Influences of sub-atmospheric pressure on downward flame spread over typical insulation material with parallel glass curtain wall structure in underground buildings

Downward flame spread over insulation materials by restriction of parallel glass curtain wall is an important potential scenario in modern underground building fire accidents. Meanwhile, considering the continuous challenge of research efforts in low pressure fire behavior, experimental investigation was conducted for the individual and coupling effects of pressure and curtain wall spacing on flexible polyurethane (FPU) downward flame spreading characteristics. Three pressures (70/85/100 kPa) below or near atmospheric pressure, and six kinds of spacings (4.5/6.0/8.5/11.0/14.0 cm and non-curtain wall) were applied to give a phenomenological study on the complicated variations of pyrolysis front, melting and dripping process, burning rate, flame spread velocity, flame height and pulsation frequency. First, evolution of vertex angle in pyrolysis front was observed to be stable and insensitive to both ambient pressure and interlayer spacing. Also, three dripping formations of molten fuel were found, and enhanced by decreasing pressure and spacing. Second, comprehensive understandings of coupling effects on burning rate, spread velocity and various heat transfer components were performed and formulated with semi-quantitative correlations theoretically. Finally, nonmonotonic tendencies of spacing vs. flame height or pulsation frequency were interpreted based on classical fluid and fire dynamic theory. This work could be valuable for fire hazardous analysis of modern building under sub-atmospheric environment.