Inclusion of differential diffusion in the conditional source-term estimation (CSE)-direct chemistry framework applied to turbulent hydrogen flames

The objective of the present work is to further investigate and assess the recently developed Conditional Source-term Estimation (CSE)-direct chemistry framework applied to a series of hydrogen flames including differential diffusion (DD). The proposed strategy is to include DD in the Favre-averaged species transport equations and consider detailed kinetics in CSE without tabulation. The molecular diffusion terms in the transport equations of species mass fraction, mean mixture fraction and its variance, and enthalpy are closed consistently when considering DD. CSE simulations of three hydrogen flames are performed with and without including DD. A mechanism with 13 reversible reactions and 8 species is used. Near the inlets, DD effects are shown to be significant. Simulations assuming a unity Lewis number present notable discrepancies in temperature profiles compared to measured data, whereas those incorporating detailed molecular diffusion match measurements much more closely. The present results demonstrate that CSE-direct chemistry can effectively capture DD effects. This is essential not only for hydrogen flame simulations but also for flames involving soot or complex fuel mixtures, which cannot be easily modelled using a tabulated chemistry approach.