Effect of sodium chloride on the evolution of size, mixing state, and light absorption of soot particles from a smoking laminar diffusion flame

In this study, the effect of sodium chloride (NaCl) on the evolution of size, mixing state, and optical properties of particle emissions was investigated. A smoking laminar diffusion flame of methane with flame length of similar to 100 mm was used, with and without NaCl particles introduced into the fuel stream. Extractive probe sampling with high dilution was used to sample particles and characterize their size and mixing state. Particle size distribution results showed that for the methane-only flame, the soot particles underwent typical processes of formation and coagulation in the flame, partial oxidation near the flame tip, and further coagulation in the post-flame region. In the case of the methane-NaCl flame, the coagulation rate between soot particles decreased in the agglomeration region of the flame, resulting in smaller soot particles with higher number concentration. Near the open tip of the flame, the NaCl inhibited soot oxidation, leading to larger particles compared to the methane-only flame due to continued coagulation and reduced oxidation of soot particles. In the post-flame region, where the temperature was sufficiently low, NaCl particles nucleated from its vapour phase, which was a distinct mode from that of soot in the particle size distributions. Electron microscopy images showed that these nucleated NaCl particles were mostly externally mixed and occasionally internally mixed with soot particles. Thermal-optical analysis of emissions sampled from post-flame region showed that the mass concentration of elemental carbon increased by a factor of similar to 3 when NaCl was added to the flame. However, addition of NaCl did not affect the mass-specific absorption coefficient of particle emissions, which was within previously reported values in the literature. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.