Detection of Turbulent Combustion Mechanism in NH₃/H₂ Stratified Premixtures Using Chemical Explosive Mode Analysis

The turbulent combustion process of NH₃/H₂/air stratified premixtures is simulated by DNS, which shows the existence of diverse local modes and structures in the domain. The chemical explosive mode analysis (CEMA) diagnostic tool, which has systematic and rigorous diagnostic capabilities, is employed to clarify the fundamental physical mechanics underlying the local modes, structures, and events. It is found that a small temperature fluctuation corresponds to the spontaneous ignition mode, and an intermediate or large temperature fluctuation corresponds to the deflagration propagation mode. The dependence of the combustion mode on composition stratification is unimportant in the simulated cases. Compared to the spontaneous ignition mode, the combustion/flame interaction in the deflagration mode is inhibited due to the mitigation of thermal dilation, thereby leaving much finer structures in the domain. The "diffusion-assisted spontaneous ignition mode", where diffusion ahead of the reaction front promotes spontaneous ignition and flame propagation, is a unique behavior in stratified NH₃/H₂/air combustion at elevated temperature fluctuation conditions. It is characterized by distinct structures that differ from the traditional autoignition mode and laminar deflagration. The preferential diffusion of H₂/H radicals from the tailing-rich front to the leading leaner zone enhances the upstream ignition reactions, which is the underlying physics for the unique "diffusion-assisted spontaneous ignition mode" surviving in stratified NH₃/H₂ combustion. The local modes ahead of the ignition front are primarily determined by the flame curvature. More specifically, for ignition fronts with negative curvature, the concave front wrinkling to the unburnt side promotes spontaneous ignition; while for those with positive curvature, the tangential diffusion with a negative contribution to ignition appears just ahead of the propagating fronts, thereby leading to the presence of an extinction mode. For the stratified NH₃/H₂ mixture combustion, the preferential diffusion associated with H₂/H radicals and tangential diffusion with a positive curvature are the most effective in promoting the local reaction rates ahead of the ignition front.