Combustion and Flame 210 (2019) 114–125 Contents lists available at ScienceDirect Combustion and Flame journal homepage: www.elsevier.com/locate/combustfame Influence of water-vapor in oxidizer stream on the sooting behavior for laminar coflow ethylene diffusion flames Francisco Cepeda a , Alejandro Jerez a , Rodrigo Demarco a , Fengshan Liu b , Andrés Fuentes a,∗ a Departamento de Industrias, Universidad Técnica Federico Santa María, Av. España 1680, Casilla 110-V, Valparaíso, Chile b Measurement Science and Standards, National Research Council, Building M-9, 1200 Montreal Road, Ottawa, Ontario K1A 0R6, Canada a r t i c l e i n f o Article history: Received 22 May 2019 Revised 25 June 2019 Accepted 20 August 2019 Keywords: Water-vapor addition Soot volume fraction Oxygen index Chemical effects PAH-based sectional soot model Chemical pathways a b s t r a c t The effects of adding water-vapor to the oxidizer stream on soot production in laminar coflow diffu- sion flames under different oxygen indices (OI) were investigated both experimentally and numerically. A modified coflow Gülder-type burner was employed to produce the laminar ethylene flames for fifteen different conditions of the oxidizer stream from oxygen-deficient (OI 17%) to oxygen-enriched (OI 25%) conditions, and also without and with adding water-vapor into the oxidizer stream up to 10% on mole basis. The measured soot volume fractions were compared with numerical predictions obtained using the CoFlame code and a chemical kinetic mechanism that consists of reaction pathways up to 5-ring PAHs. The sectional soot model used to simulate the soot particles dynamics considers soot nucleation, surface growth, PAH condensation, oxidation, particle coagulation and fragmentation. Fairly good agreement be- tween experimental and numerical results was found, as close as 1% and 5% of difference in the peak soot volume fraction for OI21% with 0% and 10% of water-vapor addition, respectively. Clear trends of in- creasing the soot volume fraction (peak and also overall) was observed with increasing OI, while a signif- icant reduction was obtained with the addition of the water-vapor. For the cases analyzed, a reduction of the total soot content was up to 60%. The chemical effects were numerically isolated using non-reacting water-vapor and analyzed, contributing especially to the soot oxidation rates. Finally, a study of the main reaction pathways was performed to better understand the chemical effects of water vapor. The results show that water vapor addition affects the concentrations of H and OH radicals and alters the formation and oxidation of soot precursors. © 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved. 1. Introduction Water is an important active agent in fire prevention. Com- monly applied as a liquid spray to suppress fires, water is expected to remove heat from the flame due to the vaporization processes, resulting eventually in the extinction of fires. However, wa- ter, once vaporized, also has significant influences on chemical reactions, combustion species, and heat transfers within the flame [1–4]. Several studies have proven the effectiveness of wa- ter in extinguishing fires in real configurations such as pool fires [5–8], rack fires [9], and even kitchen fires [10]. Although investiga- tions of these practical configurations of highly transient turbulent flames are very useful to understand the interactions between fires and water mist, it is difficult to gain detailed understanding of the different effects of water-vapor addition (namely thermal, dilution and chemical) on flame structure and soot formation. Well ∗ Corresponding author. E-mail addresses: andres.fuentes@usm.cl, andres.fuentes@polytech.univ-mrs.fr (A. Fuentes). controlled laminar flames, either premixed [11–13] or diffusion [14–18], have often been used to gain fundamental understanding of the effect of a certain physical or chemical factor. For example, Atreya et al. [14] used a counterflow laminar diffusion flame to examine the chemical and physical gas-phase fire suppression mechanism of water-vapor and found that water reduced soot concentration and enhanced the oxidation of CO to CO 2 . Laminar diffusion flames generated in coflow burners have been frequently employed to study the impact of water-vapor on com- bustion. Ndubizu et al. [15] examined the relative contribution of the latent heat, heat capacity, and oxygen dilution effects of wa- ter mist on the suppression of non-sooting methane-air diffusion flames, finding that the first two factors had the most significant effect. On the other hand, numerical studies have focused on demon- strating the chemical or physical effects of water added to the oxidizer stream [19–25]. Lentati and Chellia [20,21] studied the influence of water droplet size on flame suppression in coun- terflow non-sooting flames. Prasad et al. [22] used a numerical model to estimate the impact of different factors, such as droplet https://doi.org/10.1016/j.combustflame.2019.08.027 0010-2180/© 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.