LES of non-premixed NH₃/H₂/N₂-air jet flames at elevated pressure with differential diffusion

Cracking NH₃ into a mixture of NH₃/H₂/N₂ effectively addresses its low reactivity when used as a fuel. In this study, large eddy simulation (LES) and experiments are conducted for non-premixed NH₃/H₂/N₂-air jet flames with simulated cracking ratios of 14 % and 28 % at an elevated pressure of 5 bar. Detailed experimental data on the flow field are provided for assessing turbulence models. A recently proposed species-weighted flamelet/progress variable (SWF) model considering differential diffusion is adapted for simulating cracked NH₃ flames. The effects of differential diffusion on scalar structures are further analyzed. The SWF model, incorporating differential diffusion, achieves good agreement with experiments in predicting velocity, mean temperature, major species mass fractions, differential diffusion parameters, and NO formation. The model also qualitatively captures variations in localized extinction along the flame height. In contrast, the unity Lewis number flamelet/progress variable (ULF) model predicts the occurrence of localized extinction too upstream, significantly overestimating its occurrence in the near field where differential diffusion is significant. Moreover, NO formation in the cracked NH₃ flame is well predicted by the SWF model using flamelet tables, and slightly overpredicted by the ULF model without differential diffusion.