The reconstruction of fires involving highly flammable hydrocarbon liquids
Highly flammable hydrocarbon liquids are involved in a high percentage of building fires, whether those fires are accidental or incendiary in origin. Their mere presence is often taken as proof of a particular fire cause by some investigators despite their limited knowledge of the behaviour of the vapours from these fuels as they spread and diffuse. They are sometimes assumed to vaporize completely and instantly upon exposure and to diffuse uniformly through any compartment. The available models address large scale spills in ambient conditions of sun and wind, which do not apply to typical building fires. This study addressed the problem of modelling the spread of vapours from small-scale (less than four litre) spills of highly flammable liquids by means of a series of overlapping and complementary experiments, all of which dealt with the conditions found in most interior building fires (moderate temperatures, still air, and no sun). It was determined that the surface area produced by a given quantity of liquid could be predicted for smooth, flat floors whose surfaces could be classified as non-porous (vinyl or painted wood), semi-porous (unfinished concrete or wood), or porous (carpet or sand). The type of surface also controlled the evaporation rate (per unit area of the pool). Evaporation rates from surfaces such as carpet saturated with pentane were 1.5 times the rate for a free-liquid pool at the same temperature. A granular substrate such as sand produced a pentane evaporation rate twice that of a pentane liquid pool. This effect is not related to the roughness of the surface itself, but rather to the capillary drive within the matrix. Such a drive is stronger for granular matrices with a small void space (high packing density) and lower for those with larger void space. The size of the pool also controls the evaporation rate (the mass loss rate per unit surface area). Smaller pools (0.05 - 0.1m diameter) exhibit much higher rates than do the larger ones (0.3m) in this study. This is due to the enhanced evaporation due to lateral flow of vapours from the edges of the pools. Larger pools have a large central quiescent area that does not contribute to the overall evaporation. Smaller pools have no such quiescent area and a higher initial rate. There are also predictable losses due to pouring and splashing of volatile fuels that are closely related to the vapour pressure of the liquid involved. Vertical diffusion of n-pentane and hexane vapours is very slow when the vapours are being generated by evaporation from a pool. The heat lost to evaporative cooling results in a pronounced thermal gradient in the atmosphere above a pool that suppresses the vertical diffusion. The diffusion rates of pentane, hexane, and octane vapours can be predicted and the height at which an ignitable vapour/air mixture is present can be calculated. The vapours also exhibit a pronounced advective flow which spreads the vapours in a viscous, laminar fashion. The spread rate of this advective flow can be calculated and agrees well with experimental data. The evaporation of n-pentane, hexane, and n-octane were found to be predictive of the evaporative behaviour of petrol and camping fuels, two of the consumer products more commonly encountered in fires. Petrol, with its high concentration of pentane-like hydrocarbons, evaporates at the same rate as does n-pentane, at least for the first 10 -15min. Camping fuels are dominated by hexanes and their evaporative behaviour is very similar to that of the hexane studied in detail here. Octane contributes very little combustible vapour at typical room temperatures due to its very low evaporation rates at these temperatures. The behaviour of the flame propagation in vapour/air mixture layers is predictable. Layer ignition is found to produce some characteristic features that may be observed by a witness to the fire or that may produce burn patterns that survive the fire to be found by a diligent investigator. Unfortunately, estimates of the quantity of flammable liquid present and its distribution prior to the fire cannot be reliably made by examination of the burn patterns on carpet or floors after the fire, particularly if the fire was not suppressed for some time after ignition. Finally, an operational model based on these findings is offered for the use of fire investigators. This model, while limited to incidents in closed compartments with no mechanical ventilation and limited activity, offers a means by which the physical distribution of ignitable vapours can be predicted as it varies with time. This enables the investigator to explore the viability of various hypotheses about the quantity and distribution of flammable liquids prior to a fire, the relative location (both vertical and horizontal) of a potential ignition source, and, most importantly, the time factors involved in the evaporation of a flammable liquid and distribution of its vapours.