Experimental and kinetic modeling study of premixed o-xylene flames

Three premixed o-xylene/O2/Ar flames with various equivalence ratios (0.75, 1.00 and 1.79) have been carried out at low pressure (4.0 kPa). Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) was used for the identification of flame species and the measurement of their mole fractions. A detailed kinetic model consisting of 236 species and 1331 reactions was developed and validated against the measured mole fraction profiles of flame species. According to the rate of production analysis, o-xylene mainly decomposes via radical attack reactions, including the H-abstraction and ipso-addition reactions. o-Xylyl radical is yielded from the H-abstraction of o-xylene, and is a key intermediate leading to the formation of smaller species. In the formation of polycyclic aromatic hydrocarbons (PAHs), the structure of adjacent methyl groups facilitates the formation of bicyclic aromatic species such as indane and 1,4-dihydronaphthalene, and leads to several fuel-specific pathways for the formation of indene and naphthalene in o-xylene combustion. Consequently, relatively high concentration levels of small PAHs are produced in o-xylene combustion while phenyl and benzyl radicals cannot be sufficiently produced, which explains the comparable sooting tendency of o-xylene to those of toluene and ethylbenzene.