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This item is the archived peer-reviewed author-version of:
Study of vertical upward flame spread on charring materials-Part II: Numerical simulations
Wasan, S.R., Rauwoens, P., Vierendeels, J. and Merci, B.
In: Fire and Materials 35 (5), 261-273, 2011.
To refer to or to cite this work, please use the citation to the published version:
Wasan, S.R., Rauwoens, P., Vierendeels, J. and Merci, B (2011). Study of vertical upward
flame spread on charring materials-Part II: Numerical simulations. Fire and Materials 35 (5)
261-273.
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Abstract
Simulation results, obtained by means of application of an enthalpy based pyrolysis
model, are presented. The ultimate focus concerns the potential of the model to be used in
flame spread simulations. As an example we discuss vertically upward flame spread over
a charring material in a parallel plate configuration. Firstly, the quality of the pyrolysis
model is illustrated by means of cone calorimeter results for square (9.8 cm x 9.8 cm
exposed area), 1.65cm thick, horizontally mounted MDF samples. Temperatures are
compared at the front surface and inside the material, for different externally imposed
heat fluxes (20 kW/m
2
, 30 kW/m
2
and 50 kW/m
2
), for dry and wet samples. The mass
loss rate is also considered. Afterwards, vertically upward flame spread results are
reported for large particle board plates (0.025 m thick, 0.4 m wide and 2.5 m high),
vertically mounted face-to-face, for different horizontal spacing between the two plates.
The simulation results are compared to experimental data, indicating that, provided that a
correct flame height and corresponding heat flux are applied as boundary conditions,
flame spread can be predicted accordingly, using the present pyrolysis model.
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1. Introduction
In part I [1], the outcome of an experimental campaign was reported. In the present paper,
we apply a simple pyrolysis and evaporation model, based on enthalpy [2], to the same
configurations.
Firstly, we discuss the one-dimensional cone calorimeter configurations.
Afterwards, vertically upward flame spread in a parallel plate configuration is considered.
By no means, it is our intention to introduce a (semi-empirical) flame spread model, to be
used for other configurations than the specific one considered here. The only objective is
to illustrate that the developed pyrolysis model is ready-to-use for such configurations
and that reasonably accurate results can be obtained, provided an appropriate value for
incoming heat flux onto the solid material is provided. This heat flux could stem from
CFD (Computational Fluid Dynamics) in the gas phase, where the turbulent combustion
is simulated. However, we do not use CFD in the present paper, as uncertainties in CFD
would distract the attention from our objective as mentioned. Rather, we use
experimental data [1] to estimate the heat fluxes that serve as boundary conditions for the
simulations. In this sense, the set-up is somehow a sophism, but this suffices for the sake
of the present paper, as explained above. The major advantage is that the strong
sensitivity of flame heat fluxes to e.g. fuel sootiness [3, 4] is avoided. To summarise,
expressions as developed in [3] for a similar set-up as the one under study in the present
paper, are not applied here, but the present paper is not intended to provide an alternative
for such relationships.
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2. Numerical simulations set-up
2.1 Model description
In [2, 5], the model, along with the solution procedure, is extensively described and
applied to some basic configurations. The reader is referred to those references for all
details. We only recall here that the model relies on a fixed computational mesh, which
can be relatively coarse. On this mesh, the energy equation is solved numerically.
Pyrolysis (and evaporation) is modelled as an infinitely fast irreversible process, taking
place at an infinitely thin front at ‘pyrolysis’ (resp. ‘evaporation’) temperature. Thus,
fronts are moving through the solid material. As the evaporation front passes, wet virgin
material becomes dry virgin material. As the pyrolysis front passes, dry material becomes
char. In the present simulations, the water vapour and pyrolysis gases are assumed to
leave the solid instantaneously. They are in thermal equilibrium with the solid. More
advanced pyrolysis modelling (e.g. [6]) is possible, but this is beyond the scope of the
present paper.
2.2 Cone calorimeter set-up
We first discuss the results for the cone calorimeter set-up of [1]. We consider one-
dimensional heat transfer in the solid [1]. The computational mesh in the solid, with
thickness 1.65 cm, contains 33 cells. The time step size is set to 0.5s. It has been verified
that the results presented do not vary when more cells or smaller time steps are chosen.
The externally imposed heat flux
''
ext
q
equals 20 kW/m
2
, 30 kW/m
2
or 50 kW/m
2
. The
experiments were performed in open atmosphere, so that flames appear when the
volatiles are ignited with a spark ignition placed above the retainer frame. These flames
provide additional heat flux to the solid during pyrolysis. In the simulations, this is
