Revisiting performance of reactivity stratification with hydrogen addition for ammonia combustion

As a promising energy vector for a carbon -neutral and sustainable future, ammonia (NH 3 ) combustion has attracted considerable attention. Due to the low reactivity of ammonia, NH 3 /H 2 blended fuel becomes a promising alternative, with enhanced flame speeds and wider flammability ranges. Accordingly, premixed NH 3 /H 2 /air flames have been extensively investigated in recent years. Recently, enhanced NH 3 combustion performance was observed using reactivity stratification with hydrogen addition. Following this idea, the present study revisits the performance of reactivity stratification by varying stratification thickness and equivalence ratio. Special attention is paid on the flame speed and pollutant emissions in the stratified layer. Interestingly, it is found that the stratified flame speed (relative to the unburned gases) is reduced and subsequently increased compared to the corresponding homogeneous flame due to preferential diffusion of H 2 ahead of the preheat zone. This finding is slightly different from that in the previous literature, showing that the stratified flame is not really superior in terms of flame speed compared to the premixed NH 3 /H 2 /air flame. A detailed analysis is provided to explain this point. Furthermore, a thicker stratification thickness would cause the stratified flame speed to come closer to the homogeneous flame. The well-known back -support effect in the burnt gas is counteracted by H 2 diffusion on the unburnt gas side. Additionally, an increased equivalence ratio of H 2 amplifies the back -support effect. A decreased equivalence ratio of NH 3 facilitates the combustion in the posterior section (NH 3 side) of the stratification mixture. Finally, NO production rate in the stratified flame is significantly lower compared to that in the homogeneous flame, and this phenomenon is enhanced for rich H 2 /air mixtures. These observations suggest advantages for a H 2 -NH 3 reactivity -stratification flame from rich H 2 /air mixtures propagating into lean NH 3 /air mixtures, which would lead to a higher flame speed on the NH 3 side and reduced emissions, i.e., NO and N 2 O.