Modeling turbulence–chemistry interaction in lean premixed hydrogen flames with a strained flamelet model

Premixed flames in the thin and broken reaction zones regime exhibit strong non-linear turbulence–chemistry interactions and are hence challenging to model. In the present study, a direct numerical simulation (DNS) database for a hydrogen–air flame is used to understand the effect of turbulence–chemistry interactions on the flame speed. To this end, statistics of the local flame front displacement speed are investigated and the roles of flame stretch due to curvature and strain are analyzed. The local flame speed is found to be on average around 30% lower than the laminar burning velocity, which is shown to be a consequence of strain. Strain effects are then modeled by a recently proposed strained flamelet model, which is validated a priori against the DNS in terms of the flame speed and the reaction source term. The strained flamelet model leads to a moderate improvement of the flame speed prediction and is able to reproduce the decrease of the local flame speed. Moreover, it significantly improves the source term closure, which is demonstrated on the basis of the optimal estimator approach and, overall, describes the investigated DNS flame with very good accuracy.