A Theoretical Model to Understand Some Aspects of Firebrand Pile Burning

Firebrand piles are known to ignite combustible infrastructure resulting in significant damage; however, the parameters that impact the heat transfer from firebrand piles to a combustible surface are not well understood. Heat transfer from a firebrand pile is directly related to the local firebrand temperatures which can vary significantly due to changes in the burning behavior. A two-phase flow analytical model was developed that includes time varying firebrand diameter, reradiation effects between firebrands, gas temperature and species evolution, and pile porosity effects. The analytical model was used to quantify the temperature of cylindrical firebrands and explore the effects of changing firebrand diameter, firebrand aspect ratio (pile porosity), gas velocity, and local oxygen mass fraction. All of these parameters were found to impact firebrand temperatures. Firebrand pile porosity has a significant impact on the velocity within the pile, with a decrease in pile porosity resulting in lower velocities and lower firebrand temperatures. The modeling results were used to explain the trends in heat transfer measured for firebrand piles on a horizontal plate.