Ignition of premixed hydrogen/air by heated counterflow

The inert temperature required to ignite a lean premixed hydrogen/air mixture in a counterflow was determined experimentally and numerically using detailed chemistry and transport. It was found that above φ = 0.2, the ignition temperatures increased with increasing equivalence ratio. This effect is due to the fact that the ignition kernel is located on the hot, inert side of the flow and preferential diffusion of hydrogen makes the flow self-stratifying, resulting in a rich mixture in the ignition kernel even for a very lean freestream mixture. The dearth of O₂ in the kernel reduces the reaction rates to the point where diffusive loss becomes significant relative to the rates of kinetic production and consumption. In the presence of this significant transport loss mechanism, premixed ignition temperatures are much higher than non-premixed ignition temperatures and the influence of the strain rate is likewise increased. Adding a few percent of O₂ to the hot inert side of the flow lowers the kernel equivalence ratio and increases the reaction rates to the point where diffusive effects are no longer of the same order as kinetic effects. In these cases, the ignition temperatures drop significantly to values close to those of non-premixed ignition even though the free-stream flow is still predominantly premixed.