Soot and nanoparticle formation in ethylene counterflow diffusion flames: effects of nitrogen dilution and strain rate

Stricter health regulations now cap sub-23 nm particle emissions from all combustion devices. High strain rate (α) and inert dilution each curb soot formation. To our knowledge, however, their combined effect on nanoparticle inception and soot growth has never been quantified in a single, well-characterised burner. To decouple these effects, we systematically investigated ethylene (C2H4) counterflow diffusion flames (CDFs) at three nitrogen (N2)-dilution levels (baseline SF21, and the more heavily diluted SF26 and SF18) while changing the α from 26 to 103 s⁻¹. Dual-wavelength laser induced fluorescence UV-LIF (at 350 nm) and laser induced incandescence LII (at 500 nm) yielded spatially resolved nanoparticle and soot maps. Across all dilutions, increasing the α from 26 to 103 s⁻¹ lowered the peak soot-volume fraction (SVF) by 78–82 % and reduced the maximum LII signal proportionally, confirming the sensitivity of soot growth to the residence time. Nanoparticle LIF behaved asymmetrically: in the fuel-rich (pyrolytic) zone, it fell by ≈ 30 %, whereas on the oxidizer side, it plunged by ≈ 62 %. At the same time, the axial location of the maximum rate of soot formation advanced about 1.0 ± 0.1 mm toward the stagnation plane (SP), and sectional modelling reproduced this shift within 15 %. Dilution amplified the strain-induced attenuation, especially in the oxidative zone, but alone could not match the reduction achieved at the highest α. Moderate N₂ dilution combined with high α clarifies how reduced residence time and enhanced mixing jointly suppress nanoparticle inception and soot growth. Importantly, this approach does so without changing the fuel composition or compromising flame stability.