DNS of soot formation and growth in turbulent non-premixed flames : Damköhler number effects and Lagrangian statistics of soot transport By

An analysis of soot formation and growth, based on direct numerical simulation of n-heptane/air turbulent nonpremixed jet flames, is presented. A detailed chemical mechanism, which includes polycyclic aromatic hydrocarbons, and a high-order method of moments for soot modeling are employed for the first time in a three-dimensional simulation of turbulent sooting flame. It is shown that soot nucleates and grows mainly in the region between 0.3 and 0.5 in mixture fraction space, and it is spread out in the whole mixture fraction space owing to differential diffusion effects. From the analysis of Lagrangian trajectories, it appears that the presence of soot at large values of mixture fraction is related to turbulent fluctuations whose overall effects cannot be captured considering only the average mixing process. Soot moving toward the flame is oxidized, therefore soot mass is negligible for a mixture fraction smaller than 0.25. Finally, it is observed that decreasing by a factor of two the Damköhler number has negligible effects on the overall soot number density while it causes a decrease of a factor of four in the soot volume fraction.

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