Jose Torero (UCL)

Understanding Extinction of Charring Materials

Timber structural elements need to be designed to retain their structural function until the burn-out of the building’s fuel, therefore, if the structural timber does not stop burning naturally, then timber structures will be inevitably less safe than non-combustible structures. Timber stops burning only if the supply of gaseous fuel descends below a critical mass flux, and this naturally occurs as timber chars. Hence, the design and implementation of timber structures entirely depends on the charring capacity of timber. The complex degradation process of timber is dominated by the gas phase combustion chemistry as well as exothermic and endothermic reactions occurring in the solid phase. The rate of these reactions is governed by a heat balance at the surface and at the interface between the timber and the char. In turn, the gas phase chemistry is dominated by the supply of gaseous fuel and therefore the solid phase chemistry. A detailed analysis of all the different processes was conducted experimentally and combined with a high-activation energy asymptotic analysis to elucidate the key processes controlling the quenching of the gas phase reaction. The results reveal that the mechanism controlling the extinction of the gas phase reaction is the quenching of the char oxidation chemistry. This allows for the char thickness to increase, the fuel supply to the flame to decrease and as a result the Damköhller number of the gas phase chemistry to drop below the critical value for quenching. This fundamental study represents the centrepiece of a new design methodology for complex timber structures.