Burnout

The parameterization for burnout is based on \citealt{tse1998flight} and \citealt{bhutia2010comparison}.
As the firebrand is transported by the ambient flow, its mass and volume (and consequently its diameter) decrease with increasing time due to pyrolysis, or the decomposition of material at high temperature. In the case here, woody material is pyrolyzed through both heterogeneous (glowing) combustion on the outer surface and homogeneous gas-phase oxidation of volatiles diffusing from within the firebrand \cite{albini1979}. The pyrolysis front, which marches inward toward the particle center as woody material is converted into char, closely marks the region where heterogeneous combustion occurs. This demarcation is used to track what is referred to as the effective mass diameter, \(d_{eff}\),  as described in \citealt{tse1998flight,bhutia2010comparison}. It is implemented as