NIST Time|NIST Home|About NIST|Contact NIST

HomeAll Years:AuthorKeywordTitle2005-2010:AuthorKeywordTitle

Ignition and Subsequent Flame Spread Over a Thin Cellulosic Material.


pdf icon Ignition and Subsequent Flame Spread Over a Thin Cellulosic Material. (632 K)
Nakabe, K.; Baum, H. R.; Kashiwagi, T.

Microgravity Combustion Workshop, 2nd International. Proceedings. National Aeronautics and Space Administration, NASA Lewis Research Center. Proceedings. September 15-17, 1992, Cleveland, OH, 167-179 pp, 1992.

Keywords:

cellulosic materials; ignition; flame spread; equations; vapor phases; conservation

Abstract:

Both ignition and ffame spread on solid fuels are processes that not only are of considerable scientific interest but that also have important fire safety applications. Both types of processes, ignition and flame spread, are complicated by strong coupling between chemical reactions and transport processes, not only in the gas phase but also in the condensed phase. In most previous studies, ignition and flame spread were studied separately with the result that there has been little understanding of the transition from ignition to fiame spread. In fire safety applications this transition is crucial to determine whether a fire will be limited to a localized, temporary bum or will transition into a growth mode with a potential to become a large fire. In order to understand this transition, the transient mechanisms of ignition and subsequent flame spread must be studied. However, there have been no definitive experimental or modeling studies, because of the complexity of the flow motion generated by buoyancy near the heated sample surface. One must solve the full Navier-Stokes equations over an extended region to represent accurately the highly unstable buoyant plume and entrainment of surrounding gas from far away. In order to avoid the complicated nature of the starting plume problem under normal gravity, previous detailed radiative ignition models were assumed to be one-imensional or were applied at a stagnation point. Thus, these models cannot be extended to include the transition to ffame spread. The mismatch between experimental and calculated geometries means that theories cannot be compared directly with experimental results in normal gravity. To overcome the above difficulty, theoretical results obtained without buoyancy can be directly compared with experimental data measured in a miaogravity environment. Thus, the objective of this study is to develop a theoretical model for ignition and the transition to flame spread and to make predictions using the thermal and chemical characteristics of a cellulosic material which are measured in normal gravity. The model should take advantage of the miaogravity environment as much as possible in the gas phase instead of modifying a conventional normal-gravity approach. A thermally-thin cellulosic sheet is eonsidered as the sample fuel, which might ignite and exhibit significant flame spread during test times available in NASA's drop towers or in the space shuttle, without requiring a pilot flame. This last situation eliminates many complicating parameters such as pilot flame location, temperature, and size.