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پایان نامه با موضوع Kinetic Studies of Non-equilibrium Plasma-assisted Ignition and Combustionدر سطح دکتری تخصصیدانشگاه Drexel University سال 2013خلاصهThe application of thermal plasmas in combustion has a long history which dates back at least as far as the early spark ignition engines for automobile applications. In recent decades, a great amount of experimental data has demonstrated promising results for nonthermal plasma application in high speed flow and automotive engines. However, the mechanisms of plasma assisted combustion still remain unclear. The first part of this thesis presents a computational study in understanding the physics and chemistry of plasma-assisted combustion at temperatures above the auto-ignition threshold. The energy costs in generation of chemically active species by different discharges were calculated. The optimal physical parameters were determined in terms of the energy efficiency. The role of singlet oxygen molecules was numerically studied in promoting ignition of a hydrogen-oxygen mixture. The major reaction pathways have been identified. The second portion of this study, which is the major emphasis of this thesis, shifted the research focus to the investigation of plasma chemical kinetics at temperatures below auto-ignition threshold. An experimental installation was designed, fabricated and calibrated for this purpose. Three types of laser-based optical diagnostics were applied to investigate the hydroxyl (OH) radical dynamics in the afterglow of a pulsed nanosecond discharge. Experiments were carried out using a premixed lean fuel-air mixture (φ=0.1) at atmospheric pressure for temperatures ranging from 300 K to 800 K (below the autoignition threshold). The fuels were methane, ethane, propane, butane, hydrogen and hydrogen-carbon monoxide. The nanosecond pulsed discharge was formed in a pin to pin electrode system. During the discharge, atomic oxygen and hydrogen are generated by direct electron impact and dissociative quenching of excited nitrogen. The results from laser induced fluorescence (LIF) have shown that after generation by the plasma the OH persists at significant levels for a long time that lengthens with increasing temperature. The ~100 μs-long plateau clearly indicates the existence of chain reactions at low temperature (starting at 500 K for alkanes mixtures, 400 K for hydrogen mixtures), which are not predicted in current kinetic models. The results from the planar laser-induced fluorescence (PLIF) study have confirmed the unique phenomena and also demonstrated uniform OH radical distribution along the discharge channel. Comparison of OH radical emission dynamics with discharge emission dynamics from excited nitrogen revealed a close similarity in spatial distribution and allowed clarification of the mechanisms of atomic oxygen formation. The third laser diagnostics, Cavity Ring-Down Spectroscopy (CRDS), which is an absorption spectroscopy, demonstrated consistent results and further validated the unpredicted results. The temperature measurements through Optical Emission Spectroscopy (OES) of the second positive nitrogen system revealed no significant heating by the plasma. Further studies were focused on identifying the key reaction pathways to reveal the chemical mechanism of the unpredicted phenomena. The roles of nitric oxide and vibrationally excited nitrogen have been experimentally studied. The results have ruled out the possibility of nitric oxide as the key species and have provided support suggesting that vibrationally excited nitrogen could play a major role in the observed OH plateau effect.
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پایان نامه ,
دکتری تخصصی ,
پلاسما ,
احتراق ,
موتور ,
موتور احتراق داخلی ,
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تاریخ انتشار : چهار شنبه 13 اسفند 1395 |
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پایان نامه با موضوعNon-equilibrium Plasma-Assisted Combustionدر سطح دکتری تخضضخلاصهAs a promising method to enhance combustion, plasma-assisted combustion has drawn considerable attention. Due to the fast electron impact excitation and dissociation of molecules at low temperatures, plasma introduces new reaction pathways, changes fuel oxidation timescales, and can dramatically modify the combustion processes. In this dissertation, the radical generation from the plasma and its effect on flame extinction and ignition were investigated experimentally together with detailed numerical simulation on a counterflow CH4 diffusion flame. It was found that the atomic oxygen production played a dominant role in enhancing the chain-branching reaction pathways and accelerating fuel oxidation at near limit flame conditions. To understand the direct coupling effect between plasma and flame, a novel plasma-assisted combustion system with in situ discharge in a counterflow diffusion flame was developed. The ignition and extinction characteristics of CH4/O2/He diffusion flames were investigated. For the first time, it was demonstrated that the strong plasma-flame coupling in in situ discharge could significantly modify the ignition/extinction characteristics and create a new fully stretched ignition S-curve. To understand low temperature kinetics of combustion, it is critical to measure the formation and decomposition of H2O2. A molecular beam mass spectrometry (MBMS) system was developed and integrated with a laminar flow reactor. H2O2 measurements were directly calibrated, and compared to kinetic models. The results confirmed that low and intermediate temperature DME oxidation produced significant amounts of H2O2. The experimental characterizations of important intermediate species including H2O2, CH2O and CH3OCHO provided new capabilities to investigate and improve the chemical iv kinetics especially at low temperatures. A numerical scheme for model reduction was developed to improve the computational efficiency in the simulation of combustion with detailed kinetics. A multigeneration Path Flux Analysis (PFA) method for kinetic mechanism reduction is proposed and validated. In this method, the formation and consumption fluxes of each species at multiple reaction path generations were analyzed and used to identify the important reaction pathways. The comparisons of the ignition delays, flame speeds, and flame structures showed that the PFA method presented a higher accuracy than that of current existing methods in a broad range of initial pressures and temperatures.
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پایان نامه ,
پلاسما احتراق ,
احتراق و پلاسما ,
موتور احتراق داخلی ,
دکتری تخصصی ,
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تاریخ انتشار : چهار شنبه 23 بهمن 1395 |
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نام کامل پایان نامه: COMBUSTION INITIATION BY ELECTRICAL-DISCHARGE-INDUCED PLASMA IN LEAN AND DILUTE METHANE-AIR MIXTURE: EXPERIMENTAL AND MODELING INVESTIGATIONدر سطح دکتری تخصصیدانشگاه MICHIGAN TECHNOLOGICAL UNIVERSITY سال 2014خلاصه:This dissertation represents experimental and numerical investigations of combustion initiation trigged by electrical-discharge-induced plasma within lean and dilute methaneair mixture. This research topic is of interest due to its potential to further promote the understanding and prediction of spark ignition quality in high efficiency gasoline engines, which operate with lean and dilute fuel-air mixture. It is specified in this dissertation that the plasma to flame transition is the key process during the spark ignition event, yet it is also the most complicated and least understood procedure. Therefore the investigation is focused on the overlapped periods when plasma and flame both exists in the system. Experimental study is divided into two parts. Experiments in Part I focuses on the flame kernel resulting from the electrical discharge. A number of external factors are found to affect the growth of the flame kernel, resulting in complex correlations between discharge and flame kernel. Heat loss from the flame kernel to code ambient is found to be a dominant factor that quenches the flame kernel. Another experimental focus is on the plasma channel. Electrical discharges into gases induce intense and highly transient plasma. Detailed observation of the size and contents of the discharge-induced plasma channel is performed. Given the complex correlation and the multi-discipline physical/chemical processes involved in the plasma-flame transition, the modeling principle is taken to reproduce detailed transitions numerically with minimum analytical assumptions. Detailed measurement obtained from experimental work facilitates the more accurate description of initial reaction conditions. The novel and unique spark source considering both energy and species deposition is defined in a justified manner, which is the key feature of this Ignition by Plasma (IBP) model. The results of numerical simulation are intuitive and the potential of numerical simulation to better resolve the complex spark ignition mechanism is presented. Meanwhile, imperfections of the IBP model and numerical simulation have been specified and will address future attentions.
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پایان نامه ,
مهندسی مکانیک ,
دکتری تخصصی ,
پلاسما ,
موتور احتراقی ,
موتور احتراق داخلی ,
CI ,
SI ,
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تاریخ انتشار : چهار شنبه 23 دی 1395 |
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