An Integral Model for the Transient Pyrolysis of Solid Materials

Abstract

The modelling of the spread of fire and its extinguishment still represents a significant challenge. As part of a combined experimental and computational study of fires we have developed a general Computational Fluid Dynamics (CFD) model of fire spread and extinguishment. The primary objective was to produce a flexible computational tool which can be used by engineers and scientists for design or research purposes. The present paper deals with the description and validation of a solid pyrolysis model which has been applied, as a sub-model, in this general computer fire code. The pyrolysis model has been formulated using the heat-balance integral method. The model can be applied to slabs of char forming solids, such as wood, as well as non-charring thermoplastic materials, such as PMMA. Results are compared with analytical solutions, numerical simulations and experimental data. In all cases the integral model performs well.

Generalized pyrolysis model for combustible solids

“…An exception is the asymptotic analysis of Wichman and Atreya [57] wherein approximate formulas are developed for the mass loss rate of a charring solid in the limit of large activation energy. In the simplest class of numerical models for charring pyrolysis, it is assumed that an infinitely thin reaction zone (or pyrolysis front) separates the char layer from the virgin material [58][59][60][61][62][63][64][65][66][67][68], analogous to the Stefan problem where phase change occurs at a thin interface. This is a reasonable approximation at high heat 23 flux levels, but can become questionable at lower heat fluxes.…”

“…A single reaction is considered, and infinitely fast or finite rate kinetics can be used. In some models, the conversion of virgin material to char is assumed to occur at a fixed pyrolysis temperature [58][59][60][61][62][63][66][67] and the velocity at which the front propagates into the solid is determined by a heat balance at the pyrolysis front. Thus, the kinetics are infinitely fast, analogous to the thermoplastic ablation models discussed earlier.…”

“…A recent paper [68] compared the infinite-kinetics (fixed pyrolysis temperature) approach to the finite kinetics approach. Integral models [58,59,61,62,66,67,69] have the advantage that the governing partial differential equations are transformed to ordinary differential equations, but numerical solution is generally still required.…”

Pyrolysis Characterization of a Lineal Low Density Polyethylene

“…A range of empirical formulations based on experimental data obtained from the cone calorimeter or furniture calorimeter tests, to semiempirical models [1][2][3] that consider transient heat conduction, to comprehensive models [4,5] based on advanced descriptions of the in-solid heat and mass transfer processes, have been developed. Comprehensive pyrolysis models adopt a material science perspective, and describe the heat-driven physical-chemical transformation of the virgin solid into solid, liquid and gaseous products.…”

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

“…During the past two decades, several numerical models were developed for pyrolysis, of charring materials with different levels of complexity, such as: Arrhenius-type models [5]; 'integral' models [6][7][8][9]; an 'extended' integral model [10]; a moving mesh model [11]; a dual mesh model [12]. Reviews on pyrolysis modelling have been provided in e.g.…”