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 greater than unity and the Zeldovich number is large, pulsating modes of propagation have been observed through computational simulation using detailed chemistry and transport descriptions with and without radiative heat loss. The present study extends the previous investigation 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 H₂/O₂ oxidation. Results show that the critical equivalence ratio separating steady from pulsating propagation decreases monotonically throughout this pressure range, and can be predicted quite well by the local maximum of the mass burning, rate versus pressure in steady propagation. This maximum corresponds to the transition from a state of strong chain-branching to that of weak chain-branching, while the stability boundary is minimally affected by radiative heat loss. However, the oscillation becomes more intense as the equivalence ratio is increased further for a given pressure. As a result, the radiactive heat loss becomes significant to extinguish the flame at an equivalence ratio much smaller than that of the steady-state flammability limit. The flammability limit as determined for both the steady and unsteady propagation exhibit non-monotonic behavior; decreasing with increasing pressure from 1 to 5atm, and then increasing, from 5 to 20atm,’ which translates to a broader flammable range for the unsteady flames with increasing pressure. Thus, the extinction mechanism remains the same in unsteady as in steady flames, but the pulsating instability effectively lowers the fundamental flammability limit by allowing the flame to extinguish during the weakly burning stage of its oscillations.