Analysis of firebrand release on the spot fire mechanism

Abstract

Eucalyptus trees are among the most important sources of firebrands with potential to produce spot fires which are one of the most relevant manifestations of extreme fire behaviour. The mechanism of spot fires has several sequential phases starting in the release of a burning ember which is lofted and transported by the airflow, to land in a fuel bed causing a new ignition. In the present study, dedicated to the phase of firebrand release, an analysis of the firebrands released in a process of burning of eucalyptus trees is carried out. Barks of eucalyptus trees have a great potential to produce firebrands during a forest fire. Sometimes these fuels are lay down on the ground or are attached to the trunk and to large branches of the canopy. In this study, the number and size distribution of firebrands released during the burning of these fuels for different scenarios are analysed. Three tests were made varying the location of the barks (suspended and lay down on a fuel bed) and the orientation of suspended barks (vertical and horizontal). An additional similar test was performed with a fuel bed composed of shrubs. A particle image velocimetry system available in the Forest Fire Research Laboratory of ADAI was used to analyse the release of firebrands. Additionally, the convective up flow velocity and the temperature 2m above the tree, as well as the weight loss decay were measured. The final results among the several parameters controlled are compared.