The piloted ignition of wood by thermal radiation

Abstract

Previous experimental studies on the ignition of dry wood by thermal radiation show poor agreement because all of the primary variables have not been monitored, and consequently they are not included in the resulting correlations. Although the density of the wood, the thickness of the specimen, and the magnitude of the incident irradiance have generally been recognized as principal parameters, there is little evidence in the literature that the spectral absorptance of the wood relative to the spectral quality of the incident radiation has been appreciated as a dominant variable. Therefore, the purpose of this investigation was to examine the significance of all of these variables. Piloted ignition data were obtained for thirteen dry wood species having thicknesses from less than 0.2 cm to more than 2.5 cm and densities from less than 0.1 g/cm 3 to more than 1.0 g/cm 3 . Diffusion flames and high-temperature tungsten lamps were used as sources of thermal radiation at incident irradiances from about 0.6 cal/cm 2 -sec to about 3.5 cal/cm 2 -sec. The spectral absorptances of the wood samples were measured in order to calculate an average absorptance over the range of spectral quality of the incident radiation. The ignition data were correlated on the basis of relationships suggested by considering the wood specimens as infinite slabs of inert, opaque material. It was found that, when the density, thickness, and absorbed irradiance were included in the correlation of the experimental data, a unique ignition correlation could be defined.