Effects of ambient CO2 and H2O on soot formation in n-dodecane spray combustion
In this study, large eddy simulation (LES) is performed to investigate the effects of ambient carbon dioxide (CO2) and water (H2O) on the soot formation in an n-dodecane spray flame. A two-equation soot model, in which acetylene (C2H2) is set as the soot precursor and surface growth species, while OH is selected as the one of the soot oxidizers, is implemented here. The ambient oxygen (O2) level and temperature are fixed at 15% (mole basis) and 900K, respectively. The predicted ignition delay, lift-off length, and soot distributions show good agreement with experimental data. The effects of ambient CO2 and H2O on the soot formation can be separated into thermal and chemical effects. For the thermal effects, the ambient CO2 and H2O enhance the formation of C2H2 but reduce the formation of OH radicals by lowering the flame temperature. This leads to a higher soot mass formed. Conversely, the ambient CO2 and H2O reduce the soot formation due to their chemical effects. The reaction CH2* + CO2 ↔ CH2O + CO is found to be main pathway for reducing C2H2 formation when the ambient CO2 is present. The ambient H2O results in a lower C2H2 mass formed due to a higher amount of OH radicals produced. As a result, these collectively lead to a lower soot mass formed.