Plasma enhanced combustion using microwave energy coupling in a re-entrant cavity applicator

An atmospheric compact high-Q microwave applicator is used to couple electromagnetic energy directly into the reaction zone of a premixed laminar methane-oxygen flame for energetic enhancement. At low microwave powers (1 to 5 W), the flame is influenced by an electromagnetic field only. As power is increased, the reactive gases in the flame break down and ionize into a plasma plume with significant increase in the flammability limit. 2-D laser induced fluorescence imaging of hydroxyl radicals (OH) is conducted in the reaction zone over this transition, as well as spectrally resolved flame emission measurements to monitor excited state species and derive rotational temperatures using OH chemiluminescence. Measurements are made for two equivalence ratios (φ = 0.9 and 1.1) and two gas flow rates (60 sccm and 100 sccm). In the electromagnetic field only phase (1 to 5 W), flame stability, excited state species, and temperature slightly increased with power while no significant change in OH radicals was detected. With the onset of a plasma plume, a significant rise in both excited state species and OH radical number density was observed.