Dispersion and deposition of smoke plumes generated in massive fires

Abstract

Massive fires resulting from the uncontrolled burning of crude oil from spills or industrial accidents produce large smoke-laden buoyant plumes which rise in the wind direction before they equilibrate within a stably stratified atmosphere. Beyond this point, the plume material cools by entrainment and the plume becomes negatively buoyant due to the heavy smoke loading. The trajectory of the descending plume, which determines the ground distribution of smoke, is the subject of this paper. A computational model for the simulation of large-scale smoke plumes resulting from such fires is developed and applied to investigate the effects of the plume initial properties on its trajectory and smoke deposition patterns. Attention is focused on the descent and dispersion of wind-driven plumes in a homogeneous atmosphere, and the smoke deposition on flat terrain. Results show that the plume dynamics in the cross-wind direction are dominated by two buoyantly generated, coherent, streamwise vortices which distort the plume cross section into a kidney-shaped structure. The strength of the two vortices and their separation increase as the plume falls. The plume width grows under the action of these vortices at a rate which increases as the plume settles on the ground, leading to a smoke footprint which does not resemble the prediction of Gaussian dispersion models. The effects of the injection altitude and the initial shape of the plume cross section on the transport and dispersion of the negatively buoyant smoke plume are investigated. Plumes falling from higher elevations disperse more in the vertical direction while those falling from lower elevations disperse further in the horizontal cross-wind direction. Plumes with circular cross-sections reach the ground faster and disperse horizontally further than plumes with elliptical cross-sections with the minor axes in the vertical direction. Vertical plume dispersion is weakly dependent on the shape of its initial cross-section.