Assessment of first-order conditional moment closure for lean H2-air turbulent premixed flames using direct numerical simulation data

An a priori assessment of first-order Conditional Moment Closure (CMC) for turbulent lean H 2 –air premixed flames was conducted using Direct Numerical Simulation (DNS) data for statistically planar flames with equivalence ratios of 0.4 and 0.7 under varying turbulence intensities. A presumed β-Probability Density Function (PDF) has been shown to capture the qualitative behaviour of the Favre-conditioned PDF of the reaction progress variable, although quantitative discrepancies remain. Existing models for conditional mean velocity perform satisfactorily, while conditional scalar dissipation rate models reproduce DNS trends qualitatively but with quantitative discrepancies linked to inaccuracies in Favre-averaged scalar dissipation rate and PDF predictions. First-order CMC predicts conditional mean reaction rates accurately for H 2 and H 2 O but overpredicts O 2 reaction rates due to elevated conditional variance. DNS data also reveal counter-gradient conditional scalar fluxes associated with sub-unity Lewis number effects, highlighting the need for improved scalar flux closures.