Structure and Propagation of Methanol–Air Triple Flames

Abstract

The structure and propagation for a methanol (CH 3OH)–air triple flame are studied using direct numerical simulations (DNS). The methanol (CH 3OH)–air triple flame is found to burn with an asymmetric shape due to the different chemical and transport processes characterizing the mixture. The excess fuel, CH 3OH, on the rich premixed flame branch is replaced by more stable fuels CO and H 2 which burn at the diffusion flame. On the lean premixed flame side, a higher concentration of O 2 leaks through to the diffusion flame. The general structure of the triple point features the contribution of both differential diffusion of radicals and heat. A mixture fraction–temperature phase plane description of the triple flame structure is proposed to highlight some interesting features in partially premixed combustion. The effects of differential diffusion at the triple point add to the contribution of hydrodynamic effects in the propagation of the triple flame. Differential diffusion effects are measured using two methods: a direct computation using diffusion velocities and an indirect computation based on the difference between the normalized mixture fractions of C and H. The mixture fraction approach does not clearly identify the effects of differential diffusion, in particular at the curved triple point, because of ambiguities in the contribution of carbon and hydrogen atoms’ carrying species.