Deposition of large particles from warehouse fire plumes—a small-scale wind tunnel model study

Abstract

The report describes measurements of the deposition of large particles from a small scale wind tunnel model of a chemical warehouse fire plume. A common feature of such fires is the discharge of relatively large particles with falling speeds of the order of m s −1, partly generated by mechanical damage, which can fall out of the fire plume in a different pattern to that of the gases and fine particles. These large particles may also contain toxic components, so it is desirable to know their fallout pattern. The deposition of large particles from fire plumes has therefore been modelled directly as an adjunct to earlier small scale wind tunnel experiment on gaseous plume dispersion [1]using the same experimental conditions, so that the relative behaviour of gas plume and heavy particle dispersion could be compared. As far as we are aware, this is the first experiment of this type to be carried out. There are constraints on the range of deposition conditions that can be modelled, due to scaling problems. However, it proved possible to develop a viable technique and some useful data was obtained over a range of particle and plume conditions of interest. Nonetheless, the work should be regarded mainly as a proving trial of the technique and an indicator of the significant parameters as the measurements made were too few in number (essentially of three particle deposition conditions) to provide a broad-based indication of large particle deposition. Experiments were carried out for two gas plumes, buoyant and non-buoyant, both with and without a building shell around the source. Large particle concentrations near the surface proved markedly different to those for a gaseous plume, showing a much more rapid reduction in concentration with increasing distance. Both particle falling angle and particle inertia affected the particle plume dispersion, the highest concentrations at the ground occurring for the case with small falling angle and inertia, the lowest concentrations for the case with large falling angle and moderate inertia. Concentrations for the remaining condition, with large falling angle and high inertia, fell between these two, probably because the high inertia constrained the rate of lateral dispersion of the particles. Compared with the effects of particle falling angle and inertia, the effects of a building shell around the source were relatively limited. This was greatest for the non-buoyant plume where the building shell tended to act as a trap for large particles. The buoyant plume tended to carry particles beyond the building shell, so that in this case its effects were quite limited.