On the mechanisms affecting soot production in oxygen-depleted buoyant flames

Abstract

The objective of this article is to apply a detailed polycyclic aromatic hydrocarbons (PAH)-based soot production model to investigate the effects of oxygen depletion on the overall soot production processes in laminar coflow ethylene diffusion flames. This task is accomplished by simulating the flames, studied experimentally by Sun et al. (Combust. Flame 211:96–111, 2020), burning under oxygen concentration in the oxidizer down to 16.8% and characterized by a constant volumetric stoichiometric air to fuel ratio to keep the residence time unchanged. This configuration allows isolating the effects of reducing oxygen on both soot formation and oxidation processes and avoiding the complex interactions between soot production and turbulence. Model predictions are in reasonable agreement with the experiments in terms of temperature, soot volume fraction, and primary particle diameter and capture quantitatively the effects of oxygen depletion on soot production. In all the flames, both the H-abstraction and acetylene addition (HACA) and PAH condensation contribute significantly to the soot mass growth, with the HACA slightly dominating the overall process. Reducing the oxygen concentration affects similarly all the formation and oxidation processes with a reduction in the range 30–40% from 21.0% to 18.9% and in the range from 60 to 70% as it is further reduced to 16.8%. Among the formation processes, the PAH condensation and surface growth by HACA have a similar sensitivity to oxygen depletion, followed by soot inception.