Firebrand ignition : characterization of heat transfer mechanisms




Wessies, Savannah Storm

Journal Title

Journal ISSN

Volume Title



Every year, there are thousands of wildland fires instances. These fires burn vegetation and built structures. In many cases, the wildfires burn at the wildland-urban interface (WUI) where homes and other manmade structures can be found. One of the major ways manmade structures are ignited is through firebrand spotting downwind of the fire front. Ignition by firebrands is a complex process involving environmental factors, geometric considerations, and characteristics of the firebrands and substrates. Experiments were designed to investigate the ignition mechanisms and heat transfer processes involved in firebrand deposition to better understand the threat firebrand deposition poses to the built environment in the WUI. In experiments with attic insulation, the effects of airflow condition, substrate material, and firebrand configuration (single v. multiple) on ignition by firebrands were studied. These experiments revealed that increased airflow led to increased firebrand temperature and decreased time to ignition. In general, these experiments also showed that a relatively smaller pile of firebrands lead to quicker ignition times than a larger single firebrand for the same substrate material and flow conditions. In addition to the experiments, a heat and mass transfer model was developed, which matched well with experimental results. To better understand the heat transfer associated with a small, high temperature object deposited on a much larger, cool substrate, experiments were conducted with a variable flow air jet impinging on a high temperature cylinder cooling on a calcium silicate substrate. Using the experimental temperature data, experimentally determined heat transfer coefficients, and known radiative properties, the parameters for a conductive heat transfer model were deduced by optimization. Through this modeling process, the relative contribution of the different modes of heat transfer was determined. This understanding of how the heat transfer processes evolve is important to understanding the hazard posed by a firebrand. Additional experiments with the impinging jet flow were conducted on reacting firebrands placed on an inert substrate to understand the differences in the heat transfer processes with a reacting source. Higher flow conditions led to higher firebrand temperatures. The geometrical configuration between the firebrand and the substrate influenced the firebrand temperatures and the thermal propagation into the substrate. Experiments with hot, surrogate firebrands on non-flame retardant and flame retardant mulch fuel beds were conducted to explore how flame retardants affect ignition by a hot body under various impinging jet flows. The presence of flame retardants was found to delay time to ignition and decrease ignition propensity. Finally, experiments were conducted with reacting firebrands and cellulose insulation fuel beds under an impinging jet flow. These experiments allowed for the exploration of characteristic ignition signatures with in the fuel bed. The reacting area growth rate for these experiments provided an indication of flaming or smoldering ignition, which agreed well with visual observations.


LCSH Subject Headings