The Correlation Between External Heat Flux and Time to Ignition

Abstract

AbstractThe action of an external source of thermal radiation on the surface of a combustible material causes a number of physical and chemical processes. Depending on the properties of the material and the ambient conditions, its temperature is increased until a point is reached where the amount of gaseous combustion products released is sufficient to initiate the flame. This temperature is called the ignition temperature. With a uniform heat flux, the time required to achieve it is also a significant characteristic. It is called time to ignition and is important information pertaining to the initiation process. Based on its dependence on the external heat flux density, mathematical relationships can be compiled that allow the ignition to be characterised. A relatively large number of such relationships are proposed, each of which is suitable for certain materials or measurement conditions on the basis of specific simplifications. In the first part of this chapter, the physical processes associated with the impact of thermal radiation on the surface of the material are presented. The basic mathematical relationships that describe these physical processes are explained. For a clearer understanding of the text, the characteristics of selected plastics, which are necessary in the calculations, are also given. The second part focuses on suggested correlations. It provides an overview of the various correlations proposed in the scientific literature. Their order is chronological and thus, to a certain extent, also provides some insight into the development of ideas about the initiation of flame combustion.KeywordsExternal heat fluxHeat transferTime to ignition