On the nature, influence and magnitudes of flame heat transfer during surface flame spread

Abstract

The basic equation describing steady surface flame spread on solid surfaces indicates that the primary characteristics describing surface flame spread are flame heat flux, forward heating length scale and a thermally affected depth of solid material. There is however great variability in the reported estimations of the average flame heat flux incident over the effective heated length scale, particularly for opposed flow flame spread. In addition, there has been little consideration given in the literature as to how flame heat fluxes derived from different methodologies relate to each other and to the actual processes of surface flame spread. This paper addresses these issues with particular regard to opposed flow flame spread. It is suggested that the controlling mechanisms of wind aided and opposed flow flame spread are similar despite the obvious difference in geometric view factors associated with each mode of flame spread. Thus, the attainment of a strong, steady, piloted ignition condition occurring during opposed flow flame spread results in transition flame heat spreading fluxes that are of similar magnitudes to those which control steady wind aided flame spread rates, i.e., 16–33 kW m −2. It is concluded that the transition ignition and flame propagation heat fluxes controlling opposed flow flame spread encompass a range of 5–70 kW m −2. The major source of difference between the two generic modes of surface flame spread lies in the definition and magnitude of a critical flame heat influenced length scale, l h which is defined for the case of opposed flow flame spread in this paper.