Thermal dynamics of deposited firebrands using phosphor thermometry

Abstract

A series of experiments were carried out to identify and better understand the thermal mechanisms which govern the ignition propensity of firebrands. This work leverages the temperature-sensitive phosphorescence of YAG:Dy to non-intrusively measure and spatially resolve firebrand contact temperature along a 6.1 cm diameter area. The highly coupled firebrand heat transfer problem is studied by depositing smoldering firebrands onto a flat inert substrate and subjecting them to flow speeds of (1.5 ± 0.15) m/s. The temporal evolution of the thermal footprint is described and analyzed. Quasi-steady peak contact temperatures ranged between 500 K and 850 K while brief peaks exceeding 1150 K were also observed. The large difference in contact temperatures alludes to the significance in factors such as firebrand-fuel contact resistance, firebrand geometry, firebrand-flow orientation, fuel type (density and heat of combustion), and mass. Furthermore, the findings shed light on secondary heating events associated with shifting of the firebrand contact area and/or fragmenting, which can impart peak net heat flux equivalent to 70 % of the initial deposition. Lastly, considering the sensitivity of ignition on temperature, the work provides a brief discussion on the presence of unimodal and bimodal contact temperature distributions.