Direct numerical simulations of methane, ammonia-hydrogen and hydrogen turbulent premixed flames

Three turbulent premixed flames with the same unstretched laminar flame speeds and thicknesses are analyzed and compared using 3D Direct Numerical Simulation (DNS) in a slot burner configuration at atmospheric conditions: a CH4/ air flame at 1) = 1 , an NH3 - H2/ air (46% vol. H 2 ) at 1) = 1 and an H2/ air flame at 1) = 0 . 45 .While both stoichiometric methane and ammonia-hydrogen flames behave similarly, the lean hydro-gen flame brush is twice as short with less flame surface, and exhibits significant alteration of its local flame structure: for the H 2 flame, the thickness of flamelets decreases significantly while burning rates increase drastically. This is observed for flame elements convexly curved with respect to fresh reactants (positive curvature) because of preferential diffusion, where thermo-diffusive instabilities generate long-tail structures that continuously head toward the fresh gases, and also for near-flat flame elements be -cause of local strain effects. Opposite behaviors are observed for flame elements concavely curved (neg-ative curvature) where fuel depletion caused by unfocusing of hydrogen and strain effects are unable to increase the consumption, even leading to near-extinction of the flame. The turbulent methane and ammonia-hydrogen flames studied in this work do not exhibit the instabilities seen in the pure hydrogen flame. However, local flame-flame interactions are observed in the cusp regions of all three flames, which significantly increase the displacement speed as well as flame surface destruction.A 1D-3D comparison indicates that strain effects in regions of low curvature and preferential diffusion effects in regions of positive curvature can be modeled using counterflow flamelets to account for stretch effects on the turbulent hydrogen flame propagation in addition to turbulent wrinkling.& COPY; 2023 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Automated summary

Three turbulent premixed flames, including methane/air, ammonia-hydrogen/air, and hydrogen/air flames, were analyzed and compared using 3D Direct Numerical Simulation. The results showed that the hydrogen flame exhibited significant alteration of its local flame structure, with decreased flame length and surface area, and increased burning rates. In contrast, the methane and ammonia-hydrogen flames behaved similarly.