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.
