A Numerical Study on Premixed Turbulent Planar Ammonia/Air and Ammonia/Hydrogen/Air Flames: An Analysis on Flame Displacement Speed and Burning Velocity

Abstract

The economic storage and transportation of ammonia (\(\hbox {NH}_3\)), and its capability to be thermally decomposed to hydrogen (\(\hbox {H}_2\)) make it a potential carbon-free synthetic fuel for the future. To comprehend the fundamental characteristics of \(\hbox {NH}_3\) as a primary fuel enriched with \(\hbox {H}_2\) under low turbulent premixed flame conditions, three quasi direct numerical simulations (quasi-DNS) with detailed chemistry and the mixture-averaged transport model are conducted under decaying turbulence herein. The Karlovitz number is fixed to 4.28 for all the test conditions. The blending ratio (\(\alpha\)), specifying the hydrogen concentration in the ammonia/hydrogen mixture, varies from 0.0 to 0.6. The results reveal that the mean value of the density-weighted flame displacement speed (\(S_{\textrm{d}}^{*}\)) is similar to (higher than) the unstrained premixed laminar burning velocity (\(S_{\textrm{L}}^{0}\)) for \(\hbox {NH}_3/\)air flame (\(\hbox {NH}_3/\hbox {H}_2/\)air flames). Furthermore, the performance of two extrapolation relations for estimating \(S_{\textrm{d}}^{*}\) as linear and non-linear functions of flame front curvature is discussed thoroughly. The performances of both models are almost similar when evaluating the data near the leading edge of the flame. However, the non-linear one offers more accurate results near the trailing edge of the flame. The results show that the mean flame stretch factor increases with increasing the blending ratio, suggesting that the mean flamelet consumption velocity deviates from \(S_{\textrm{L}}^{0}\) by enriching the mixture with \(\hbox {H}_2\). The mean value of the local equivalence ratio (\(\phi\)) for the turbulent \(\hbox {NH}_3/\)air flame is almost equal to its laminar counterpart, while it deviates significantly for \(\hbox {NH}_3/\hbox {H}_2/\)air flames. In addition, the local equivalence ratio for the flame front with positive curvature values is higher than the negatively curved regions for \(\hbox {NH}_3/\hbox {H}_2/\)air flames due to \(\hbox {H}_2\) preferential diffusion. Furthermore, the results indicate that hydrogen is consumed faster in positively curved regions compared to the negatively curved zones due to enhanced reaction rates of specific chemical reactions.