Forced Ignition of NH3/H2 Blends in a Mixing Layer with Different Air Temperatures

Hydrogen addition has attracted significant attention due to its ability to improve the combustion performance of ammonia. However, the forced ignition mechanism of ammonia blended with hydrogen with various air temperatures under nonpremixed conditions remains unclear at present. The forced ignition process of ammonia-hydrogen blended fuel in a mixing layer is systematically investigated using numerical simulations. Three air temperatures, TO = 450, 750, and 1050 K, are considered. The results show that for the cases with TO = 450 and 750 K, a tetrabrachial flame structure, consisting of two premixed branches and two nonpremixed branches, forms in the mixing layer after ignition. The flame is dominated by a propagating mode. When the air temperature is increased to 1050 K, the edge flame initially exhibits a tetrabrachial flame structure dominated by propagation, but later an autoignition-dominated branch appears upstream, resulting in a pentabrachial flame structure. The OH transport-budget analysis indicates that the autoignition branch is dominated by chemical reactions, with diffusion playing a comparatively minor role. In addition, HO2 and H2O2 serve as key markers for the formation of an autoignition branch. Analysis of the chemical reactions in the different flame branches shows that heat release in the lean nonpremixed branch is dominated by the hydrogen oxidation reaction system, while the rich nonpremixed branch is controlled by ammonia oxidation and nitrogen chemical reaction pathways. The reactions in the autoignition branch are primarily associated with the hydrogen–oxygen chain reactions. The thermal contributions of each branch further indicate that the premixed branch is the primary source of heat release for the edge flame. Increasing the air temperature not only enhances the heat release intensity of each propagation branch, but also activates the thermal contribution of the autoignition flame branch.