Fluid-mechanical structure of methane-air and propane-air turbulent premixed flames

Abstract

The flow fields of lean and rich propane-air and methane-air turbulent premixed flames of the identical conditions of laminar burning velocity and characteristic of turbulence have been closely examined by using a three-color six-beam LDV system. New finding of the present measurements is the now-well-established clear difference between the turbulent flame structure of the rich propane flame and the others. Hypothesized in an earlier paper, on the basis of measurements of flamelet motion, the present work verifies the existence of this phenomenon through measurements of gas velocities. The data for the rich and lean methane flames are essentially the same, not only are the burnt-gas-conditioned radial velocity components for the rich propane flame higher than those for the methane flames, but also the same conditioned values for the lean propane flame are appreciably lower than those of the methane flames. When the deficient reactant has a higher diffusion coefficient it will diffuse preferentially to turbulence-generated wrinkles, thereby increasing the local burning velocity there, with a resulting positive increment in the turbulent burning velocity, while if the deficient reactant has a lower diffusion coefficient, its diffusion to a wrinkle is preferentially retarded, thereby decreasing the local burning velocity and contribution a negative increment to the turbulent burning velocity. Preferential diffusion, thus, exerts measurable influences on turbulent flame structures in the reaction sheet regime.