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 extinction and flame bifurcations of the cylindrical premixed CH₄/air flames are numerically investigated with a detailed chemistry. The interaction between radiation heat loss and flame curvature is emphasized. The results show that a mixture below the standard limit can burn in the cylindrical flame configuration by imposing a moderate stretch rate. This flame is quenched either by radiation heat loss at low stretch rate or by incomplete combustion with an excessive stretch. As the fuel concentration increases, it is found that two kinds of flames, normal flame and weak flame, can exist at the same boundary conditions. A G-shaped extinction curve showing the flammable regions of these two flame regimes is obtained. The relation between the flammability limit of the cylindrical flame and the standard limit is discussed. Furthermore, comparisons between the cylindrical flame and the counterflow flame are made. The results show that the interaction of flame curvature and radiation heat loss greatly affects the flame strength and extinction. It is shown that flame curvature extends the radiation extinction limit and accelerates the opening up of the sublimit flame branch. Comparisons between the predicted results with the experimental data show good agreements both in the extinction limit and in the extinction flame diameter.