posted on 2022-12-21, 21:05authored byKevin Gleason, Alessandro Gomez
We
perform spatially resolved measurements of temperature,
gaseous
species up to three-ring Polycyclic Aromatic Hydrocarbons (PAHs),
and soot in atmospheric pressure counterflow diffusion flames. First,
we characterize fully a baseline ethylene flame and then a toluene-seeded
flame in which an aliquot of ethylene in the feed stream is replaced
with 3500 ppm of prevaporized toluene. The goal is twofold: to investigate
the impact of a common reference fuel component of surrogates of transportation
fuels and bypass the main bottleneck to soot formation from aliphatic
fuels, that is, the formation of the first aromatic ring. The composition
of the fuel and oxidizer streams are adjusted to maintain a constant
stoichiometric mixture fraction and global strain rate, thereby ensuring
invariance of the temperature–time history in the comparison
between the two flames and decoupling the chemical effects of the
fuel substitution from other factors. Major combustion products and
critical radicals are fixed by the baseline flame, and profiles of
critical C2–C5 species precursors to aromatic formation are
invariant in both flames. On the other hand, doping with toluene boosts
the aromatic content and soot volume fraction, increasing the mole
fraction of benzenoid structures and soot volume fraction by a factor
of 2 or 3, relative to the baseline ethylene flame. This finding is
consistent with the expectation that the formation of the first aromatic
ring is no longer a bottleneck to soot formation in the doped flame.
In addition, toluene bypasses completely benzene formation, opening
a radical recombination pathway to soot precursors through the production
of C14H14 (via dimerization of benzyl radical)
and pyrene (through dimerization of indenyl radical).