Combined effects of radiation, flame curvature, and stretch on the extinction and bifurcations of cylindrical CH4/air premixed flame

The combined effects of flame radiation, stretch, and curvature on the exti nction and flame bifurcations of the cylindrical premixed CH4/air flames ar e numerically investigated with a detailed chemistry. The interaction betwe en radiation heat loss and flame curvature is emphasized. The results show that a mixture below the standard limit can burn in the cylindrical flame c onfiguration by imposing a moderate stretch rate. This flame is quenched ei ther by radiation heat loss at low stretch rate or by incomplete combustion with an excessive stretch. As the fuel concentration increases, it is foun d that two kinds of flames, normal flame and weak flame, can exist at the s ame boundary conditions. A G-shaped extinction curve showing the flammable regions of these two flame regimes is obtained. The relation between the fl ammability limit of the cylindrical flame and the standard limit is discuss ed. Furthermore, comparisons between the cylindrical flame and the counterf low flame are made. The results show that the interaction of flame curvatur e and radiation heat loss greatly affects the flame strength and extinction . It is shown that flame curvature extends the radiation extinction limit a nd accelerates the opening up of the sublimit flame branch. Comparisons bet ween the predicted results with the experimental data show good agreements both in the extinction limit and in the extinction flame diameter. (C) 1998 by The Combustion Institute.