Ignition/non-ignition phase transition: A new critical heat flux estimation method

Abstract

Flaming ignition properties of dead and live thin wildland porous fuels submitted to different incident heat flux intensities are examined experimentally using a cone calorimeter. Data are compared to analytical and numerical results of a model that uses an energy balance and includes energy and temperature ignition criteria. The model provides the linear trend of ignition time for large heat flux intensities, but fails to reproduce the behavior of data near threshold ignition, because it does not include explicitly volatiles emission at pyrolysis. A new method is proposed for the estimation of the critical heat flux for ignition of porous fuels, based on the ignition time behavior as a power-law that characterizes phase transitions theory. The critical heat flux for ignition thus estimated has been found small compared to literature data. The discrepancy is due to the probabilistic ignition behavior observed in the critical region and ignored by literature that uses deterministic methods for the estimation of the critical heat flux for ignition. The heterogeneities of fuel particles composition and arrangement could be the major causes of such a probabilistic aspect.