On soot inception in nonpremixed flames and the effects of flame structure

A simplified three-step model that describes the oxidation, soot formation and soot consumption reactions has been employed with high activation energy asymptotics to study soot inception in nonpremixed counterflow and spherical flames. Emphasis has been made on the understanding of the effects of hydrodynamics and transport on the soot inception processes. The resulting scheme yields three distinct reaction regions: (1) a fuel oxidation region wherein the fuel and oxidizer react to form product and radical, (2) a soot/precursor formation region where the radical reacts with fuel to form “soot/precursor”, and (3) a soot/precursor consumption region where soot/precursor reacts with the oxidizer to form product. The kinetic scheme allows for the coupling between soot inception and flame structure to be assessed. The analysis yields the solution of flame temperature, flame location, and soot/precursor concentrations as functions of the Damköhler number of the soot formation reaction and the mass flow rate issued from the burner for the spherical flames. The flame temperature indirectly indicates the total amount of soot/precursor production because as soot/precursor is formed less heat is released.; Two limiting cases, the fuel/air flame and diluted-fuel/oxygen flame, were studied for both the counterflow and spherical flame configurations. The effects of flame structure, initial reactant concentrations, Lewis numbers of the radical and the soot/precursor, as well as the rates of soot formation and consumption reactions on the sooting behavior were investigated. Results show that the diluted-fuel/oxygen flame produces much lower soot/precursor than its fuel/air counterpart. Because the production of soot/precursor is more difficult to occur for the flames in which inert is supplied with the fuel as indicated by the much lower soot/precursor production, soot inception can be reduced or completely suppressed by the redistribution of inert from the oxidizer side to the fuel side even when the flow direction does not favor soot oxidation. This suggests that flame structure represented by the stoichiometric mixture fraction play a more important role on soot inception in diffusion flames than hydrodynamics.