Multi-scale modelling of wood degradation using thermally thin wood plates

Abstract

The mass loss rate is an indicator which defines the mass source term of combustible gases that supply the flames and influences the propagation of wildland fires. In this work, we investigated the thermal degradation of two woods (oak and eucalyptus) using a multi-scale approach. The originality of this work lies in the fact that we used thermally thin plates to carry out the experiments at different scales. At matter scale, experiments were conducted using a thermogravimetric analyser (TGA) under oxidizing atmosphere at several heating rates. We focused on temperatures ranging from 150°C to 650°C in order to study the thermal degradation of dry wood and thus, avoid the dehydration of free water. A kinetic mechanism was determined by using the thermogravimetric results. It consists of four steps, each one based on an Arrhenius law. This mechanism assumes that dry wood is composed of hemicellulose, cellulose and lignin. Each of these components degrades into char, which is then oxidized and releases ashes. The kinetic parameters of the four reactions were calculated by using the gradient descent algorithm method. The kinetic mechanism gives an accurate representation of the mass loss of both woods obtained with TGA. The kinetic mechanism was then tested at material scale using thermally thin wood plates. The experiments were carried out with a cone calorimeter at higher heating rates than used in TGA. Radiant heat fluxes ranging from 20 to 28 kW/m2 were imposed at the top of the fuel sample in order to avoir igntion. The mass loss was recorded as well as the temperature at the back surface of the wood, which was supposed to be equal to the temperature of the whole plate. For a radiant heat flux of 20 kW/m2, the predicted mass loss and mass loss rate are close to the experimental results for temperatures lower than 400°C. Beyond this value, the prediction underestimated the mass loss. For a radiant heat flux higher than 22 kW/m2, the model did not correctly predict the thermal degradation of the plate. This difference is probably due to the assumption of an equilibrium temperature within the plate. Above this level of heat flux, the assumption of a thermally fine plate is no longer valid an it is necessary to take into account the temperature gradient within the plate.