Soot Evolution and Microstructural Trends in Ethylene/Propane Flames under Oxygen-Enriched Simulated Exhaust Gas Recirculation Conditions

Oxygen-enriched combustion in simulated exhaust gas recirculation (EGR) atmospheres can alter soot formation and oxidation. This study investigates soot evolution in ethylene/propane co-flow laminar diffusion flames under a fixed simulated EGR background (N2:CO2 = 7:1) across Oxygen Indices (OI) of 21%–50%. Experimental diagnostics and numerical simulations were combined to examine changes in soot structure, the thermochemical environment, and the spatial distribution of soot. XPS and FT-IR measurements reveal changes in soot chemical characteristics, with the XPS-derived sp3/sp2 peak-area ratio reaching 1.165 at 30% OI before decreasing to 0.369 at 50% OI. These observations are discussed in relation to the modeled thermal and radical fields, but they do not uniquely establish a single microstructural mechanism. The simulations indicate that increasing OI changes the gas-temperature and radical distributions, consistent with a change in the oxidation environment and contraction of the soot-containing region. Oxygen enrichment is the primary controlled variable in this study; CO2 is considered as a component of the prescribed simulated EGR environment, and its independent effect is not isolated. These results provide condition-specific insight into soot evolution under oxygen-enriched simulated EGR conditions.