Temperature and methyl radical measurements in NRP glow discharges for combustion enhancement*

While nonequilibrium plasma discharges have showed promising abilities to enhance combustion, the mechanisms of their effect on the reactive front remain under investigation. These effects can be classified into three main categories, thermal, chemical, and transport, that are usually coupled. In this study we investigate in a canonical configuration the thermal effect as well as the production of methyl radical (CH₃) by nanosecond repetitively pulsed (NRP) glow discharges, applied across a laminar methane-air flame at atmospheric pressure. A wall-stabilized configuration with the discharges generated on the symmetric axis of the burner is chosen to allow phase-locked averaging as well as comparisons with two-dimensional axis symmetric simulations. Hybrid fs/ps rotational Coherent Anti-Stokes Raman Scattering (CARS) measurements are used to determine the temperature of ground state nitrogen and the oxygen-to-nitrogen concentration ratio in the discharge region. Photofragmentation Laser Induced Fluorescence (LIF) is utilized to image the spatial distribution of methyl in the discharge region. Results show that in this configuration, the thermal effect is minimal while the production of methyl radical upstream of the flame front can reach 1200 ppm, i.e., twice the concentration that is predicted in the flame without plasma. These results are put into perspective to previous measurements performed on this system, more specifically on the spatial distribution of atomic oxygen (O), atomic hydrogen (H), hydroxyl radical (OH), and methylidyne radical (CH). A discussion on the chemical pathway of plasma-assisted oxidation of methane is then proposed.