Investigation of spark ignition processes of laminar strained premixed stoichiometric NH3-H2-air flames

A knowledge of the ignition properties of ammonia (NH3)/hydrogen (H2) mixtures is important because of their abundance in chemical engineering processes, and also because of their prospective role as fuels in future energy systems. In particular, the question arises if and how important characteristics like ignition limits and minimum ignition energies in NH3/H2 mixtures are related to the physical conditions. To address this question, this work studies ignition process in ammonia/hydrogen mixtures by numerical simulations. These track the evolution of ammonia/hydrogen mixtures during and after the deposition of a certain ignition energy, using a detailed treatment of chemical reactions and molecular transport. Studies on the influence of different system parameters on the minimally required ignition energy are performed. These are the strain rate, hydrogen content, pressure and initial (pre-ignition) temperature. Significant findings include a quasi-linear correlation between the transition strain rate, defined as the strain rate below which no external energy is required to initiate successful ignition (auto-ignition) and a characteristic reaction rate, defined as the inverse of ignition delay time in homogeneous, quiescent mixtures. Also, the relative decay of minimum ignition energy with increasing hydrogen content is less pronounced for higher pressures. Analysis of the results supports a knowledge-based approach towards fail-proof ignition devices and reliable prevention of hazards. The simulations are used for assessing the ignitability of ammonia and its mixtures with hydrogen.