Developing Extinction Criteria for Fires.
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.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899