Measured velocity and temperature profiles near flames spreading over a thin combustible solid

Abstract

To explore the flame-spread mechanism over thin solid-fuel sheets, the gas velocity and temperature profiles in the vicinity of spreading flames over paper at three different angles have been examined by using particle tracer techniques and fine-wire thermocouples. The gas flow in the vicinity of a spreading flame, which depended on the inclined angle, was found to be closely related to the heat-transfer phenomena to the unburnt material. In front of the pyrolysis region of a vertical downward spreading flame, the heat transfer to the unburnt material was confirmed to take place mainly through the gas phase. More than 80% of the heat needed for initiation of the pyrolysis reaction in this case was transferred within 0.1 cm in front of the pyrolysis region. In the case of a flame spreading downward at 30° from horizontal, a small vortex was occasionally observed in front of the leading edge of the bottom flame. The repeated appearance of this vortex was considered to increase the heat flux to the unburnt material and, thus, the flame-spread rate. In the case of a flame spreading horizontally, the heat was found to be transferred to the unburnt material mainly from underheath, where the gas was flowing in the same direction as the flame spread. Further discussion on gas behavior points out that, even near its leading edge, the characteristics of the flame were far from those of a premixed flame.