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An atmospheric high-Q re-entrant cavity applicator is used to couple microwave (2.45 GHz) electromagnetic energy directly into the reaction zone of a premixed laminar methane–oxygen flame for flame enhancement. As microwave energy increases, a transition from electric field enhancement to microwave plasma discharge is observed. At low microwave powers (1–5 W), the flame is influenced by an electromagnetic field only. When power is increased, ionization and eventually breakdown of gas molecules result in a plasma plume with significant increase in the flammability limit. 2-D laser induced fluorescence imaging of hydroxyl radicals (OH) and carbon monoxide (CO) are 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 for a range of equivalence ratios ( ϕ = 0.9–1.1) and total flow rates. In the electromagnetic field only phase (1–5 W), flame stability, excited state species, and temperature slightly increased with power while no significant change in OH number density was detected. With the onset of a plasma plume, a significant rise in both excited state species, CO and OH number density was observed. The importance of in-situ fuel reforming in plasma coupled flames is shown through the concentration of CO, which increases ∼18% with 30 W microwave power. |
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