A computational study of differential diffusion characteristics in NH3/H2/N2-air non-premixed jet flames

Abstract

Partial cracking of ammonia presents a practical solution to the challenges posed by the low laminar burning velocity and high autoignition energy of pure ammonia. On the other hand, the presence of hydrogen in the fuel can induce significant differential mass diffusion in turbulent non-premixed flames. In this study, a direct numerical simulation (DNS) of a temporally evolving planar jet NH₃/H₂/N₂-air non-premixed flame was conducted to investigate the characteristics of differential diffusion in partially cracked ammonia flames. A recently proposed extended mixture fraction definition was employed to consider the effect of nitrogen element. The results indicate that the conditional mean temperature and mass fractions of major species consistently fall between those predicted by the two laminar flamelets, calculated using the mixture-averaged diffusion model and the unity Lewis number. Differential diffusion was quantitatively characterized by the difference in the mixture fractions of hydrogen and nitrogen elements. In the turbulent non-premixed flame with a high Reynolds number of 23,000, the extent of differential diffusion still reaches 60% of that observed in laminar condition. Moreover, a spatial filtering analysis in the context of large eddy simulation (LES) confirmed that differential diffusion is not reflected in the subgrid species flux, as it arises from the convection term rather than the diffusion term.