Ignition of a spot smolder in a moist fuel bed by a firebrand

Abstract

Ember/firebrand spotting is an important mechanism by which wildland fires can rapidly spread. This process occurs when firebrands from a fire upwind land on a fuel downwind. These firebrands may initiate a flaming fire directly or a smolder fire, which may later transition to flaming in the fuel. The fuel moisture content (FMC) plays an important role in determining whether a firebrand will ignite the fuel. In this work, the effect of FMC on the smoldering ignition of coastal redwood sawdust by glowing firebrands of different sizes is studied experimentally. The results show that larger firebrands are capable of igniting sawdust with a higher FMC. The maximum FMC at which any test resulted in ignition was 40%. The results also showed that for the present experiments, single firebrands smaller than 3.17 mm in diameter were unable to initiate a smolder in a dry sawdust bed. The ignition boundary is predicted using an energy model which agrees qualitatively with a multivariate logistic regression of the experimental data using the same functional form.