Effects of Equivalence Ratio and Inlet Temperature on Combustion and NOx Emissions of Partially Cracked Ammonia in a Cavity Micro-Combustor

This work numerically examines the premixed combustion of partially cracked ammonia/air in a cavity-stabilized micro-combustor. Effects of the equivalence ratio (Φ) and inlet temperature (Tin) on the combustion features, flame–wall heat transfer and nitrogen-containing emissions are investigated quantitatively at a cracking ratio of 0.6. Results show that increasing Φ from 0.8 to 1.2 shifts the high-temperature region downstream and causes it to elongate axially. This spatial expansion decreases peak temperatures and distributes heat release over a longer distance. Mean wall temperature and overall heat loss are thus decreased due to weakened near-wall thermal interaction. NO formation closely follows the high-temperature and OH-rich zones. However, at Φ = 1.2, oxygen limitation suppresses NO production and redirects fuel-bound nitrogen towards N2O, enhancing its outlet emissions. As Tin increases from 300 K to 500 K, the improved reactivity of the mixture promotes an upstream shift of the main reaction zone. The reaction zone becomes more concentrated within the cavity. Such structural changes intensify NO formation but simultaneously compress the high-temperature zone, which reduces the wall-averaged temperature and overall heat loss. In the extended downstream post-flame region, lower temperatures and limited radical activity suppress NO2 formation and N2O decomposition. As a result, NO2 emissions decrease monotonically, while N2O emissions exhibit a gradual increase. These findings provide useful insights into the effects of operating parameters on combustion stability, heat transfer and nitrogenous pollutant evolution in microscale partially cracked ammonia flames.