Effects of stretch-chemistry interaction on chemical pathways for strained and curved hydrogen/air premixed flames

Abstract

Abstract In most combustion scenarios, stretch-chemistry interaction can directly alter the flame structure and combustion properties of a flammable mixture. In this study, the effects of stretch-chemistry interaction on chemical pathways of hydrogen oxidation are numerically investigated by considering laminar Inwardly Propagating Flames (IPF) and Outwardly Propagating Flames (OPF). One-dimensional transient simulations considering detailed chemistry and transport are conducted for different hydrogen/air mixtures with Lewis numbers well below and above unity. It is observed that the chemical pathways are affected by both flame stretch and flame propagation process. The relative roles of dominant elementary reactions vary with the Lewis number. In IPF and OPF, the negative and positive stretch rate are found to have opposite effects on the chemical pathway. When the relative importance of a reaction is enhanced by the positive stretch in OPF, it is weakened by the negative stretch in IPF. Furthermore, the chemical pathways obtained in this study are compared with other canonical flame configurations in previous work. It is observed that the relative importance of individual reaction is similar in different flame configurations, while disagreement is noticed in the quantitative contributions to the heat release and radical production. The disagreement is due to the different combinations of strain and curvature embodied in the stretch rate. To illustrate the individual role of the two stretch components respectively originating from hydrodynamic strain and flame curvature, a recently developed composition space model is used to decouple the influence of strain and curvature. It is found that these two stretch components have different effects upon chemical pathways. Overall, the hydrogen chemical pathway is more sensitive to the stretch originating from the curved flame propagation than to the flow field strain rate. Though the stretch rate is sufficient to characterize the flame chemistry for fuel-rich hydrogen/air flames with a negative stretch rate, more specific information on strain and curvature is required for fuel-lean and stoichiometric hydrogen/air flames.