A novel approach to determine charring of wood in natural fire implemented in a coupled heat-mass-pyrolysis model

Robert Pečenko,Tomaž Hozjan

doi:10.1515/hf-2020-0081

Abstract

Abstract The paper presents a novel approach to determine charring of wood exposed to standard and natural fire that is based on a new numerical model named PyCiF. The new model couples an advanced 2D heat-mass model with a pyrolysis model. A new charring criterion based on a physical phenomenon is implemented in the PyCiF model to determine charring of wood. This presents the main advantage of the new PyCiF model in comparison to common modelling approaches, which require an empirical value of the charring temperature that is often called the char front temperature. The fact that the char front temperature is not an explicit value as assumed by the isotherm 300 °C is advantageously considered in the presented approach where an assumed empirical value of the char front temperature is not directly required to determine the thickness of char layer. The validation of the PyCiF model against experimental results showed great model accuracy, meaning that the model is appropriate for the evaluation of charring depths of timber elements exposed to the standard fire as well as the natural fires. Additionally, as shown in the case study, the presented approach also enables to determine the char front temperature for various natural fire exposures. This will be especially important for the upgrade of the new design methods for fire safety of timber elements exposed to natural fire given in the various design codes such as Eurocode 5.

Highlights

The behaviour of timber structures in fire largely depends on charring, which causes the reduction of the strength and increases the deformability of the structure (Moraes et al 2004; Sinha et al 2011)
The process of pyrolysis in point P1 ends after 28.1 min of fire exposure, when the cellulose and active cellulose are fully decomposed and the char reaches its final yield which represents 12.6% of the solid
The model is based on a thorough description of the multiphase mass transfer and the sorption phenomena, coupled by the heat transfer and pyrolysis processes in timber structural elements exposed to fire

Summary

Introduction

The behaviour of timber structures in fire largely depends on charring, which causes the reduction of the strength and increases the deformability of the structure (Moraes et al 2004; Sinha et al 2011). A large number of experiments have been carried out to provide the empirical expressions to calculate charring depth of timber under standard fire exposure (Lawson et al 1952; Schaffer 1967; White and Nordheim 1992), while the experiments to determine charring depths for non-standard fire exposures (Lau et al 1999; Mikkola 1990) were less numerous The observations from these experimental investigations were adopted in standards such as EN 1995-1-2 (2005) and AS 1720 (1990), where empirical expressions to determine charring depths can be found. The standard EN 1995-1-2 (2005) provides expressions to calculate charring depth for parametric fire curves, which represent the simplest way of describing natural fire, but their use is limited and cannot be generalized

Objectives

Results

Conclusion