Stabilization of premixed NH3/H2/air flames via bluff-body flame holders

The stabilization mechanisms of fully premixed NH 3 /H 2 /air flames anchored near a bluff-body are investigated using experiments and direct numerical simulations. Particular attention is given to the interplay between preferential diffusion, heat release, flow recirculation, and turbulence–flame interaction. Comparison between non-reactive and reactive configurations shows that thermal expansion significantly alters the flow field, increasing the recirculation zone length by about 40% and the shear layer width by roughly 50% at the end of the recirculation region. Excellent agreement between measurements and simulations for mean and fluctuating axial velocities validates the numerical framework. Detailed analysis of the flame structure reveals a distinctive stabilization mechanism at the flame root: preferential hydrogen diffusion produces a localized diffusion flame branch that enhances radical production and strengthens anchoring. The chemical structure follows a sequential combustion process, with hydrogen primarily consumed in the shear layer, followed by ammonia consumption and the main heat release zone. Two distinct flame zones are identified: a recirculation zone flame governed by high tangential strain and moderate positive curvature, where preferential hydrogen diffusion contributes to flame stabilization, and a turbulent flame zone dominated by increasing turbulent fluctuations and negative curvature. Near the bluff-body, heat release is concentrated on the inside of the shear layer, while downstream it increasingly coincides with regions of elevated turbulence. The roles of curvature and strain are quantified to assess stretch effects along the flame front. Convex curvature near the flame root promotes hydrogen enrichment and locally increases burning rates, reinforcing stabilization, whereas curvature and elevated turbulence downstream increasingly weaken the flame and indicate a transition towards a turbulence-dominated regime. Probability density functions of strain and curvature further distinguish the dominant stretch mechanisms across the two flame zones. Overall, stabilization arises from coupled feedback between recirculation-induced heat exchange and rapid hydrogen oxidation, amplified by hydrogen diffusion, sustaining an intermediate ammonia reaction zone. These findings provide insight into carbon-free NH 3 /H 2 flame stabilization and guidance for robust, low-emission combustion system design. Novelty and significance statement This work presents the first combined DNS and experimental investigation of a premixed NH 3 /H 2 /air flame stabilized behind a bluff-body. While bluff-body flame stabilization has been extensively studied for conventional fuels, the staged combustion behavior of ammonia–hydrogen mixtures, where ammonia must first react to produce hydrogen, introduces fundamentally different stabilization mechanisms. Understanding these effects is crucial for developing robust and efficient combustors that can operate with carbon-free fuels across a wide range of conditions relevant to future applications. By resolving the detailed flame structure and reaction pathways, this study provides new insights into how ammonia consumption and hydrogen release interact with recirculation and mixing processes, extending classical stabilization theory to carbon-free, staged combustion systems. Detailed analysis of turbulence-flame interaction reveals different mechanisms that govern flame stabilization. Two fundamentally different flame regions are identified. The recirculation zone is governed by high tangential strain and moderate positive curvature, promoting preferential hydrogen diffusion, while the downstream turbulent flame zone is dominated by negative curvature induced by increasing turbulent fluctuations and low tangential strain. This distinction provides a physical basis for understanding how turbulence and preferential diffusion govern flame stabilization and the downstream evolution of the ammonia-hydrogen flame.