Experimental study of extinction and its quantification in laminar and turbulent counterflow CH 4–N 2/O 2–N 2 nonpremixed flames

Abstract

We investigated and quantified the extinction phenomena of laminar and turbulent counterflow CH 4/N 2–O 2/N 2 nonpremixed flames. Experimental conditions such as the stoichiometric mixture fraction, the molar stoichiometry, and the turbulent flow characteristics are widely modulated. For a given set of experiments, the molar ratio of CH 4–O 2–N 2 is held fixed, hence fixing the adiabatic flame temperature. The bulk velocity gradient at extinction, i.e., the critical strain rate, increases monotonically with increasing stoichiometric mixture fraction regardless of the initial turbulent flow property. For all the experimental conditions, extinction is more sensitive to variation in the mass fraction of oxygen in the oxidizer stream than that of methane in the fuel stream. If the extinction limit is quantified by the bulk velocity gradient, turbulence levels affect the extinction limit significantly when higher turbulence encounters the reaction zone directly. Estimations of the scalar dissipation rate at extinction are obtained from the experimentally observed quantities founded on the theoretical consideration of the flame characteristics. For the laminar counterflow nonpremixed flames, the critical value of the scalar dissipation rate at extinction, χ q, scaled with that of the CH 4–air laminar counterflow nonpremixed flames by the ratio of the bulk velocity gradient at extinction. This scaling is a suitable approximation for the present study. As to the turbulent flames, the scalar dissipation rates at extinction of the same stoichiometric mixture fraction are estimated including the contributions of both turbulent effect and mean flow effect. These show good agreement regardless of the initial turbulent flow condition.