The deceleration mechanism and the critical extinction angle of downward flame spread over inclined cellulosic solids

To investigate the behaviors of flame spread over inclined surfaces, a series of flame spread experiments over whitewood with various inclinations were carried out. The development characteristics of the pyrolysis front with time are discussed, and four different states of flame spreading were observed under different ranges of surface inclination, including the accelerating flame spread region (θ ≥ 10°), steady flame spread region (−20° ≤ θ < 10°), decelerating flame spread region (−45° < θ < −20°) and extinction region (θ ≤ −45°). Among them, the decelerating state is a special flame spread behavior, and has never been reported in previous researches. Subsequently, the deceleration mechanisms of downward flame spread were explored in this study through a balance analysis of the heat and mass transfer between the solid pyrolysis and the gas combustion. An empirical formula of the critical extinction angle was established to predict the occurrence of extinction.