On the radiation extinction of opposed flame spread over curved solid surface in low flow velocity conditions

Abstract

The controlling mechanisms of the radiation extinction of the flame spreading over cylinders in low flow velocity conditions are investigated with an analytical approach. Attention is focused on the interaction among solid surface curvature, solid surface radiation, and gas-phase volumetric radiation heat loss in the flame spread process. The analysis uses a classical thermal model, which considers the heat balance on the unburned fuel surface near the flame front. A non-dimensional number that evaluates the effects of volumetric radiation heat loss and finite rate chemistry on the flame temperature is also proposed. It is found that solid surface radiation cannot be a dominant mechanism for the radiation extinction of a flame spreading over a thin cylinder. Furthermore, in the case of flame spread over a cylinder, flame spread rate significantly increases with decreasing opposed flow velocity due to curvature effect, thus the consideration of the relative gas flow velocity seen by the spreading flame becomes essential in low flow velocity conditions. However, when the volumetric radiation heat loss in the gas phase is taken into account in the thermal model, flame spread rate for a thin cylinder once increases with decreasing opposed flow velocity and then turns to decrease. As a result, the flame spread rate of a thin cylinder shows a peak value in low flow velocity conditions. This trend is in good agreement with previous microgravity experiments (Fujita et al., 2002 [1]). Moreover, the limit condition of flame spread appears as a bifurcation point where the stable and unstable solutions of flame spread rate coincide in the low flow velocity condition. The analytical results indicate that it is essential to consider the volumetric radiation heat loss in the gas phase to predict the flame spread over a curved surface in low flow velocity conditions.