Flame stabilization low velocity

In the introduction to this chapter a combustion wave was considered to be propagating in a tube. When the cold premixed gases flow in a direction opposite to the wave propagation and travel at a velocity equal to the propagation velocity (i.e., the laminar flame speed), the wave (flame) becomes stationary with respect to the containing tube. Such a flame would possess only neutral stability, and its actual position would drift [1], If the velocity of the unbumed mixture is increased, the flame will leave the tube and, in most cases, fix itself 

When the flow velocity is increased to a value greater than the flame speed, the flame becomes conical in shape. The greater the flow velocity, the smaller is the cone angle of the flame. This angle decreases so that the velocity component of the flow normal to the flame is equal to the flame speed. However, near the burner rim the flow velocity is lower than that in the center of the tube at some point in this area the flame speed and flow velocity equalize and the flame is anchored by this point. The flame is quite close to the burner rim and its actual speed is controlled by heat and radical loss to the wall. As the flow velocity is increased, the flame edge moves further from the burner, losses to the rim decrease and the flame speed increases so that another stabilization point is reached. When the flow is such that the flame edge moves far from the rim, outside air is entrained, a lean mixture is diluted, the flame speed drops, and the flame reaches its blowoff limit. 

however, the flow velocity is gradually reduced, this configuration reaches a condition in which the flame speed is greater than the flow velocity at some point across the burner. The flame will then propagate down into the burner, so that the flashback limit is reached. Slightly before the flashback limit is reached, tilted flames may occur. This situation occurs because the back pressure of the flame causes a disturbance in the flow so that the flame can enter the burner only in the region where the flow velocity is reduced. 

FIGURE 4.30 Formation of atilted flame (after Bradley [1]). 

Because of the constraint provided by the burner tube, the flow there is less prone to distortion so further propagation is prevented and a tilted flame such as that shown in Fig. 4.30 is established [1], 

---