A Detailed Analysis of Mixture Stratification on Flame Displacement Speed for Syngas Combustion

Abstract

Gasoline direct injection engines can provide higher thermal efficiency and lower emissions than that for engines using conventional combustion techniques. Compositional stratification inside the combustion chamber opens possibilities for ultra-lean and low-temperature combustion. To explore this further, 2D direct numerical simulation (DNS) has been performed to investigate the propagation of syngas flame in an equivalence ratio (ϕ) stratified medium. Several aspects of flame propagation, such as effect of integral scale of mixing (l_ϕ) on the non-monotonic behavior of flame propagation, contribution of each chemical reaction to heat release rate (HRR), and the effect of differential diffusion were analyzed using DNS-data. A spherically expanding flame has been initiated with a hotspot at the center of the square domain of size 2.4 × 2.4 cm². The variations in the degree of stratification were simulated varying l_ϕ and fluctuations ϕ for initial mixture distribution. Further this DNS-data has been used to analyze effects of stratification on flame displacement speed (S_d) and its components, viz. reaction rate (S_r), normal diffusion (S_n), tangential (S_t), and inhomogeneity (S_z). The results reveal that stratification-induced variations in thermal diffusivity resulted in thermal runaways. These thermal runaways influence the extent of burning for simulated cases. The increase in degree of stratification resulted in flame preferably propagating towards leaner ϕ, causing reduction in components of S_d. The preferential propagation of flame also resulted in shifting of peak reaction rate for fuel species (c^*) to a higher reaction progress variable (c). This shifting of c^*, lead to a reduction in the HRR contribution of reactions that attain their peak near the production zone of H and OH species. For unity Le simulations, S_n was observed to be reduced drastically compared to cases with differential diffusion, resulting in an overall reduction in S_d.