Firebrand showers have been identified as the primary mechanism of fire spread and cause of destruction in wildland-urban interface fires. The heat transfer processes that follow deposition and lead to ignition have received little attention until recently. While conventional methods of measuring temperature and heat flux provide bulk-averaged information regarding deposited firebrand piles, they fall short of elucidating essential information such as surface contact areas and heat feed-back mechanisms, that are necessary for understanding ignition propensity and developing high-fidelity physics-based models. This work describes the development and implementation of a spatially resolved optical technique for quantifying heat flux and thermal footprint of firebrand piles using laser induced phosphorescence of YAG:Dy. The technique involves coating the surface of a UV-transparent substrate with YAG:Dy phosphors to yield surface temperature. Heat flux is inferred through simultaneously measuring of the top and bottom substrate surface temperatures after they are coated with a staggered grating pattern. An intensity ratio over two separate spectral regions of emission is used to calibrate the optical measurement system and infer temperature from 300 K - 1100 K over a 7 cm diameter interrogation area. The uncertainty in temperature and heat flux are discussed in detail. The spatial resolution of the temperature and heat flux measurements are 0.5 mm and 4 mm, respectively.
Technical Note (NIST TN) - 2052
firebrand, heat flux, thermal footprint, surface contact area, thermographic phosphors, thermometry