Experimental and numerical research on the effects of pressure and CO2 dilution on soot formation in laminar co-flow methane/air diffusion flames

Abstract

An experimental and numerical investigation was conducted to examine the formation of soot in methane/air laminar diffusion flames under varying CO₂ dilution ratios, ranging from 0% to 40%, and pressures between 5 and 10 atm. The experimental methodology incorporated diffuse-light two-dimensional line-of-sight attenuation (diffuse 2D-LOSA) to ascertain the volume fraction and peak temperature distribution of soot within the flames. For the numerical methodology, CoFlame—an open-source computational code—was utilized to calculate the detailed flame temperature, soot volume fraction, and the mole fractions of key intermediate species pivotal to soot generation. The study reveals that an increased dilution ratio of CO₂ can reduce flame temperature and the molar fraction of hydrogen (H), while simultaneously increasing the molar fraction of hydroxyl (OH). This shift in chemical composition results in a reduced rate of soot nucleation and an intensified oxidation process during the later stages of soot development, thereby diminishing the overall soot volume fraction. An increase in pressure significantly boosts the processes of soot nucleation, HACA surface growth, and PAH condensation, thereby promoting the formation of soot. Elevated pressure corresponds to an increase in flame temperature and a narrower soot formation region. Additionally, the inhibitory effect of CO₂ dilution on soot formation is mitigated under increased pressure. The findings from this research are expected to provide valuable insights and strategic guidance for the management and control of pollutants in the context of hydrocarbon fuel combustion, particularly when CO₂ dilution is employed.