Effects of preferential diffusion and flame stretch on FGM method for numerical simulations of ammonia/air premixed combustion

Importance of the considerations of preferential diffusion and flame stretch effects in the flamelet-generated manifold (FGM) method on the prediction accuracy is investigated by two-dimensional numerical simulations of cylindrical NH₃/air premixed flames, under the conditions of an unburnt gas temperature of 673 K, an ambient pressure of 2 MPa, and equivalence ratios of 0.8 to 1.2. Results of the numerical simulations using the detailed chemistry, in which 32 species and 204 reactions are directly solved in the physical space without the FGM method, show that the mixture fraction in the burnt gas increases from the unburnt gas value when considering the preferential diffusion effect, whereas it remains flat when assuming the unity Lewis number. This means that assuming the unity Lewis number causes the underprediction and overprediction of the burnt gas temperature under fuel-lean and fuel-rich conditions, respectively. Results of the numerical simulations using the FGM methods show that considering the preferential diffusion and flame stretch effects in the FGM method is important for accurate prediction of the flame propagating speed, and the effectiveness is more evident for the flame stretch effect than for the preferential diffusion effect for the NH₃/air premixed flames.