25-01-2013, 04:07 PM
Flame Stabilization
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When S > U, the flame propagates into the barrel to bring about flash-back. When the flow is very strong, U > S, the flame blows-off. The critical criterion for blow-off arises when the u0 and us profiles are tangential
The velocity gradient (at the barrel wall near its mouth) has a definite effect on the flame stability; if it is too small, the flame flashes back; if it is too large, blow-off occurs
In fact, measurements show the critical velocity gradient correlates well with the fuel oxidant ratio as qualitatively shown in Figure 8.27.
In order to accomplish commercial combustion, the supply velocity of the reactant mixture is desired to be extremely high; it is not unusual for this velocity to be as high as 10 X the maximum possible laminar flame speed of a given mixture.
Experience shows that the flame is blown away when the supply velocity exceeds the flame speed
Stability of a Bunsen Flame
indicated the mechanism which determines the shape of a Bunsen flame. This mechanism can explain the phenomena of blow-off and flash-back.
flame speed and normal component of the supply velocity us in four different situations. Only the region close to the wall is shown since it is here that the flame is anchored to the burner.
By Recirculation
When the solid bluff body discussed above is of finite length in the direction of flow, the pressure distribution prevents the high velocity flow from keeping attached to the solid surface.
Increasing pressure separates the boundary layer and causes eddy shedding in the "wake."
Under sufficiently fast flow conditions a (symmetric) recirculation pattern of flow is established behind the blunt body as shown in Figure 8.31. The recirculation zone provides a station where reactions can take place.