The Vortical Structure of Flame Spreading over Liquid Fuels

Abstract

Understanding flame spreading is a matter of both fundamental interest and crucial practical importance, mainly for its relevance to safety issues, as it is the base for the knowledge and ultimate control of fire propagation/suppression mechanisms. The fuel vapor surrounding a condensed fuel in an oxidizing atmosphere can support the propagation of a flame over the fuel surface. If the fuel temperature, and therefore the fuel vapor pressure, is high enough, then propagation occurs in the premixed (triple) flame regime [1] through the fuel vapor-air mixture. If the fuel temperature is decreased down to values that make this combustion regime impossible, due to heat loses towards the fuel and/or to adverse kinetics conditions, then the chemical reaction can still proceed, provided the chemical heat release is high enough to vaporize such an amount of fuel as to increase locally the Damkholer number above its extinction value. In this case, flame spreading relies on relatively slow mechanisms of heat and mass transfer across the fuel-gas interface, which render spreading slower than it was at high temperatures. When the fuel is a liquid, these mechanisms include convection, induced by thermocapillarity and/or buoyancy, which leads to characteristic propagation regimes [2] absent for solid fuels; see [3] for a review. Thermocapillary convection around a fluid-fluid interface occurs when temperature differences along the interface establish an imbalance of surface forces by inducing changes in the surface tension