Hydrogen addition in methane-oxygen laminar inverse diffusion flames: A study focused on free radical chemiluminescence and soot formation

This study investigates the impact of H2 addition on soot formation in inverse diffusion flame (IDF) and explores the underlying mechanism of flame structure on soot formation. Experimental measurements and simulation kinetic calculations were performed to explore the influence of free radical (OH*/CH*/C2*/CO2*) chemiluminescence on soot formation mechanism. The investigation ultimately determines the relationship between free radical chemiluminescence and soot formation. The findings reveal that similar to the simulation results, the peak soot volume fraction (SVF) is reduced by approximately 60 % when the hydrogenation increases to 40 %, and the initial soot production position moves downstream of the flame. The free-radical chemiluminescence distribution is mainly concentrated at the burner exit, where the increase in hydrogen concentration leads to a decrease in CH* and C2* and an increase in OH* content in the flame. The blue region on the fuel side is consistent with the peak CH* and C2* emission region. In the soot formation, the chemiluminescence signal of free radicals is suppressed, indicating that soot formation and free radical chemiluminescence are antithetical. The results of the component yields indicate that soot formation and chemiluminescence are fueled by gas phase components (CH3, C2H3, C2H2, etc.) and there are generated in opposite pathways, which supports the above statement. Furthermore, the increase of hydrogen content inhibits the production of C2H and CH2, resulting in the decrease of C2*, CH* and CO2* radical chemiluminescence by 77.0 %, 75.1 % and 81.2 %, respectively. The decrease of soot concentration is primarily due to the increase of H2, which hinders the formation of C3H3 and C4H5-2.