Ignition and Subsequent Flame Spread Over a Thin Cellulosic Material.
Ignition and Subsequent Flame Spread Over a Thin
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.
cellulosic materials; ignition; flame spread; equations;
vapor phases; conservation
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.