The Role of Radiative Heat Loss and Collisional Energy\nTransfer in the Flammability Limits of NH<sub>3</sub> and NH<sub>3</sub>–H<sub>2</sub> Mixtures

Understanding\nthe fundamental factors that define the flammable\nrange of ammonia is essential for developing predictive tools to identify\nsafe storage conditions, thereby facilitating its widespread use as\nan energy carrier. In this work, a numerical framework is established\nto predict and analyze the flammable range of ammonia and ammonia-hydrogen\nmixtures. To achieve this, a freely-propagating-flame method, adapted\nto account for radiation heat loss from nitrogen-containing species,\nis combined with an established ammonia oxidation mechanism, and the\nroles of initial temperature, pressure, and dilutant concentration\nare thoroughly investigated. It is found that, especially at the rich\nextinction limit, ammonia-related radiation plays a dominant role\nin causing an earlier extinction, especially at high initial temperatures.\nAlso, the upper flammability limit is found to be particularly sensitive\nto third-body reactions. Incorporating ammonia collision efficiency\ninto kinetic models is recommended as a key next step in enhancing\nthe predictive accuracy of numerical tools.