Fire-suppression characteristics of CF3H in a cup burner

A numerical investigation is performed to understand the inhibition characteristics of CF₃H in a periodically oscillating methane-air jet diffusion flame formed over a cup burner. A detailed chemical kinetic mechanism having 82 species and 1510 elementary-reaction steps is used. Calculations made without adding agent yielded an oscillating flame with a flicker frequency of 11 Hz, which compared well with that obtained in the experiment. The minimum concentration of agent required for extinguishing the cup-burner flame is determined by adding CF₃H to the air stream and by increasing its volume fraction gradually until the flame is completely extinguished. Addition of CF₃H at volume fractions up to 10.05% did not affect the cup-burner flame temperature significantly. Extinction of a cup-burner flame took place as the base of the flame became destabilized, and the unstable flame base moved downstream in search of a new stabilization location. The predicted minimum concentrations of CF₃H for extinguishing the flame obtained by (1) replacing the air with CF₃H and (2) replacing the N₂ in the air with CF₃H are 10.1 and 19.2%, respectively. These concentrations compare favorably with the corresponding measured values of 11.7 and 20.3%, respectively. For validation, calculations are also made for the steady counterflow diffusion flames with different concentrations of CF₃H in the air stream and the predicted volume fractions of agent at extinction are in good agreement with the experimental values published in the literature. Examination of the reaction rates for the cup-burner flames indicates that the reactions with fluorinated species reduce the concentration of chain-carrying radicals in the flame. The effect is stronger at the flame base than further up in the trailing part of the flame, leading to destabilization at the flame base prior to extinction in the trailing region, and yielding the observed blowoff-type extinction.