Numerical study on forced ignition and flame propagation in a counterflow of nitrogen-diluted hydrogen versus air

Hydrogen is widely perceived as the next generation of clean energy due to its zero carbon emissions. However, there is an escalating concern regarding hydrogen safety. A deeper understanding of the ignition characteristics of hydrogen in inhomogeneous environments is imperative for enhancing our physical comprehension and conducting accurate risk assessments. The purpose of this study is to delve into the ignition phenomena of hydrogen and air in a non-premixed counterflow. Transient two-dimensional simulations are performed, considering detailed chemistry and transport. The focus is placed on the effects of strain rate, mixture stratification, and ignition position on ignition and flame propagation. The first part studies the ignition process at varying strain rates while the ignition position is fixed at the stagnation point. The results show that a low strain rate has a minimal impact on ignition outcome, whereas a high strain rate hinders ignition by reducing mixing layer thickness and residence time. As a result, the minimum ignition energy (MIE) barely changes at a relatively low strain rate but rises rapidly as the strain rate approaches the extinction limit. Subsequently, ignition is studied at different positions along the symmetry axis, where the flow field and mixture composition vary simultaneously. It is demonstrated that ignition is more feasible on the lean side than on the stoichiometric and rich sides, and a high strain rate can expand the ignitability limit of the mixture fraction under extreme off-stoichiometric conditions. The separate contributions of forced convection and mixture inhomogeneity are further assessed through simulations with and without counterflow and stratified mixture. The results suggest that the significance of the forced convection is contingent upon the timescale competition between flow and ignition, while the mixture stratification determines the evolution of the ignition kernel into either an outwardly propagating premixed flame or a triple flame. The MIEs at premixed and non-premixed conditions are examined, and the influence of fuel stratification is analyzed. In the end, a regime diagram is presented, illustrating the comprehensive roles of strain rate and mixture fraction during ignition. This study provides invaluable insights into the non-premixed ignition of hydrogen in complex flow conditions.