Extinguishment of Methane Diffusion Flames by Carbon Dioxide in Coflow Air and Oxygen-Enriched Microgravity Environments.
Extinguishment of Methane Diffusion Flames by Carbon
Dioxide in Coflow Air and Oxygen-Enriched Microgravity
Environments.
(3126 K)
Takahashi, F.; Linteris, G. T.; Katta, V. R.
Combustion and Flame, Vol. 155, No. 1/2, 37-53, October
2008.
Sponsor:
National Aeronautics and Space Administration,
Washington, DC
Keywords:
diffusion flames; microgravity; extinguishment; methane;
carbon dioxide; air; spacecraft; reaction kernel;
experiments; equations; extinguishing; extinction;
blowoff; flame structure; reduced gravity; oxygen;
volume; radiative heat loss; fire suppression;
extinguishing limits
Abstract:
Microgravity experiments and computations have been
conducted to elucidate stabilization and extinguishment
mechanisms of methane diffusion flames, in the
cup-burner configuration, with CO2 added gradually to a
coflowing air or oxygen-enriched stream. The minimum
extinguishing concentration of CO2 under low oxidizer
velocities (<20 cm/s) was measured in microgravity
achieved by parabolic flights of the NASA Reduced
Gravity Aircraft. Transient computations with full
chemistry and a gray-gas radiation model were performed
to reveal the detailed flame structure and
extinguishment processes. To compensate for the
overestimation of radiative heat losses at high
concentrations of radiating CO2, the Planck mean
absorption coefficient was multiplied by a correction
factor. The fuel-lean peak reactivity spot (the
so-called reaction kernel) at the flame base stabilized
the trailing diffusion flame. The calculated temperature
along the trailing flame decreased downstream due to
radiative cooling, leading to local extinction at <1300
K and flame tip opening. As CO2 was added to the
oxidizer: (1) the calculated maximum flame temperature
decreased toward a threshold; (2) the reaction kernel
weakened (i.e., lower heat release rate) but nonetheless
remained at a nearly constant temperature; (3) the flame
base stabilized increasingly higher above the burner
rim, parallel to the axis; until finally, (4)
blowoff-type extinguishment occurred. In the lifted
flame, the broadened reaction kernel supported a
super-lean reaction branch on the oxidizer side as well
as the trailing diffusion flame on the fuel-rich side
(no triple flame structure was formed).
