On the validity of flamelet based mean reaction rate closures in lean H2-air turbulent premixed flames

The validity of the flamelet-based mean reaction rate closures for lean hydrogen-air premixed flames using Flame Surface Density (FSD) and Scalar Dissipation Rate (SDR) approaches is investigated in this study. Analysis of Direct Numerical Simulation (DNS) data for equivalence ratios of 0.4 and 0.7 under different turbulence intensities reveals that the SDR-based closure tends to underpredict the mean reaction rate for H 2 , O 2 , and H 2 O mass fraction-based reaction progress variables. Moreover, incorporating the effects of local equivalence ratio variations does not significantly improve the performance of the SDR-based closure. It is demonstrated that the FSD-based reaction rate closure requires accurate modelling of the stretch factor. Furthermore, the choice of the reaction progress variable that yields optimal performance for the mean reaction rate closure depends on the specific closure approach adopted. • Effects of equivalence ratio and turbulence intensity on flamelet closure analysed. • Statistically planar H 2 -air flames of equivalence ratios of 0.4 and 0.7 considered. • Differential diffusion effects are stronger for lower equivalence ratios. • Scalar dissipation rate-based closure underpredicts mean reaction rate. • Flame Surface Density based closure with unity stretch factor is unsuitable.