Physical, Thermal, and Chemical Effects of Fine-Water Droplets in Extinguishing Counterflow Diffusion Flames.
Physical, Thermal, and Chemical Effects of Fine-Water
Droplets in Extinguishing Counterflow Diffusion Flames.
(844 K)
Lentati, A. M.; Chelliah, H. K.
Combustion Institute, Symposium (International) on
Combustion, 27th. Proceedings. Volume 2. August 2-7,
1998, Boulder, CO, Combustion Institute, Pittsburgh, PA,
2839-2846 pp, 1998.
Sponsor:
National Institute of Standards and Technology,
Gaithersburg, MD
Keywords:
combustion; droplets; water; counterflow flames;
diffusion flames; physical properties; thermal
properties; chemical properties; extinguishing; halon
Abstract:
A numerical method, based on a hybrid
Eulerian-Lagrangian formulation for gas and droplet
phase, is used here for the analysis of physical,
thermal, and chemical effects of water droplets in
extinguishing a methane-air non-premixed flame. The flow
field considered here is a steady, laminar counterflow
with monodisperse water droplets introduced with the air
stream. The droplet sizes considered range from 5 to 50
mum, with maximum water-mass fraction in the condensed
phase of 3%. The physical effects are analyzed by
modifying the droplet trajectory and dilution. When the
droplets are assumed to follow the gas exactly, the
flame extinction results are shown to deviate
considerably from the predictions obtained previously,
where the flame extinction strain was shown to have a
nonmonotonic dependence on droplet size. By decoupling
the chemical effects associated with water, mainly the
three-body recombination effects, it is shown here that
the evaporated water vapor has less than 10% effect on
the flame-extinction condition. In contrast, the
thermal effects, mainly through the latent heat of
vaporization, is shown to influence the flame-extinction
condition significantly. Detailed comparisons of the
flame structure obtained with water droplets and with
that obtained using chemical agent halon 1301 are shown
here to illustrate the completely different
flame-suppression mechanisms of the two classes of
agents.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899