Creeping flame spread: Energy balance and application to practical materials

Abstract

Creeping flame spread has been extensively investigated analytically, experimentally, and numerically. Test method have also been developed for measuring material properties neededto predict creeping flame spread speeds. We offer, in this work, a new interpretation of the physics, we develop an energy balance equation that includes all the creeping flames spread physics, and we illustrate how this equation can be used to determine material properties in a way that eliminates several shortcomings of previous research and practice. An important term in the new energy balance equation is the energy input in the pyrolyzing region near the front owing to external radiatio (including flame radiation) and reradiation losses: lack of consideration of this term has led to misinterpretation of data in research projects and in standard test methods for evaluating creeping flame spread for practical materials. By using the new formulation of the enery eqution, two parameters can be used to characterize theoretically and experimentally creeping flame spread: one related to a thermal diffusion length associated with the flow and the other related to reaction dynamics and heat transfer near the pryolysis front. No assumptions or approximations for gaseous properties are needed to correlate and interpret experimental data. The proposed methodology allows consideration of finite material thickness, profile of opposed velocity, chemical kinetic effects, transient effects, and geometric effects (e.g., plane or cylindrical geometry), as well as various levels of external heat flux (including flame radiation) imposed on the pyrolyxing region near the flame front.