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

TL;DR

This study investigates how bluff-body flame holders stabilize premixed NH3/H2/air flames through experiments and simulations, revealing the roles of preferential diffusion, heat release, and flow recirculation in flame anchoring.

Contribution

It uncovers the stabilization mechanism involving preferential hydrogen diffusion and flow recirculation, providing new insights into ammonia/hydrogen flame anchoring behind bluff bodies.

Findings

Recirculation zone length increases by 40% due to thermal expansion.

Hydrogen diffusion creates a localized diffusion flame branch at the flame root.

Heat release concentrates within the recirculation zone, influencing flame stability.

Abstract

The stabilization mechanisms of fully premixed NH3/H2/air flames anchored behind a bluff body are investigated using combined 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 cases shows that thermal expansion strongly alters the flow field, increasing the recirculation zone length by about 40% and the shear layer width by roughly 50% near the end of the recirculation region. Excellent agreement between measurements and simulations for mean and fluctuating axial velocities validates the numerical approach. Analysis of the flame structure reveals a distinctive stabilization mechanism at the flame root: preferential hydrogen diffusion generates a localized diffusion flame branch that enhances radical production and promotes robust anchoring. Combustion proceeds sequentially, with hydrogen mainly consumed in the shear layer, followed by ammonia cracking and the main heat-release region. Near the bluff body, heat release is concentrated within the recirculation zone, while downstream regions are increasingly influenced by turbulence and velocity fluctuations. The roles of curvature and strain are quantified to assess stretch effects along the flame front. Convex curvature near the flame root enhances hydrogen enrichment and locally increases burning rates, reinforcing stabilization. In contrast, concave curvature and higher stretch near the end of the recirculation zone weaken the flame and mark a transition toward a turbulence-dominated regime. Overall, stabilization results from a coupled feedback between recirculation-driven heat exchange and rapid hydrogen oxidation, sustaining an intermediate ammonia reaction zone and enabling robust anchoring of carbon-free NH3/H2 flames.