Publication date: Available online 19 May 2017
Source:Fire Safety Journal
Author(s): James P. White, Salman Verma, Elizabeth Keller, Ailing Hao, Arnaud Trouvé, André W. Marshall
The present study seeks to measure suppression effects in a canonical experimental configuration, featuring the exposure of a buoyant, turbulent, methane-fueled diffusion flame to a co-flowing oxidizer laden with a fine water mist (Sauter mean diameter of 6.6µm). The carefully designed facility features well-characterized inlet and boundary conditions suitable for accurate representation in numerical simulations. Suppression performance is monitored via combustion efficiency (ηcomb), measured using species-based calorimetry techniques. As the mass fraction of water mist in the oxidizer (Ywmox) is increased, visible flame emissions are reduced due to diminishing soot incandescence coupled with a reduction in visibility due to Mie scattering effects in the mist. Global flame extinction is observed at Ywmext=0.093, a condition corresponding to 44% less water mass in the oxidizer than required for extinction due to dilution effects alone, suggesting that significant latent cooling effects contribute to the observed extinction limit. Despite visually observed suppression effects, ηcomb≈1 across all tested Ywmox, decreasing abruptly only at the extinction limit. Numerical simulations of the experiments are conducted using FireFOAM, an open source large eddy simulation solver incorporating a flame extinction model based on a critical Damköhler number and a flame reignition treatment based on a critical flame temperature. Though numerical results show qualitative agreement with the observed extinction behavior, the simulated extinction limit of Ywmext=0.20 significantly exceeds that measured in the experiment, but is itself consistent with an extinction limit due only to dilution effects.
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