Spatial Dependence of Polycyclic Aromatic Compounds Growth in Counterflow Flames

TL;DR

This study combines computational and experimental methods to analyze how flame conditions influence the growth mechanisms of soot precursors in counterflow flames, highlighting the importance of 3D modeling for accurate understanding.

Contribution

It introduces a combined modeling and experimental approach to identify key reaction pathways of soot precursor growth in counterflow flames, emphasizing spatial dependence.

Findings

Hydrogen-abstraction-acetylene-addition and oxygen-insertion reactions drive molecular growth.

Oxygenated species are prevalent in high-temperature, high-oxygen regions.

3D modeling is essential to accurately capture spatial variations in growth mechanisms.

Abstract

The formation mechanisms of aromatic compounds in flame are strongly influenced by the chemical and thermal history that leads to their formation. Indeed, the complex environments that characterize combustion systems do not only affect the composition of gas-phase species, but they also determine the structure and the characteristics of the soot precursors generated. To illustrate the importance of these effects, in this work we investigate the growth mechanisms of soot precursors in an atmospheric-pressure ethylene/oxygen/argon counterflow diffusion flame, using a combination of computational and experimental techniques. In diffusion flames, flow characteristics play an important role in the formation, growth, and oxidation of particles, and soot precursors are strongly affected by the flame location. Fluid dynamics simulations and stochastic discrete modeling were employed together to identify key reaction pathways along various flow streamlines. The models were validated with experimental mass spectra obtained using aerosol mass spectrometry coupled with vacuum-ultraviolet photoionization. Results show that both the hydrogen-abstraction-acetylene-addition mechanism and oxygen-insertion reactions are responsible for the molecular growth, and their relative importance is determined by the flame conditions along the streamlines. Oxygenated species were detected in regions of high temperature, high atomic oxygen concentration, and relatively low acetylene abundance. This study also emphasizes the need to model the counterflow flame in three dimensions to capture the spatial dependence on growth mechanisms of soot precursors.