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Suppression of Cup-Burner Flames.


pdf icon Suppression of Cup-Burner Flames. (1555 K)
Takahashi, F.; Linteris, G. T.; Katta, V. R.

Scale Modeling, 4th International Symposium (ISSM-IV). International Symposium on Scale Modeling (ISSM). Proceedings. September 17-19, 2003, Cleveland, OH, 45-54 pp, 2003.

Keywords:

fire suppression; flame stabilization; halon alternatives; diffusion flames; flame structures; flame extinction

Abstract:

The unsteady suppression process of a laminar methane-air co-flow diffusion flame formed on a cup burner has been studied experimentally and numerically in normal earth gravity. The computation uses a time-dependent direct numerical simulation with detailed chemistry. A fire extinguishing agent (CO2 or CF3H) was introduced into a coflowing oxidizer stream by gradually replacing the air (the standard method) or the nitrogen in the air (the constant oxygen concentration method). The agent concentration required for extinguishment was constant over a wide range of the oxidizer velocity, showing a so-called plateau region. The measured extinguishing volume fractions of the agents were: CO2 replacing air, (15.9 0.6)%; CF3H replacing air, (11.7 0.8)%; CO2 replacing N2, (40.2 2.0)%; and CF3H replacing N2, (20.3 1.5)%. The cup-burner flame without agent flickered at ~11 Hz or ~15 Hz, depending on the oxidizer velocity. The flame base sometimes oscillated at half the flickering frequency just before the flame blew off. The suppression of cup-burner flames occurred via a blowoff process (in which the flame base drifts downstream) rather than the global extinction phenomenon typical of counterflow diffusion flames. The numerical simulations predicted the suppression limits and the flickering frequency with good agreements with the experimental observations and, more importantly, revealed the detailed mechanisms of the flame-base oscillation and subsequent blowoff phenomena.