Influence of pressure on the structure and chemistry of counterflow diffusion flames from 1 to 6 bar

The chemical structure of soot forming (SF) counterflow diffusion flames (CDFs) is explored in a combined experimental and numerical study for pressures ranging from 1 bar to 6 bar. The experiments are designed to maintain normalized temperature–time histories at increasing pressures for three dilution levels of the fuel and oxidizer stream, resulting in moderately to highly sooting flames with soot volume fractions ranging from f v ≈ 0 . 1 − 20 ppm. Measurements of f v and dispersion exponents ( β ) are compared to numerical simulations in OpenSMOKE++ with detailed finite-rate chemistry, a sectional soot model and tracking of the hydrogen/carbon ratio in particulates. Soot production rates are evaluated and a Soot-based Global Pathway Analysis is utilized to reduce chemical complexity and identify the dominant pathway for carbon transfer from fuel to soot at a given location. Results show that increasing peak flame temperatures and pressure causes a reduction in the dispersion exponents, while soot concentrations and production rates increase. The increase in pressure is noted to affect soot production rates through an increase in precursor concentrations. Meanwhile, fuel and oxidizer dilution affects soot production through changes in temperature and precursor concentrations. PAH condensation was noted to be the major contributor for mass addition to soot, second to the inception and HACA growth routes. The dominant global pathways (GPs) progressed as . The relevant PAH for GPs changed from acepyrene ( ) in the soot inception zones to pyrene ( ) and acenaphthylene ( ) in the soot growth regions and benzene near the particle stagnation plane, highlighting a contrast with coflow flames. For increasing pressures, the dominance of the acepyrene route ( ) is noted to decrease, which is then substituted by the pyrene-dominated routes ( ). For flames with lowest peak flame temperatures, GPs are not as dominant and multiple pathways shows similar carbon flux transfer from fuel to soot. Novelty and significance statement This work advances the understanding of pressure and dilution effects on soot formation in SF CDFs using a combination of multi-wavelength absorption emission diagnostic, detailed soot modeling, and a soot-based global pathway analysis. Results reveal distinct pathways compared to coflow flames, with larger PAHs driving inception and smaller PAHs supporting growth. The chemical sooting structure underscores the accentuated role of acetylene and pyrene in soot production rates. Numerical insights highlight the need for improved modeling of dehydrogenation and pressure-dependent reactions, offering refined mechanistic understanding of soot chemistry under elevated pressures.