Experimental study of PAH formation in laminar counterflow CH4N2/O2N2 and C3H8N2/O2N2 nonpremixed flames

We studied polycyclic aromatic hydrocarbon (PAH) formation in laminar counterflow CH4N2/O2N2 and C3H8N2/O2N2 nonpremixed flames using a gas chromatograph-mass spectrometer (GC-MS) under widely varying experimental conditions. In this paper we describe the effect of varying the flame-controlling parameter on the formation of PAHs. The formation of most PAHs is enhanced by increasing the strain rate regardless of fuel species and the stoichiometric mixture fraction, Zst. Increasing the dilution levels slightly suppresses PAH formation. However, the effect is quantitatively weak. PAH formation also is suppressed as Zst increases. However, the quantitative dependence of PAH formation on Zst variation is different between CH4 flames and C3H8 flames. Naphthalene is the most abundant compound under all experimental conditions in CH4 flames. On the other hand, for C3H8 flames, naphthalene is the most abundant compound in the range of smaller Zst while acenaphthylene is abundant under larger Zst conditions. For all experiments, under small Zst conditions, small-molecular-mass PAHs including naphthalene have significant concentration. Large-molecular-mass PAHs including acenaphthylene, which are mostly 2- and 3-ring, are produced mainly for larger values of Zst, when nonpremixed flames are established in the fuel stream. The formation behavior of PAHs in the counterflow nonpremixed flame is classified into two groups for C3H8 flames. One is the PAH group that has formation behavior similar to naphthalene, and the other is the PAH group that has formation behavior similar to acenaphthylene. It is experimentally observed that the effect of varying the initial C/O ratio on the PAH formation of nonpremixed flames is similar to that of previously reported numerical predictions for C2H2 premixed flames.