Paper # 070FR-0155 Topic: Fire 8 th U. S. National Combustion Meeting Organized by the Western States Section of the Combustion Institute and hosted by the University of Utah May 19-22, 2013 Impact of an Oxidative Pyrolysis Model for Charring Wood in Fire Simulations Ragini Acharya 1 Meredith Colket 1 Paul Papas 1 Joseph Senecal 2 1 United Technologies Research Center, East Hartford, CT 2 Kidde Fenwal Inc., Ashland, MA A general method for modeling wood burning within the framework of fire simulations is to apply a specified burning rate on a surface. This burning rate does not depend upon surface temperature. In order to predict mass loss rate of wood and reduction in mass loss rate due to surface cooling provided by watermist, it is required that a closed-loop coupling must exist between the heat feedback from the gaseous flame to solid surfaces, via radiation and convection. A pyrolysis model that couples the burning rate of a solid with the surface temperature via the Arrhenius law can be applied to achieve the closed loop coupling. The Fire Dynamics Simulator (FDS) code contains the entire infrastructure to this complex non-linear feedback and hence provides an excellent modeling environment for implementing an enhanced wood combustion model. While such a model is implemented in the FDS code, the pyrolysis model does not include a char oxidation model, which should account for the effect of oxygen concentration near the surface. In absence of such a model, the char oxidation reaction was treated as a kinetically-controlled reaction that is independent of the local oxygen level, which is in contradiction with the experimental observations for wood-based char oxidation by air (Kashiwagi, Ohlemiller, & Werner, 1987). A single-step reaction scheme with non-reacting char residue was found inadequate for large and medium-scale fire simulations. Therefore, an enhancement was made to FDS (svn no. 10095), to represent the role of oxygen in char oxidation using a power-law based model. In addition, the model for charring wood pyrolysis was compared against the experimental data of Janse et al. (1998) and Santangelo et al. (2012). 1. Introduction Wood-based fire is a major area of research in fire suppression. Wood packages are commonly used to represent solid fuel sources in practical fires and certification tests. For modeling fire suppression by water-based systems, it is imperative to understand and have a capability to accurately simulate the wood burning rate. Introduction of watermist in the wood-based fire can result in flame cooling and quenching, which can reduce the heat feedback from the flame to the burning surfaces of the wood. Reduced heat feedback from the flame can reduce the surface temperatures of wood, and thereby, lower the burning rates. Therefore, the wood burning rate must be coupled with the solid temperature of wood for the purposes of accurate fire suppression modeling, as shown in Fig. 1. A pyrolysis model that couples burning rate of solid with the surface temperature via Arrhenius law can be applied to achieve such closed loop coupling. Figure 1: Flow chart description of closed-loop coupling between gas-phase and solid surface Net Heat flux to fuel surface Gas-phase (via radiation and convection) Other hot surfaces (via radiation) Calculation of T surf Calculation of ( ) surf fuel T m & Kinetic parameters for surface reactions via gas-phase heat release