Experimental Study of the Effects of Nanosecond-Pulsed Non-equilibrium Plasmas on Low-Pressure, Laminar, Premixed Flames.

摘要

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/N 2 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), multiline, 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.