Chemically-active fire suppressants may, due to their properties or the means by which they are added to flames, have strong inhibition effects in particular locations in a flame. To study the spatial effects of chemically-active inhibitors, numerical experiments are conducted in which the rates of reactions of model inhibitors are varied in spatial regions defined by temperature. The influence of three types of regions are investigated, those with the inhibitor: 1) active only within a narrow temperature band (off-on-off), 2) active below a cut-off temperature (on-off), and 3) active above a cut-off temperature (off-on). The effect of several localized chemical perturbations on the burning velocity are studied, including the variation of the H+O2=OH+O or the CO+OH=CO2+H reaction rate, and catalytic scavenging of radicals by an idealized perfect inhibitor or by CF3Br (halon 1301). The results indicate that the flame speed is reduced most when the perturbation location corresponds to the regions of maximum radical mole fraction or maximum chain-branching reaction rates. Each of the perturbations has a negligible effect below 1200 K. Calculations for CF3Br-inhibited flames indicate a temperature of maximum influence that is higher than previous suggestions for Br-based inhibitors. Calculations for flames with the H+O2 rate perturbed or with addition of the perfect inhibitor indicate that the important region for flame inhibition in lean, rich, and stoichiometric flames corresponds to the position of the peak H-atom mole fraction. The results of this work demonstrate that the burning velocity is sensitive to inhibition over a relatively small spatial region of the flame. Simulations with stepwise activation and deactivation of an inhibitor show that the effect of the inhibitor is small when the activation or deactivation temperature is below 1700 K.
Citation: Proceedings of the Combustion Institute
Pub Type: Journals
chemical inhibition, flame chemistry, halon replacements