Examining preferential diffusion effects in flamelet-generated manifold on the turbulent flame modelling

Hydrogen (H₂) has been regarded as the most promising sustainable energy. Reliable numerical prediction of its combustion is one of the vital steps towards an ultra-clean energy system. This work was intended to examine the preferential diffusion (PD), which is a major feature of the H₂ fuel, using the Flamelet-Generated Manifold (FGM) combustion model. The major focuses were on the reduced treatment of the PD effects and the impacts on turbulent flame modelling with and without H₂ addition. It is found that the PD hardly influences the velocity prediction even with H₂ enrichment, which can be well modelled merely using a unity Le assumption. The quick burning of H₂ can be captured only when the PD effects are included, which is observed to reinforce the burning intensity near the flame root. The PD terms in the control variable equations are found to be less dominant and the by-effects are twofold: (1) the performance of different reduced PD models is validated to be close; (2) ignoring the PD terms and including the PD effects only in the flamelet-tabulation are acceptable. Nevertheless, for scalar predictions in the recirculation zone, preserving the PD terms is found to be essential. It is observed that a more complete PD model can increase the computation cost up to 11%, but a reduced method can save nearly a half of the cost increment. These observations are significant for accurate and efficient treatment of the PD effects in the turbulent flame simulation.