Pressure effects on incipiently sooting partially premixed counterflow flames of ethylene

Abstract

We studied the effect of pressure on the flame structure of incipiently sooting partially premixed counterflow ethylene flames. Oxygen was added to the fuel stream of a purely diffusion flame and removed from the oxidizer side to lower the overall equivalence ratio at constant stoichiometric value of the mixture fraction. These rich conditions lead to a two-stage combustion mode, with substantial amounts of CO and H2 in the post-premixed flame region burning in the diffusion flame part. Soot was formed in the region straddling the gas stagnation plane and sandwiched between the two flame components. We quantified profiles of species as large as 3-ring aromatics. Using C6H6 as a qualitative marker of the flame soot load, we observed that under partially premixing an eightfold pressure increase lead to a substantial increase in the concentration of C6H6 despite the fact that the flame peak temperature was deliberately decreased by hundreds of degrees to preserve conditions of incipient sooting compatible with gas microsampling. Part of the increased soot load with pressure was attributed to a diminished back diffusion of oxidizing radicals like OH from the diffusion flame side towards the gas stagnation region where the soot was formed. Benzene mole fraction and visible soot luminosity increased with the lowering of the equivalence ratio regardless of the pressure because of an increase in temperature in the region between the two flame components of the double flame structure, with attending enhancement of pyrolytic growth reactions. A comparison of the experimental results with one-dimensional modeling of the flames, revealed good agreement for major species, some key intermediates and benzene, in the latter case for just one of the two tested chemical kinetic mechanisms. Reaction path analysis revealed an increased role of the C2/C4 path in the formation of C6H6 as the pressure increased.