Dynamic Characteristic Of Melting Layer In Flame Front During Downward Flame Spread Of XPS Foam

Melting layer is formed in the flame front region during the downward flame spread of the thermoplastic polymeric material(XPS).In addition,the melting liquid may drip and ignite the unburned fuel under certain conditions,which enlarge the fire rapidly.Therefore,the flame spread behaviours of thermoplastic material are much more complicated and hazardous than that of the ordinary solid fuel.In this paper,the formation mechanism,morphology and heat transfer of the melting layer during the downward flame spread of thermoplastic materials were investigated in detail.Additionally,the influences of accumulation and dripping of the melting liquid on the downward flame spread of thermoplastic materials were also analyzed.This article focuses on the following aspects:The melting and downward flame spread of XPS sheet were abstracted as"growth and downward movement of a narrow strip melting liquid pool fire at the top of solid fuel",and the relationship among melting liquid layers,flame spread rate and burning mass loss rate was established.The mass and the average thickness of the melting layer accumulated in the flame front during the downward flame spread of XPS sample were obtained,based on the experimental measurements of the flame front position and burning rates.The results showed that about 80%of the generated hot liquid fuel remained in the pool fire during a certain period,and most of the consumed solid XPS fuel didn't really burn away but transformed as the liquid fuel in the downward moving pool fire.In addition,the flame morphology and flame height were analyzed by statistical analysis of the image brightness.It showed that the relationship between flame height and mass loss rate satisfied the power function during the steady increasing of the molten liquid.Based on the image processing algorithm,the image of the flame front movement during the downward flame spread of XPS is obtained by stripping the flame front curve in the video image.The dynamic characteristics of the i'rregular flame front curve are studied.In addition,by longitudinally dividing the video image of downward flame spread,the influence of melting and dripping on the downward spread of thermoplastic materials was studied.The results showed that the lagged sub-flames might suddenly spread downward fast to catch up with their neighbors.Moreover,because of the dripping and accumulating of liquid fuel,the peaks of flame height and the flame spread rate increased with oscillating,and the local peaks of the flame spread velocity arrived later than the flame height.Additionally,the downward flame spread rate model with the inclined flame leading edge was established,which reveals that the local downward flame spread rate increases with the inclination angle ? of the local flame leading edge.The results also showed that the relationship between the non-dimensional local peak sub-flame flowing velocities and the corresponding non-dimensional peak sub-flame heights can be well described by a power function.Thermal structure of in the condensed phase during downward flame spread was experimentally measured with optimized thermocouple arrays.The temperature measured by the thermocouple was converted into a space-dependent temperature along the direction of flame propagation.The temperature field of the condensed phase,the thickness and the shape of the molten liquid of XPS samples were obtained.The results showed that the solid-liquid interface near the back wall was significantly higher than that near the outer surface,and the heat transfer in the condensed phase was more concentrated in the area near the outer surface.Additionally,the thicknesses of the melting layer for each thick case were less than 5mm near the surface and increased from the surface to the back wall.Furthermore,the molten liquid adhered to the back wall,which was much more obvious for the thicker samples.The reconstructed temperature field showed that the solid-liquid interface near the outer surface was flatter than that near the back wall,especially for the thicker samples.Consequently,molten liquid generated inside the sample may flow toward the flat area close to the outer surface,which caused the melting flow and rapid flame spread.