Numerical study on propagation and NO reduction behavior of laminar stratified ammonia/air flames

In recently developed ammonia combustion technologies such as swirl flow and staged combustion, locally rich or lean pockets of unburned mixtures may occur due to insufficient mixing. This results in a premixed flamelet propagating into a gradually leaner or richer mixture. The present study aims to numerically investigate the propagation of laminar ammonia/air premixed flames in compositionally stratified mixtures. Results indicate that the flame speed in a rich-to-lean stratified mixture is increased from that in corresponding homogeneous mixtures at each local equivalence ratio. In contrast, the stratified flame speed is decreased in a lean-to-rich stratified mixture. The response of the stratified flame speed is attributed to variation in the amount of H₂ in the burned gas. In the rich-to-lean stratified flame, an extra amount of H₂ diffuses into the reaction zone to enhance chain-branching reactions which produce H radicals. The increased H radicals then promote dehydration reactions, resulting in an increased fuel consumption rate. In the lean-to-rich stratified flame, the opposite process takes place. The above mechanism is similar to the so-called back-support effect observed in methane/air stratified flames. Meanwhile, in both rich-to-lean and lean-to-rich stratified flames, additional NO reduction occurs in the stoichiometric region of the burned gas. This is facilitated by unburned ammonia diffusing from the neighboring rich burned gas mixing with O/H radicals diffusing from the neighboring lean burned gas, resulting in a production of extra NH_i radicals which readily reduce NO. Therefore, NO emission in stratified flames is expected to be lower than that estimated from the emission characteristics in homogeneous mixtures. Although similar to the well-known thermal DeNOx mechanism, the current NO reduction process occurs under a much higher temperature due to the abundance of radical species. A similar phenomenon is expected to be observed in a triple flame configuration, which requires future investigations.