Enhanced burning rates in hydrogen-enriched turbulent premixed flames by diffusion of molecular and atomic hydrogen

• Published in: Combustion and flame 2022-05, Vol.239, p.111740, Article 111740
• Main Authors: Rieth, Martin, Gruber, Andrea, Williams, Forman A., Chen, Jacqueline H.
• Format: Article
• Language: English
• Subjects: Adiabatic conditions; Ammonia; Burning rate; Diffusion effects; Diffusion rate; Direct numerical simulation; Enrichment; Flame curvature; Flame propagation; Hydrogen; Hydrogen-enriched fuels; Mixtures; Molecular diffusion; Nuclear fuels; Premixed flames; Reaction kinetics; Species diffusion; Trailing edges; Turbulence intensity; Turbulent flames
• ISSN: 0010-2180; 1556-2921
• DOI: 10.1016/j.combustflame.2021.111740

Direct numerical simulations (DNS) of fuel-lean turbulent premixed hydrogen-enriched flames are analyzed to improve understanding of local effects of molecular diffusion of fast-diffusing, low-Lewis-number hydrogen species (H2 and H) on the overall burning rates of reactant mixtures. Although it is often assumed that the importance of molecular diffusion decreases with increasing turbulence intensity, our analysis reveals that, on the contrary, diffusion of molecular and atomic hydrogen can remain the rate-controlling processes even at high Karlovitz numbers (Ka≫1). Three-dimensional DNS with detailed chemical kinetics and species transport of turbulent premixed hydrogen-air and hydrogen/ammonia-air flames in the regimes of thin and distributed reaction zones are performed and analyzed at different reactant temperatures and pressure levels. The DNS data reveal a significant impact of H2 and H diffusion in all flames analyzed. In particular, the magnitude of H2 diffusion, occurring from low progress variable near the flame elements of the turbulent reaction front that exhibit convex (positive) curvature towards the fresh mixture, is greatly enhanced at high pressure through increased spatial gradients, resulting in localized equivalence-ratio enrichment and super-adiabatic conditions that accelerate and strengthen the flame front. Moreover, for higher reactant temperatures, back diffusion of atomic hydrogen, transported from the reaction zone into unburnt regions from negatively curved reaction layers surrounding them, facilitates the occurrence of localized spontaneous ignition events at the trailing edges of reaction fronts. For certain conditions these events, induced by differential diffusion, can become the dominant process controlling the overall rate of reactants consumption.