Limitations of simplified models to predict soot formation in laminar flames

Abstract

Soot formation and radiation are important aspects for combustion problems. In this work, numerical simulations of ethylene coflow laminar flames are used to evaluate soot formation and radiation processes under different modeling approximations. Priority was given for models that were capable of producing detailed information with reduced computational requirements. So, the objective of this work is to show and quantify the importance of heat loss by gas and soot radiation and to quantitatively show the impact of different transport models (a detailed and a simplified) in soot predictions. For soot modeling, a semiempirical two-equation model is chosen for predicting soot mass fraction and number density. The model describes particle nucleation, surface growth and oxidation. For flame radiation, the radiant heat losses (gas and soot) are modeled by using the gray-gas approximation with optically thin approximation. For the chemical kinetics, a detailed approach is employed. It is found that gas and soot components of the radiative heat loss are comparable, with the gas radiation being larger (65%). To capture 99.9% of the total heat loss, the numerical domain has to be extended to 2.4 times the flame length based on the stoichiometric mixture fraction. Radiation modeling has a large impact on soot predictions. An error of 19% in the peak soot volume fraction is found when radiation is neglected. Errors due to simplified transport properties are also around 21%.