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.
