In-Depth Temperature Profiles in Pyrolyzing Wood
The move towards performance-based design of the fire resistance of structures requires more accurate design methods. An important variable in the fire performance of timber structures is the in-depth temperature distribution, as wood is weakened by an increase of temperature, caused by exposure to high heat fluxes. A proper prediction of temperature profiles in wood structural elements has become an essential part of timber structural design. Current design methods use empirically determined equations for the temperature distribution but these assume constant charring rates, do not account for changes in the heating conditions, and were obtained under poorly defined boundary conditions in fire resistance furnaces. As part of this research project, a series of experimental in-depth temperature measurements were done in wood samples exposed to various intensities of radiant heat fluxes, with clearly defined boundary conditions that allow a proper input for pyrolysis models. The imposed heat fluxes range from 10 kW/cm^2, which generates an almost inert behaviour, to 60 kW/cm^2, where spontaneous flaming is almost immediately observed. Mass loss measurements for all the imposed heat fluxes were also performed. The second part of this project dealt with the modelling of the pyrolysis process, with an emphasis placed on temperature prediction. The main objective was to identify the simplest model that can accurately predict temperature distributions in wood elements exposed to fires. For this, an analysis of the different terms which have been included by several models in the energy equation has been done, by quantifying its magnitude. Five models with different degrees of simplification have been developed. Comparison with the experimental data has shown that a simple and accurate model of temperature profiles must include the rise in the solid sensible heat, the heat transferred by conduction, the heat of moisture evaporation, the heat of pyrolysis reaction and the effect of char oxidation.
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