Steady and pulsating propagation and extinction of rich hydrogen/air flames at elevated pressures

In near-rich limit hydrogen/air flames, for which the Lewis number is great er than unity and the Zeldovich number is large, pulsating modes of propaga tion were observed in a recent study through computational simulation with detailed chemistry and transport descriptions, with and without radiative h eat loss. The study further showed that the stability boundary is minimally affected by radiative heat loss, while the concentration limit at which th e pulsating flame extinguishes due to radiative heat loss is reduced from t hat of the steady-state limit. The present study extends this previous inve stigation to include the effect of elevated ambient pressure, for pressures ranging from 1 to 20 atmospheres, with emphasis on the influence of chain mechanisms in H-2/O-2 oxidation. Results showed that the critical equivalen ce ratio separating steady from pulsating propagation decreases monotonical ly with increasing pressure throughout this pressure range, that this onset of instability corresponds to the transition from the state dominated by H -O-2 branching to that of H-O-2 termination, and that a second stable regim e was observed at high pressures and equivalence ratios where the weak HO2 chain-branching pathway is dominant. Furthermore, the extinction limits as determined for both the steady and unsteady propagation exhibit nonmonotoni c behavior: decreasing with increasing pressure from I to 5 atm, and then i ncreasing from 5 to 20 atm. This translates to a broader flammable range wi th increasing pressure for pressures in the range of 5 to 20 atm. (C) 2001 by The Combustion Institute.