The Influence of Particle Size in Influencing Smoldering Behavior Through Porous Woody Fuel Beds

Abstract

ABSTRACT Understanding how the size of fuel particles influences the initiation and spread of smoldering fronts is an important consideration in the spread of wildfires. Natural fuels often consist of layers of organic materials of varying sizes, yet the influence of the size of the particle on the burning behavior is not well understood. Changes in the size of particles can influence how readily oxygen transports to the reaction zone, how particles heat and burn, and how much heat transfers between particles. With this background, the objective of this study was to identify sensitivities of smoldering behavior to the size of particles and elucidate changes in heat transfer and oxygen transport in causing observed sensitivities. To achieve this objective, experiments were conducted where fuel beds of granulated Douglas-fir wood pellets of varying particle sizes were put in contact with an ignition source. The propensity to sustain horizontal and downward smoldering combustion and the resulting propagation rates and temperatures were quantified. Tests of homogenous and mixed particle sizes were investigated. In general, particle sizes with characteristic diameters (d) > 1.27 mm did not transition to self-sustained smoldering, while particle sizes with d < 1.27 mm transitioned to self-sustained smoldering, irrespective of the ignition temperature. Increasing the size of particles reduced the spread rate and surface temperature, despite having lower overall densities. Larger particles could sustain smoldering if sufficient concentrations of smaller particles were present in the pores. Fuel beds with increased porosity and permeability (i.e., larger particles) had reduced propensity to sustain smoldering showing that how readily oxygen can be transported to the reaction zone is not a limiting condition for particles of this size when burns are near the surface. A simplified heat transfer analysis was conducted to consider the contributions of conduction, convection, and radiation in transferring heat between particles. Increasing particle size decreases the heat transfer rate between particles as a result of greater heat loss due to convection. Additionally, smaller particles have similar Biot numbers resulting in increased heating rate, ignitability and spread rates. The results from this work provide insights into the important influences that the size of the fuel particles can play in smoldering near surfaces (i.e., up to 8 cm evaluated).