Simultaneous CH3 and CH2O imaging in premixed turbulent flames for instantaneous flame structure visualization

Photofragmentation laser-induced fluorescence (PF-LIF) of methyl radical (CH3) and planar laser-induced fluorescence (PLIF) of formaldehyde (CH2O) were simultaneously carried out in premixed methane-air turbulent flames to visualize flame structure and extract quantitative features of CH3 and CH2O layers. A CH3 layer is located along the edge of a CH2O layer, and they partially overlap with each other. CH3 is closer to the reaction zone of a premixed flame compared with CH2O that is regarded as a typical marker of the preheating zone. The results show that the CH3 can be served as a flame marker for the transitional zone between the preheating zone and the reaction zone. The thicknesses of CH3 and CH2O layers and their distances were then quantitatively estimated to reduce the influence of layer merging and strong curvature. It was found that the broadening of CH2O layers against turbulence is more sensitive than that of CH3 layers. In stoichiometric and rich flames, layer broadening was clearly observed even with a slightly increased Karlovitz number.