Turbulent flame speed and morphology of pure ammonia flames and blends with methane or hydrogen

Ammonia appears a promising hydrogen-energy carrier as well as a carbon-free fuel. However, there re -main limited studies for ammonia combustion especially under turbulent conditions. To that end, using the spherically expanding flame configuration, the turbulent flame speeds of stoichiometric ammonia/air, ammo-nia/methane and ammonia/hydrogen were examined. The composition of blends studied are currently being investigated for gas turbine application and are evaluated at various turbulent intensities, covering different kinds of turbulent combustion regimes. Mie-scattering tomography was employed facilitating flame struc-ture analysis. Results show that the flame propagation speed of ammonia/air increases exponentially with increasing hydrogen amount. It is less pronounced with increasing methane addition, analogous to the be-havior displayed in the laminar regime. The turbulent to laminar flame speed ratio increases with turbulence intensity. However, smallest gains were observed at highest hydrogen content, presumably due to differences in the combustion regime, with the mixture located within the corrugated flamelet zone, with all other mixtures positioned within the thin reaction zone. A good correlation of the turbulent velocity based on the Karlovitz and Damkohler numbers is observable with the present dataset, as well as previous experimental measure-ments available in literature, suggesting that ammonia-based fuels may potentially be described following the usual turbulent combustion models. Flame morphology and stretch sensitivity analysis were conducted, re-vealing that flame curvature remains relatively similar for pure ammonia and ammonia-based mixtures. The wrinkling ratio is found to increase with both increasing ammonia fraction and turbulent intensity, in good agreement with measured increases in turbulent flame speed. On the other hand, in most cases, the flame stretch effect does not change significantly with increasing turbulence, whilst following a similar trend to that of the laminar Markstein length. & COPY; 2022 The Authors. Published by Elsevier Inc. on behalf of The Combustion Institute. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ )

Automated summary

Ammonia shows promise as a hydrogen-energy carrier and carbon-free fuel, but there is limited research on ammonia combustion under turbulent conditions. This study examines the turbulent flame speeds of ammonia/air, ammonia/methane, and ammonia/hydrogen using a spherically expanding flame configuration. The results show that the flame propagation speed of ammonia/air increases exponentially with increasing hydrogen amount, and that the turbulent to laminar flame speed ratio increases with turbulence intensity. Flame morphology and stretch sensitivity analysis indicate that flame curvature remains relatively similar for pure ammonia and ammonia-based mixtures, and the wrinkling ratio increases with both increasing ammonia fraction and turbulent intensity.