Lateral flame spread over thermally thin Fuels: Theoretical prediction of spread rate and experimental validation with PMMA and cellulosic fuels

Abstract

Flame spread over solid fuel is a fundamental problem in fire safety science. Although it has been studied in different orientations – downward, upward, horizontal, and lateral– closed-form formulas for predicting the flame spread rate were first proposed for the opposed-flow configuration and experimentally verified in downward configuration, a special case of opposed-flow flame spread. In this work we present a simplified theory to predict the flame spread rate when a vertical flame spreads sideways (laterally) on both sides (symmetrically) of a thermally thin sample held horizontally in a quiescent normal-gravity environment. The ratio of flame spread rates between the lateral and downward orientations are shown to depend only on the flame coefficient, a non-dimensional known temperature ratio, and the Prandtl number. Experiments are performed in both downward and lateral configurations with a series of PMMA (Poly-methyl metacrylate) samples with fuel thickness ranging from 400 through 1500 μm and width from 10 through 60 mm as parameters. Similar experiments are also performed with ashless filter paper. The spread rate ratio between the two configurations is compared to the theoretical prediction with reasonable agreement. As predicted by the theory, the spread rate ratio is found to be independent of fuel thickness and fuel width for laminar flame spread.