Fire Propagation in Concurrent Flows. Final Progress Report. September 1, 1992-August 31, 1993.
Fire Propagation in Concurrent Flows. Final Progress
Report. September 1, 1992-August 31, 1993.
(2022 K)
Fernandez-Pello, A. C.
NIST GCR 94-644; 66 p. June 1994.
Sponsor:
National Institute of Standards and Technology,
Gaithersburg, MD
Available from:
National Technical Information Service
Order number: PB94-193844
Keywords:
fire spread; buoyant flow; fire research; flame size;
polymethyl methacrylate; turbulent burning; turbulent
flow; turbulent heat transfer; oxygen concentration
Abstract:
A research program has being conducted to study the
mechanisms controlling the spread of flames in an
oxidizing gas flow moving in the direction of flame
propagation. During this reporting period research has
been conducted to study the effect of the oxidizer flow
characteristics on the concurrent flame spread over
thick PMMA sheets. The parameters varied in the
experiments are the oxidizer flow velocity, turbulence
intensity and oxygen concentration, and the geometrical
orientation (floor and ceiling). Their effect on the
flame spread process is studied by measuring the rate of
flame spread, flame length, surface heat flux, products
of combustion and soot. The results of the experiments
show that the combined effect of flow velocity,
turbulence intensity, and oxygen concentration has a
complex influence on the flame spread process. At low
flow velocity, the flame spread rate increases
monotonically with turbulence intensity. At high flow
velocity, however, the flame spread rate increases with
flow turbulence at low turbulence intensities, but it
decreases at high turbulence intensity values. The
effect is more pronounced at high oxygen concentration.
These trends appear to be due to a strong influence of
the turbulence intensity on the flame temperature and
length, and on the heat flux from the flame to the solid
fuel. Turbulence enhances mixing, which increases the
flame temperature and then the heat flux. The effect of
turbulence on the flame length comes from two opposing
factors. In one hand the enhanced mixing results in a
stronger reaction with faster reactant consumption,
which tends to produce a shorter but hotter flame. On
the other hand, the higher flame temperature results in
an increased mass burning rate, which tends to increase
the flame length. It appears that at low flow
turbulence, the latter effect dominates and thus there
is an increase in the flame length. As the turbulence
level continues to rise, the reactant consumption
dominates, which leads to a decrease in the flame
length. For the present experiments, the transition
between the two regimes shifts from u'/U=5% at U=2.0
m/s, to u'/U=15% at U=1.0 m/s, and no transition point
is observed at U=0.5 m/s within our experimental
conditions. The flame spread rate is the outcome of the
combined effect of the flame length and the heat flux.
Under all flow velocities and turbulence intensities,
the flame spread rate increases with the oxygen
concentration. For low oxygen concentrations, a linear
dependence is observed between the flame spread rate and
the oxygen concentration. For high oxygen
concentrations, the dependence of the flame spread rate
on the oxygen concentration follows a second power law.
By comparing the floor and ceiling results, it is found
that buoyancy has two opposite effects, one is enhancing
the heat transfer to the surface by reducing the flame
stand-off distance and the other reducing the chemical
reaction completeness by intensifying the flame
quenching at the wall. The overall buoyancy effect on
the flame spread and mass burning processes depends on
the flow condition.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899