Experiments and Large Eddy Simulation of Underventilated Pool Fires

Large Eddy Simulations
Numerical modeling of underventilated pool fires
was performed using Fire Dynamics Simulator (FDS),
Version 5, developed at NIST, with the accompanying
Smokeview viewer for visualization of results [7-9].
Earlier, FDS was successfully applied to modelling of
underventilated burner fires [6]. In this work, the fire
source was provided by evaporating liquid fuel, and the
fuel supply rate is determined by the heat flux on the
pool surface due to conduction and radiation. Thus, the
evaporation rate cannot be set ad hoc, rather, it is determined
in the course of the solution.
The combustion model used in all LES simulations
is that available in FDS by default and based on the
mixture fraction approach in which a transport equation
for the mixture fraction (defined to be 1 in the fuel and 0
in the oxidizer) is solved [7, 8]. The state relations for
the fuel (heptane was used to simulate the petroleum
ether which is a mixture of several hydrocarbons) are
applied to restore the species concentrations and assess
the volumetric heat release rate. The reaction is assumed
to proceed at an infinite rate, so that the fuel and oxidized
cannot co-exist on both side of the surface where
the mixture fraction corresponds to the stoichiometric
conditions. It should be noted that, according to this
model, the fuel ignites immediately upon entering the
atmosphere, rather that after reaching the ignition
source.
The calculations have shown that simulations by
FDS give adequate qualitative picture of the process, as
was the case when burner fires were modeled previously
[6]. However, quantitative characteristics of the process,
most importantly, time to flame exhaust, are very sensitive
to such parameters as absorption coefficient and
thermal properties of pool liquid, which are somewhat
uncertain because petroleum ether is a mixture of several
hydrocarbons. Also, convection in the burning pool is
known to affect the burning rate, but is not taken into
account in the FDS model. More detailed studies are
required to assess the sensitivity of flame exhaust time
to each parameter. In this paper, we present the results
of preliminary calculations which demonstrate the predictive
capabilities of LES model.
In Figs. 11 and 12, the results obtained for the geometry
of fire box used in the experiments described in
the previous sections are presented. The opening was
chosen to be 0.18 m high and 0.1 m wide, the fuel
source was a square pan with the side of 150 mm.
In Fig. 11, the distributions of volumetric heat release
rate are presented (visualized by Smokeview
viewer [9]) at four time instants, representing the main
stages of the process: initial fuel-controlled combustion
at t = 7 s, oxygen starvation and slumping of flame to
the floor where oxygen is still available at t = 70 s,
flame propagation towards the ventilation opening and
its emergence outside the fire box at t = 150 s, and,
finally, developed external flaming at t = 240 s. The
temperature fields in the plane of symmetry are shown
at the same instants in Fig. 12. The time to flame exhaust
obtained in the calculations (about 140 s) is of the
same order as observed experimentally with the same
pool size for openings 2 and 3, but more detailed comparison
and analysis have yet to be done.
Conclusions
Experiments with underventilated pool fires have
shown that the time to flame exhaust strongly depends
on the size of the fuel source, compartment geometry
and ventilation factor. The temperatures measured inside
the box during the experiments were relatively low
(in the range of 220÷310°C). The height of the fuel pan
walls also affects the time to flame exhaust.
In the pool fire experiments it was found that flame
exhaust process is much more repeatable than in the
burner fire experiments. It seems that pool fires are
“self-organizing”: the fuel supply rate is controlled by
the radiative feed from the flame. For the same fire box
configuration, it results in more stable and repeatable
flame development, at least in terms of flame exhaust.
CFD modeling is capable of capturing the main features
of underventilated pool fires, although detailed
parametric study of coupled physical phenomena has
yet to be done.
Acknowledgments
The research was supported by EPSRG (Grant No.
GR/S69122/01) and Russian Science Support Foundation.
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