Stirred reactor calculations to understand unwanted combustion enhancement by potential halon replacements q Gregory T. Linteris a,⇑ , Donald R. Burgess b , Fumiaki Takahashi c , Viswanath R. Katta d , Harsha K. Chelliah e , Oliver Meier f a Fire Research Division, National Institute of Standards and Technology, Gaithersburg, MD 20899-8665, USA b Chemical and Biochemical Reference Data Division, National Institute of Standards and Technology, Gaithersburg, MD 20899-8320, USA c Case Western Reserve University, Cleveland, OH 44106, USA d Innovative Scientific Solutions, Inc., Dayton, OH 45440, USA e University of Virginia, Charlottesville, VA 22904, USA f The Boeing Company, Seattle, WA 98124, USA article info Article history: Received 4 March 2011 Received in revised form 20 September 2011 Accepted 22 September 2011 Available online 7 November 2011 Keywords: Fire suppression Flame inhibition CF 3 Br C 2 HF 5 Halon replacements Cargo-bay fire suppression abstract Several agents are under consideration to replace CF 3 Br for use in suppressing fires in aircraft cargo bays. In a Federal Aviation Administration performance test simulating the explosion of an aerosol can, how- ever, the replacements, when added at sub-inerting concentrations, have all been found to create higher pressure rise than with no agent, hence failing the test. Thermodynamic equilibrium calculations as well as perfectly-stirred reactor simulations with detailed reaction kinetics, are performed to understand the reasons for the unexpected enhanced combustion rather than suppression. The high pressure rise with added C 2 HF 5 or C 3 H 2 F 3 Br is shown to be dependent upon the amount of added agent, and can only occur if a large fraction of the available oxidizer in the chamber is consumed, corresponding to stoichiometric proportions of fuel, oxygen, and agent. Conversely, due to the unique stoichiometry of CF 3 Br, this agent is predicted to cause no increase in pressure, even in the absence of chemical inhibition. The stirred-reactor simulations predict that the inhibition effectiveness of CF 3 Br is highly dependent upon the mixing con- ditions of the reactants (which affects the local stoichiometry and hence the overall reaction rate). For C 2 HF 5 , however, the overall reaction rate was only weakly dependent upon stoichiometry, so the fuel– oxidizer mixing state has less effect on the suppression effectiveness. Published by Elsevier Inc. on behalf of The Combustion Institute. 1. Introduction Because of its destruction of stratospheric ozone, production of the effective fire suppressant CF 3 Br has been banned by the Mon- treal Protocol. Although a critical-use exemption has been granted to the aviation industry for use of recycled halon in cargo bay fire suppression, the European Union requires replacement of halon in new design aircraft by 2018, and in existing aircraft by 2040. Sev- eral replacements have been proposed, but they have all been found to produce enhanced burning in the FAA Simulated Aerosol Can test [1], and hence they fail FAA’s Minimum Performance Stan- dard [2]. In particular, C 2 HF 5 ,C 3 H 2 F 3 Br, and C 6 F 12 O all produce higher peak pressures in a simulated cargo bay as compared to no added agent, when they are added at concentrations less than that required to completely suppress the explosion. (Names and chemical formulas are listed in Table 1.) The agent CF 3 Br, at sub- suppressing concentrations, does not cause the overpressure. The Aerosol Can Test [1] simulates the situation in which a fire in an aircraft cargo bay container heats an aerosol can (e.g., hair spray) causing it to burst, creating an explosion. In the test, a heated container at about 16 bar, releases its contents (propane, ethanol, and water) as a two-phase impulsive spray via a fast- acting valve. A continuous DC arc across electrodes located about 1 m downstream of the valve ignites the mixture. The fireball expands into the chamber atmosphere of ambient air and water vapor and premixed suppressant, and the temperature and pressure in the chamber increases (over a time on the order of a second). During each test, instruments record the pressure, tem- perature, visual images, and concentrations of agent and oxygen. Unfortunately, when added at sub-inerting concentrations, the final pressure rise in the chamber with any of the halon replace- ment agents is higher than in the absence of agent. The agent C 2 HF 5 , added at a volume fraction of 13.5%, suppressed the explo- sion; however, when added at volume fractions of 6.2%, 8.9%, and 11.0%, the peak pressure rise was about 3.6 bar, or about twice that 0010-2180/$ - see front matter Published by Elsevier Inc. on behalf of The Combustion Institute. doi:10.1016/j.combustflame.2011.09.011 q Official contribution of NIST, not subject to copyright in the United States. ⇑ Corresponding author. Fax: +1 301 975 4052. E-mail address: linteris@nist.gov (G.T. Linteris). Combustion and Flame 159 (2012) 1016–1025 Contents lists available at SciVerse ScienceDirect Combustion and Flame journal homepage: www.elsevier.com/locate/combustflame