On the use of PIV, LII, PAH-PLIF and OH-PLIF for the study of soot formation and flame structure in a swirl stratified premixed ethylene/air flame

Abstract

Soot formation and oxidation are investigated in swirl stratified premixed ethylene/air flames at atmospheric pressure. The effects of both swirl and stratification are studied to understand the relationship between the flame structure, soot precursors and soot. The topology of the flame is obtained with particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) on hydroxyl radical (OH). The production of polycyclic aromatic hydrocarbons (PAHs) is investigated using PLIF by mainly probing the aromatic compounds with two benzene rings (i.e. naphthalene) that are known to actively participate in soot nucleation and growth. Soot production is investigated using laser-induced incandescence (LII), giving quantitative data on the soot volume fraction. Extensive information on the flame structure and the mechanisms of formation/consumption of soot is gathered based on the coupling of these laser diagnostics. An image analysis of velocity, OH, PAHs and soot distributions enables us to propose a scenario that describes the link between the inception, growth, aggregation and oxidation processes. In particular, the data reveal the presence of distinct zones for these processes: a thermal decomposition region in which PAHs contributes to nascent soot formation, organised as filaments along the interface between the PAH region and the inner recirculation zone (IRZ); a mixing region controlled by large moving structures that favour the growth and aggregation of nascent soot into mature soot; and an oxidation region leading to the fast consumption of soot particles. These processes are impacted to varying extents by the intensities of swirl and stratification.