Effects of Partial Fuel Cracking on the Forced Ignition and Spherical Flame Propagation in Ammonia/Air Mixtures

As a zero-carbon fuel, ammonia has promising applications and thereby has received great attention recently. However, compared to traditional hydrocarbon fuels, ammonia has very low laminar flame speed and extremely high minimum ignition energy (MIE). A feasible method to promote ammonia combustion is partial fuel cracking, during which ammonia undergoes decomposition into hydrogen and nitrogen. This work reports a computational study on forced ignition and spherical flame propagation in partially cracked ammonia/air mixtures. The objective is to assess the effects of fuel cracking on the MIE and flame propagation. It is found that even a slight amount of ammonia cracking can significantly reduce the MIE and promote ignition. In addition, the influence of positive stretch rate on the spherical flame propagation speed, which is quantified by the Markstein length, is shown to be greatly affected by ammonia cracking. The mechanism of ignition promotion and change in spherical flame propagation by ammonia cracking is interpreted and discussed.