Effects of differential diffusion on turbulent burning velocity and flame surface area in NH3/H2/N2-air premixed flames

This study investigates the influence of differential and preferential diffusion on turbulent burning velocity and flame surface area in premixed NH₃/H₂/N₂-air flames using a three-dimensional Direct Numerical Simulation (DNS) database of statistically planar flames. The analysis covers three fuel blends—pure NH₃-air, 60 %NH₃/25 %H₂/15 %N₂-air, and 40 %NH₃/45 %H₂/15 %N₂-air—all at an equivalence ratio of 0.81, falling within the thin reaction zones regime on the Borghi-Peters diagram. Both the normalised flame surface area (by the projected area in the mean propagation direction) and the turbulent-to-laminar burning velocity ratio are generally comparable (but not equal) across the different fuel blends. However, the normalised flame surface area is found to be smaller than the corresponding normalised turbulent burning velocity, especially in cases involving non-unity Lewis numbers. This discrepancy highlights the impact of differential and preferential diffusion. Among the mixtures studied, the NH₃-air flame exhibits the highest effective Lewis number and correspondingly the lowest values of normalised flame surface area and turbulent burning velocity ratio. Despite these variations, the similarity between the normalised surface area and burning velocity across cases lends partial support to Damköhler’s first hypothesis. Moreover, the second Damköhler hypothesis reasonably estimates the order of magnitude for the normalised turbulent burning velocity. Overall, the findings suggest that conventional turbulent combustion models relying upon Damköhler’s hypotheses may remain valid for hydrogen-enriched ammonia-air flames under the studied conditions, despite the presence of differential and preferential diffusion effects.