Study On Flame Spread Characteristics Of Rigid Polyurethane And Polystyrene Foams At A High Altitude

There are numerous historical relics and ancient buildings on the Tibetan plateau, and it is hard to carry out rescue operations when the fire occurs due to the low economic development level and vast territory, and it is more likely to cause a large number of casualties. Many fire accidents which occurred in civil buildings on the plateau can bring huge harms, additionally, fire accidents often happen in ancient buildings with high density of population such as ancient temples and palaces which can lead to immeasurable losses. Therefore, the progress of fire safety science and fire prevention technology on the plateau need more attentions. In recent years, as the requirement of development of architectural energy conservation technique, building insulation materials began to be used on the Tibetan plateau, but the flame spread models developed in normal pressure environment can not apply to the low oxygen concentration and low ambient pressure environment on the plateau, so it is of great importance to investigate flame spread mechanism of typical building insulation materials on the Tibetan plateau.Rigid polyurethane (PUR) foam and polystyrene (PS) foam (which mainly includes molded polystyrene, EPS and extruded polystyrene, XPS) are two kinds of most widely used building insulation materials in China, which both have excellent thermal insulating capabilities. However, according to China’s existing code of design on building fire protection and prevention, the lowest fire resistance rating of them is B2(combustible). Therefore, the PUR and PS foams constitute significant fire hazards in the applications due to the facts that they are easy to ignite, support rapid flame spread over their surfaces and generate large amounts of light-obscuring and toxic smoke. The two typical building thermal insulation materials are selected as experimental samples in this study. PUR foam is a kind of thermosetting and charring material, and PS foam is thermoplastic, and thermal behaviors of the two materials have great differences. In this article, a series of bench-scale flame spread experiments of PUR and PS foams have been carried out both on the Tibetan plateau (Lhasa, at the altitude of3658m) and in the Middle-Lower Yangtze Plain (Hefei, at the altitude of30m), and the key parameters such as flame length, flame height, flame spread rate, temperature distribution in the gaseous and solid phase and pyrolysis mass flow rate are obtained in the experimental process, then the influences of sample width, external radiation flux, sample inclination angle, atmospheric pressure and absolute oxygen concentration on both of the two insulation materials have been analyzed and discussed combining with theoretical knowledge such as combustion science, heat and mass transfer principle and fire dynamics theories and so on. Finally, the laws and effects of the above parameters on the two insulation materials are revealed and relevant models of flame spread are achieved.Firstly, the horizontal flame spread process of PUR foam with different sample widths, ambient pressure and absolute oxygen concentration is studied, then the influences of sample width and ambient pressure on flame height and flame spread rate of PUR foam are discussed, and the power exponent’s experimental value in the correlation of flame spread rate and ambient pressure is obtained by verification.Secondly, the influences of external radiation flux, ambient pressure and absolute oxygen concentration on heat transfer mechanism during flame spread process of PUR and EPS foams are studied in this paper, and the experimental results indicate that the negative one half power of the flame spread rate of PUR and EPS foams are both in negative linear relationships with external radiation intensity. The theoretical value of parameter C of PUR foam in the plain is also obtained by calculation. The heat transfer processes of horizontal flame spread of PUR and EPS foam are analyzed using the method of energy conservation in control volume, and two simplified expressions of horizontal flame spread rates of PUR and EPS foam under external radiation flux are established, respectively.Finally, the effects of sample inclination angle on both upward and downward flame spread processes of PUR and PS foams are analyzed in details. The fitting correlations between flame length and pyrolysis length, heat release rate per unit width are firstly obtained during the upward flame spread process of PUR foam with different orientations, and the flow conditions and exponents of fitting correlations of upward flame spread process of PUR foam can be classified by the value of modified Rayleigh number. An expression of critical modified Rayleigh number when flame spread cannot sustain is deduced from the expression of mass flow rate and critical flame extinction conditions. The rates of downward flame spread and steady mass flow of PUR foam both have good linear relations with the cube root of sine of sample inclination angle. At last, the influence mechanisms of sample surface collapse when heating and fluidity of molten mass on both upward and downward flame spread behaviors and characteristics of PS foam with different inclination angles are revealed.