Reactivity stratified flame to enhance ammonia combustion with hydrogen: Effects of temperature, pressure, downstream mixture, and molecular diffusion

To assist ammonia combustion and reduce pollutant emissions, a reactivity stratified hydrogen-ammonia flame has recently been identified to be promising, with enhanced flame speed and reduced NO/N 2 O production. The previous studies on reactivity stratified flames are rare and primarily focused on standard working conditions, e.g. with unburned gas temperature of 300 K, pressure of 1 bar and from pure hydrogen/air mixture to pure ammonia/air mixture. Therefore, it is meaningful to check effectiveness of the reactivity stratified configuration under various working conditions. The present study investigates the flame speed and NO/N 2 O productions of the reactivity stratified flames with unburned temperature ranging from 300 K to 600 K, pressure ranging from 1 bar to 5 bar, and downstream mixture hydrogen volume ratio ranging from 0 to 0.4. It is found that the reactivity stratified configuration becomes less beneficial at higher temperature, higher pressure and with larger hydrogen volume ratio in the downstream mixture, because the differential diffusion effect becomes less pronounced under these conditions. The effect of molecular diffusion is evaluated. Differential diffusion (mainly the preferential diffusion of H and H 2 ) is found to be important for the flame speed and pollutant production in the stratified flame, while thermal diffusion (Soret) is less significant. These findings are critical to identify efficient designs using reactivity stratification to enhance ammonia combustion.