The structure of toluene-doped counterflow gaseous diffusion flames

Abstract

The structure of gaseous counterflow diffusion flames perturbed with the addition of hundreds of ppm of prevaporized toluene is studied in two distinct flame environments: a blue methane flame stabilized on the fuel side of the gas stagnation plane and an incipiently sooting ethylene flame stabilized on the oxidizer side. The goal is to provide a well-defined testbed in terms of temperature–time history, major species and part of the radical pool, for the examination of reference fuels that are critical components of practical fuel blends. Gas samples are extracted from the flame with fused silica microprobes for subsequent GC/MS analysis and thermocouples and thin filament pyrometry are used to characterize the temperature field. Profiles of critical toluene pyrolysis products and stable soot precursors are compared with computational models using two semi-detailed chemical mechanisms. Results show that in the methane flame some oxygen containing radicals like O and OH are contributing early on to the toluene destruction path. In the incipiently sooting ethylene flame, the primary attack is from H alone. This finding confirms the different challenges that such flames pose to the validation of a chemical kinetic mechanism. The onset of toluene decay in these flames begins at relatively modest temperatures, on the order of 800K. This reactivity is captured reasonably well by both chemical mechanisms in the methane flame, in the absence of reactants larger than C2, but not so in the ethylene flame, in the presence of a richer, more complex mixture. The aromatic ring opening mechanisms are not adequately modeled in either case. This discrepancy has implications for the modeling of practically relevant fuel blends with both aliphatic and aromatic compounds. The dominant species larger than toluene in the doped methane flame is ethylbenzene, which at least one of the mechanisms reproduces quite well. The largest measured species in the incipiently sooting flame is indene, whose concentration increase due to toluene addition is properly captured by one of the models. The experimental dataset reported here may help identifying future improvements to chemical kinetic mechanisms and complement other reactor datasets lacking the coupling of kinetics and transport of flame environments.