge. Subsequently, ignition research is conducted to combustible mixtures, in constant volume chambers, to demonstrate the ignition capability of corona discharge under both quiescent and forced flow conditions. The preliminary test results of plasma-enhanced combustion are discussed to help future studies on clean combustion innovations

Investigation of the multi-physics of laser-induced ignition of transportation fuels

Cleaner and more efficient combustion systems are expected to operate at conditions where successful spark ignition is difficult to achieve. Laser ignition is a proposed alternative ignition system capable of stable engine performance under these conditions. Fundamental studies are needed to fully characterize the complex, multi-physics nature of the laser ignition process. This thesis is a contribution in that direction, also characterizing the ignition and flame behavior of some engine-relevant fuels. This work investigates experimentally the early stages of the laser ignition process, characterizing breakdown and laser-induced shock waves. It then explores self-sustained flame behavior from early flame emergence to complete propagation or quenching. Regarding the early stages of laser ignition, the influence of focusing optics, thermodynamic conditions, and chemical structure of fuels on optical breakdown threshold is examined. These results are presented in a universal representation of the breakdown threshold, facilitating their comparison. The results agree with previous studies and new data sets are generated. Thermomechanical differences between breakdown in non-reactive and reactive mixtures are quantified, isolating the effect of exothermicity on plasma and shock wave propagation. The thermodynamic conditions of the gas near the focal volume are investigated and quantified using two-color interferometry. This information is applied toward developing accurate initial conditions for simulations based on absorbed laser energy and early kernel geometry. With respect to flame propagation, schlieren and interferometric imaging techniques are used to examine early flame behavior, especially near flammability limits. This provides insight into the mechanisms controlling quenching of fuel-lean laser ignited flames as well as the time-scales involved. Four fuels (methane, biogas, iso-octane, and E85) are characterized, highlighting thermochemical effects which control their flame kernel development, the dynamics, and fate of initially sustained flames. Laser ignition is further put into context by contrasting with the better established spark ignition process. The duration of energy deposition and heat transfer to the spark plug electrodes are found to be the main reasons for differences between laser and spark ignited flames. By examining these different physical aspects of laser ignition, this thesis advances understanding of forced ignition, consolidating this by contrasting with spark-ignition behavior. The results are useful for the design of fuel-flexible and lean combustion technologies. The data set is also useful for CFD simulations and simplified modeling of the ignition process