Ignition of premixed hydrogen/air by heated counterflow under reduced and elevated pressures

The temperature of an inert jet required to ignite a counterflowing lean premixed hydrogen/air jet was experimentally determined over the pressure range of 0.6 to 7 atm and computationally simulated using detailed chemistry and transport. Results show that, compared to the homogeneous explosion limits, ignition takes place at higher temperatures and exhibits five limits over the pressure range investigated. The first and second ignition limits resemble the corresponding first and second homogeneous explosion limits, except they have steeper slopes in the pressure-temperature response, with the first limit being affected by the significant transport loss of the H radical and the second limit modified by the activation of the otherwise metastable HO₂ radicals by the diffusively enriched H₂. The third and fifth ignition limits are respectively manifestations of the low- and high-pressure responses of the third homogeneous explosion limit behavior, which is nevertheless punctuated by the fourth ignition limit characterized by the HO₂-H reactions. Furthermore, the fourth ignition limit runs fairly parallel to the crossover temperature, but is shifted to lower temperatures. An explicit expression, 2k₁={2k₁₀/(k₁₀+k ₁₁)}k₉[M], was derived and found to describe well this limit as well as the extended second limit observed in previous flow reactor studies. It is further shown that, since transport effects are inherently important for the present premixed system because of the diffusive loss of H to the hot, inert side of the counterflow, the ignition temperature increases substantially with increasing strain rate at all pressures and that such a sensitivity can be moderated by doping the inert flow with a small amount of oxygen.