THE ROLE OF KINETIC VERSUS THERMAL FEEDBACK IN NONPREMIXED IGNITION OF HYDROGEN VERSUS HEATED AIR

System response S-curves for a hydrogen-air diffusion flame have been simulated numerically using detailed chemistry and transport. In parti cular, the globally nonpremixed ignition state has been studied in thr ee distinct ignition regimes at pressures of 0.1, 1, and 10 atm. The r ole of heat release in providing ''thermal feedback'' at the ignition turning point is examined in detail for all three regimes. Contrary to classical notions based upon one-step overall chemistry, thermal feed back is shown to play essentially no or minimal role in the steady-sta te solution at the ignition turning point-either in its character or p arametric dependence. In the majority of cases studied, turning point and S-curve behavior are found to exist in the complete absence of hea t release, driven solely by ''kinetic'' feedback provided by nonlinear ities in the coupled chemical kinetics. As a result, the location of t he ignition turning point, which depends parametrically upon global va riables such as air temperature, strain rate, pressure, and fuel conce ntration, is essentially governed by the kinetics of gain versus loss of key radicals in the ignition kernel. One cause of this phenomenon i s the extremely small size of the radical pool at the ignition turning point, which necessarily limits the degree of localized heat release and temperature perturbation. The small radical pool is also found to decouple the problem such that, on the lower branch and around the ign ition turning point, the temperature and possibly major species profil es may be solved independently of the complex chemistry involving the minor species. Furthermore, it is also suggested that when heat releas e is not significant at the ignition turning point, the transient igni tion process (from the turning point to a diffusion flame) must begin with an induction period wherein the radical pool increases via essent ially isothermal chemical kinetics before thermal feedback can ensue.