Effects of Ambient Atmosphere on Flame Spread over Solid Fuel : Effects of Ambient Temperature and Limiting Oxygen Concentration for Downward Flame Spread

The downward flame spread over solid fuel has been studied. Paper sheets are used as samples, and experiments are conduced in a combustion chamber with a vertical duct to obtain uniform ambient atmosphere, keeping the inflow velocity constant. The gas composition and temperature can be changed to examine the effects of ambient atmosphere. The downward flame spread rate has been measured at lower oxygen concentration and preheated temperature (∼150°C). Results show that, flame spread rate is increased with an increase of ambient temperature. As the oxygen concentration is decreased, the flame spread rate is decreased monotonically. Then, the flame can not propagate and the flame extinction occurs at some oxygen concentration. We define this condition as the limiting oxygen concentration for downward flame spread. It can be reduced as the ambient temperate is increased. To discuss these experimental facts, we estimate the net heat flow into the preheat region, which is an important factor to sustain the flame spread. The net heat flow into the preheat region is decreased as the oxygen concentration is smaller. It is interesting to note that this heat flow near the flame extinction is smaller with an increase of the ambient temperature. These experimental data are useful to make clear the controlling mechanism of flame spread.