Abstract
Ammonia’s carbon-free promise as fuel is offset by low reactivity and a strong propensity for fuel-NOx. This study presents a CFD comparison of non-premixed (diffusion) and premixed ammonia combustors operating in a staged micro gas turbine combustion chamber, operating at two pressure levels (1 and 3 bar) and at different overall equivalence ratios (ϕoverall=0.124∼0.5), under a rich-lean combustion strategy (ϕprimary=1.1). Using a flamelet-generated manifold coupled with a realizable k-ε turbulence model, outlet temperature, thermal power, NO/NOx emissions, and NH3/H2 slip are quantified, and their underlying mechanisms are analyzed through mid-plane streamlines and the Ux=0 iso-surface (Central Recirculation Zone, CRZ boundary) visualized using NH3 concentration. Premixed operation yields higher outlet efficiency NH3/H2 slip (especially at 3 bar), but NO and dry-corrected NOx rise with overall equivalence ratio and pressure because high-temperature stoichiometric layers are contiguous and NHx availability for re-burning is minimal. Non-premixed operation shows complementary behavior: lower outlet temperature/power and higher NH3/H2 slip, while dry NOx decreases with overall equivalence ratio, most prominently at 3 bar; the streamline topology is mixing-limited and the Ux=0 surface is NH3-laden, sustaining NHx-mediated NO re-burning. The O2 fields reconcile the opposite NOx trends: in premixed, the increase in in-flame fuel-NO outpaces the reduction of the 15% O2 correction factor as overall equivalence ratio rises; in non-premixed, modest gains in raw NO are overwhelmed by (i) stronger re-burning in the NHx-rich CRZ and (ii) a shrinking correction factor as outlet O2 drops with overall equivalence ratio. These results establish aerodynamics as the dominant lever linking mixing mode and pressure to efficiency and emissions in pure-ammonia micro-turbine combustion.