Experimental Analysis of Critical Acceleration Condition for Two-Sided Upward Flame Spread Over Inclined Thin Fuel Surfaces

Abstract

Most of previous work focused on the one-sided upward flame spread over inclined surfaces. However, few investigations have systematically addressed the dependence of spread rate on the inclination angle for two-sided upward flame spreading. The present paper investigates the two-sided upward flame behaviors over inclined surfaces by performing experiments using 0.255 mm thick, 100 cm tall and 5 cm wide cotton sample sheets with various inclination angles varying 0° to 90° from the horizontal. The pyrolysis spread rate, pyrolysis length, preheating length, ignition time, flame tilt angle and standoff distance are obtained and analyzed. The corresponding results are as follows: As the inclination angle increases, the pyrolysis spread rate, pyrolysis length and preheating length increase, but the ignition time decreases. One transition zone is observed around 10° to 15° for flame spread rate, pyrolysis length and preheating length, which is an external manifestation of the change of flame spread from steady state to acceleration. Two parameters of tilt angle and standoff distance are used to qualitatively modify the heat flux profiles ahead of the flame front, which control the flame spread rate. Generally, the tilt angle and standoff distance of upper flame decrease as a function of inclination angle. On the contrary, the standoff distance shows an opposite trend with inclination angle. The combined effects of radiation and convection of upper and lower flames result in a sharp increase in net heat flux, and correspondingly a transition zone occurs around 10° to 15°. The results of this study have implications concerning designs for fire safety and may help advance understanding of two-sided flame spread over inclined surfaces.