Flamelet models with differential diffusion effects for large eddy simulations of ammonia/hydrogen/nitrogen-air partially premixed jet flames

Abstract

Blending ammonia with hydrogen and partially cracking ammonia are promising strategies to enhance the combustion performance of ammonia. Accurate prediction of ammonia/hydrogen blend combustion behavior requires careful consideration of differential diffusion effects associated with hydrogen. In this study, differential diffusion effects are assessed considering various flamelet-based modeling approaches: the unity Lewis number flamelet/progress variable (ULF) model, variable Lewis number flamelet/progress variable (VLF) model, and species-weighted flamelet/progress variable (SWF) model. The latter one incorporates weighting between two flamelet datasets based on the unity Lewis number assumption and the mixture-averaged diffusion models. An a priori analysis based on direct numerical simulation (DNS) data and an a posteriori analysis involving large eddy simulation (LES) of turbulent partially premixed NH₃/H₂/N₂-air jet flame are conducted. The analysis confirms the presence of strong differential diffusion in turbulent partially premixed NH₃/H₂/N₂-air flames and demonstrates the feasibility of weighted flamelet models for properly capturing the differential diffusion effects in different levels of turbulence. Moreover, due to the longer chemical time of NO formation on the fuel-lean side, adding the NO mass fraction in the definition of the progress variable effectively improves NO predictions. In the LES simulations, it is found that the SWF model performs well by considering both turbulent diffusion and molecular diffusion. The predictions of the SWF model fall between those of the ULF and VLF models and align closely with the measurements on fuel-rich side, indicating the significant roles of both turbulent diffusion and molecular diffusion in these regions.

Novelty and Significance Statement

This study innovates by conducting comprehensive a priori and a posteriori analyses on modeling differential mass diffusion using flamelet-based models. The a priori analysis based on the DNS data confirms the strong differential diffusion in turbulent partially premixed NH₃/H₂/N₂-air flames, as well as the feasibility of using weighted-based flamelet models for the modeling of differential diffusion. LES simulations further reveal that differential diffusion is more pronounced on the fuel-rich side than on the fuel-lean side.