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Developing Extinction Criteria for Fires.


pdf icon Developing Extinction Criteria for Fires. (259 K)
Williamson, J. W.; Marshall, A. W.; Trouve, A.

Volume 2;

Interflam 2007. (Interflam '07). International Interflam Conference, 11th Proceedings. Volume 2. September 3-5, 2007, London, England, 849-860 pp, 2007.

Sponsor:

National Institute of Standards and Technology, Gaithersburg, MD

Keywords:

extinction; scalar dissipation; time scale; experiments; vitation; laminar flames; flame temperature; compartment fires; equations

Abstract:

The scalar dissipation rate is widely used in combustion analysis to describe a characteristic transport time scale. This time scale is often used with Damkohler number arguments (describing the ratio of the mixing time to the chemical time) to determine if kinematic conditions within reacting flows will cause extinction. However, for extinction analysis in fires, transport time scales are assumed to be relatively long and flow effects are generally ignored. In the current study, transport time scales in fires and their associated scalar dissipation rates are explored analytically and computationally to determine if extinction events should be expected in fires and under what conditions they may occur. Particular attention is given to the compartment fire scenario where air vitiation effects will weaken flames and increase the probability of extinction. A model is presented which uses reactant composition and temperature in the vicinity of the flame to determine a modified (for vitiation) critical scalar dissipation rate for extinction. This model is based on vitiated laminar flame experiments and OPPDIF 1-D flame analysis conducted over a wide range of thermal, composition, and flow conditions. The experiments are performed with a novel counterflow slot burner producing flame sheets approaching extinction. The model is used to produce extinction maps for comparison with FDS to evaluate the validity of critical flame temperature models and the benefits of more general scalar dissipation rate based models for fire applications.