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Effects of Finite Sample Width on Transition and Flame Spread in Microgravity.


pdf icon Effects of Finite Sample Width on Transition and Flame Spread in Microgravity. (331 K)
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, 2000.

Keywords:

combustion; microgravity; flame spread; numerical models; flame behavior; ignition

Abstract:

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.