Competition between flame stretching and flame distortion in controlling early flame kernel development for turbulent lean premixed NH3/H2/air flames

The development of flame kernels in turbulence is critical in internal combustion engines and gas turbines. This study investigates the competitive effects of flame stretching/wrinkling (indicated by the integral Reynolds number Re t ) and inner flame structure distortion (indicated by the Karlovitz number Ka) on extinction of premixed lean NH 3 /H 2 /air flame kernels using three-dimensional direct numerical simulations (DNS) with detailed chemistry and molecular transport models. Four DNS with varying Re t and Ka are performed. It is found that the flame kernels extinguish mainly due to inner flame structure distortion at high Ka number. The local flame reactivity is found to be controlled by preferential diffusion of H 2 at both positive curvature region (due to curvature-induced flame tangential diffusion) and negative curvature region (due to strain-induced flame normal diffusion). It is shown that, at high Ka number, the intense turbulence at higher Re t leads to an earlier extinction of the flame kernel, because of a faster penetration of small-scale eddies into the flame structure. The enstrophy field represents well the suppression effect of turbulent eddies on differential diffusion. In contrast, at low Ka number, more intense flame stretching/wrinkling at higher Re t enhances the flame reactivity due to increased probabilities of positive curvature and tangential strain rate. Early flame kernel/turbulence interactions also accelerate the flame kernel development due to steeper species gradients. These results provide an improved understanding of the extinction and propagation mechanism of lean turbulent premixed NH 3 /H 2 /air flame kernels. Novelty and significance Turbulence is commonly assumed to stretch (resulting in excessive strain rate) and wrinkle (resulting in local flame shortening) the flame at high Reynolds number, as well as to disturb the inner flame structure at high Karlovitz number. The dominant mechanism of turbulence for global flame extinction remains incompletely understood, especially for lean NH 3 /H 2 /air flames, which are highly relevant for carbon-free combustion. Through DNS of such flames at various Ka and Re t numbers, the present study systematically quantifies the effects of flame stretching and wrinkling as well as inner flame structure distortion on flame extinction (resp. enhanced burning). The results provide novel insights on what controls early flame kernel development, why the thermochemical effect (due to differential diffusion) is more important than the aerodynamic effect (due to flame stretch) for lean NH 3 /H 2 /air flames, how turbulent stretching or wrinkling enhances flame extinction (for the broken flame structure at high Ka numbers) or promotes flame burning (for the thin flame structure at low Ka numbers).