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

International audienceAmmonia appears a promising hydrogen-energy carrier for renewable energy as well as a carbon-free fuel. However, there remain limited studies for ammonia combustion under turbulent conditions. To that end, using the spherically expanding flame configuration, the turbulent flame speeds of stoichiometric ammonia/air, ammonia/methane and ammonia/hydrogen, with mixture composition of blends studied currently being investigated for gas turbine application, were evaluated at various turbulent intensities, covering different kinds of turbulent combustion regimes. Mie-scattering tomography was employed facilitating flame structure diagnosis. Results show that the flame propagation speed of ammonia/air increases exponentially with increasing hydrogen amount, analogous to behavior displayed in the laminar regime. The turbulent to laminar flame speed ratio increases with turbulence intensity, however, smallest gains were observed at the highest hydrogen content, presumably due to differences in the combustion regime, with the mixture located within the flamelet zone, with all other mixtures positioned within the thin reaction zone. A good correlation of the turbulent velocity employing the Karlovitz and Damköhler numbers is observable with the present dataset, as well as previous experimental measurements 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, revealing that flame curvature remained relatively similar for pure ammonia and ammonia-based mixtures. The wrinkling ratio was 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, flame stretch does not change significantly with increasing turbulence, whilst following a similar trend to that of the laminar Markstein length