CHEMICAL INHIBITION OF NONPREMIXED METHANE FLAMES BY CF₃Br

ABSTRACT

Experimental, numerical, and analytical studies are carried out to elucidate the mechanisms of inhibition of nonpremixed methane flames by bromotrifluoromethane (CF₃Br). Experiments are performed in the counterflow configuration. In this configuration the reactants are injected into a mixing layer from two ducts. A fuel stream made up of methane and nitrogen (N₂) is injected from one duct, and an oxidizer stream of oxygen and N₂ is injected from the other duct. The inhibitor CF₃Br is either added to the oxidizer stream or the fuel stream. A key objective of this study is to compare chemical inhibition of methane flames when CF₃Br is added to the oxidizer stream with that when this inhibitor is added to the fuel stream. Critical conditions of extinction are measured. The concentrations of reactants at the boundaries are so chosen that the values of stoichiometric mixture, ξ_st, and adiabatic temperature, T _st, are the same. Numerical calculations are performed using a detailed chemical-kinetic mechanism. A reduced chemical-kinetic mechanism made up of eight global steps is derived from this detailed mechanism. Rate-ratio asymptotic analysis is carried out using this reduced mechanism. Experimental data, numerical calculations, and results of asymptotic analysis show that CF₃Br is more effective in inhibiting methane flames when it is added to the oxidizer stream when compared with its effectiveness when it is added to the fuel stream. The elementary reaction H + Br₂ → HBr + Br plays a central role in the inhibition when CF₃Br is added to the oxidizer stream. This reaction, however, has negligible influence on critical conditions of extinction when CF₃Br is added to the fuel stream.

Keywords:

• inhibition
• rate-ratio asymptotic analysis
• laminar diffusion flames

Notes

Units are moles, cubic centimeters, seconds, kJoules, Kelvin.

Third body collision efficiencies are [M] = 6.5 [CH₄] + 1.5[CO₂] + 0.75[CO] + 0.4[N₂] + 6.5[H₂O] + 0.4[O₂] + 1.0 [Other].