Response of a counterflow diffusion flame over a methanol pool to suppressant doping was investigated to understand inhibition processes. Experimentally, flame temperatures, locations, and fuel gasification rates were measured as a function of nitrogen concentration added to air, while flame-extinguishing concentrations of nitrogen, CHF_3, and C_3HF_7 were measured as a function of oxidizer velocity, with two different thermal conditions of the fuel pool. Computations were performed using a counterflow flame model with fuel gasification and heat gain/loss at the pool surface and with detailed chemistry and transport in the gas phase. Flame responses to nitrogen, CO_2, CHF_3, C_3HF_7, and CF_3Br addition were computed at constant oxidizer velocities. The model well predicts the measured flame-extinguishing concentrations. The maximum flame temperature was reduced monotonically with the addition of nitrogen in both the experiment and computation, while no remarkable reduction was computed with CHF_3, C_3HF_7, or CF_3Br doping, resulting in significant difference in the flame temperature at extinction. Nevertheless, the condition of the vaporizing pool was insensitive to the suppressant variation. The computation further shows that the addition of CHF_3 and C_3HF_7 promotes dominant chain branching reaction on the oxidizer side of the flame, while simultaneously inhibiting it in the fuel oxidation region. None of iocal flame parameters such as peak H, O, or OH concentration reflects universally the suppressant performance in the flame, while the total integrated amount of these flame radicals is reduced linearly over the entire range of the suppressant concentrations and shows excellent agreement at the extinction turning points. A concept of limit integrated amount of the reactive flame radicals is suggested. Difference between the suppressants in the sensitivity of the flame-extinguishing concentrations to the heat balance at the fuel pool is also demonstrated and interpreted based on the flame temperature effect on the suppressant performance.
展开▼
