Effects of ambient CO₂ and H₂O on soot processes in n-dodecane spray combustion using large eddy simulation

In this study, large eddy simulations, coupled a two-equation soot model, are performed to investigate the effects of ambient carbon dioxide (CO₂) and water (H₂O) additions on the soot formation and oxidation processes in an n-dodecane spray flame. In the soot model, acetylene (C₂H₂) is soot precursor and surface growth species, while hydroxyl radical (OH) and oxygen (O₂) are soot oxidizers. The effect of ambient CO₂ and H₂O additions on soot formation/oxidation can be separated into thermal and chemical effects. For the thermal effects, the ambient CO₂ and H₂O additions increase C₂H2 but reduce OH formation by lowering the flame temperature. This leads to a higher soot mass formed. On the contrary to the thermal effects, the ambient CO₂ and H₂O additions reduce the soot formation due to their chemical effects. The reaction CH₂^∗ + CO₂ ↔ CH₂O + CO is found to be responsible for reducing C₂H₂ formation. The ambient H₂O addition
results in a higher OH but lower the C₂H₂ mass formed owing to the reverse reactions H₂ + OH ↔ H₂O + H and OH+OH ↔ H₂O+O. Furthermore, the chemical effects is more significant than the thermal effects under the tested conditions. This leads to a lower soot mass formed when adding ambient CO₂ and H₂O.