Ignition phenomena in hydrogen-air mixtures

Abstract

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.