A stabilization condition for the premixed flame in a tube was investigated for the propane/air and methane/air flames. The stabilized flames were classified into two regimes, one-dimensional regime and two-dimensional regime, by both the shape and the mass consumption rate. In the case of propane/air premixed flame, the stabilized flame at the lean flammability regime (φ<0.6) has one-dimensional flame characteristics. In this regime the radius of curvature of the flame is sufficiently larger than the flame thickness, and the mean velocity of unburned mixture is similar to the laminar burning velocity of the previous research. In the case of methane-air flame, cellular shaped flame is observed in the one-dimensional regime when the equivalence ratio is larger than 0.61. And within some range of equivalence ratio both the one-dimensional and the two-dimensional stabilization conditions were satisfied at the same equivalence ratio. In this condition, a small increase in the mean velocity could cause a sudden transition from the one-dimensional flame to the two-dimensional flame and the flame skirt seemed to rotate azimuthally.
A mean velocity variation larger than the burning velocity was introduced to the stabilized flame for a period longer than the reaction time scale in order to examine the unsteady behavior of flame propagation. The magnitude and period of the mean velocity variation were treated as experimental parameters. When the large velocity variation was introduced in the same direction as the initial mean velocity, the extinction behaviors were observed and systematically classified into two groups: extinction by boundary layer and extinction by acoustic instability. We found out that there exists a critical velocity variation and a critical time above which the extinction region develops to the center of the tube and extinguishes the flame. With the velocity variation in the opposite direction of the initial mean velocity, the flame was not extinguished near the wall and the characteristics of the flame propagation were similar to those of earlier studies on the flame propagation. The mechanism of the extinction near the wall is explained by the flame stretch theory, which provides a clue to the stretch effects on the finger flame.
The effects of non-unity Lewis number on the flame extinction was negligible in the extinction by the boundary layer, but was important in the extinction by the acoustic instability. When the Lewis number is smaller than 1, it was found that there exists a critical velocity variation above which the eventual extinction takes place by the acoustic instability.
