Soot primary particle radial profiles in laminar diffusion flames for Jet A-1/SAF fuels: A SAXS study

Sustainable aviation fuels (SAF) are designed to progressively replace conventional Jet A-1 fossil fuel, with documented lower soot emissions. However, detailed understanding of how SAF blending affects soot formation mechanisms remains limited. The present study employs Small-Angle X-ray Scattering (SAXS) to characterize soot primary spheres in laminar diffusion flames burning ethylene and Jet A-1/HEFA-SPK blends and to analyze their spatial distribution. Using a recently developed Spline-Based Abel Transform (SAT) method for signal deconvolution and an improved SAXS model, spatially-resolved radial profiles of primary particle size distributions are determined across the flame. The analysis reveals distinct differences between pure Jet A-1 and HEFA-SPK-blended flames: Jet A-1 produces larger primary particles (maximum $D_{p,geo}$ of $27\pm 1.5\mathrm{nm}$) with sharp transitions between growth and oxidation regions, while HEFA-SPK blends show smaller particles (maximum $D_{p,geo}$ of $16\pm 2.5\mathrm{nm}$) with more gradual transitions. This suggests that HEFA-SPK addition fundamentally alters soot formation dynamics rather than simply reducing particle size. The Porod invariant, which is proportional to the soot volume fraction, shows systematic decreases in soot volume fraction with increasing HEFA-SPK content while maintaining similar particle size distribution patterns. This work presents the first comprehensive dataset of spatially-resolved primary particle characteristics in aviation fuel flames, offering valuable insights for soot formation modeling and clean combustion technology development.