Effects of Pyrolysis Kinetics on Opposed-Flow Flame Spread Modeling

Abstract

ABSTRACT–Fuel pyrolysis is an important step in gas-phase flame spread over condensed fuels. First generation models treat pyrolysis as a vaporization phenomenon and assign a constant vaporization temperature, an experimentally determined unknown, to the pyrolyzing fuel. In second generation models,and Arrhenius expression is used to evaluate the surface mass flux. The kinetic parameters, i.e., the pre-exponential factor and the activation energy, replace the vaporization temperature as unknown constants. The values used for these parameters for a particular fuel vary over a wide range in the literature.Here a parametric study is presented to evaluate the importance ofthese two kinetic parameters in relation to flame spread over a thin cellulosic fuel. An increase in the pre-exponential factor or a decrease in the activation energy causes the vaporization temperature to decrease, which in turn affects the flame structure and the spread rate. However, the two kinetic parameters do not act independently, and sets of kinetic parameters are found to beequivalent for a given environmental condition. A dimensionless parameter, comprising the kinetic constants as well as several environmental variables, that is related to the vaporization temperature through a simple formula has been identified. This formula predicts the vaporization temperature for a given pair of kinetic constants and given environmental conditions. The effect of the pyrolysis kinetics on flame spread can be reproduced by a constant-vaporization-temperature model by using the developed correlation between the vaporization temperature and the kinetic constants.