Fire Safety Science Digital Archive

When the predominating mode of heat transfer is by radiation, the rate at which flame spreads sideways across a vertical thick board, but not the furthest extent of spread, is independent of the height of the board. By assuming that wood is an inert totally absorbing material, the time taken for a flame to reach a given position in spreading over the surface of boards of density varying from 0.13 to 1.00 g/cm^3 with a nominal moisture content of 15 per cent, has been correlated satisfactorily using simple heat transfer theory with intensities of radiation ranging from 0.5 to 1.0 cal cm^-2s^-1. In this range, spread of flame occurs when the surface temperature, at a point as the flame reaches 300C. Independent measurements of the surface temperature, at a point as the flame reaches it, have confirmed that this value is of the right order but suggest that there is a slight fall as the intensity of radiation falls. This fall becomes more rapid below intensities of about 0.5 cal cm^-2s^-1 and a fixed temperature criterion no longer applies. Spread of flame continues to occur at intensities of radiation well below that expected from the correlating temperature (0.25 cal cm^-2s^-1). The lowest intensity of radiation at which spread of flame is possible (the threshold intensity) is related to the density; the lighter the wood the lower the intensity at which spread can occur.