Backdraft Phenomena. Final Report. 1990-1992.
Backdraft Phenomena. Final Report. 1990-1992.
(7480 K)
Fleischmann, C. M.
NIST GCR 94-646; 238 p. June 1994.
Sponsor:
National Institute of Standards and Technology,
Gaithersburg, MD
Available from:
National Technical Information Service
(NTIS), Technology Administration, U.S. Department of
Commerce, Springfield, VA 22161.
Telephone:
1-800-553-6847 or 703-605-6000;
Fax: 703-605-6900; Rush
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Website:
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Order number: PB94-193927
Keywords:
backdraft; building fires; deflagration; fire research;
fireballs; glass; gravity; room fires; ventilation;
windows; gravity current
Abstract:
The purpose of this project was to develop a fundamental
physical understanding of backdraft phenomena. The
research was divided into three phases: exploratory
simulations, gravity current modeling, and quantitative
backdraft experiments. The primary goal of the first
phase was to safely simulate a backdraft in the
laboratory. A half-residential-scale compartment was
built to conduct exploratory experiments. The initial
experiments concluded with a scenario describing the
fundamental physics of backdrafts. The importance of
the gravity current which enters the compartment after
opening was identified. In the second phase, the
gravity current speed and the extent of its mixed region
was investigated in a series of scaled salt water
experiments. The scaled compartment (0.3m x 0.15m x
0.15m) was fitted with a variety of end openings: full,
slot, door, and window. Video and photo data indicate
that the mixing layer which rides on the gravity current
in the full opening case, expands to occupy nearly the
entire current in the partial opening cases. The Froude
number and nondimensional head height are independent of
beta and are in good agreement with numerical
simulations and special limits from the literature. In
the final phase, 28 backdraft experiments were conducted
in a 1.2m by 1.2m by 2.4m compartment. A methane burner
was ignited inside a closed compartment and allowed to
burn as long as oxygen was available. After the flame
extinguished due to oxygen starvation, the burner was
left on to allow the unburned fuel fraction to increase.
Upon opening the hatch a gravity current enters the
compartment and travels across the floor to the ignition
source. After ignition a deflagration rips through the
compartment and out the opening culminating in a large
fireball. Histories recorded included: fuel flow
rates, upper layer temperatures, lower layer
temperatures, opening velocities, compartment pressures,
upper layer species concentrations for O2, CO2, CO, and
HC. Results indicate that unburned fuel mass fractions
>15% are necessary for a backdraft to occur and that the
backdraft severity strongly depends on the delay time
and species concentrations.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899