Laser-induced breakdown spectroscopy as an analytical tool for equivalence ratio measurement in methane-air premixed flames

Laser-Induced Breakdown Spectroscopy has been applied to laminar premixed methane-air flames in order to obtain, for the first time, quantitative measurements of atomic species densities in a reacting flow. The electron density and the plasma temperature were determined and it was shown that the hypothesis of local thermodynamic equilibrium is valid under the present experimental conditions. It was further shown, that the equivalence ratio ϕ of methane-air mixtures can be quantitatively determined from the ratio of the number densities NH/NO, the latter being obtained from the ratio of the hydrogen (Hα at 656 nm) and oxygen (O(I) at 777 nm) atomic lines intensities.