Effects of non-equilibrium plasmas on low-pressure, premixed flames. Part 1: CH* chemiluminescence, temperature, and OH

Abstract

In this paper, we investigate 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 using CH* chemiluminescence and quantitative OH laser-induced fluorescence (LIF) diagnostics. Two different plasma sources, both of which generate uniform, low-temperature, volumetric, non-equilibrium plasma discharges, are used to study changes in chemiluminescence, temperature, and OH concentration when non-equilibrium plasmas are directly coupled to conventional hydrogen/hydrocarbon oxidation and combustion chemistry. Qualitative imaging of CH* chemiluminescence indicates that during plasma discharge, the luminous flame zone is shifted upstream towards the burner surface with little change in the CH* zone thickness. For the same plasma discharge and flame conditions, quantitative results using spatially-resolved OH LIF and multi-line, OH-LIF thermometry show significant increases in ground-state OH concentrations in the preheating zones of the flame. More specifically, for a particular axial position downstream of the burner surface, the OH concentration increases, which can be viewed as an effective “shift” of the OH profiles towards the burner surface. The increase in OH concentration is conceivably due to an enhancement of the lower-temperature kinetics including O atom, H atom and OH formation kinetics and temperature rise due to the presence of the low-temperature, non-equilibrium plasma.