Creeping flame spread: Some new results and interpretation for material flammability characterization

Abstract

An important component for material flammability is the propensity of flames to spread in horizontal or downward orientation, a situation which is usually called creeping or lateral flame spread. Interpretation of test methods for creeping flame spread is difficult because it involves heat transfer at the tip of the flame, quenching of the flame, the formation of a triple-premixed-diffusion flame, as well as solid pyrolysis kinetics. Separation of these effects theoretically, numerically or experimentally is required for deducing relevant flammability properties. New results presented in this paper on this subject are related to a modification of the initial formulation of the creeping flame spread problem (deRis). In that work and subsequent analysis by other people, it was assumed that the flame leading edge (for infinitely fast chemical kinetics) coincides with the location of the beginning of pyrolysis, i.e., the separation distance between them l = 0. This assumption led to a well defined problem and exact solutions for flame spread on thick and thin materials. Inspired by a recent paper, we have found that the separation distance, l, between the flame leading edge and the beginning of pyrolysis can be arbitrarily set. Then, the creeping flame spread rates for both thin and thick materials have been found (by using the Wiener-Hopf decomposition method) to depend on this separation distance. It is also discussed how this separation distance could be related to the quenching distance of a triple flame near the location of the onset of pyrolysis. For a thermally thick solid, a new analytical solution for creeping flame spread is derived which, moreover, also accounts for the exact nonlinear mass transfer relationship in pyrolyzing region. It is briefly described how these results can be used to obtain relevant flammability parameters for creeping flame spread by comparison with experiments.