Turbulent, Concurrent, Ceiling Flame Spread: The Effect of Buoyancy.
Turbulent, Concurrent, Ceiling Flame Spread: The Effect
of Buoyancy.
(1037 K)
Zhou, L.; Fernandez-Pello, A. C.
Combustion and Flame, Vol. 92, No. 1-2, 45-59, 1993.
Sponsor:
National Institute of Standards and Technology,
Gaithersburg, MD
Keywords:
flame spread; ceilings; air flow; polymethyl
methacrylate; turbulence; air; buoyancy; flame fronts
Abstract:
Experiments have been conducted to study the effects of
forced air flow velocity and grid-generated turbulence
on the flow-assisted flame spread over a flat solid
combustible surface in a ceiling configuration. The
tests are conducted with thick PMMA sheets as fuel, and
air as oxidizer. Flame spread rate, flame length,
surface heat flux, and products composition are obtained
for air flow velocities ranging from 0.25 to 4.5 m/s and
turbulence intensities the 1% to 15%. It is found that
for all turbulence intensities the ceiling flame spread
rate increases with the flow velocity, and that the flow
turbulence retards the flame spread for flow velocities
larger than 1 m/s and enhances it at lower velocities.
The flame length and the surface heat flux exhibit power
law correlations with the fuel pyrolysis length, and the
flame spread rate data can be correlated with an
expression deduced from a simplified heat transfer
analysis of the process. In order to determine the
effect of buoyancy on the flame spread processes, data
from the ceiling configuration experiments are compared
with data from floor tests conducted previously. The
experimental results indicate that in ceiling spread,
buoyancy has two main competing effects. One is an
enhancement of the heat transfer from the flame to the
solid surface because the flame stands closer to the
surface, the other is an incomplete (larger than 1 m/s),
the enhanced heat transfer is found to be dominant and
results in a faster flame spread in the ceiling than in
the floor. For small flow velocities, the incomplete
combustion becomes more important and the opposite
result is observed. The species concentration data show
that in general the combustion reaction is less complete
in ceiling spread than in floor spread, and that
significant amounts of CO and unburned hydrocarbons are
produced in ceiling flame spread.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899