A global model for the combustion of gas mixtures released from forest fuels

Abstract

In the most detailed models of forest fires, the combustible part of the devolatilization products are generally considered to be carbon monoxide burning in air. The aim of this study is to improve the combustion mechanisms used in these models by providing a simple oxidation scheme in the gas phase. To proceed, the degradation gases released by three Mediterranean forest fuels were determined. They mainly consist of CO 2, CO, CH 4 and H 2O. Then, an experimental device was built to produce laminar, axisymmetric, time-varying and non-premixed flames from these vegetative fuels previously crushed. The collected data (temperature, flame radius and mass flow rate) were used to test different models of combustion. Using numerical methods, the transient equations for the conservation of mass, momentum, energy and chemical species were solved for the flame as well as the radiative transfer equation. Three combustion mechanisms were tested: a skeletal mechanism, a global mechanism currently used in detailed models that incorporates only carbon monoxide and a global mechanism with two reactions including methane and carbon monoxide. The model containing only carbon monoxide underestimates significantly the temperature in the fire plume whereas the results obtained with the skeletal and the global mechanisms including both methane and carbon monoxide are in good agreement with the experimental data. This underlines the necessity of including methane to model the gas phase combustion in forest fires.