Cup-Burner Flame Extinguishment by CF3Br and Br2.
Cup-Burner Flame Extinguishment by CF3Br and Br2.
(1666 K)
Linteris, G. T.; Takahashi, F.; Katta, V. R.
Combustion and Flame, Vol. 149, No. 1/2, 91-103, April
2007.
Sponsor:
National Aeronautics and Space Administration,
Washington, DC
Keywords:
cup burners; flame extinguishment; fire suppression;
hqalon alternatives; experiments; diffusion flames;
flame structures
Abstract:
Experiments and calculations have been performed for a
methane-air coflow diffusion flame, in the cup-burner
configuration, with CF3Br or Br2 added to the air
stream. The time-dependent, two-dimensional numerical
code, which includes a detailed kinetic model and
diffusive transport, has predicted the flame extinction
within 4 or 8% for each. Analysis of the flame structure
has allowed the mechanisms of flame weakening in the
base and trailing flame regions to be compared. The
agents CF3Br and Br2 behave very similarly with regard
to flame extinguishment: both raise the temperature in
the flame everywhere, as well as lower radical volume
fractions in the trailing diffusion flame and at the
peak reactivity spot (the "reaction kernel") at the
flame base where the flame is stabilized. The mechanism
of lowered radical volume fractions is shown primarily
to be due to a catalytic cycle involving bromine species
in both regions of the flame, with small contributions
from radical trapping by fluorinated species in the
trailing diffusion flame. In the reaction kernel, the
radical volume fractions are reduced more, and the
catalytic radical recombination cycles are shown to be
more effective as compared to in the trailing diffusion
flame. At the latter location, the effectiveness of the
agents is reduced because the hydrocarbon species, which
are necessary for the regeneration of HBr, are scarce at
the location of the peak radical volume fraction (i.e.,
at the flame zone), a limitation which does not exist in
the reaction kernel, where there is good upstream mixing
of the fuel and oxidizer because the base is lifted.
That is, the premixed character of the reaction kernel
actually allows the HBr in the catalytic cycle to be
more effective there because of the effective overlap
between the Br and the hydrocarbon species, which allows
efficient regeneration of HBr.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899