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Finger-Like Smoldering Over Thin Cellulosic Sheets in Microgravity.


pdf icon Finger-Like Smoldering Over Thin Cellulosic Sheets in Microgravity. (2996 K)
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.

Keywords:

combustion; microgravity; cellulose; stability; smoldering; lamps

Abstract:

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.