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 particular, the globally nonpremixed ignition state has been studied in three distinct ignition regimes at pressures of 0.1, 1, and 10 atm. The role 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 feedback is shown to play essentially no or minimal role in the steady-state solution at the ignition turning point-either in its character or parametric dependence. In the majority of cases studied, turning point and S-curve behavior are found to exist in the complete absence of heat release, driven solely by ''kinetic'' feedback provided by nonlinearities in the coupled chemical kinetics. As a result, the location of the ignition turning point, which depends parametrically upon global variables such as air temperature, strain rate, pressure, and fuel concentration, is essentially governed by the kinetics of gain versus loss of key radicals in the ignition kernel. One cause of this phenomenon is 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 ignition turning point, the temperature and possibly major species profiles may be solved independently of the complex chemistry involving the minor species. Furthermore, it is also suggested that when heat release is not significant at the ignition turning point, the transient ignition process (from the turning point to a diffusion flame) must begin with an induction period wherein the radical pool increases via essentially isothermal chemical kinetics before thermal feedback can ensue.