A comparison between fossil and synthetic kerosene flames from the perspective of soot emissions in a swirl spray RQL burner

Swirl-stabilised turbulent spray flames operating at atmospheric pressure, with both a conventional Jet-A kerosene (`A2') and a synthetic fuel blend (`C5') and radially-injected dilution air were investigated from the perspective of soot emissions in a model Rich-Quench-Lean (RQL) combustor. The primary and downstream dilution air flow split was varied while keeping the global equivalence ratio constant ($\phi_g =$ 0.4). 2.5 kHz PIV, 5 kHz OH-PLIF, and simultaneous 10 Hz Mie scattering and LII were used. Results revealed a correlation between OH and soot distributions. With no dilution air, soot volume fractions (\textit{$f_v$}) peaked in a v-shaped cone between the OH-containing regions along the spray cone and those anchored to the bluff body. With dilution air at 20\% of total air flow, \textit{$f_v$} shifted to the central axis as OH distributions widened. At 40\% dilution, soot initiation began farther downstream along the spray cone, OH regions merged into one, and \textit{$f_v$} peaked farther downstream at the edge of the Central Recirculation Zone (CRZ). With no air dilution, peak time-averaged \textit{$f_v$} was 20\% lower in the C5 flame compared to A2, despite C5 having a higher aromatics content. \textcolor{black}{However, at 20\% dilution \textit{$f_v$} in C5 increased by 75\% compared to the no dilution case.} \textit{$f_v$} in the C5 flame dropped by 12.5\% in the high dilution case compared to 40\% reductions in A2. \textcolor{black}{Mie scattering showed that the C5 spray was shorter and more resistant to change from dilution air compared to A2. The different spray behaviours were due to C5's flat boiling range, lower viscosity and density, leading to higher evaporation rates. Detailed chemistry laminar flamelet simulations of A2 and C5 showed that peak soot precursor species mass fraction values and temperatures decreased as the scalar dissipation increased and a significantly stronger presence of toluene in the C5 flame compared to A2. Toluene's soot yielding behaviour may cause C5's increased \textit{$f_v$} with moderate dilution compared to A2. C5 demonstrates the importance of the fuel's chemical and physical properties when developing future sustainable aviation fuels for practical applications to maximize their soot mitigation potential.