Further examinations on extinction and bifurcations of radiative CH ₄/air and C₃H₈/air premixed flames

The extinction and flame bifurcations of radiative CH₄/air and C₃H₈/air free propagating flames and counterflow flames are investigated numerically with detailed chemistry as well as the optically thin and spectral emission-absorption statistical narrow band models. For counterflow CH₄/air flame, a G-shaped extinction curve is obtained. It is shown that flame bifurcations and extension of the flammability limit due to the interaction of radiation and stretch at subunity Lewis number exist regardless the radiation model used. Furthermore, it is shown that even for small-scale flame, radiation reabsorption greatly extends the flammable regions of the weak flame (WF) and the near-stagnation flame (NSF). For counterflow C₃H₈/air flame, a K-shaped extinction curve is obtained. It is concluded that multiple flame bifurcations are the physically intrinsic phenomena of the practical stretched flames. It is shown that the extrapolation of the stretched extinction limit to zero stretch rate is possible only when the extinction curve is K-shaped. Transition from the G-shaped extinction curve to the K-shaped one, as a result of the competition between radiation heat loss and the Lewis number effect, is examined by replacing the nitrogen gas in CH ₄/air mixture with helium. The critical Lewis number for the transition of the extinction curve is obtained. Results of the present study show good agreement with theory and experiment.