Flow-field effects on soot formation in normal and inverse methane-air diffusion flames

We investigate the effects of the flow-field configuration on the sooting c haracteristics of normal and inverse coflowing diffusion flames. The numeri cal model solves the time-dependent, compressible, reactive-flow, Navier-St okes equations, coupled with submodels for soot formation and thermal radia tion transfer. A benchmark calculation is conducted and compared with exper imental data, and shows that computed peak temperatures and species concent rations differ from the experimental values by less than 10%, while the com puted peak soot volume fraction differs from the experimental values by 10- 40%, depending on height. Simulations are conducted for three normal diffus ion flames in which the fuel/air velocities (cm/s) are 5/10, 10/10, and 10/ 5, and for an inverse diffusion flame (where the fuel and air ports have be en reversed) with a fuel/air velocity of 10/10. The results show significan t differences in the sooting characteristics of normal and inverse diffusio n flames. This work supports previous conclusions from the experimental wor k of others. However, in addition, we use the ability of the simulations to numerically track soot parcels along pathlines to further explain the expe rimentally observed phenomena. In normal diffusion flames, both the peak so ot volume fraction and the total mass of soot generated is several orders o f magnitude greater than for inverse diffusion flames with the same fuel an d air velocities. In normal diffusion flames, soot forms in the annular reg ion on the fuel-rich side of the flame sheet, while in inverse flames, the soot forms in a fuel-rich region on top of the flame sheet. Surface growth is the dominant soot formation mechanism (compared to nucleation) for both types of flames; however, surface growth rates are much faster for normal d iffusion flames compared to inverse flames. Soot oxidation rates are also m uch faster in normal flames, where the dominant soot-oxidizing species is O H, compared to inverse flames, where the dominant soot-oxidizing species is O-2. In the inverse flames, surface growth continues after oxidation has c eased, causing the peak soot volume fraction to be sustained for a long per iod of time, and causing the emission of soot, even though the quantity of soot is small. Comparison of soot formation among the three normal diffusio n flames shows that the peak soot volume fraction and total mass of soot ge nerated increases as the fuel-to-air velocity ratio increases. A larger fue l-ah velocity ratio results in a longer residence time from the nucleation to the oxidation stage, allowing for more soot particle growth. When the fu el-to-oxidizer ratio decreases, there is less time for surface growth, and the particles cross the flame sheet (where they are oxidized) earlier, resu lting in decreased soot volume fraction. Published by Elsevier Science Inc.