Since being discovered, combustion of fuels, especially fossil fuels in the last centuries, has been the dominant source of energy for human life. However, over the years, the adverse effects and shortcomings caused by the vast utilization of these energy sources have been observed; the three most important of which are unreliable resources, unfavorable natural outcomes, and limited performance. Using biofuels is one of the well-established proposed solutions to the scarcity and environmental issues of fossils as they are sustainable sources of energy with acceptable and even superior combustion characteristics. As a second-generation biofuel, anisole has shown promising results with high flame speed and high knock resistance. Therefore, the first chapter of this thesis is focused on experimental investigation of anisole laminar burning speed and stability properties so that it can be used as a benchmark for future kinetic mechanism validations. Stability is another important parameter in combustion systems, especially in diffusion jet flame combustion as used in many applications like thrusters or burners. Different methods are applied to improve the stability of such diffusion flames in propulsion systems, e.g., changing geometrical or flow characteristics of the burner. Most of these efforts have not been practically successful, due to the cost and compatibility issues. Another technique which minimizes such problems is to use electron impact excitation, dissociation and ionization and generate highly concentrated charged/excited species and active radicals. These methods include microwave, dielectric barrier, and repetitive nanosecond pulsed (RNP) discharge and the latter has shown promising results as one of the most effective low-temperature plasma (LTP) methods. In chapters 3 to 5, the benefits and issues associated with using RNP discharge in a single-element concentric methane-air inverse diffusion jet flame are discussed. It has been shown that RNP discharge with adequate discharge properties (voltage and repetition) can increase the stability of the flame and expand the flammability of the jet toward leaner compositions. However, the effectiveness is significant in a certain voltage-frequency ranges which results a non-thermal spark discharge mode. Hence, different modes of discharge were investigated and a parametric study on the transition between these modes were done.

Current concepts of flame propagation in premixed, turbulent gas streams are examined. This leads to the conclusion that the link between theory and experiment is entirely inadequate and incapable of improvement by existing methods. A series of new method is implemented in an attempt to short-circuit the unprofitable chain of hypothesis and experiment which has hampered the identification of dubious steps. Methods of introducing uniform turbulence at relatively slow flows and improvements in light sources allow analysis of the approach flow by photographing particles illuminated by an interrupted Tyndall beam. Three new optical deflection methods are used to give a measure of the randomness of flame-front orientation, of the time-mean structure of the flame and of the instantaneous shape of the corrugated front. It is found that this corrugated surface propagates at a velocity considerably in excess of the normal laminar burning velocity. Quantitative analysis of the frequency of 'peaks' and 'valleys' on the surface, together with comparative data from the apex of laminar flames, suggests an explanation in terms of the effects of curvature and, secondarily, of the influence of small scale turbulence. (Author).

"Alternative fuels and alternative refrigerants have attracted a lot of attention as many are deemed to be environmentally friendly. Consequently, the combustion behavior of fuels such as Syngas, Biogas, Liquified petroleum gas (LPG), and Gas to liquid (GTL) premixed flames were studied. In this investigation, the laminar burning speed and the flame instability of alternative fuels and refrigerants and different diluents (Exhaust gas recirculation (EGR), Helium, CO2) mixtures were evaluated both experimentally and numerically. Experiments were conducted using a spherical vessel to measure laminar burning speed and a cylindrical vessel to investigate flame instability. The cylindrical vessel is set up in a Z-shape Schlieren system, coupled with a high-speed CMOS camera that is used to capture evolutionary behavior of flames at up to 40,000 frames per second (around 2000 frames per second in general case). Upon ignition, the pressure rises as a function of time, during flame propagation in the spherical chamber, is the primary input of a multi-shell thermodynamic model, used to calculate the laminar burning speed for smooth flames. Power law correlations were developed for experimental burning speed results of different combustible mixtures over a wide range of equivalence ratios, temperatures, pressures, and diluent concentrations. For the onset of flame instability, a correlation for the ratio of critical pressure to initial pressure of syngas/air/diluent flames over a wide range of initial temperatures, initial pressures, equivalence ratios, diluent concentrations, and hydrogen percentages were developed. Kinetics simulations calculated by 1-D steady state flame code from CANTERA were compared with various experimental burning speed results"--Author's abstract.

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.