Characterization and Identification of Super-Effective Thermal Fire Extinguishing Agents. First Annual Report.
Characterization and Identification of Super-Effective
Thermal Fire Extinguishing Agents. First Annual Report.
(5691 K)
Pitts, W. M.; Yang, J. C.; Huber, M. L.; Blevins, L. G.
NISTIR 6414; 58 p. October 1999.
Available from:
National Technical Information Service
(NTIS), Technology Administration, U.S. Department of
Commerce, Springfield, VA 22161.
Telephone:
1-800-553-6847 or 703-605-6000;
Fax: 703-605-6900.
Website: http://www.ntis.gov
Order number: PB2000-100889
Keywords:
fire extinguishing agents; databases; diluents; diluent
gases; fire extinguishment; reaction kinetics; surface
cooling; temperature effects
Abstract:
The use of halons for fire fighting is being phased out
due to their deleterious effects on stratospheric ozone.
This report summarizes the first-year findings of a
three-year study designed to characterize and identify
super-effective thermal fire-fighting agents as possible
replacements for these effective compounds. Three
distinct aspects related to the effectiveness of
potential thermal agents have been considered. First,
existing thermodynamic databases maintained by NIST have
been searched in order to identify chemical compounds
which are predicted to extract large amounts of heat
from a combustion zone. Second, detailed chemical
kinetic modeling has been used to characterize the
effects of thermal agents on an idealized flame system,
namely, a methane/air counterflow diffusion flame.
Third, empirical heat transfer correlations for spray
cooling of a surface have been used to estimate the
efficiencies of surface cooling by thermal agents. The
database search used two primary sources--the Design
Iustitute for Physical Properties database containing
1458 compounds from 83 family types and a smaller
database, REFPROP, containing 43 compounds which is
tailored to refrigerant applications. Additional
substances were included which are not well represented
in these databases. Compounds having 1) high heats of
vaporization, 2) liquid-phase heat capacities, and 3)
total heat absorption due to phase changes (if
applicable), heating of a liquid (if applicable), and
the heating of the gas phase to combustion temperatures
were identified. The results are reported in tables of
compounds ordered in terms of their ability to extract
heat. Detailed chemical kinetic modeling of opposed
flow methane diffusion flames burning in air and air
diluted with thermal agents has been used to obtain
insights into the effectiveness of thermal agents and
their mechanisms of flame extinction. Values of fuel and
air velocities which induce flame extinction were
determined as a function of agent concentration.
Comparison of the calculated results for burning in two
types of oxidizer, air diluted with added nitrogen and a
synthetic "air" having nitrogen replaced by argon and
diluted with additional argon, with corresponding
experimental measurements of the concentrations
necessary to extinguish a counterflow diffusion flame
showed that extinguishment occurs when the maximum
calculated flame temperature drops to approximately 1550
K for fuel and oxidizer velocities of a few tens of
cm/s. Using this result, extinguishing calculations were
then estimated for carbon dioxide, argon, helium, and
water. Published experimental extinguishment
concentrations for these thermal agents are unavailable
for methane flames, but a strong correlation was found
with agent extinguishing concentrations determined in
cup burner tests using liquid heptane as fuel. A series
of calculations were performed for one of compounds
identified as likely to be particularly effective at
extracting heat during the database search,
methoxy-nonafluorobutane. An extinguishing concentration
of 5.5% was predicted, which is close to unpublished
experimental cup burner values of 6.1%. An advantage of
detailed kinetic modeling studies is that surrogate
agents having properties which are not physically
realizable can be used to investigate specific details
concerning extinguishment. A surrogate agent was
specified which reacted over different temperature
ranges to extract a predetermined amount of heat. The
calculations showed that the effectiveness of this agent
was independent of the location of beat extraction
relative to the flame zone. In a second series of
calculations a surrogate agent was used to isolate the
role of dilution on extinguishment. When the agent,
which was incapable of extracting heat, was added to the
air, much higher concentrations were required to
extinguish the flame than when heat was extmcmd. Details
of the calculations revealed that extinguishment
ultimately occurred due to oxygen passing through the
flame zone. Calculations of droplet evaporation times
using the classical d2-law for the five fluids (water,
lactic acid, C3F5H30, HFE7100, and R338mccq) identified
as having the highest latent heat of vaporization (per
unit mass) by the database searches were performed as
part of the surface cooling studies. Empirical heat
transfer correlations from the spray surface quenching
literature were used to assess the surface cooling
characteristics of these fluids for various heat
transfer regimes. Based on these calculations, water
and lactic acid appear to be more effective than the
other three fluids for surface cooling applications.
Recommendations are included for additional studies
during the second year of the project.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899