Ignition in a counterflowing non-premixed CO/H₂-air system

This paper presents a computational study of ignition in nonpremixed counterflowing CO/H₂ versus heated air. To understand the chemical and dynamical mechanisms governing CO/H₂ ignition, we have examined the following features: the effect of H₂ concentration on the system's ignition characteristics, the dominant chemical kinetics just prior to ignition, the role of convection and diffusion with respect to chemical kinetics, and the dependence of ignition temperature on pressure variation. Three ignition regimes were identified, based upon the influence of H₂ concentration, which we have designated: 1) H₂-dominating, 2) transition and 3) H₂-catalyzing. In the H₂-dominated regime, relevant for high H₂ concentrations, the controlling ignition kinetics involve that of H₂ chemistry with minimal CO influence. In the transition regime, the participation of CO kinetics, especially through its exothermicity, allows the CO/H₂ system to be ignitable even when the analogous N₂/H₂ system ceases to be ignitable. Sensitivity analysis shows that it is the competition between H and HO₂ chemistry in the two systems that determines whether or not the system is ignitable. Finally, in the H₂-catalyzed regime, relevant for low H₂ concentrations, the dominant kinetics are found to include both H₂ and CO chemistry. Furthermore, by studying the effects of pressure on the ignition temperature in the of H₂-catalyzed regime, three iginition limits corresponding to those of H₂ explosion limits are identifed, as is reasonable to expect.