Extinction and Stabilization of a Diffusion Flame on a Flat Combustible Surface with Emphasis on Thermal Controlling Mechanisms

Abstract

The process of extinction and stabilization of a diffusion flame on a flat combustible surface in a mixed convective, oxidizing, gas flow parallel to the fuel surface is studied theoretically in detail and in a more simplified approach experimentally. The Navier-Stokes elliptic set of conservation equations, including finite rate kinetics, is solved numerically to analyse the structure of the flame near its leading edge and to calculate the extinction or stabilization distance as a function of the velocity and oxygen concentration of the gas flow. A global one-step chemical reaction is used to describe the chemical kinetic mechanisms. The analysis, therefore, emphasizes the thermo-physical mechanisms controlling the extinction process. The calculations are performed for a representative solid combustible (PMMA) and liquid fuel (heptane). The experiments are performed in a small scale combustion tunnel. The extinction distance for a Heptane flame, stabilized on a porous, vertical surface, is measured for several conditions of flow velocity and oxygen concentration. The experimental results agree qualitatively with the theoretical predictions, verifying the concept that the flame stabilizes itself at a position on the fuel surface where the flow and chemical times are of the same order of magnitude. The flame has a premixed character at its leading edge and a diffusional one downstream.