Direct-Coupled Plasma-Assisted Combustion Using a Microwave Waveguide Torch

A tunable microwave waveguide is used to initiate and enhance combustion by coupling an atmospheric plasma discharge to a premixed methane/air flame. The absorbed microwave power ranges from 60 to 150 W, which was generated from a continuous source operating at 2.45 GHz, whereas combustion power ranges from 200 to 1000 W. OH radical number densities were measured using planar laser-induced fluorescence (PLIF), and temperatures were measured using Rayleigh scattering thermometry for various flow rates, equivalence ratios, and power levels. Increases in reaction volume, OH density, and temperature were observed as power increased. In the plasma-coupled premixed flame, OH number densities, which are quantified on the order of 10¹⁶ cm^-3, increased by up to 50%, and temperature ranging from 2000 to 3000 K increased by up to 40% as the absorbed microwave power was increased from 60 to 130 W. Air-only plasma discharges exhibited a much greater temperature increase, i.e., up to 190%. The power associated with the measured temperature increases varied greatly with flow and input power but are typically three to four times greater in the air-only plasma compared to the flame coupled plasma, demonstrating a greater degree of nonthermal mechanisms present in plasma-enhanced flame discharge.