Investigation of Extinguishment by Thermal Agents Using Detailed Chemical Modeling of Opposed-Flow Diffusion Flames.
Investigation of Extinguishment by Thermal Agents Using
Detailed Chemical Modeling of Opposed-Flow Diffusion
Flames.
(1223 K)
Pitts, W. M.; Blevins, L. G.
Halon Options Technical Working Conference.
Proceedings. HOTWC 1999. April 27-29, 1999,
Albuquerque, NM, 145-156 pp, 1999.
Sponsor:
Deparment of Defense, Washington, DC
Order number: AD/A-379530
Keywords:
halon alternatives; diffusion flames; extinguishment;
combustion; diluent gases; diluents; fire extinguishing
agents; fire suppression; laminar flames; reaction
kinetics; temperature effects
Abstract:
The manufacture of the halons widely used in fire
extinguishing systems was banned in 1994 due to their
deleterious effect on stratospheric ozone. Since the
late 1980s there have been ongoing research efforts to
identify replacement agents having comparable
properties. This search has proven difficult and
continues today with a large directed effort known as
the Next Generation Fire Suppression Technology Program
(NGP). As part of the NGP, the National Institute of
Standards and Technology is investigating whether highly
effective thermal agents are feasible. Thermal agents
are defined as those that obtain their effectiveness
solely by heat extraction and dilution. Excluded from
investigation are species that directly or indirectly
disrupt the combustion chemistry such as halons, which
derive much of their effectiveness by the release of
bromine atoms that catalytically remove hydrogen atoms
in the flame zone. A great deal is known about the
effects of thermal agents on flames. The paper by
Sheinson et al. provides a good introduction. A number
of endothermic physical processes can extract heat from
a gaseous flame zone, thus lowering the temperature and
ultimately leading to flame extinguishment. These
include simple heating (i.e., heat capacity) of an
agent, phase changes such as vaporization of a liquid or
sublimation of a solid, endothermic molecular
decomposition (which is classified as a physical process
as long as the initial agent and its products do not
participate in the combustion chemistry), and simple
dilution, which can modify flame temperatures by
spreading the heat release over larger volumes and by
affecting three-body flame reactions. The flame
temperature is also expected to be a function of the
thermal diffusivity of an agent. During the past two
decades the understanding of the chemical kinetics
involved in combustion has reached the point where
realistic detailed mechanisms involving large number of
reactants and reactions can be written for simple
combustion systems, and mathematical techniques have
been developed for simultaneously solving the large
number of differential equations that result. While
still involving significant approximations, such
modeling has advanced to the point where it can be used
to gain useful insights into the behavior of practically
relevant flames. This paper describes the results of a
detailed chemical kinetic modeling investigation of
laminar opposed-flow methane/air diffusion flame
designed to provide an improved understanding of the
extinguishment of fires by thermal agents. A particular
focus was to test the hypothesis that the effectiveness
of a thermal agent depends on the location of heat
absorption relative to the flame zone. An internal
report has been prepared, which summarizes the kinetic
modeling in detail and also includes the results of an
extensive database search of potential thermal agents
and modeling results for the effectiveness of thermal
agents in cooling liquid surfaces.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899