The effect of preferential diffusion on the soot initiation process in ethylene diffusion flames

The influence of differential diffusion of chemical species in the soot initiation process in turbulent flows is investigated through Direct Numerical Simulations coupled to a compact global chemical mechanisms for ethylene (C ₂H₄) flame combustion (Løvås et al., Combust Sci Tech 182(11):1945-1960, 2010) featuring the important reaction steps for acetylene production. Our focus is on the formation of acetylene (C ₂H₂) which is one of the most important species indicative of soot formation layers, especially in relation to the location of the H and H₂ layers. The effect of preferential diffusion is assessed by comparison of results from unity and non-unity Lewis number simulations. The results indicate that under moderate turbulent conditions, where preferential diffusion effects become prominent, and with the global scheme used preferential diffusion greatly enhances the spread of the radical H whose peak value in mass fraction is reduced by a factor of about two; the spread of H₂ is also enhanced though to a lesser extent. Importantly, the H and H₂ spread into a range of mixture fraction Z between 0.2 and 0.3 which contains the soot formation range, supporting the hypothesis that soot formation is enhanced by preferential diffusion. Nevertheless, the acetylene formation layers themselves show little adjustment in the presence of non-unity Lewis numbers suggesting that the acetylene formation is dominated under the current conditions by the direct thermal decomposition of ethylene to acetylene in the global chemistry used. The specific F_i factors that appear in flamelet models are explicitly computed; only F_H, F_H2 and F_CO show appreciable differences on the fuel lean range of mixture fraction due to non-unity Lewis numbers, suggesting that the effects of non-unity Lewis numbers could be incorporated by a selective inclusion of only a few of the F_i factors in order to save computational time.