A simplified wood combustion model for use in the simulation of cooking fires

Abstract

Paper presented at the 5th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 1-4 July, 2007.

Wood combustion was studied with the intention of developing a simplified model of fuel burn-rate in small cooking fires, for inclusion in a CFD model of a whole cook-stove. The investigation included collecting experimental data on fuel burn-rate, model development and comparison of experimental and simulation results. In the experimental phase, regular blocks of wood were arranged in a lattice or crib with a range of volumes, void fractions and specific surface areas. The burning cribs yielded 3-40 kW fires. The simplified model assumed an unreacted core of virgin wood surrounded by char. It included considerations of heat transfer through the fuel by conduction; thermal decomposition of the virgin fuel into char and volatile gases, limited by the supply of heat to the pyrolysis region; the surface combustion of char limited by the diffusion of oxygen through the species boundary layer and impeded by the counter-flow of volatile gases. The model predicted the change of burn-rate with crib volume, porosity and surface area shown by experimental data, though it does incur significant errors, due to the assumption of one-dimensional behaviour within the crib, and neglecting spatial and temporal variations in boundary conditions. It was concluded that accuracy of the model could be improved by developing it to two or three dimensions, and that the easiest way to do this was through CFD. The model was sufficiently accurate to be used as a source of wood volatiles when modelling small fires in cookstoves, with the aim of investigating the effect of design changes on stove efficiency.