Fire Safety Science Digital Archive

The work of the Fire Research Station on the ignition of materials by radiation is summarised. A range of intensities of radiation has been obtained from three different sources and methods of measuring and detecting these intensities have been developed. Three types of ignition have been studied; spontaneous ignition where the flame appears without an external source, pilot ignition where ignition starts about a smaIl flame in the volatile stream, and surface ignition where the pilot flame is on the surface of the irradiated material. A number of factors which affect the onset and occurrence of ignition have been examined; two of particular importance are the size of the area irradiated and the absorptivity of the surface. Dimensional analysis of the thermal balance of the irradiated solid has been used to derive dimensionless groups in which to correlate experimental results. In this way, the empirical use of fixed temperature criteria in the solid has been shown to be adequate for correlating ignition times on cellulosic materials over a wide range of experimental conditions, e.g. intensities of radiation, densities and moisture contents with a different temperature for each type of ignition. In particular, spontaneous transient ignition of thermally thick solids occurs at a fixed surface temperature of about 500C and the spontaneous sustained ignition of thermally thin solids occurs at a fixed mean temperature also of about 500C. The method has been extended to a pulse of radiation varying with time, and good agreement has been obtained between the calculated threshold energies for ignition by these pulses and those determined experimentally. From these correlations, the threshold energies for ignition of both thermally thick and thin materials for a range of nuclear explosions have been derived. Correction factors to allow for the effect of different colours are also given. Correlations based on a fixed temperature criterion break down at low rates of heating, probably because of the limited supply of volatiles and at very high rates of heating probably because the time taken to form a flammable mixture is comparable with the heating time.