Effects of velocity boundary layer on opposed flame spread over a thermally thin solid fuel in microgravity

Abstract

Upstream (opposed) flame spread over a thermally thin cellulosic sheet in various imposed flow speed and ambient oxygen concentration under microgravity environment is studied numerically. Two-dimensional time-dependent phenomena, from spontaneous ignition to the flame spread in a fully-developed velocity boundary layer formed over a flat cellulosic sheet, are simulated with a well-developed numerical model. 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 flow speed is very slow/high and lower oxygen concentration. This unsteady flame spread behavior is due to the boundary layer effect; when the flame spreads to upstream (i.e. thin boundary layer zone), local flow speed at the flame front is not constant but increased. The local flow speed has excellent correlation with the local flame spread rate. Plots of the flame temperature vs. the local flame spread rate show their linear relationship for all conditions considered in the present study, indicating that classical deRis's theory still be valid for present unsteady (quasi-steady) flame spread mode.