On the flight pahts and lifetimes of burning particles of wood

Abstract

Results of a research program on the burning properties, flight paths and lifetimes of buring particles of wood are given in the paper. This study is related to fire spread produced by fire brands, which is the dominating propagation mechanism in major forest fires. The laws of variation of the aerodynamic drag and weight of firebrands as functions, of both time and relative wind speed are obtained experimentally in a wind tunnel. From these data the flight paths and corresponding lifetimes of the firebrands are calculated for giving horizontal wind conditions and for a certain model of the convection column above a fire. The conclusion was drawn that it is an excellent approximation to assume that the firebrands always fly at their final or terminal velocity of fall. This assumption considerably reduces the experimental work as well as the theoretical studies. The results obtained permit the valuation of the dangerous areas of possible fire propagation. Comparative results of the potential danger of firebrands according to size, shape, kind of wood, moisture content, and wind conditions are shown. A preliminary study of the general laws governing combustion of wood with forced convection is also included.