Extinguishment Mechanisms of Coflow Diffusion Flames in a Cup-Burner Apparatus.
Extinguishment Mechanisms of Coflow Diffusion Flames in
a Cup-Burner Apparatus.
(1021 K)
Takahashi, F.; Linteris, G. T.; Katta, V. R.
Volume 31; Part 2;
Combustion Institute, Symposium (International) on
Combustion, 31st. Proceedings. Volume 31. Part 2.
August 5-11, 2006, Heidelberg, Germany, Combustion
Institute, Pittsburgh, PA, Barlow, R. S.; Sick, V.;
Glarborg, P.; Yetter, R. A., Editor(s)(s), 2721-2729 pp,
2007.
Sponsor:
National Aeronautics and Space Administration,
Washington, DC
Keywords:
combustion; fire research; diffusion flames;
extinguishment; burners; fire suppression; flame
stability; gravity; experiments; fire extinguishing
agents; methane; heat capacity; flame temperature
Abstract:
The extinguishment processes of methane-air coflow
diffusion flames formed on a cup burner in earth gravity
have been investigated experimentally and
computationally. As a gaseous fire-extinguishing agent
(CO2, N2, He, Ar, CF3H, CF3Br, or Br2) was introduced
gradually into a coflowing oxidizer stream, the base
(edge) of the flame detached from the burner rim,
oscillated, and eventually extinguished. This
extinguishment occurred via a blowoff process (in which
the flame base drifted downstream) rather than the
global chemical extinction typical of counterflow
diffusion flames. The agent concentration in the
oxidizer required for extinguishment was nearly
independent of the mean oxidizer velocity over a wide
range, exhibiting a plateau region. Numerical
simulations with full chemistry revealed the unsteady
blowoff process and
predicted the minimum extinguishing concentration (MEC)
of each agent in good agreement with the measurement.
The calculations indicated that flame stabilization at
the flame base depended upon diffusion of radicals and
heat from the trailing diffusion flame upstream into the
peak reactivity spot (i.e., reaction kernel). For
physically acting agents, the flame blew off as the
trailing diffusion flame temperature decreased at which
point the back-diffusion of heat and chain radicals into
the flame stabilizing region was sufficiently reduced.
Consequently, the relative ranking of inert agent
effectiveness depended primarily on the heat capacity of
the agent-laden oxidizer. Nonetheless, for helium, the
MEC was lower than that of argon (which has the same
specific heat). The numerical results showed that
addition of helium leads to greater heat losses from the
downstream diffusion region of the flame than addition
of argon because helium addition raised the thermal
conductivity of the gas mixture relative to argon
addition. The results highlight the importance of the
downstream diffusion flame conditions for supporting the
flame stabilization which ultimately occurs at the
reaction kernel.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899