TED-AJ03-234 EFFECTS OF VELOCITY BOUNDARY LAYER ON OPPOSED FLAME SPREAD OVER A THERMALLY THIN SOLID FUEL IN MICROGRAVITY

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Upstream flame spread over a thermally thin cellulosic sheet in various imposed wind speed and ambient oxygen concentration under microgravity environment is studied numerically. Two-dimensional time-dependent phenomena, from spontaneous ignitionto the flame spread in a fully-developed velocity boundary layer formed over a flat cellulosic sheet, are simulated with a well-developed numerical model (Figure A-1). The model predicts that the upstream flame spread in the boundary layer is essentially time-dependent phenomena, and the unsteadiness is pronounced when the imposed wind speed is very either low or high (i.e. near the extinction limits) with lower oxygen concentration. This time-dependent flame spread behavior is due to the boundary [figure]layer effect; when the flame spreads to upstream (i.e. thin boundary layer zone), local flow velocity at the flame front is not constant but increased as the time elapsed (see Figure A-2). The local flow speed has excellent correlation with the local flame spread is caused by the boundary layer effect. Plots of the local flame temperature vs. the local flame spread rate show linear relationship for all conditions considered in the present study, indicating that classical DeRis's theory still be valid for the present unsteady (quasi-steady)flame spread mode instantaneously.[figure]
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