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Characterization and Identification of Super-Effective Thermal Fire Extinguishing Agents. First Annual Report.


pdf icon 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.