Pulsating instability in near-limit propagation of rich hydrogen/air flames

The adiabatic and radiation-affected unsteady planar propagation of rich hydrogen/air flames of near-limit concentrations in the doubly infinite domain is computationally simulated with detailed chemistry and transport. Results for the adiabatic propagation show that, with progressive increase in the fuel richness, the mode of propagation changes from steady state to oscillatory with a single period, to oscillatory with double periods, and to oscillation separated by increasingly long periods of dormant chemical reactivity. In the presence of radiative loss, propagation with the first three modes are minimally affected, whereas extinction readily occurs, with precipitous drop in the flame temperature, during the dormant period of the last mode. Because the state for the onset of the last mode is at a leaner concentration than that of the nonadiabatic steady-state propagation limit the use of the steady-state result provides a conservative estimate for the rich fundamental flammability limit. The study also shows that, by using appropriately extracted Lewis and Zeldovich numbers characterizing the steady, adiabatic flame propagation, the transition boundary from steady to pulsating propagation can be adequately described by the criterion derived by Sivashinsky based on one-step chemistry.