The effects of fire-extinguishing agent C2HF5 (pentafluoroethane, HFC-125) added to a coflowing airstream on the structure and extinguishing processes of microgravity cup-burner flames have been studied numerically. Propane and a propane-ethanol-water fuel mixture, prescribed for a Federal Aviation Administration (FAA) aerosol can explosion simulator test, were used as the fuel. The time-dependent, two-dimensional numerical code, which includes a detailed kinetic model (177 species and 2986 reactions), diffusive transport, and a gray-gas radiation model, revealed unique flame structure and predicted the minimum extinguishing concentration of agent. The peak reactivity spot (i.e., reaction kernel) at the flame base stabilized a trailing flame. The calculated flame temperature along the trailing flame decreased downstream due to radiative cooling, causing local extinction at <1250 K and flame tip opening. As the mole fraction of C2HF5 in the coflow (XC2HF5) was increased gradually: (1) the flame base stabilized increasingly higher above the burner rim (XC2HF5 > 0.08), parallel to the axis until finally blowoff-type extinguishment occurred; (2) the reaction kernel weakened (i.e., lower heat release rate) but nonetheless formed at higher temperature; (3) the calculated maximum flame temperature remained at nearly constant ( 1700 K); and (4) the total heat release of the entire flame increased (i.e., combustion enhancement). In the lifted flame base, H2O formed from hydrocarbon-O2 combustion was converted further to HF and CF2O through exothermic reactions, thus resulting in a heat-release rate peak. In the trailing flame, two-zone flame structure developed: CO2 and CF2O were formed primarily in the inner and outer zones, respectively, while HF was formed in both zones. As a result, the combustion enhancement due to the C2HF5 addition occurred primarily in the trailing flame.
Citation: Proceedings of the Combustion Institute
Pub Type: Journals
Aircraft cargo-bay fire suppression, Halon replacement, Diffusion flame stabilization, Reaction kernel, Microgravity combustion