Inhibiting effect of brominated compounds on oxidation reactions

Abstract

Halon 1301 (CF3Br) is a well-known flame inhibitor, the understanding of which can guide the search of novel compounds with similar capabilities. Despite its importance as a benchmark for new fire suppressants, there are comparatively few detailed data for CF3Br that can be used to fine tune its chemical kinetics or to use as a baseline for candidate suppressants, particularly when interacting with hydrocarbon fuels. This paper presents new experimental data on ignition delay times and laminar flame speeds for mixtures containing CH4, C2H6, or C3H8 over a range of equivalence ratios with various amounts of CF3Br added to the mixture. Reflected-shock experiments were performed with fuel–O2 mixtures highly diluted in Argon (98%) for temperatures between 1250 and 2250 K at an average pressure of 1.4 atm and equivalence ratios of 0.5, 1.0, and 2.0; Halon 1301 was added at levels up to about 10% of the fuel for each hydrocarbon studied. Data obtained from the shock-tube experiments included ignition delay times and OH* time histories. Laminar flame speeds were measured using a windowed, constant-volume bomb at initial pressures of 1 atm for fuel–air mixtures at equivalence ratios between 0.7 and 1.3; CF3Br was added at levels of 0.5% and 1.0% to each mixture. A chemical kinetics mechanism was used herein to compare with the data, assembled from a recent, well-studied C0–C5 hydrocarbon mechanism from NUI Galway and a recent CF3Br mechanism. The model is very accurate with regard to the methane ignition behavior (with CF3Br actually accelerating ignition). However, for ethane and propane, although the model correctly predicted that the halon should slow down ignition, the level of inhibition was drastically under-predicted for the entire range of conditions studied. For the OH* time histories, the correct trends with respect to the addition of CF3Br were predicted, but major improvements can be made for the relative effect of the suppressant. With regard to the laminar flame speed results, the model also correctly predicted the basic trends of lowering flame speed with increasing levels of CF3Br addition, but it rather poorly predicted the magnitude of the impact for all three fuels. Using the assembled mechanism, the relative thermal and chemical flame inhibition contributions of CF3Br were estimated for each fuel, with the experimental chemical effect ranging from 72% to 87%, being much larger than the thermal effect (from 13% to 28%). Finally, ignition delay time and flame speed sensitivity analyses were conducted to identify the rate-controlling reactions for the systems analyzed. Given the importance of CF3Br as a model fire suppressant, and based upon the results of this study, further improvements in its kinetics mechanism are warranted.