On the relative contributions of soot to radiative heat transfer at different oxygen indices in ethylene – O2/CO2 laminar diffusion flames

Abstract

Soot concentration measurements in ethylene-O2/CO2 diffusion flames (fuel Re 120) encompassing oxidizer oxygen mole fractions (oxygen index, OI) of 35% to 90% were employed to validate a methodology for modeling soot production. A skeletal 10-step reaction mechanism and modeling the soot nucleation parameter (Cα) in the Moss-Brookes soot model as a function of OI provided adequate agreement with measurements. Cross-validation of the OI based Cα model was ascertained through ethylene-O2/CO2 diffusion flame simulations in a different experimental configuration (fuel Re 43, 35%-50% O2) by ensuring that peak soot concentrations were in good agreement with previously published results. A mesh resolution of 0.0625 mm within the flame was deemed necessary to obtain grid independent results. Despite increases in peak soot volume fractions and flame temperatures with OI, the radiant fraction (χR) (in-flame) decreases due to decreasing flame lengths. Although in-flame radiation is dominated by gases, the spectrally-continuous soot radiation plays an important role on χR (when it is assessed across the entire domain) due to the attenuation of flame radiation by the cold, radiatively-participating, co-flow. Consequently, in-flame radiation show a strong sensitivity to gas-radiative property models whereas χR when assessed across the entire domain is impacted by the choice of soot radiative property model.