Effects of Continuous Volumetric Direct-Coupled Nonequilibrium Atmospheric Microwave Plasma Discharge on Swirl-Stabilized Premixed Flames

Abstract

The effect of continuous volumetric direct-coupled nonequilibrium atmospheric microwave plasma discharge on swirl-stabilized premixed methane–air flames was investigated using quantitative OH planar laser-induced fluorescence and spectrally resolved emission. The plasma discharge was found to influence the dynamics of flame stabilization, i.e., plasma-assisted flames stabilized in the quiescent center body wake were relatively stable while swirl flames stabilized in the active inner shear layer were prone to local extinction due to aerodynamic shear. At coupled plasma powers corresponding to less than 3% of the thermal power output, in addition to the improved flame stability, significant improvement in the lean blow-out limit ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 43$ </tex-math></inline-formula> %) and OH number density ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 150$ </tex-math></inline-formula> %) was observed. The enhancements are shown to be nonequilibrium plasma effects and not predominantly ohmic heating as significant equivalence ratio dependence of OH number density in plasma-assisted flames was observed. Spectrographic measurements indicated nitrogen vibrational temperatures as high as 6100 K, suggesting both vibrational and electronic excitation of nitrogen molecules in the presence of a plasma discharge. The activation of highly reactive species through vibrational–vibrational relaxation and direct impact dissociation accelerates the combustion chemistry. It is demonstrated that microwave direct plasma coupling can drastically enhance the dynamic flame stability of swirl-stabilized flames, especially at very lean operating conditions.