The effects of turbulence on the flame structure and NO formation of ammonia turbulent premixed combustion at various equivalence ratios. (15th January 2023)

• Record Type: Journal Article
• Authors: Tian, Tingquan; Song, Chengbin; Wang, Haiou; Xu, Chao; Luo, Kun; Fan, Jianren
• Is Part Of: Fuel. Volume 332(2023)Part 2
• Publication Date: 2023-01-15
• Subjects: Turbulent ammonia flames -- Flame structure -- NOx emission -- Direct numerical simulation; Fuel -- Periodicals; Coal -- Periodicals; Coal; Fuel; Periodicals; 662.6
• DOI: 10.1016/j.fuel.2022.126127
• Languages: English
• ISSNs: 0016-2361

Abstract

Abstract: Ammonia is carbon-free and is regarded as a potential fuel to address global warming issues. In this work, three-dimensional direct numerical simulations (DNS) of ammonia/air turbulent premixed flames were performed to explore the influence of turbulence and equivalence ratio on the flame structure and NO formation characteristics. Two equivalence ratios were considered, i.e. ϕ = 0.9 and ϕ = 1.1. The general flame structures were presented and species distributions were examined. The NO mass fraction was found to be the highest in the product for the lean case and in the reaction zone for the rich case. The conditional mean values of species mass fractions and reaction rates were compared with those of the unstrained and strained laminar premixed flames to explore how well the laminar flame structures can approach those of the turbulent flame. The budget analysis of the species transport equations showed that turbulent diffusion plays an important role in species transport. The turbulent diffusivity D T was estimated using the gradient hypothesis based on the DNS data. Various laminar flame simulations with different diffusivities were carried out. It was shown that the conditional means of the DNS agree well with those of the laminar flames including D T in the transport property calculation. The global and local NO formation characteristics were investigated. It was found that the mean NO production conditioned on the progress variable is lower compared with the corresponding laminar flame in the rich case. However, the relative contributions from various NO pathways are rarely affected by turbulence. The NO mass fraction is higher in negative curvature regions compared with positive curvature regions of the flame surface for the rich case, which is due to the preferential diffusion of H 2 and other radicals and the enhanced NO pathways in negatively curved regions. Highlights: NO is the highest in the product of lean case and in the reaction zone of rich case. Turbulent diffusion plays an important role in species transport. Turbulence has evident impacts on the NO pathways globally and locally. The enhanced NO production due to preferential diffusion is observed for rich case.