Pulsating instability in rich hydrogen/Air counterflow flames

Pulsating instability in adiabatic, rich hydrogen/air counterflow flames was computationally simulated with detailed chemistry and transport. Results demonstrate that for large Lewis number flames, thermal-diffusive pulsating instability is promoted by the positive stretch of the counterflow flames, that oscillation is initiated at an equivalence ratio much smaller than the onedimensional rich threshold, and that the critical strain rate leading to pulsation is smaller than the corresponding static extinction limit. Furthermore, similar to the one-dimensional, unstretched cases, with progressive increase in the strain rate for a sufficiently rich mixture, the pulsation mode changes from that of monochromatic, to period doubling, and to pemlanent extinction. It is also seen that the pulsating flames are quasi-steady in nature in that the period of oscillation is larger than the characteristic flame time. As such, the unsteady flame cannot recover once the instantaneous flame temperature is reduced below the corresponding steady-state extinction temperature. Since pulsating extinction occurs at a smaller strain rate than the steady extinction limit, it implies that the flame extinguishes in the pulsating instead of the steadily propagating mode, and the flammable range is accordingly narrowed. It is expected that when radiative heat loss is considered, the onset of pulsation will be facilitated such that the regime in strain rate for steady burning will be further narrowed. This pulsation-induced extinction can be a major factor accounting for the discrepancy in the extinction strain rate betlveen the computed values based on static extinction turning points and the experimental data, for large Lewis number mixtures.