Rate of Burning of Wood (A Simple Thermal Model)

Abstract

In the design of reliable fire hazard ratings and effective fire detection/protection systems, a clear understanding of the interaction between the fire and fuel is greatly desirable. While the fuel supplies combustible vapors to the flames, the flames feed heat back to the fuel for more vapors to be generated. Assuming that (a) the temperature profiles in the interior of a wood-like solid heated externally are linear, (b) the solid pyrolyzes abruptly at a critical temperature, (c) the pyrolysis products are removed out of the solid instantaneously upon their release and (d) the thermal properties of char and wood depend only on the solid density but not on temperature, a simple energy balance is made to deduce the influence of surface heat flux, specimen thickness, pyrolysis endothermicity, internal convection and thermal properties (of the virgin solid and char) on the rate of mass loss of an infinite slab and a cylinder in a fire. The predictions are in good agreement with the available experimental data. Precise measurement of the surface heat flux condition and inclusion of the resistance to the vapor flow in the char is expected to bring the predictions even closer to the measurements.

Specifically, the theory explains the high apparent heat of vaporization measured by Thomas and indicates priorities for future experimental work. Measurements of the (a) exposure condition in terms of the incident heat flux, (b) thermal properties of partially charred wood, (c) latent heat of pyrolysis, (d) specific heat of the pyrolysis products and (e) parameters associated with the vapor flow through the hot porous char are recommended by this work in that order of priority.