Evaluation of the sooting properties of real fuels and their commonly used surrogates in a laminar co-flow diffusion flame

In an effort to help determine the fidelity of simple surrogate fuels to represent real fuel chemistry in computational fluid dynamic simulations of engines, quantitative two-dimensional soot volume fraction measurements were made in a laminar coflow methane–air diffusion flame seeded with approximately 2200 ppm of real and surrogate fuels. A combined laser extinction and laser-induced incandescence (LII) method was used to measure soot volume fraction. Additionally, soot particles were thermophoretically sampled from the flame and soot morphology data were collected with a transmission electron microscope (TEM). Vaporized liquid fuels were seeded at low concentrations to maintain constant thermodynamic conditions for each experiment. In all, 14 different fuels were investigated, including: three real fuels (gasoline, diesel, and jet fuel), two alkanes, and a variety of simple surrogate fuels. A toluene reference fuel (TRF) (30% aromatics) and gasoline (28% aromatics) were found to have similar soot volume fractions and soot morphology. The addition of toluene to the long-straight chain of n-tetradecane in similar concentrations (30 vol.%) as the aromatic concentration of diesel (31.1 vol.%) resulted in soot volume fractions that were very similar, although the primary particle size and mass-weighted radius of gyration were both smaller for the surrogate than for the conventional diesel fuel. Finally, the jet-fuel surrogate tested was found to have a lower sooting tendency than the jet-A fuel despite the jet-A fuel having a lower concentration of aromatics than the surrogate. Soot morphology between jet-A and the jet-fuel surrogate were the same within experimental uncertainty. The current work provides an experimental dataset for validation of fuel-surrogate chemistry and soot models.