Study of Microwave Plasma Enhanced Methane Flame at Atmospheric Pressure*

Non-thermal plasma assisted combustion can provide potential accommodation in improving fuel efficiency, contaminant reduction, faster ignition time, etc. A 2.45 GHz microwave (MW) plasma source was used with a premixed He/CH$_{4}$ gases to study the effect of MW power coupling and hence OH radical generation. UV pulsed laser cavity ringdown spectroscopy was employed to measure absolute number density of OH (A-X) (0-0) band in plasma enhanced flame. Emission species such as OH(A-X), N$_{2}$(C-B), N$_{2}^{+}$(B-X) and C$_{2}$ swan band were observed using optical emission spectroscopy. Depending on the mixing ratio of CH$_{4 }$and MW power, two kinds of CH$_{4}$ flames were obtained; (1) Flame with coupled MW energy but no pre-flame (flame and plasma interaction region); at high CH$_{4}$/He mixing ratios and low MW energies, detached flame were obtained with detaching gap depended on MW power. (2) Flame with visible pre-flame region: at low CH$_{4}$/He mixing ratios and high MW energies. In both the cases total flame volume increased with increase in MW energy. Compared to the flame, OH concentration was higher in the pre-flame.