Flammability limit of lean ammonia premixed flames in a Micro-Planar combustor

• Ammonia/air combustion simulated in a micro-planar combustor across Re = 15–609 and ϕ = 0.65–1.0. • Flammability limits mapped for the first time in this configuration. • Flame shape, temperature, and OH profiles used to identify flame structure. • NH and HNO pathways analysed for NO formation. • Enthalpy flux and radiative heat exchange were resolved under adiabatic wall conditions. • Reynolds number and equivalence ratio control flame shape, radical distributions, and NO formation, highlighting key flow-chemistry coupling in ammonia micro-combustion. • The kinetic study showed that NO formation depends more on reaction competition than on intermediate species levels. • Results offer parametric maps to guide ammonia micro-combustor design. This study numerically investigates ammonia/air premixed combustion in a micro-planar combustor over a range of Reynolds numbers (Re = 15–609) and equivalence ratios ( ϕ = 0.65–1.0). The simulations employ a steady laminar 2D solver in ANSYS Fluent using finite-rate chemistry and a discrete ordinates radiation model. Results reveal stable flame propagation down to Re = 15 for all equivalence ratios, with upper blow-off limits at Re = 114, 267, 572, and 609 for ϕ = 0.65, 0.7, 0.8, and 1.0, respectively. Flame structure transitions are observed: flat flames form at Re = 15, U-shaped flames at Re = 38, wrinkled flames emerge at Re = 114 ( ϕ = 0.65) due to hydrodynamic instabilities, and parabolic flames dominate at higher Re. OH radical concentration contours are used to track flame position and intensity, revealing broader and less intense reaction zones for lean mixtures and sharper flame fronts at stoichiometric conditions. NO formation is primarily governed by fuel-NO x pathways, with HNO and NH acting as key intermediates. Peak NO emission is observed at ϕ = 0.8 and Re = 114, with a mole fraction of 93.9 × 10 −4 . This is attributed to a reduced influence of the De-NOx reaction NH + NO = N 2 O + H (R-36), which otherwise suppresses NO formation, as demonstrated through the kinetic rate study conducted. NO formation peaks at intermediate Re, where residence time is sufficient for reaction completion, but declines at higher Re due to limited conversion time for intermediates. Radiation and convective heat flux analysis demonstrates a transition from radiation-dominated losses at low Re to convective dominance at high Re. Incident radiation and radiative heat flux peak at intermediate Reynolds numbers, then decline as the flame shifts downstream. The findings contribute to a detailed understanding of ammonia flame dynamics, blow-off behaviour, and NO control strategies in micro-scale combustion systems.