An unsteady one-dimensional model
for the solid phase is applied
to simulate the spontaneous ignition and burning of thick wood samples
with grain orientation parallel or perpendicular to incident heat
fluxes in the range 18–40 kW/m2, that is, in the
absence of flame. The description of heat and mass transfer processes,
at constant gas pressure, is combined with global volumetric rates
of wood decomposition and char oxidation. Surface regression occurs
for a limit value of char density while a critical surface temperature
describes ignition. Good quantitative predictions are obtained for
the ignition times and the surface temperature and mass loss rate
profiles during burning. Conversion always consists of three main
stages. The first short transients correspond to the formation of
a relatively thin charred surface layer and glowing ignition. The
second much longer stage represents a pseudo-steady-state burning,
where the rates of advancement of the decomposition and oxidation
zones are approximately constant as well as the surface temperature
and the global rate of mass loss. Finally, the last short stage, resulting
from the adiabatic bottom condition, is the enlargement of the decomposition
zone to the entire sample. The first transient stage and the ignition
times are noticeably affected by the external heating conditions whereas
the characteristics of the pseudo-steady-state burning are mainly
dependent on the char oxidation rate, which is controlled by the oxygen
diffusion rate.
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