On the Influence of H2 Addition on NH3 Laminar Flame Speed under Engine-like Conditions

As zero-carbon fuels, hydrogen and ammonia are of great interest in the transition toward a climate-neutral transportation system. In order to use these fuels and their blends in reciprocating engines, a characterization of the combustion of NH3/H2/air mixtures at high pressures and temperatures is needed. The aim of this work is to compute the Laminar Flame Speed (LFS) of NH3/H2/air mixtures by varying the thermochemical conditions of the reactants. For this purpose, several simulations have been carried out using different kinetic reaction mechanisms. The accuracy of the model has been assessed by comparing the results with experimental data available in the scientific literature. Finally, the influence of mixture composition and thermodynamic conditions of the reactants on LFS has been assessed by considering temperature and pressure values relevant to automotive applications and not yet explored in the literature. By adding H2 to NH3/air mixtures, LFS increases exponentially. By plotting the logarithm of LFS as a function of the H2 mole fraction, the numerical results are well fitted by using a second-degree polynomial regression. However, a linear regression is accurate enough if the H2 mole fraction does not exceed 0.6. Regarding the effect of pressure, the decrease in LFS with increasing pressure is less important as pressure increases. On the other hand, LFS increases with temperature, and this effect is more pronounced as the H2 mole fraction decreases and pressure increases.