Chemical effects of 1,2,4-trimethyl benzene addition in counterflow gaseous diffusion flames

Abstract

The structure of gaseous counterflow diffusion flames perturbed with the addition of hundreds of ppm of prevaporized 1,2,4-trimethyl benzene (TMB) is studied in two distinct flame environments: a blue methane flame and an incipiently sooting ethylene flame. The two flames provide well defined temperature-time histories and chemical environments to investigate the behavior of complex fuels and complement other reacting environments lacking the coupling of kinetics and transport that is typical of flame environments. Profiles of critical pyrolysis products and of some stable soot precursors are determined from GC/MS analysis of gas samples extracted from the flames and compared with results from the OPPDIFF model using a semi-detailed chemical mechanism. Experimentally, because of the presence of aliphatic fragments, TMB reactivity is enhanced in these flames with the onset of TMB decay beginning at relatively modest temperatures, on the order of 800K. The dominant path to stable species is driven by H radical attack. It leads in sequence to xylenes, toluene (through benzyl radical) and benzene formation. This enhanced reactivity is captured reasonably well by the model in the methane flame, but not in the ethylene flame, in the presence of a richer, more complex mixture. The model does not reproduce accurately the pathway yielding C3 and some C4 species from TMB cracking. Aromatic ring opening is the bottleneck in the TMB cracking process in the methane flame but not in the ethylene one. Indene, an important soot precursor for monoaromatic fuels since the second aromatic ring formation is considered to be a bottleneck in the process, is measured in the ethylene flame in poor agreement with the model predictions. The dataset presented here and available supplemental data online may help identifying improvements to the chemical kinetic mechanism of this reference fuel.