A Critical Discussion of Theories of Flame Spread across Solid and Liquid Fuels

Abstract

Mathematical descriptions of flames spreading over liquid and solid fuels are obtained, using basic assumptions derived from observations or physical reasoning. A review of existing theories shows that they are incomplete in that they either treat an uncoupled problem of the condensed phase where the spreading rate and heat flux at the surface are given, or they merely determine the spreading rate in terms of a new vaguely defined eigenvalue. An important difference between the liquid and solid cases, due to convection, is pointed out, and it is shown that solid-fuel flame-spread theories which claim to apply to the liquid case, in reality do not apply to it. A mathematical formulation and a method of solution are presented for the phenomenon of flame spread over solid fuels with forward heat conduction in both the solid and the gas. The method uses an energy integral over the field to determine the spreading rate in terms of the basic properties of the fuel and air.