Visualization of soot inception in turbulent pressurized flames by simultaneous measurement of laser-induced fluorescence of polycyclic aromatic hydrocarbons and laser-induced incandescence, and correlation to OH distributions

Distributions of polycyclic aromatic hydrocarbons (PAH) and their correlation with soot formation were studied in ethylene-air swirl flames stabilized in a gas turbine model combustor at increased pressure. The combustor can be operated with secondary air injection to study the influence of soot oxidation. We employed PAH laser-induced fluorescence using UV excitation simultaneously with IR-excited laser-induced incandescence to identify soot. PAH signatures typically appear discontinuous unlike OH, yet similar to soot but exhibit more uniform intensity and larger size. The correlation of both diagnostics allowed identification of a wide range of soot formation progress, including isolated soot or PAH, as well as PAH transitioning into soot. The occurrence of soot, PAH and OH and their spatial variations are strongly dependent on the properties of the flow field. In the bottom part of the inner recirculation zone and for the reference case, a rich flame with additional oxidation air, soot levels are relatively high, while PAH intensities in this region are minimal. This correlates well with high temperatures in this region published recently, which are unfavorable for soot formation as the precursors, PAH, decompose. Consequently, soot presence here is attributed to transport. In contrast to OH and soot distributions which change significantly upon addition of secondary air downstream of the primary combustion zone, PAH distributions for both cases look relatively similar. This is attributed to a downstream consumption of PAH by different processes. Without oxidation air, PAH completely transform into soot, while additional oxidation air leads to their oxidation.