The Effect of Thermoplastics Melt Flow Behaviour on the Dynamics of Fire Growth
The UK Health & Safety Executive are responsible for advising on ways to ensure the safety of employees within the workplace. One of the main areas of concern is the potential problem of unwanted fire, and it has been identified that within the area of large-scale storage in warehouses, there is an uncertainty posed by large quantities of thermoplastic. Some forms of thermoplastic exhibit melt-flow behaviour when heated, and a large vertical array exposed to a fire may melt and ignite forming a pool fire in addition to a wall fire. This project is largely experimental, and aimed at quantifying the effect of a growing pool fire fuelled by a melting wall on overall fire growth rate. The pool fire has been found to increase melting and burning rates, producing a much faster growing fire. It has also been found that - 80% of flowing and burning material will enter a potential pool fire, with only 20 - 25% of total mass loss actually burning from the original array. During the project 400+ small-scale tests and several medium-scale experiments have been undertaken at both Edinburgh University and the HSE's Fire & Explosion Laboratory, Buxton. The experiments have confirmed the main parameters governing pool fire development are molecular weight degradation rate and mechanism, which control flow viscosity. There have also been investigations into other influences, the most significant of which was found to be flooring substrate. These parameters then form the basis of a simple 1-D model. A semi-infinite heat transfer approximation is used to determine temperature profile through a thermoplastic exposed to its own flame flux, with extrapolated temperature dependant material properties. The derived profile is then inserted into a gravity driven flow model, to produce estimates of flow rate and quantity for plastics undergoing either random or end chain scission thermal degradation processes. The model identifies property data which are required to permit its use as a hazard assessment tool.