Effects of Finite Sample Width on Transition and Flame Spread in Microgravity.
Effects of Finite Sample Width on Transition and Flame
Spread in Microgravity.
Mell, W. E.; Kashiwagi, T.
Combustion Institute, Symposium (International) on
Combustion, 28th. Proceedings. Volume 2. July
20-August 4, 2000, Edinburgh, Scotland, Combustion
Institute, Pittsburgh, PA, Candel, S.; Driscoll, J. F.;
Burgess, A. R.; Gore, J. P., Editor(s)(s), 2785-2792 pp,
combustion; microgravity; flame spread; numerical
models; flame behavior; ignition
In most microgravity studies of flame spread is assumed
to be two-dimensional and two-dimensional models are
used to aid data interpretation. However, since limited
space is available in microgravity facilities the flames
are limited in size. It is important, therefore, to
investigate the significance of three-dimensional
effects. Three-dimensional and two-dimensional
simulations of ignition and subsequent transition to
flame spread were performed on a thermally-thin
cellulosic sample. Ignition occurred by applying a
radiant flux in a strip across the center of the sample.
The sample was bounded by an inert sample holder. Heat
loss effects at the interface of the sample and the
sample holder were tested by varying the
thermal-physical properties of the sample holder.
Simulations were also conducted with samples of
different widths and with different ambient wind speeds.
The width of the sample affected both the duration of
the flame transition period and the post-transition
flame spread rate. Finite width affects were most
significant when the ambient wind was relatively small.
In such environment, the velocity due to thermal
expansion reduced the net inflow of oxygen enough to
significantly affect flame behavior. Since the expansion
velocity depended on the flame size its effects on the
net inflow decreased with increasing ambient wind speed.
Thus, for a given sample width, with increasing ambient
wind speed both the transition and flame spread behavior
of the 3D flame tended to that of the 2D flame. Heat
losses to the sample holder were found to affect the
flame spread rate in the case of the narrowest sample
with the slowest ambient wind.