  The formation mechanisms of aromatic compounds in flame are strongly
influenced by the chemical and thermal history that leads to their formation.
Indeed, the complex environments that characterize combustion systems do not
only affect the composition of gas-phase species, but they also determine the
structure and the characteristics of the soot precursors generated. To
illustrate the importance of these effects, in this work we investigate the
growth mechanisms of soot precursors in an atmospheric-pressure
ethylene/oxygen/argon counterflow diffusion flame, using a combination of
computational and experimental techniques. In diffusion flames, flow
characteristics play an important role in the formation, growth, and oxidation
of particles, and soot precursors are strongly affected by the flame location.
Fluid dynamics simulations and stochastic discrete modeling were employed
together to identify key reaction pathways along various flow streamlines. The
models were validated with experimental mass spectra obtained using aerosol
mass spectrometry coupled with vacuum-ultraviolet photoionization. Results show
that both the hydrogen-abstraction-acetylene-addition mechanism and
oxygen-insertion reactions are responsible for the molecular growth, and their
relative importance is determined by the flame conditions along the
streamlines. Oxygenated species were detected in regions of high temperature,
high atomic oxygen concentration, and relatively low acetylene abundance. This
study also emphasizes the need to model the counterflow flame in three
dimensions to capture the spatial dependence on growth mechanisms of soot
precursors.
