Incipient soot formation in halogen bearing premixed laminar flames
Abstract
The results of an experimental and numerical study of the effects of halogen on soot inception in premixed flames are presented. Soot thresholds of pure hydrocarbon premixed flat flames are reported as functions of flame temperature, fuel type, diluent type, and dilution ratio. These are contrasted to measurements in similar halogen-doped flames where flame temperature, halogen loading, halogen type, and halogen source are varied. All variables are significant, except halogen source and diluent type which have no measurable impact on the soot threshold. The measurements are correlated using a new, two-parameter global model for soot inception. This model improves upon similar prior models by accounting for dilution effects in both of the important competing processes, soot precursor formation by pyrolysis and precursor destruction by oxidation. The model successfully correlates variations in temperature (above 1700 K), fuel type (for aliphatics above CH$\sb4$), and small variations in dilution. Most of the observed fuel-dependent sooting behavior of aliphatics is apparently due to dilution of the available carbon for soot inception and growth, by both inerts and combustion products. While all the aliphatics considered (except CH$\sb4$) correlate with a single pair of fitting parameters, methane, benzene, and halogen-doped fuel mixtures require unique parameter sets. This suggests that the kinetics controlling soot inception differ for these fuels. A previously developed kinetic mechanism is used to contrast sooting behavior in flames with and without chlorine. In methane/air flames at the visible soot threshold, the concentration profiles of hydroxyl and acetylene are synchronous through the preheat, reaction, and postflame regions. Chlorine destroys this synchronization by delaying production of radical oxidative species and promoting early formation of acetylene. Delaying radical production to a region of higher temperature causes formation rates to increase. The peak concentration of hydroxyl is actually higher with chlorine than without, in contrast to predictions based on chemical equilibrium. It is thought that early production of acetylene and late oxidizer formation allow precursors to grow large enough to resist oxidative attack. This is supported by prior observations of soot luminosity below the reaction zone in heavily chlorinated flames.
Degree
Ph.D.
Advisors
Senser, Purdue University.
Subject Area
Mechanical engineering|Chemistry|Chemical engineering
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