Sooting transition diagnostics in counter-flow flames of C4 isomer fuels

Abstract

Macroscopic transition processes of soot formation for 1-butene, isobutene, n-butane and isobutane counterflow diffusion flames from sooting-free to heavy-sooting were studied by varying fuel concentration (XF) in the fuel stream. A novel optical diagnostic algorithm was proposed to classify different sooting transition stages in flames. Effects of saturated/unsaturated bonds and linear/branched structures of fuel molecules on the sooting transition were investigated in details. Flame species were detected and quantified by on-line gas chromatography and also kinetic analysis was performed. KL factor was obtained by optical diagnostics to qualitatively compare soot concentrations. Results showed that XF required for 1-butene flame to reach the critical state of macroscopic sooting transition was the lowest, followed by isobutene, isobutane and n-butane flames. Fuels with higher sooting tendencies could achieve entire sooting transition process with a narrower range of XF. Linear butane was more difficult to achieve the sooting transition than branched butane, while linear 1-butene was easier to form soot than branched butene, reflecting the difference between isomeric structures acting on C4 alkenes and alkanes. These might be largely due to the close association of allyl (C3H5-A) with 1-butene. Higher C3 species content in isobutane flame made it have higher sooting tendency than n-butane.