Effect of the Methyl Substitution on the Combustion of Two Methylheptane Isomers: Flame Chemistry Using Vacuum-Ultraviolet (VUV) Photoionization Mass Spectrometry

Alkanes with one or more methyl substitutions
are commonly found
in liquid transportation fuels, so a fundamental investigation of
their combustion chemistry is warranted. In the present work, stoichiometric
low-pressure (20 Torr) burner-stabilized flat flames of 2-methylheptane
and 3-methylheptane were investigated. Flame species were measured
via time-of-flight molecular-beam mass spectrometry, with vacuum-ultraviolet
(VUV) synchrotron radiation as the ionization source. Mole fractions
of major end-products and intermediate species (e.g., alkanes, alkenes,
alkynes, aldehydes, and dienes) were quantified axially above the
burner surface. Mole fractions of several free radicals were also
measured (e.g., CH3, HCO, C2H3, C3H3, and C3H5). Isomers of
different species were identified within the reaction pool by an energy
scan between 8 and 12 eV at a distance of 2.5 mm away from the burner
surface. The role of methyl substitution location on the alkane chain
was determined via comparisons of similar species trends obtained
from both flames. The results revealed that the change in CH3 position imposed major differences on the combustion of both fuels.
Comparison with numerical simulations was performed for kinetic model
testing. The results provide a comprehensive set of data about the
combustion of both flames, which can enhance the erudition of both
fuels combustion chemistry and also improve their chemical kinetic
reaction mechanisms.