Experimental study on thermal safety analysis of flexible polyurethane at various facade inclined structures under low ambient pressure condition

Experimental and theoretical investigations are conducted to analyze the influences of low atmospheric pressure on burning behavior of building facade insulation material flexible polyurethane (FPU). Comparison experiments for downward flame spread process over FPU under various facade inclined structures were performed at Hefei (99.8 kPa) and Lhasa (66.5 kPa), respectively. Characteristic parameters including burning rate (i.e. mass loss rate), flame spread velocity and flame height were studied in this paper. Firstly, morphological experimental results exhibited that the combustion was slower in reduced pressure condition and wider specimens showed more gradual flame spreading. Secondly, an inverted V-shaped spreading pyrolysis front was observed, and a “two-terminal” combustion behavior was found for narrow FPU board by partial molten flow combustion, which is critical to practical fire rescue techniques. Thirdly, power-law progressions of burning rate vs. pressure was proposed with an exponent ranging from 0.61 to 1.39, which can be illustrated by utilizing classical theory for sub-atmospheric pressure pool fire. Further, the increasing facade inclination angle significantly enhanced the flame spreading velocity, which suppressed the effect of pressure at high inclination angles. The flame temperature was measured to be increased at the lower pressure causing faster flame puffing. Finally, a linear relationship was determined between the pressure index values obtained by flame height vs. pressure and the inclination angle.