Radial Flame Spread Behavior Characteristics Over Typical Thermoplastics Materials XPS

As a thermal insulation material for exterior walls,thermoplastics materials are widely used in construction now,especially for extruded polystyrene(XPS).Because of its low density,low thermal conductivity and low price,XPS is an excellent thermal insulation material.However,because of its low ignition energy and high combustion heat,with the melting and dripping behaviors happening and toxic gases released in the combustion considered,it is consumed to be a highly dangerous material.For example,the CCTV's building fire was spreading quickly by XPS foam which eventually caused a huge loss in 2009.The previous research on thermoplastic materials was carried out with strips,which is different from the actual fire scene.Therefore,the square XPS foam was used to study the flame spread characteristics and laws under horizontal and inclined conditions in this paper,and the theoretical analysis was conducted.In the horizontal flame spread experiments,XPS foam with different thicknesses(1.1cm,1.6cm,2.4cm,3.4cm and 4.4cm)was used to analyze the flame morphology,flame spread rates,mass loss and melting liquid pool mass.The results show that after the central ignition of XPS foam,the flame front maintains a circle in the outward expansion,and the melting liquid pool was formed on the spread path,with its size positively correlated to the thickness of the XPS foam.The flame spread velocity was obtained by the image proceeding algorithm,and it was found that the thinner XPS foam has a decelerating flame spread,while the thicker XPS foam will have an accelerating one.Besides,based on the mass loss and the spread radius,the mass of the melting liquid pool can be calculated,and the percentage of the remaining liquid mass to the total melted liquid mass during the spread is finally maintained at about 75%.Finally,based on the partial differential equation about the conduction in solid-phase,a simplified model of the flame spread velocity was obtained,which shows that the spread velocity is not only controlled by the heat release of the melting liquid pool,but also affected by the flame spread radius.The model explains the difference in the trend of flame spread velocity at different thicknesses.When the XPS material is thin,the increase in the radius of the liquid pool is dominant for the weakening of heat transfer,so the flame spread will slow down.When the XPS foam sheet is thick,the increase in the size of the oil pool has a greater impact on the preheated zone,so an accelerating flame spread appears.In the inclined flame spread experiments,a series of experimental studies were conducted to research the development of the upward,downward and lateral flame spread of the material after the central ignition for the 1.6cm thick square thermoplastic XPS foam with different inclination angles.The results show that the downward flame spread can be continually sustained after the central ignition of XPS foam with different inclination angle.However,compared with traditional solid materials,the upward flame spread characteristics are significantly different.That is,when the inclination angle is less than 60 degrees,an upward flame spread can be formed and sustained,and the spread velocity increases as the inclination angle increases.But the upward flame spreads briefly after the ignition and then extinguishes quickly when the inclination angle is greater than 60 degrees.For the downward flame spread,the spread velocity decreases first and then increases with the increase of the inclination angle.The sustainability of upward flame spread under different inclination angle conditions depends on the competitive mechanism of shrinkage,melting and pyrolysis strength during heating in the flame front region.Finally,based on the internal temperature field measurement and analysis of the XPS foam flame spread,the results show that the flame spread is sustainable if the melting temperature is quickly reached after heating.On the contrary,it is easy to extinguish if the XPS foam shrinks significantly with a slow temperature rising.