- Author
- Kee, R. J.
- Title
- Structure and Extinction of Premixed Methane-Air Flames in the Presence of Fine Water Mists. BFRL Fire Research Seminar. VHS Video.
- Coporate
- Colorado School of Mines, Golden
- Report
- Video, May 23, 2001,
- Keywords
- water mist | premixed flames | flame structure | flame extinction | fire suppression
- Abstract
- Water mist is increasingly being considered as a fire suppression agent, replacing environmentally harmful fluorocarbon-based compounds. With the objective of elucidating the fundamental fluid, thermal, and chemical interactions between a flame and a mist cloud, we have developed a computational model to d escribe the structure and extinction characteristics of freely propagating premixed flames. The model is an extension of the CHEMKIN PREMIX software, in which coupled two-phase flow behavior is incorporated. The gas-phase conservation equations are described in an Eulerian framework using adaptive-mesh strategies to resolve details of the flame structure. The droplets are modeled in a Lagrangian framework. Droplet motion considers viscous and thermophoretic forces and the droplet temperature and evapor ation rates depend on the local temperature and composition field. Arc-length continuation procedures are used to determine flame-extinction limits. The model predicts that flame structure and extinction limits depend strongly on the droplet size and the net water loading, with small droplets (less than 30 microns) being much more effective for flame extinction than larger droplets. One interesting result of the work is the prediction of abrupt turning-point extinction behavior, which is qualitatively different than that observed with a gaseous suppression agent, such as water vapor alone. The mist cloud can be a reasonably strong absorber of thermal radiation, especially in the wavelengths associated with water bands. The radiation can serve to preheat and pre-evaporate the droplets upstream of the flame itself, altering the flame structure and extinction. The mist also acts as a radiation shield that attenuates radiation through its thickness. We have characterized the interaction of radiation wit h the mist and the flame in terms of droplet size and number density. The spectrally dependent emission of gas-phase radiation is modeled using exponential wide-band theory. The radiation-droplet interaction is modeled using Mie-scattering theory, wherein the absorption behavior of the water mist is incorporated considering the spectral complex refractive index for liquid water.