Modelling the Generation of Toxic Combustion Products and its Transport in Enclosure Fires

Arun Mahalingam

2007

Combustion products generated in enclosure fires can be transported throughout the enclosure causing death and injury to occupants and a great deal of damage to property and the environment. The ability to estimate the generation and transport of toxic combustion products in real fire scenarios involving common building materials is of great importance to fire protection engineers in producing detailed quantified risk assessment and in the design of fire-safe buildings. Most common building materials are polymer based. Thus toxic products evolving from burning polymers is the single most important factor in fire fatalities. Fire hazard calculations require modelling of heat generation, toxic combustion products generation and its transport in realistic building scenarios involving common building materials. However, the thermal decomposition, combustion behaviour and chemical kinetics for common polymers like wood, plastics, rubber and textiles are extremely complex. In the present study, a methodology (STEM-LER: the Scalar Transport Equation based Model using the Local Equivalence Ratio concept) based on solving separate transport equations for the species and using the yield correlations obtained from bench-scale experiments to model the source terms is proposed to predict the products generation and its transport during enclosure fires. Modelling of complex solid phase degradation and chemical kinetics of polymers is bypassed by measuring the product yields as a function of equivalence ratio by burning the samples in a bench-scale combustion apparatus called Purser furnace. Since the accuracy of prediction depends upon the quality of the yield data obtained from the Purser furnace, attempts were also made to numerically investigate this bench-scale toxicity test method in order to understand its modus operandi.

Also, large-scale fire tests were carried out involving combustible cable materials to generate the validation data for the combustion and toxicity models developed in the current work. Simulations were carried out using the proposed methodology to simulate the large-scale cable fires and validated with the experiments. Irrespective of the errors involved in approximating the complex physical and chemical processes, the STEM-LER methodology is able to model the combustion products generation near the fire and its transport to distant locations with reasonable accuracy.

Finally, a preliminary assessment on the effect of cable fires on building evacuation for the simulated fire scenarios were carried out using a sophisticated evacuation model.