Investigating the Thermal Effect of Nanosecond Repetitively Pulsed Glow Discharges on a Methane-Air Flame by Coherent Anti-Stokes Raman Scattering and Optical Emission Spectroscopy

Nanosecond Repetitively Pulsed (NRP) glow discharges have been shown to alter combustion characteristics such as ignition, flame speed, lean stability and thermoacoustic instability, with a minimal electrical power. However, the pathways in which the discharges affect the flames are not fully understood. In this work, the thermal effect of NRP glow discharges applied to a methane-air flame is investigated. 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. The temperature of nitrogen in the discharge region was also measured using Optical Emission Spectroscopy (OES) of the second positive system of nitrogen. The spatial profile of temperature and the oxygen-to-nitrogen concentration ratio in the discharge region as well as the special and temporal profiles of the vibrational temperature of electronically excited nitrogen N2(C) are reported. The temperature and oxygen-to-nitrogen concentration ratio profiles in the discharge region were found to be in steady state during and in between discharges within the uncertainty of the measurements. The results suggest that NRP glow discharges have a negligible thermal effect on flames, indicating that the discharges influence flame characteristics through non-thermal pathways.