Flame stability and NOx pathways in ultra-lean NH3/H2 micro-combustion

• Flame stability with heat transfer and NOx pathways of ultra-lean NH 3 /H 2 blends. • Hydrogen enrichment extends ammonia’s lean flammability limit. • Radical pool strength (H, O, OH) increases with Re and ϕ. • Intermediate species (NH 2 , NH, HO 2 , HNO) exhibit distinct equivalence-ratio trends. Ammonia’s narrow flammability and low reactivity limit its use in micro-combustors, but hydrogen enrichment offers a pathway to stable ultra-lean operation. Hydrogen enrichment has been proposed as a viable strategy to overcome these limitations, yet systematic mapping of lean NH 3 /H 2 flames under micro-scale confinement remains scarce. In this work, a two-dimensional numerical study was conducted on a planar micro-combustor fuelled with an NH 3 /H 2 blend (10/90 vol%) to quantify flame stability, heat transfer, and NOx chemistry over a broad range of Reynolds numbers Re = 191–1330 and ϕ = 0.65–0.20. Hydrogen addition extends the lean limit from ϕ = 0.65 to ϕ = 0.2, with ultra-lean flames ( ϕ = 0.20–0.25) stabilized only at Re = 572 and 381, respectively. Outlet temperatures rise with Re and approach adiabatic values, while heat loss ratio ( Q loss /HoR) reduces below 0.065 once Re exceeds 953 across the entire equivalence ratio range. Radiative coupling provides an additional stabilizing effect, with incident radiation increasing from 3.6 × 10 4 W/m 2 ( ϕ = 0.20, Re = 572) to 76 × 10 4 W/m 2 ( ϕ = 0.65, Re = 1330). Radical pool analysis revealed that H, O, and OH intensify with both Re and ϕ , sustaining chain branching and flame anchoring, while intermediate species showed distinct behaviours. Kinetic pathway analysis confirmed that NO formation is dominated by HNO decomposition and NO 2 reduction, whereas consumption proceeds mainly through reactions with HO 2 and HNO cycling. The findings provide the first integrated stability–heat transfer–reaction pathway analysis of NH 3 /H 2 micro-planar flames, demonstrating how hydrogen extends ammonia’s lean limit to ultra-lean regimes while reshaping the NO/N 2 O distribution through radical-driven chemistry.