Definition of Partial Mixture Fractions for Estimating Differential Diffusion Effects in Non-Premixed Ammonia-Hydrogen Flames

Blended fuels, such as hydrogen-ammonia, are of current research interest for the development of renewable energy. However, some challenges are encountered upon evaluation of differential diffusion effects in hydrogen-ammonia non-premixed flames. First, due to the presence of H and N atoms in multiple species, the conventional differential diffusion parameter based on the element mixture fractions becomes invalid. Secondly, conventional stoichiometry-preserving mixture fractions, such as Bilger’s formulation, cannot identify the individual stoichiometric locations of each fuel oxidation under differential diffusion. To address these issues, the present study improves on a recently proposed mixture fraction formulation that individually accounts for stoichiometry of each fuel component, by introducing a new tabulation approach to estimate the magnitude of differential diffusion. The ability to estimate the individual stoichiometric locations of each fuel oxidation was examined. In counterflow non-premixed flames, the extent of differential diffusion was reasonably represented by the difference between the two partial mixture fractions. In addition, the stoichiometric locations of ammonia and hydrogen oxidation were individually estimated for a wide range of hydrogen content under a moderate strain rate. For a higher strain rate, the combined effect of reactant leakage and differential diffusion of H atoms resulted in broadening of the heat release zone in the mixture fraction space, leading to higher inaccuracies in the estimated stoichiometric locations. Meanwhile, in co-flow non-premixed flames, the partial mixture fraction resulted in lower accuracy in the estimated stoichiometric locations due to strong multidimensional diffusion effects. Specific strategies for further improvement were also discussed.