Combustion and Soot Formation From Sustainable Aviation Fuels Near the Lean Sooting Limit

Sustainable aviation fuels (SAFs) present an opportunity to lower the carbon footprint while meeting current criteria and standards for aircraft fleets. This study investigates the spray combustion of SAFs in an optically-accessible high-pressure and high-temperature chamber, where conditions representative of modern and next generation aero-engine are emulated. Three novel cyclo-alkanes that were blended with hydroprocessed esters and fatty acids synthetic paraffinic kerosene (HEFA-SPK), were compared to neat HEFA-SPK, n-dodecane, n-butylcyclohexane and low-sulfur Jet A. Three highspeed optical diagnostics were applied to understand the spray, combustion, and soot formation processes of SAFs: diffuse back illumination extinction imaging, planar laser-induced fluorescence, and OH* chemiluminescence. While the results confirmed the fair correlation between flame stability and derived cetane number under some conditions, the cetane number failed to capture reactivity across the board and proved not to be an appropriate metric for flame stabilization. Soot formation was consistent with literature trends regarding the classification of fuel type from low to high sooting propensity: n-alkanes, iso-alkanes, cyclo-alkanes, and aromatics. A soot production index is introduced to account for fuel reactivity. This index showed that the novel cyclo-alkane blends produced equivalent or lower soot levels compared to neat HEFA-SPK while potentially offering seal swelling characteristics in line with jet fuel requirements.