Fire-Suppression Characteristics of CF3H in a Cup Burner.
Fire-Suppression Characteristics of CF3H in a Cup
Burner.
(2106 K)
Katta, V. R.; Takahashi, F.; Linteris, G. T.
Combustion and Flame, Vol. 144, No. 4, 645-661, March
2006.
Keywords:
cup burners; fire suppression; extinction; diffusion
flames; halon alternatives; flame inhibition;
experiments; mathematical models; flame structures;
extinguishment
Abstract:
A numerical investigation is performed to understand the
inhibition characteristics of CF3H in a periodically
oscillating methane-air jet diffusion flame formed over
a cup burner. A detailed chemical kinetic mechanism
having 82 species and 1510 elementary-reaction steps is
used. Calculations made without adding agent yielded an
oscillating flame with a flicker frequency of 11 Hz,
which compared well with that obtained in the
experiment. The minimum concentration of agent required
for extinguishing the cup-burner flame is determined by
adding CF3H to the air stream and by increasing its
volume fraction gradually until the flame is completely
extinguished. Addition of CF3H at volume fractions up to
10.05% did not affect the cup-burner flame temperature
significantly. Extinction of a cup-burner flame took
place as the base of the flame became destabilized, and
the unstable flame base moved downstream in search of a
new stabilization location. The predicted minimum
concentrations of CF3H for extinguishing the flame
obtained by (1) replacing the air with CF3H and (2)
replacing the N2 in the air with CF3H are 10.1 and
19.2%, respectively. These concentrations compare
favorably with the corresponding measured values of 11.7
and 20.3%, respectively. For validation, calculations
are also made for the steady counterflow diffusion
flames with different concentrations of CF3H in the air
stream and the predicted volume fractions of agent at
extinction are in good agreement with the experimental
values published in the literature. Examination of the
reaction rates for the cup-burner flames indicates that
the reactions with fluorinated species reduce the
concentration of chain-carrying radicals in the flame.
The effect is stronger at the flame base than further up
in the trailing part of the flame, leading to
destabilization at the flame base prior to extinction in
the trailing region, and yielding the observed
blowoff-type extinction.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899