The mechanisms of lean limit extinguishment of an upward and downward propagating flame in a standard flammability tube

Abstract

The mechanisms by which lean limit upward and downward propagating flames extinguish in a standard flammability tube (51mm ID, 1.8m long) were deduced using schlieren and direct light photography and temperature measurements and by observing the behavior of the flame under transient loading. The lean methane-air system was studied. The upward propagating flame takes the shape of a stable hemispherical cap attached to a trailing skirt. This flame always extinguishes first at the holding region on the axis of the tube and a failure wave subsequently washes down the flame until extinguishment is complete. The extinguishment occurs because the flame is stretched at the tip and heat loss to the walls is not important to this extinguishment. On the other hand the extinguishment of a downward propagating flame is a multistep process. First heat loss to the walls causes the flame to extinguish near the walls and a smaller residual flame is left in the center of the tube. Once this happens, differential buoyancy of the hotter central gases and the cooler surrounding gases forces the cooler gases to move ahead of the flame. Finally, the downward propagating flame is observed to rise slightly. At this point the flame extinguishes completely.