Effect of H-atom concentration on soot formation in premixed ethylene/air flames

Abstract

Soot formation is a major challenge in the development of clean and efficient combustion systems based on hydrocarbon fuels. Fundamental understanding of the reaction mechanism leading to soot formation can be obtained by investigating the role of key reactive species such as atomic hydrogen taking part in soot formation pathways. In this study, two-dimensional laser induced incandescence (LII) measurements using λ = 1064 nm laser have been used to measure soot volume fraction (fV) in a series of rich ethylene (C2H4)/air flames, stabilized over a McKenna burner fitted with a flame stabilizing metal disc. Moreover, a comparison of UV (λ = 283 nm), visible (λ = 532 nm) and IR (λ = 1064 nm) laser excited LII measurements of soot is discussed. Recently developed, femtosecond two-photon laser-induced fluorescence (fs-TPLIF) technique has been applied for obtaining spatially resolved H-atom concentration ([H]) profiles under the same flame conditions. The structure of the flames has also been determined using hydroxyl radical (OH) planar laser induced fluorescence (PLIF) imaging. The results indicate an inverse dependence of fV on [H] for a range of C2H4/air rich flames up to an equivalence ratio, Φ = 3.0. Although an absolute relationship between [H] and fV cannot be easily derived owing to the multiple steps involving H and other intermediate species in soot formation pathways, the present study demonstrates the feasibility to couple [H] and fV obtained using advanced optical techniques for soot formation studies.