Effect of H[formula omitted] addition on NH[formula omitted] flame structure and dynamics in a mixing layer.

Hydrogen addition has been widely used to improve the combustion performance of low-reactivity fuels such as NH 3. In this study, a series of two-dimensional NH 3 /H 2 flames in mixing layers were investigated numerically. The aim is to assess and interpret the effects of H 2 addition on the structure and dynamics of ammonia flames. Detailed chemistry and transport models were considered in the simulations. It was shown that for pure hydrogen or ammonia flames, a classical tribrachial structure was observed. The non-premixed branch in the tribrachial flame of NH 3 combustion features high heat release rate (HRR), while the two premixed branches are weak. The mixture fraction corresponding to the maximum HRR of NH 3 combustion is significantly lower than the stoichiometric mixture fraction. When blending ammonia with hydrogen, a tetrabrachial flame structure consisting of two premixed branches and two non-premixed branches was observed, where one non-premixed branch corresponds to the reaction of NH 3 , while the other is formed due to the reaction of H 2. It was suggested that the formation of the tetrabrachial flame is related to the preferential diffusion of hydrogen. After decreasing artificially the diffusivity of hydrogen to suppress the preferential diffusion effect, the flame exhibits only three branches with the merging of the two non-premixed branches. By analyzing the flame dynamics at the leading edge, it was found that the flame speed (S d ∗) is negatively correlated with the flame stretch (κ) and scalar dissipation rate (χ), with S d ∗ decreasing almost linearly with κ or χ , consistent with previous studies of methane and hydrogen flames. As the H 2 concentration is increased, the values of S d ∗ increase due to the enhanced reactivity. • Effect of H 2 addition on NH 3 flame structure and dynamics in a mixing layer is studied. • Tetrabrachial flame of NH 3 /H 2 combustion is identified for the first time. • Influence of preferential diffusion of H 2 on the structure of NH 3 flame is analyzed. [ABSTRACT FROM AUTHOR]