Pump-probe strategy for instantaneous 2D detection of CH₃ in flames using a single laser

Visualization of the reaction zone of flames using CH radicals as markers is restricted by the low concentration of CH in fuel-lean conditions. To address this, methyl radicals (CH₃) are employed as a substitution of CH in premixed methane/air flames. A pump-probe method was adopted with the pump laser photolyzing CH₃ and the probe laser detecting the photolyzed CH (X²Π) fragments. Laser excitation scans were performed to ensure that the fluorescence detected was from CH only. Visualization of the reaction zone of flames was accomplished by a CH₃ photofragmentation laser-induced fluorescence technique in fuel-lean conditions (the equivalence ratio of 0.4), where CH planar laser-induced fluorescence did not work in both laminar and turbulent jet flames. The proposed pump-probe method of CH₃ can be used to visualize the reaction zone of hydrocarbon combustion under both fuel-lean and fuel-rich conditions with a superior signal-to-noise ratio.