Effects of n-propylbenzene addition to n-dodecane on soot formation and aggregate structure in a laminar coflow diffusion flame

Abstract

The effects of n-propylbenzene addition to n-dodecane on soot formation and aggregate structure in a laminar coflow diffusion flame is investigated. The goal is to gain insight on the influence of n-propylbenzene addition to n-dodecane on the underlying mechanisms of soot formation processes. The methane laminar coflow diffusion flame doped with pure n-dodecane establishes the base environment in this study. n-Propylbenzene is mixed into n-dodecane at three levels: 15%, 30% and 45% in mole fraction. The inlet total carbon flow rate is held constant for all of the flames. The combined laser extinction and two-angle elastic light scattering method is used to measure soot volume fraction, primary particle diameter and number density. As expected, soot volume fraction is shown to increase at all heights in the flames with increasing mole fraction of n-propylbenzene in the liquid fuel mixture. The differences in profiles of soot volume fraction suggest a non-linear relationship between the mole fraction of n-propylbenzene in the liquid fuel mixture and the increase in soot production. The relative importance of soot inception and surface growth affected by n-propylbenzene addition is shown to be different along the flame wing and centerline. Along the wing, the aromatic fuel molecular structure primarily influences the initial soot inception process at earlier times by providing a larger population of incipient soot particles for the subsequent surface growth. Along the centerline, the aromatic fuel chemistry effect is stronger, as it accelerates both the soot inception and the subsequent surface growth processes. The experimental data presented in this study is also useful for the validation of soot models of the liquid fuel surrogate components.