Effect of strain rate on nanoparticles and soot in counterflow flames of ethylene/ethanol and ethylene/OME3

Abstract

This study delves into the impact of strain rate on soot and nanoparticle formation in counterflow diffusion flames (CDFs). While strain rate is generally known to reduce soot, understanding of nanoparticle formation remains limited. Strain rate (up to 207 s−1) effects in ethylene flames are here investigated along with ethanol and for the first time oxymethylene ether-3 (OME3). Ethanol and OME3 are well known to reduce particulate emission in several combustion conditions, however, especially for OME3, their effects at different strain rates lack comprehensive information. In soot-forming CDFs, two distinct zones emerge: a pyrolytic zone from the fuel nozzle to the stagnation plane (S.P.), and an oxidative zone from the oxidizer nozzle to the S.P., analogous to the centerline and wings of a coflow diffusion flame. The study aims to unravel the intricate relationship between strain rate, oxygenated fuel, and soot and nanoparticle formation in these two zones. Laser-induced fluorescence, attributed to nanoparticles, laser-induced incandescence, attributed to soot, and scattering were used to track particle formation. Both nanoparticles and soot are formed in both pyrolytic and oxidative zones, with smaller sizes in the former, emphasizing dominant nucleation and coagulation processes. Findings indicate a general particle reduction with strain rate for all the fuel blends, suggesting an inhibition effect on particle nucleation and growth. The effect of strain rate differs for pyrolytic and oxidative zones, with particle reduction in the pyrolytic zone slightly more sensitive to strain rate due to lower kinetic timescales present in this zone of the flame. Oxygenated fuels generally decrease particle formation, however, in pyrolytic zones a slight increase of nanoparticles can be detected. This effect is more evident at low strain rates, and it disappears as strain rate increases. OME3 exhibits higher reactivity and lower sensitivity to strain rate. The insights from this study contribute to developing cleaner combustion technologies, especially in blending oxygenated fuels with traditional ones. Further research is recommended for broader applicability, considering high strain rates and a wider range of dilution conditions.