Chapter 6: Dispersion of Fire Plumes in the Atmosphere

Direct exposure of populations and ground deposition of contaminants from fire plumes released to the outside environment are highly dependent on the characteristics of atmospheric dispersion. This chapter describes the main determinants of fire plume dispersion and the underlying parameters and functions that can be used for plume dispersion calculations. Data and calculation results are presented for a range of scenarios with illustrations of plumes from both large-scale and small-scale wind tunnel experiments. In general, factors such as the size and area distribution of fire sources and the manner of their release, directly from the ground or associated with buildings, control the initial conditions of the fire plume. These parameters and the wind speed are important as they control the initial spreading of the plume, and its initial buoyant rise, in the area near the source where plume concentrations are high and exposure is at its greatest. At long distances these source term effects tend to disappear as the plume eventually forgets the detailed conditions of its release. Plume buoyancy from the fire heat release is, however, normally conserved indefinitely. Its effects thus persist for much longer distances, though eventually even these also disappear. Topics covered include fire plume buoyancy scaling functions, basic properties and scaling for calculation of contaminant concentration; the effect of source buoyancy, shape and size; fire plume heat release from buildings in relation to shape size and orientation and fire plume ‘lift-off’; fire plume dispersion in urban areas; and deposition to the ground of particles and other components in fire plumes.