Fire Safety Science Digital Archive

Temperature profiles around a flame stably spreading downward over a filter paper in an upward air flow were examined for further understanding of the effects of the free-stream velocity on opposed-flow flame spread over thin solid combustibles. Temperature was measured by fine thermocouple, and gas phase temperature profiles were determined as well as surface temperature distributions. In the stable flame spread region, the flame spread rate decreases gradually as the free-stream velocity U increases. Although the flame temperature at its leading edge is approximately independent of U, in the vicinity of the leading edge the flame temperature gradient along the reaction zone increases with U. As U increases, the location of the flame leading edge moves to downstream direction toward the pyrolysis zone front and also the gas-phase temperature rising zone ahead of the flame leading edge becomes shorter, with reducing the heat flow from gas phase to the preheat zone surface. The gas phase temperature gradient at the pyrolysis zone surface essentially increases with U, and the pyrolysis zone length decreases more rapidly than the flame spread rate. These suggest that as U increases the mean ejection velocity of the pyrolysis gas increases in spite of the reduction of the total mass flux of the evolved pyrolysis gas. Based on the experimental results, the effects of the free-stream velocity of opposed air flow on the flame spread processes were discussed.