Simulations of the unsteady response of biomass burning particles exposed to oscillatory heat flux conditions

Abstract

Understanding the mechanisms responsible for the ignition and pyrolysis of natural vegetation fuel is crucial in modeling the rate of spread of wildland fires. This study provides a fundamental understanding of the burning process of biomass fuel particles under oscillatory heat flux conditions; the oscillations are studied as a model to represent flapping flames or fluctuations in the irradiation above natural vegetation beds. We consider two separate cases corresponding to unsteady radiative or convective heating, with oscillations at different frequencies, and we consider charring (constant-volume) and non-charring (shrinking), elongated and square-shaped, solid fuel particles of 1 mm and 10 mm half-size. The results of the numerical simulations suggest that the particles experience a quasi-linear response in the case of fluctuating irradiation or fluctuating local gas temperature; this linear response means that the effects of oscillations can essentially be ignored. However, a non-linear response is obtained in the case of unsteady convective heating, when the local gas velocity fluctuates in phase with the local gas temperature; this non-linear response leads to a net effect of the oscillations in the form of an augmented heat transfer, higher temperatures, higher values of the fuel mass loss rate, and shorter burnout times.