Development and Validation of a Finite Element Pyrolysis Model for Simulating Fire Resistance Tests of Wood Building Elements

Modern wood buildings have to meet high standards with regard to fire resistance. This is in stark contrast to the nature of wood as a flammable material. Therefore, wooden building elements have to undergo real fire resistance tests in order to prove their performance under high thermal loads. Those tests are expensive, time consuming and are only a snapshot of a single component made of an inhomogeneous and anisotropic medium. Simulations are a way to get a better understanding of the physical processes occurring during fire resistance tests and could provide deeper insight into the importance of hard to control variables such as density variations, water content and fiber deviations. In this paper we develop a multiphase model of wood pyrolysis in which five species interact together with three different irreversible reactions. The underlying reaction mechanism is derived from previous work and complemented by supplementary physical mechanisms such as solid phase expansion and transport mechanisms of water and pyrolysis gases. Laboratory scale methods such as dilatometry and small scale fire tests in an oven with a sample size of up to \qty{0.25}{\square\metre} were used to validate the proposed model. The laboratory experiments were performed to gather thermophysical parameters and to validate the kinetic parameters of the reactions between the phases of the multiphase model while the main purpose of the fire tests was to demonstrate the applicability of the model to those tests. Both kinds of real world tests show good agreement between them and the numerical simulations.