The impact of thermal diffusion on the structure of non-premixed flames

It is well understood how the flame structure of laminar non-premixed counterflow flames is influenced by differential diffusion, which is driven by species gradients, and corresponding models are available for physical and flamelet space. Thermal diffusion, driven by temperature gradients, is another process that leads to changes in the flame structure and can cause species separation due to differences in the thermal diffusion coefficients. In the present work, a systematic study quantifying the impact of thermal diffusion on the flame structure of non-premixed flames is carried out for five fuel-diluent-oxidizer systems. We also investigate the influence of curvature and strain and present a method for easily incorporating differential diffusion and cross-diffusional effects in flamelet modeling. The study is carried out for one-dimensional counterflow flames, tubular flames and a laminar two-dimensional coflow flame. Examples of flame structures are examined and compared to results obtained with an extended flamelet model, accounting for thermal diffusion and non-unity Lewis number diffusion.