DOJ Study:
Christopher L. Mealy and Daniel T. Gottuk
Hughes Associates, Inc.
3610 Commerce Drive, Suite 817
Baltimore, MD 21227
Ph. 410-737-8677 FAX 410-737-8688
The development of a fire within an enclosure and the
corresponding impact of the enclosure
on the combustion process are dependent on numerous factors. These
factors can be grouped into
three different categories: enclosure geometry, ventilation, and
fuel. The first category that needs
to be considered is the geometry of the enclosure, which can
include the volume of the space, the
aspect ratio, and the ceiling height. When considering
ventilation, both the area of the vent as
well as the location/elevation of the vent must be considered. The
third category is fuel, which
includes the type of fuel, the total fuel surface area, the
quantity of fuel, and the combustion
characteristics of the fuel. As a fire develops within an
enclosure, the factors listed in each of
these categories begin to play a role in either the growth/decay
of the fire depending on the
combination of applicable factors. The extent to which the burning
dynamics of a fuel change
when burning in an enclosure compared to the open is dependent
upon increased thermal effects,
which result in increased burning rates, and reduced ventilation
effects, resulting in reduced
burning rates.
The purpose of this research was to characterize the changes in
burning dynamics of fuels
burning in enclosures as opposed to in the open and in doing so
provide an experimental data set
for both Class A and liquid fuels. To date, relatively little
full-scale fire testing has been
conducted to characterize these changes. This objective was
achieved by way of full-scale fire
testing and empirical-based analyses. A summary of the testing
conducted and the rationale for
their execution is provided in Table E.1.
In this work, the burning dynamics of both confined and unconfined
liquid fuel fires as well
as Class A fuel packages were characterized. The liquid fuels used
in this work were gasoline,
heptane, and denatured alcohol. These fuels were selected for
various reasons, including their
prevalence in real-world forensic fire scenarios (gasoline), their
historical presence in
experimental fire research (heptane), and their differences in
combustion chemistry (denatured
alcohol). More specifically the denatured alcohol fuel was
selected because of its negligible soot
yield, which differs from both gasoline and heptane. The Class A
materials (furniture and
flooring) used in these tests were all selected because of their
relevance to residential fires and
their use in previous research efforts [Wolfe et al. 2009, Mealy
et al. 2010] conducted which
allows for the comparison new data to existing data sets. The
enclosure used in this work was
designed to be representative of typical building spaces (i.e.,
height to width ratio of less than
one).
These tests allowed for direct comparisons between full-scale open
burning and enclosure
fire scenarios. This work provides an improved understanding of
the impact of the enclosure on
fuel burning dynamics for three different fuel scenarios and
identifies some of the key factors
that govern this impact.
For unconfined liquid fuel fires (i.e., a spill), the impact of
the enclosure was evaluated for
both vinyl and carpet flooring systems. For both open (Test Series
1) and enclosed (Test Series 4)
burning conditions, a 2.0 L (0.53 gal) gasoline spill was used as
the spill fire scenario. For both
flooring types, the enclosure fires behaved differently than the
open burning scenarios. However,
the difference was not due to enhanced burning of the liquid fuel;
instead, the primary difference
was the involvement of additional combustible material (i.e.,
adjacent flooring material outside
the initial spill area). For the period of time in which the liquid
fuel was the primary material
burning, the fires grew in a similar manner and reached peak
values that were comparable. For
the vinyl substrate, the fire in the enclosure burned at peak
values for an extended period of time
as opposed to immediately transitioning to the decay phase as was
observed in the open. Due to
the involvement of additional flooring material, the carpet
enclosure fire resulted in a larger fire
than was observed in the open. Considering the short period of
time in which the liquid fuel was
the primary material burning (1 to 2 minutes), the enclosures did
not have an effect on the spill
fire, but did contribute to the fires growing larger and involving
more material. There will be a
certain critical room volume to fire size that will dictate
whether an enclosure will lead to fires
growing beyond the initial spill areas. Additional work is needed
to identify this critical
parameter.
The impact of an enclosure on confined area liquid fuel fires
(i.e., pan fires) was determined
to be dependent on fuel type, fuel location, and ventilation
condition. Open (Test Series 2) and
enclosed (Test Series 5) tests were conducted using 0.23 m2 (2.5 ft2) and 1.0 m2 (10.8 ft2) pans
containing heptane and denatured alcohol, respectively. Pan fires
in the enclosure were evaluated
in two locations, center and corner, with full door (AH0.5 = 2.6) and slit vent (AH0.5 = 0.6)
conditions. The quasi-steady-state heat release rates from these
tests were compared.
In summary for the liquid fuel pan fires burning both in the open
and within an enclosure
(i.e., confined pool with sufficient depth to burn to
steady-state), the results clearly show that
enhanced burning occurs relative to open burning when a radiating
upper layer is created in the
compartment fire. During the initial 60–90 seconds of the heptane
fire tests, both open burning
and enclosed heat release rates were generally similar. After this
initial period of burning, the
enclosure fires continued to grow surpassing the steady-state
value achieved in the open. The
extent of this growth was dependent upon both the pan location and
ventilation condition. In
three of the four scenarios, the enclosed fires eventually reached
a quasi- steady-state burning
rate that was on average 60 percent higher than that measured in
the open. However, an increase
of only 19 percent over open burning conditions was measured for
the slit vent corner fire
scenario. The minimal enhancement observed in this test was
attributed to the vitiation of the
combustion air being entrained into the fire plume resulting in
less efficient combustion of the
heptane.
The second fuel evaluated was denatured alcohol. With the
exception of the slit vent scenario
with the fuel pan located in the corner, the denatured alcohol
fires conducted within the
enclosure were within three percent of that measured during tests
conducted in the open and
within one percent of other tests conducted within the enclosure.
Open burning and enclosed
denatured alcohol fires were comparable with respect to both fire
growth and steady-state
burning conditions. The enhanced burning observed for the
denatured alcohol pan fire located in
the corner of the enclosure with the slit vent condition was
attributed again to the vitiation of the
combustion air being entrained into the fire plume. However, in
this case, the less efficient
combustion produced a sootier, and thus a more radiative upper
layer, which in turn enhanced
the burning rate of the denatured alcohol. These tests illustrate
the varying effect that an
enclosure can have depending on the fuel that is burning within.
For a non-sooting fuel
(denatured alcohol), the enclosure/ventilation condition had only
a minimal effect on the
maximum heat release rate achieved, while for a sooty fuel
(heptane) under the same conditions,
the enclosure enhanced the burning of the fuel due to enhanced
radiation to the floor and fuel
surface.
In summary for liquid fuels in compartments:
The fire size of a spill
fire generally will not be affected by the compartment due to
the relatively quick duration of the fire.
If the fuel is contained in
a pool so that it is deep enough (5 mm or more) to burn to a
steady-state condition, a radiating upper smoke layer will
increase the burning rate.
An average 60 percent increase was observed for heptane pan fires
with a full door
vent. This increase can be moderated by restricted ventilation to
the compartment.
For the Class A fuels, the impact of the enclosure on the burning
dynamics of the fuel was
evaluated based on analysis of the mass loss rates of the
upholstered sofa which was the primary
fuel item within the enclosure. In these tests, the Class A
materials were evaluated using either a
Class A ignition source or liquid fuel spill on either the floor
of the enclosure or on an
upholstered chair opposite the upholstered sofa. These fire
scenarios were evaluated using both
full
door (AH0.5 = 2.6) and slit vent (AH0.5 = 0.6) ventilation conditions.
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