Relative polycyclic aromatic hydrocarbon concentrations in unsteady counterflow diffusion flames

Qualitative measurements of polycyclic aromatic hydrocarbon (PAH) concentrations made in counterflow diffusion flames subjected to both steady and oscillating strain rates are presented. Planar laser-induced fluorescences was used to make spatially and temporally resolved measurements of the relative concentrations of PAH. The measurements for each PAH size class were made as a function of strain rate, forcing frequency, and fuel type. The peak laser-induced fluorescence spectra from PAHs is known to be a function of the number of benzene rings that make up the particular PAH. It was found that, with increasing strain rate, the intensity of PAH fluorescence decreases dramatically, though unequally, among the three size classes measured. With an unsteady strain rate, the PAH intensity oscillates in phase for frequencies below 50 Hz. When nondimensionalized relative concentration is plotted, it is shown that the smaller PAHs continue to respond to the instantaneous strain rate at a higher frequency than larger PAHs.