Wind-aided flame spread under oblique forced flow

Abstract

he wind-aided flame spread process along a solid fuel rod under oblique forced flow is analyzed in absence of gravity or when the forced flow dominates the gravity-induced flow. The transverse velocity is large enough to ensure that mixing of the fuel vapors and air occurs in a thin boundary layer surrounding the fuel rod and we can use the boundary layer approximation to describe the gas-phase chemical reaction and downwind flame spread process. A global, second-order, Arrhenius expression is employed to describe the gas-phase reaction, while the solid surface gasification reaction is modeled in terms of a constant pyrolysis temperature. The solid is heated by the hot gases convected from the flame by the axial component of the velocity in the direction of the flame spread. The solid will be considered thermally thick, assuming the thickness of the heated layer in the solid to be small compared with the rod radius. The analysis determines the flame spread velocity and the flow structure in the flame front region. The analysis also shows that flame spread is not possible at large flow velocities due to finite rate effects, while at low velocities the gas-phase reaction is diffusion-controlled. By including radiation losses from the surface a flame spread limit, at low velocities, is also found in the present analysis. The wind-aided flame spread process along a solid fuel rod under oblique forced flow is analyzed in absence of gravity or when the forced flow dominates the gravity-induced flow. The transverse velocity is large enough to ensure that mixing of the fuel vapors and air occurs in a thin boundary layer surrounding the fuel rod and we can use the boundary layer approximation to describe the gas-phase chemical reaction and downwind flame spread process. A global, second-order, Arrhenius expression is employed to describe the gas-phase reaction, while the solid surface gasification reaction is modeled in terms of a constant pyrolysis temperature. The solid is heated by the hot gases convected from the flame by the axial component of the velocity in the direction of the flame spread. The solid will be considered thermally thick, assuming the thickness of the heated layer in the solid to be small compared with the rod radius. The analysis determines the flame spread velocity and the flow structure in the flame front region. The analysis also shows that flame spread is not possible at large flow velocities due to finite rate effects, while at low velocities the gas-phase reaction is diffusion-controlled. By including radiation losses from the surface a flame spread limit, at low velocities, is also found in the present analysis.