| Abstract: |
The structure and suppression of laminar methane air co-flow diffusion flames formed on a cup burner have been studied experimentally and numerically using physically acting fire-extinguishing agents (CO2, N2, He, and Ar) in normal earth (1g) and zero gravity (0g). The computation uses a direct numerical simulation with detailed chemistry and radiative heat loss models. An initial observation of the flame without agent was also made at the NASA Glenn 2.2-Second Drop Tower. An agent was introduced into a low-speed coflowing oxidizing stream by graduallyreplacing the air until extinguishment occurred under a fixed minimal fuel velocity. The suppression of cup burner flames, which resemble real fires, occurred via a blowoff process (in which the flame base drifted downstream) rather than the global extinction phenomenon typical of counterflow diffusion flames. The computation revealed that the peak reactivity spot (the reaction kernel) formed in the flame base was responsible for attachment and blowoff of the trailing diffusion flame. The extinguishing agent volume fractions determined experimentally in 1g were CO2, 15.7 0.6 %; N2, 25.9 1.0 %; He, 26.7 1.1 %; and Ar, 37.3 1.5 %. The numerical simulation performed thus far predicted the extinguishing agent volume fractions as: CO2, 14.5 % (or 16.1 % with different kinetic parameters for a methyl-H atom reaction step) in 1g and CO2, 19.1 %; N2, ≈38 %; He, 30.7 %; and Ar, ≈49 % in 0g. The buoyancy-induced flame flickering in 1g and thermal and transport properties of the agents affected the flame extinguishment limits. |
