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Physical, Thermal, and Chemical Effects of Fine-Water Droplets in Extinguishing Counterflow Diffusion Flames.


pdf icon 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.