Physical Modeling of Large-Area Fire Plumes
Technical rept. 1 Sep 1983-31 Aug 1986
COLORADO STATE UNIV FORT COLLINS
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The study identified four regions of distinct characteristics in large-area fires a shallow combustion layer, a radial convective boundary layer, a central core and a vertical convective plume. Vigorous mixing with the ambient air occurs in the radial convective boundary layer, where the hot burned fuel rises in the form of thermals. Such thermals are formed over continuous combustion layers as well as over large-area fires made of many distinct closely spaced, small fires. The rapid rise of the thermals induces subsidence of fresh, cool air between the thermals, as observed in atmospheric convective boundary layers. The buoyancy flux also induces radial flow toward the central core. The magnitude of the induced radial velocities are found to be smaller than those suggested in some previous studies, but are in good agreement with observations in Project Flambeau. The radial flow causes the combined plume above the combustion layer to pinch before it rises in the form of a convective plume. At large heights, the convective plume behaves as a weak plume with a virtual origin at approximately zR -1.65. This suggests that the maximum plume rise in upper stably-stratified layers would be much smaller than the maximum plume rise from a point source at the ground with the same buoyancy flux. Keywords Fires, Storms, Wind.
- Safety Engineering
- Combustion and Ignition