Effect of solid surface curvature and wall heat loss on the downward flame spread along the edge of thin PMMA sheets

Flames propagating along the sample edges (edge flame) will transform two-dimensional flame spread into three-dimensional flame spread, causing a significant interference with results of material flammability test. A potential method to minimize above effect is utilizing an inert wall to inhibit the side surface of sample with a designed air gap distance. In this work, the downward flame spread along the sample edge with various air gap distances is systematically studied. Cast PMMA with thicknesses from 0.4 to 2.5 mm were used as samples. The general trend of flame spread rate (FSR) at edge with respect to air gap distance can be summarized into three regimes: when the air gap distance is less than 2 mm, the edge flame is quenched by wall heat loss, and the overall flame spread behavior is two-dimensional. Then, the edge flame appears after the air gap distance exceeds 2 mm, resulting in a sharp increase in FSR. This critical air gap distance for the onset of edge flame is less sensitive to the sample thickness. Finally, as the air gap distance continues to increase, the FSR gradually approaches the asymptotic value of the uninhibited sample. With the increase in air gap distance, thermally-thin assumption loses validity as a result of the reduction in residence time of solid phase (larger FSR). To interpret the general trend of FSR with respect to air gap distance, a modified flame spread model based on conventional thermal theory is developed. The effect of air gap distance on flame spread along sample edge is characterized by two correction factors, one of which describes solid surface curvature at sample edge, the other of which is the effective flame temperature considering the heat loss from the spreading flame to the metal wall. The new model well reproduces the experimental results, and further supports the significance of heat loss in the trend of flame spread rate. This work helps to understand the flame spread mechanism under effects of solid surface curvature and heat loss, while also providing scientific support for optimization of material flammability testing standards.