An experimental investigation into the effect of spark gap and duration on minimum ignition energy of partially dissociated NH₃ in air

This paper presents an experimental investigation into the effect of spark gap and spark duration on the minimum ignition energy (MIE) of partially dissociated NH₃ in air at 295 K and 1 atm. The measurements were carried out using a Hartmann bomb apparatus with the degree of NH₃ dissociation varying from 0 to 10%, corresponding to 0 to 7.5%v/v H₂ in the fuel mixture with a fixed H₂/N₂ ratio of 3, and equivalence ratio varying from 0.7 to 1.2, assuming complete oxidation. Axisymmetric two-dimensional CFD modelling was performed using Ansys Fluent incorporating the Okafor mechanism to predict MIE. At a given equivalence ratio, both measured and predicted MIE values decreased significantly with increasing the degree of NH₃ dissociation, due to increased presence of H₂ in the mixture. There were optimum spark gap and spark duration for a given degree of NH₃ dissociation and equivalence ratio at which the MIE was the lowest. The increased presence of H₂ due to rising degree of NH₃ dissociation enhances the mixture's oxidation reactivity and combustion intensity, which in turn increases the flame kernel temperature and size, the rate of production of key radicals (OH, NH₂, H, and O) and rate of heat generation, leading to a more self-sustaining flame and significantly reduced MIE.