A Methodology for Experimental Quantification of Firebrand Generation from WUI Fuels

Abstract

Over the past few years, numerous large-scale disasters have occurred due to wildfires at the wildland-urban interface (WUI). In these fires, spread via the transport of firebrands (burning embers) plays a significant role. Several models have been developed to describe the transport of firebrands but few, if any, are available which can provide a quantitative means to generate firebrands at the source of a fire. In this regard, a new methodology is proposed here that uses a wind tunnel to experimentally quantify the generation of firebrands from WUI fuels under different ambient conditions. The setup allows for the collection of all generated solid firebrands and major downstream gaseous species concentrations. Unique firebrand yield correlations can then be generated for each tested fuel, while also accounting for the heat-release rate, providing unique validation targets for numerical simulations. Generation of firebrands from branches of two conifers at a fixed wind speed of 4 m/s are presented to demonstrate the capabilities of this new methodology. A carbon mass balance was utilized to analyze preliminary results and understand how much of the fuel mass transitions to firebrands vs. gases. These results provide a description of the mass burning process and ultimately tie firebrand production to a time-dependent heat-release rate for initialization of firebrand transport in numerical simulations. An average firebrand yield ranging from 3–4% of initial dry mass is ultimately presented for lodgepole pine and Douglas fir. Future work is required with larger fuel sizes pertaining to real wildfire scenarios; however, the presented methodology can provide valuable data needed to initialize numerical simulations of firebrand transport, necessary for reconstruction of WUI fires and to aid in the development of mitigation strategies for the prevention of future disasters.