An enthalpy-based pyrolysis model for charring and non-charring materials in case of fire

Abstract

In a simulation of a developing fire, flame spread must be properly accounted for. The pyrolysis model is important in this respect. To that purpose, we develop a simplified enthalpy based pyrolysis model that is extendable to multi-dimensional solid-phase treatments. This model is to be coupled to gas phase turbulent combustion simulations. The description of the pyrolysis process is simplified in order to acquire short simulation times. In this paper, first, the basic thermodynamic description of pyrolysis phenomena is revisited for charring and non-charring materials, possibly containing moisture. The heat of pyrolysis is defined and its relation to the formation enthalpies of individual constituents is explained.Solving only one equation for enthalpy on a fixed computational mesh, provides a useful description of the transport of heat and the pyrolysis process inside the solid material. Models for e.g. char oxidation or complex transport of the pyrolysis gases or water vapour inside the solid material can be coupled to the present model. Next, numerical issues and implementation are discussed. We consider basic test cases with imposed external heat flux to a solid material that can be dry or contain moisture. We illustrate that continuous pyrolysis gases mass flow rates are obtained when a piecewise linear representation of the temperature field is adopted on the fixed computational mesh. With constant temperature per computational cell, discontinuities, with sudden drops to zero, are encountered, as reported in the literature. We show that the present model formulation is robust with respect to numerical aspects (cell size and time step) and that the model performs well for variable external heat fluxes. For charring and non-charring materials, we validate the model results by means of numerical reference test cases and experimental data. By means of a numerical test case, we show that the model, when coupled to CFD calculations, is able to simulate flame spread.

Application of a simple enthalpy‐based pyrolysis model in numerical simulations of pyrolysis of charring materials

“…Clearly, the mentioned simplifications imply limitations on the validity range of the models as described. In [3,4], we extend the model for application to charring materials with moisture and noncharring materials. However, complex phenomena like melting, in depth radiation absorption, surface oxidation, smoldering, etc are not modelled.…”

“…Effects of e.g. cracking could be incorporated by introduction of a transport model for pyrolysis gases in the solid, in the model framework as described in [3]. Introduction of multiple fronts can allow modelling of e.g.…”

“…3. A piecewise linear representation of the temperature field is necessary to provide acceptable results for the mass flow rate [4].…”

“…[1,2]), we focus here on dry charring materials for the basic application of our model. In [3,4] we discuss the theoretical background in more detail and illustrate applications for charring materials with moisture content and for non-charring materials.…”

“…First of all, we provide a brief model description. A more complete description and a sensitivity study with respect to numerical aspects (grid size and physical time step) are found in [3,4].…”