Highly strained turbulent rich methane flames stabilized by hot combustion products

Abstract

The structure of rich turbulent methane flames, with an equivalence ratio of 2.0, at high strain rates has been investigated based on simultaneous and instantaneous line profile measurements of the species mass fractions of CH 4, O 2, N 2, H 2O, CO 2, CO, H 2 and OH radical, together with gas temperature. The flames are stabilized by the hot combustion products from a stoichiometric large pilot flame surrounding a Bunsen burner. Two flames have been investigated at overall stretch rates of 2500 s −1 and 4167 s −1. One-dimensional combined UV Raman, Rayleigh, and laser-induced predissociation fluorescence (LIPF) technique has been applied. Measurements at different axial positions enable the flame structure to be studied at different turbulence levels. The instantaneous species mass fractions and temperature profiles are conditioned on both mixture fraction and a reaction progress variable, namely the H 2O mass fraction, allowing a proper description of the partially premixed flame structure. The components of the scalar dissipation rate of both mixture fraction and progress variable in the radial direction are also measured. The flames are stabilized at rich conditions by radicals and heat transferred from the stoichiometric pilot flame. Stretch effects are found to be dominant in these flames, leading to local flame structures that correspond to flames in the distributed-reaction-zones regime. The pdf of the progress variable is found to be monomodal in all cases. The conditional mean scalar dissipation rate profile in mixture fraction space exhibits a local minimum at the reaction zone due to differential polynary diffusion effects. The scalar dissipation rates of the progress variable are of the same order of magnitude as those of the mixture fraction, when these are appropriately normalized. This indicates that molecular mixing in both variables is equally important. This also suggests that a two-variable formulation is necessary to model these flames.