Abstract:
This
study
describes the simulation and experimental
investigation
of a
heptane
pool
fire, burning
within a small compartment, in
which
interaction between
a number of
key
physical processes is
amplified.
In
particular, the configuration emphasises the coupling
of
buoyancy induced
ventilation, smoke production and radiation
heat transfer to the
liquid fuel
surface,
from
the luminous flame
zone,
from the smoke
filled
ceiling
layer
and
from the confining walls.
This
study contrasts with those customarily performed
for
the purpose of model validation
in
compartment
fires,
which employ gas
burners
and so
simplify much of the interaction.
Initial
experiments were carried out using a
0.23m diameter
circular pan
burning fixed
amounts of
heptane. Subsequently,
a constant supply was used with a smaller circu-
lar
pan of
0.17m in diameter, in
order to introduce
experimental
longevity
under safe,
controllable conditions whilst establishing a quasi steady-state system.
Issues
of non-
stationarity
in
relation to heat-feedback
to the fuel
surface
-
an
important
pool
fire
mech-
anism
-
are
discussed.
In
addition to probe measurements of velocity and thermocouple temperature, the smoke
yield was
determined
using a
light
extinction technique employing a 670nm
wavelength
diode laser
and photo-diode
detector, housed
within a novel
fully-traversible
water-
cooled probe.
Data from
these experiments illustrate the importance of accounting
for
room ventilation
in terms of overall production of smoke and sound a cautionary note
to the labelling
of soot
by
a convenient marker such as temperature.
Numerical simulation of the compartment fire is performed using the field
model SOFIE,
incorporating a simple evaporation model, which relates the mass-loss-rate of
fuel to the
net
heat flux to the fuel
surface and
heat
of gasification.
This
relationship assumes that
heat losses to the pan, re-radiation
by
the fuel
surface and other enthalpy
loss terms, are
small.
Simulations
of compartment
fire
scenarios using this model to calculate the rate
of
heat
release are reported.
Further
comparisons are made
between the industry
stan-
dard 'Eddy-Breakup'
combustion model and the 'Laminar Flamelet'
model.
In
general
both the eddy-breakup model and
laminar flamelet
model tend to underpredict the yields
of
CO,
whilst the eddy-breakup model over-predicts temperature and thus soot.
The
laminar flamelet
approach shows more promise and shows particularly good agreement
with the experimental measurements reported
here
under well ventilated conditions.
SORE, the predictive tool employed
in this research,
has
proved
invaluable in discern-
ing the reason
for
apparent ambiguities in the experimental measurements of soot con-
centration.
The
results suggest that an alternative simplified zone model approach would
overpredict visibility
in
smoke
in terms of concentration,
but
underpredict
in terms of
layer depth, due to its inability to capture the important
shape of the hot
upper
layer,
which varies significantly
from the homogenous, laterally
uniform
distribution
which
is
assumed.
The incorporation
of a simple evaporation model which relies on accu-
rate prediction of
heat transfer in
ultimately determining
the heat
release rate
has been
shown to be in
very good agreement with the experiments.
Despite the irregularity in
predicted
distribution
of mass loss
rate across the fuel
surface
-
caused mainly
due to the
'ray
effect' of the radiation model
-
the main trend of
lower heat transfer at the centre
of the burner is demonstrated, in
agreement with the experiments performed.
This
phe-
nomenon
is
captured
despite the lack
of
description
of
fuel
vapour radiation
blockage
above the fuel
surface, suggesting that this process may
be disregarded. The heat flux
distribution
which
is found here is in
contrast to research conducted
by
other workers
for
similar sized pans in
an open environment, which show a
higher
measured heat transfer
at the centre of the burner.
It has been
shown that significant
improvements
could
be
made
in
experimental
design
of compartment
fire
experiments if CFD
prediction
is
considered
for the determination
of suitable measurement
locations in
regions with
lower local
spatial variations.