Abstract
Fire exposure of timber leads to charring, surface cracking and timber burnout, shifting the external thermal load deeper into the timber domain. This phenomenon plays its role mainly in situations of longer fire exposure. The majority of current approaches and models assume initial geometry during the whole analysis, leading generally to the overestimation of the insulation effect of the charred layer and to a limited burnout. This paper presents a heat transport model which is supplemented with a moving boundary condition, a criterion for the finite element deactivation and the internal heat source. Comparison with experiments using a constant radiative load testifies that the moving boundary condition becomes important after approximately 10 min of fire exposure and rather leads to a constant charring rate observed in several experiments.
Highlights
Timber exposure to high temperatures triggers several multiscale thermochemical processes responsible for the timber degradation and loss of mechanical resistance [1]
The main objective of this paper aims at capturing a thinning charred layer using a moving boundary condition (MBC) and an internal heat source in the framework of finite element analysis for heat transport
The comparison with experimental data further revealed that the moving boundary condition performed well in temperature profiles, especially if fire exposures exceeded approximately
Summary
Timber exposure to high temperatures triggers several multiscale thermochemical processes responsible for the timber degradation and loss of mechanical resistance [1]. The most advanced models introduce heterogeneous kinetics for both pyrolysis (thin interface between char and wood with no oxidation) and oxidation (converting cellulose, hemi-cellulose and lignin to char and to ash) [1,10,11]. Those models can provide an accurate description of ongoing processes, they need to solve conservation equations for mass, species, energy and momentum, giving rise to a considerable amount of parameters and necessary calibrations
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