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

Abstract

Gasoline direct injection (GDI) engines can provide higher thermal efficiency and lower emissions compared to conventional combustion techniques. The direct charge injection near the ignition source forms compositional stratification inside the combustion chamber.Compositional stratification inside the combustion chamber opens possibilities for ultra-lean and low-temperature combustion. In this paper, a 2D direct numerical simulation (DNS) has been performed to investigate the propagation of syngas flame in an equivalence ratio stratified medium. A spherically expanding flame has been initiated with a hotspot at the center of thedomain. An open-source PENCIL code [Babkovskaia, 2011] is used to analyse the effect of stratification by simulating cases with varying integral scales of mixing (lϕ) and fluctuations of equivalence ratio (ϕ´). Effects of differential diffusion of species on flame propagation have also been examined by comparing results with cases with unity Lewis number (Le=1). The results show that with an increase in lϕ, flame propagation shows a non-monotonic behavior.With an increase in lϕ, the flame speed and extent of burning increase first and then decrease.With an increase in ϕ´, the flame speed and extent of burning decreased consistently. The peak reaction rate of fuel species is also observed to be shifted to a higher reaction progress variable (c) with increased stratification. The effect of stratification and differential diffusion has been analysed for four identified components of flame displacement speed (Sd) viz. reaction (Sr), normal diffusion (Sn), tangential (St), and inhomogeneity (Sz). Sr and Sn are observed to be major contributors to Sd. The magnitude of Sr shows reductions with an increase in stratification. In comparison, Sn does not show significant change with increased stratification.The variation of the contribution of chemical reactions to heat release rate with stratification is also analysed in this study. The results show that shifting of peak reaction rate of fuel species to higher c values results in variation in heat release rate contribution for chemical reactions.