Experimental and kinetic modeling of soot formation in counterflow non-premixed flames of surrogate fuel components: n-dodecane and iso-dodecane

Normal dodecane (n-C12) and iso-dodecane (i-C12) are often used as components for surrogate mixtures of aviation and diesel fuels. Although studies have been performed to understand the combustion and spray behavior of dodecane isomers, the soot formation from the combustion of n- and i-C12 in non-premixed flames is not well studied. In this work, soot volume fraction (SVF) profiles of n- and i-C12 were measured across a wide range of strain rates and mixture conditions in a counterflow burner facility at the University of Connecticut. Neat and binary mixtures of n- and i-C12 were investigated to study the influence of alkane branching on soot formation. A soot model was developed and validated in this study by extending the detailed kinetic model developed at the Lawrence Livermore National Laboratory (LLNL) for the formation of polycyclic aromatic hydrocarbons (PAHs) to simulate soot formation and growth based on the discrete sectional method. Additional gas phase reactions forming pyrene and ring enlargement reactions were added to the existing PAH model. The LLNL kinetic model was validated with SVF data obtained in this work for n-C12 and i-C12 flames along with data available in literature for ethylene, iso-butene, n-heptane, and iso-octane. The soot precursor reactions added in this work were found to play a critical role in simulating the experimentally observed non-linear trend of the peak SVF with increased branched alkanes. Reaction path analysis was conducted to illustrate the fuel structure effects on soot formation pathways in n- and i-C12 mixtures. In view of the satisfactory agreement between modeling results and experimental data, as well as capturing the non-linear variation in peak SVF with the alkane branching for the first time, further investigation within the framework of the soot model is discussed and critical insights are provided into the reaction pathways which require further attention.