Effect of Fuel/Air Ratio and Aromaticity on Sooting Behavior of Premixed Heptane Flames

Oxidation and pyrolysis products were sampled and quantified along the axis of premixed laminar flames burning n-heptane in slightly sooting (C/O=0.7) and heavily sooting conditions (C/O=0.8) by means of gas chromatographic analysis. Total particulate was extracted with dichloromethane (DCM) in order to separate condensed species (CS) including polycyclic aromatic hydrocarbons from soot. The effect of aromatic addition (10 vol % of n-propylbenzene) to n-heptane fuel in heavily sooting conditions on the distribution of light hydrocarbons was also studied. Detailed modeling extended to the formation of high-molecular-weight species and soot was performed to verify the effect of the C/O ratio and of the fuel aromatic content on soot formation. The C/O ratio was found to affect benzene formation which in turn caused an increase of condensed species and soot formation rates. The fuel aromaticity was found to shift soot inception upstream in the flame increasing soot in the oxidation region of the flame. However, aromaticity was not found to influence the ultimate soot loading because of the reduced soot growth rate due to the slightly lower temperature and lower acetylene formation in the aromatic-doped flame. The model was used to explore the effect of larger amounts of aromatics on soot formation. Increased benzene and condensed species, including polycyclic aromatic hydrocarbons, are obtained from the addition of more aromatics (>10%) to n-heptane resulting in higher ultimate soot loading, confirming the direct relation between gas-phase aromatics and final soot loading.