Large Eddy Simulation of Ammonia–Hydrogen Nonpremixed Flames Stabilized on a Bluff Body Burner

Abstract

The demand for carbon-free fuels such as ammonia (NH3) and hydrogen (H2) has been growing rapidly due to stricter environmental policies on carbon emissions. Since ammonia is easier to store and transport, it is considered the leading alternative fuel for marine, aviation, and gas turbine industries. However, the narrow flammability, low reactivity, and potential emissions of nitrogen oxides (NOx) are important challenges that need to be overcome if ammonia is to be used as a practical fuel for industrial use. Recent works have provided important experimental data on the characteristics of NH3 flames under premixed and nonpremixed combustion modes, focusing on flame speed, turbulence–chemistry interaction, and especially the dissociation/cracking of NH3 into H2 and N2. These measurements have encouraged the use of computational fluid dynamics (CFD) for the simulation and scaling of practical ammonia systems, evaluating the performance of different numerical models for NH3 combustion. In this study, large eddy simulation (LES) is utilized to predict a series of NH3/H2/N2 nonpremixed flames stabilized on a bluff body burner. The accuracy of the flamelet generated manifold (FGM) combustion model combined with LES has been examined, using a commercial CFD solver. The different cracking levels of NH3 into H2 and N2 lead to simulation cases ranging from an H2/N2 flame (i.e., “fully cracked”) to a flame with 50% NH3 (i.e., “half cracked”). The different flame shapes, temperature distributions, NOx, and unburnt NH3 emissions have all been simulated, generally matching well the available experimental data. The developed model settings and numerical workflows may be applied to simulating more complex combustors which use NH3 as a fuel.