Experimental and numerical investigation of the induced ignition process in ammonia/air and ammonia/hydrogen/air mixtures

Ammonia is a prospective fuel for sustainable and clean combustion systems. However, its low reactivity makes ignition and stable combustion difficult, and H$_2$ addition is considered as a means to enhance the combustion. This study addresses the ignition and initial flame propagation in NH$_3$ and NH$_3$∕H$_2$ mixtures by experiment and in simulations. The minimum ignition energy (MIE) is measured in an optically accessible ignition cell. Model simulations of flames evolving from an ignition source involving a detailed treatment of chemical kinetics and molecular transport are performed. Results show that hydrogen addition strongly widens the flammable range of ammonia, and also reduces the MIE. Simulations conducted with and without a radiation model show that radiation strongly influences ignition of ammonia and 10%H$_2$/ 90%NH$_3$ mixture. When radiation is included in the simulations, the predicted flammable range matches experimental observations more closely than without radiation. The results provide an overview of flame initiation and early flame propagation for ammonia/hydrogen mixtures, and highlight the flame enhancement with hydrogen, as well as the importance of radiation in the modeling of ammonia ignition.