A theoretical study on soot inception in spherical burner-stabilized diffusion flames

Abstract

The soot inception processes in nonpremixed flames have been studied in a spherical flame stabilized by a spherical porous burner to understand the effects of flame structure (stoichiometric mixture fraction) and hydrodynamics (flow direction). A simplified three-step model with high activation energies is employed to describe the fuel oxidation, soot/precursor formation, and soot/precursor consumption reactions, respectively. The fuel oxidation reaction also produces a radical necessary for the soot formation reaction. The scheme yields three distinct reaction zones, one for each of the three reactions. The spherical flame geometry is unique in being able to independently control the flow direction, either from the fuel to oxidizer or from the oxidizer to fuel, and the flame structure through inert distribution. Four limiting flames, namely the fuel/air flame, diluted-fuel/oxygen flame, air/fuel flame and oxygen/diluted-fuel flame with the first reactant being the one supplied from the burner, are studied to address the relative importance of hydrodynamics and flame structure on soot inception. The analysis yields a solution giving the flame response to the variations of the soot formation and consumption reaction rates, the mass-flow rate issued from the burner, the Lewis numbers, and the inert distribution. Results show that with negligible soot consumption reaction, soot/precursor production is greater with slower radical diffusion. In the presence of soot consumption reaction, the total amount of soot/precursor decreases with lower soot/precursor diffusion rate. The effect of burner flow rate is qualitatively similar to that of the soot/precursor formation reaction rate except that the flame temperature increases monotonically with the flow rate until the adiabatic limit is reached. The production of soot/precursor can be significantly suppressed or eliminated by redistribution of the inert gas from the oxidizer side to the fuel side regardless of the flow direction.