Fire Growth on Horizontal Surfaces of Wood

The results of an extensive experimental work conducted on transient three-dimensional fire spread on horizontal surfaces of various woods up to two feet in diameter indicate that flame spread on wood is sensitive to factors both internal and external to the sample. The transport of fuel gases from the burning to the unburned zone presents a new mechanism of flame spread unique to wood. These experiments show that (i) forward radiative heat transfer from the flame is the primary accelerating mechanism, (ii) forward gas phase heat transfer is a local phenomenon independent of fire size, (iii) conduction of heat parallel to the spread surface does not contribute significantly to the flame spread process, (iv) reradiation from the surfaces of wood and char is the primary heat loss mechanism, and (v) energetics due to the desorption of adsorbed moisture has a very noticeable effect on the flame spread process. Using these experin~ental observations and treating flame spread as a continuous ignition process a model to predict transient fire growth on horizontal surfaces of wood is developed. In this model the actual 3-D fire growth process is approximated by a 2-D axisymmetric fire growth. Also, the buoyant turbulent combustion in the gas phase is empirically treated and the sblid phase decomposition process is emphasized. The numerical calculations are in good agreement with the experimental results.