Methyl Radical Imaging in Methane–Air Flames Using Laser Photofragmentation-Induced Fluorescence

Abstract

Imaging detection of methyl radicals has been performed in laminar premixed methane–air flames at atmospheric pressure. A nanosecond Q-switched neodymium-doped yttrium aluminum garnet (Nd:YAG) laser was employed to provide the fifth-harmonic-generated 212.8 nm laser beam. The intense ultraviolet (UV) laser pulse was sent through the flame front to photodissociate the methyl (CH3) radicals in the reaction zone of the flames stabilized on a piloted jet flame burner. The emission spectra from the photodissociated fragments were collected using an imaging spectrometer with the flame-front structure spatially resolved. Combining the spatial and spectral information, we recognized that the emission from the ( A-X) methine (CH) transitions located at 431 nm was generated from the CH3 photolysis and could be used to visualize the distribution of CH3 radicals. With proper filtering, the high-power UV laser (around 15 mJ/pulse) provided by the compact Nd:YAG laser makes it possible to visualize CH3 distribution naturally generated in the reaction zone of laminar methane–air premixed flames.