Determination of methyl radical concentrations in a methane/air flame by infrared cavity ringdown laser absorption spectroscopy

Infrared cavity ringdown laser absorption spectroscopy (IR-CRLAS) is employed to determine absolute methyl radical concentrations in a 37.5 Torr laminar methane/air flame. IR-CRLAS rovibrational absorption spectra of the ${\nu }_3$ fundamental band system near $\text{3200\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}}$ are combined with ${\mathrm{N}}_{\mathrm{2}}\mathrm{-CARS}$ temperature measurements to obtain methyl radical concentrations as a function of height above the burner surface. These data are compared with flame chemistry simulations under both stoichiometric and rich flame conditions. Issues regarding the applicability of IR-CRLAS for combustion studies are discussed, including the uncertainties present for the specific case of methyl radical. These IR-CRLAS measurements indicate the ability to monitor reactants, intermediates, and products within a narrow spectral window, and, to our knowledge, constitute the first infrared detection of a polyatomic radical in a flame.