Abstract

A numerical study is conducted using the CoFlame code to investigate the sooting characteristics of ethylene-fueled laminar coflow inverse diffusion flames (IDFs), aiming to gain insights into the different flame structure and soot formation features between an IDF and a normal diffusion flame (NDF). The effects of oxygen mole fraction in the oxidizer stream, denoted as Oxygen Index (OI), on flame structure and soot production are studied over the OI range of 21% to 33%. The soot model parameters were chosen to produce overall reasonably good agreement between the predicted and measured in terms of soot volume fraction, mean primary particle size, and the mean number of primary particles per aggregate in both the normal and inverse configurations. A sensitivity study showed that soot nucleation and soot surface growth by PAH adsorption modeling should be considered simultaneously as a strongly coupled process through competing for polycyclic aromatic hydrocarbons (PAHs). The IDF configuration displays distinct characteristics from the NDF one in terms of flame structure as well as soot production. In IDFs soot is formed along the outer edge of reaction front, and does not cross the reaction front to the oxidizer, decoupling completely soot oxidation which leads to soot emission from the open flame tip. Modeling results indicate that in the IDF configuration the hydrogen abstraction acetylene addition (HACA) process is severely suppressed, while the adsorption of PAH molecules is dominant to soot surface growth at relatively low temperatures. Nucleation of new particles is significant all along the flame height, causing emission of young soot. IDF represents an ideal configuration to study soot surface growth by PAH adsorption. Increasing OI enhances all the soot processes, especially surface growth by PAH adsorption. Although soot oxidation rate by OH radicals is also strongly enhanced, it remains about two orders of magnitude lower than soot surface growth rate by PAH adsorption. Crown Copyright (c) 2021 Published by Elsevier Inc. on behalf of The Combustion Institute. All rights reserved.