PAH Formation in Counterflow Non-Premixed Flames of Butane and Butanol Isomers

Authors: Singh and C. J. Sung

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Using the planar laser-induced fluorescence (PLIF) technique in a counterflow non-premixed flame configuration, the formation of the polycyclic aromatic hydrocarbons (PAHs) for the butane isomers and the butanol isomers was investigated. For these C4 fuels, the PAHs of various aromatic ring size groups (2, 3, 4, and larger aromatic rings) have been characterized and compared in non-premixed combustion configuration. In particular, the formation and growth of the PAHs of different aromatic ring sizes in these counterflow flames was examined by tracking the PAH-PLIF signals at various detection wavelengths. The fuel structure effects on the PAH formation and growth processes were also analyzed by comparing the PAH growth pathways for these C4 fuels. Furthermore, PAH-PLIF experiments were conducted, by blending each of the branched-chain isomers with the baseline straight-chain isomer, in order to study the synergistic effects, as well as identify the relative importance of the H-abstraction-C2H2-addition (HACA) mechanism and the propargyl (C3H3) recombination/addition pathways in the PAH formation and growth processes. A chemical kinetic model available in the literature that includes both the fuel oxidation and the PAH chemistry was also used to simulate and compare the PAH species up to A4 rings. At the incipient stage of the PAH formation, the simulated results exhibited similar behavior to the experimental observations. The PAH formation pathways considered in the chemical kinetic model were further analyzed. In addition to propargyl, the present results demonstrated the important role of acetylene in the PAH formation and growth processes.

Purely curved, premixed, sooting ethylene/air flames were studied using a spherical porous burner under microgravity. These weak, fuel-rich flames were shown to be stabilized without the complications of hydrodynamic straining and conductive heat loss to the burner. Using the rainbow Schlieren deflectometry (RSD) optical system, the flames were imaged and the sooting flame speeds and flame thicknesses of ethylene/air mixtures were determined in the equivalence ratio range of 3.0–4.5, and for selected equivalence ratios, with additional nitrogen dilution. Numerical results obtained with detailed reaction kinetics and transport properties, but without considering soot chemistry and radiation, were shown to over-predict the flame intensity.

Citation: Singh and C. J. Sung, “PAH Formation in Counterflow Non-Premixed Flames of Butane and Butanol Isomers,” Combustion and Flame 170, 91-110 (2016).