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Fire Growth and Spread Models. (Model for Opposed Flow Flame Spread Over Charring Materials.) (Abstract/Presentation/Visuals)


pdf icon Fire Growth and Spread Models. (Model for Opposed Flow Flame Spread Over Charring Materials.) (Abstract/Presentation/Visuals) (252 K)
Atreya, A.; Baum, H. R.

NIST SP 998; May 2003.

Keywords:

fire growth; fire spread; pyrolysis; flame spread; charring; fire models; temperature

Abstract:

Opposed-flow flame spread over the surface of a semi-infinite solid has been a subject of much investigation in the past. Several elegant analytical flame spread models have also appeared in the literature. It is indeed fascinating to note how the authors of these and other previous studies reduced the complex flame spread problem with novel approximations to obtain analytical flame spread formulas. It is even more fascinating to see how well these formulas agree with the experimental measurements for materials for which they were designed. Previous studies, however, have focused primarily on the gas phase part of the problem and the solid was assumed to "vaporize" at a known fixed temperature. Yet, a large number of natural and synthetic polymers do not simply "vaporize" - instead, they undergo a complex thermal decomposition process which results in an insulating layer of char on the surface. This char layer thickens with time and gives rise to a two-phase moving boundary problem in the solid. Due to this difficulty, the understanding of flame spread over these solids is lacking (a noteworthy contribution toward developing a model of wind-aided flame spread over charring materials was made by Carrier, et. a. Also, to obtain an analytical solution, it was often necessary to simplify the gas-phase momentum and continuity equations by using a uniform Oseen velocity 'Ug' and the constant pressure condition. Thus, the objective of the present study is to extend the previous work on "vaporizing" solids to charring materials like wood and provide a more realistic description of the gas phase.