Finger-Like Smoldering Over Thin Cellulosic Sheets in Microgravity.
Finger-Like Smoldering Over Thin Cellulosic Sheets in
Olson, S. L.; Baum, H. R.; Kashiwagi, T.
Combustion Institute, Symposium (International) on
Combustion, 27th. Proceedings. Volume 2. August 2-7,
1998, Boulder, CO, Combustion Institute, Pittsburgh, PA,
2525-2533 pp, 1998.
combustion; microgravity; cellulose; stability;
Microgravity smolder spread over a thin cellulosic fuel
was studied with the Radiative Ignition and Transition
to Spread Investigation (RITSI) apparatus in the
Glovebox Facility on the STS-75 USMP-3 space shuttle
mission. Radiative smoldering ignition was initiated by
a focused beam from a tungsten/halogen lamp at the
center of the smolder-promoted filterr paper. The
external airflow velocity was varied from 0.5 cm/s to
6.5 cm/s. The ignition and subsequent smolder spread
events were recorded by a video camera, a 35-mm camera,
and six thermocouples (two in the gas phase and four in
the sample). Nonpiloted smoldering ignition of the paper
in microgravity by external thermal radiation was
demonstrated for the first time. Unlike the uniform
normal gravity smolder front, a complex, unexpected
finger-shaped char pattern was obsenred in microgravity.
The preferred direction of smolder finger propagation
was upstream into the fresh oridizer. Downstream smolder
was less viable and slower. Increasing external flow
velocity increased the number of localized smoldering
fronts, the number of the char fingers they left behind,
and the frequency of bifurcations from the fingers. An
analytical "oxygen shadow" model indicated that each
localized smolder front cast an oxygen shadow that
depleted the ambient oxygen in an egg-shaped region
around itself. These oxygen shadows are a plausable
explanation of the fingering smolder patterns observed
in the experiments.