CAREER: Understanding the Mechanisms of Wildland Fire Spread

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CBET-1554026, GollnerLarge wildland fires pose an enormous threat to local populations and natural resources, and contribute carbon and particulate emissions to the atmosphere. Firefighters, community planners, and local residents all depend on accurate predictions of the spread of wildland fires for operational firefighting, evacuations, community design, and prescribed fire design. It is well known in the wildland fire community that increasing winds or slope will increase the rate of spread of a wildfire; however, the physical mechanisms by which a fire spreads are not well known. This uncertainty is, in part, why no models can accurately predict the critical thresholds for fire spread. Important thresholds include when a fire will extinguish or transition from surface fire spread to crowning through the tree-tops. This research project will utilize laboratory experiments to develop quantitative models to describe the physics of fire spread. Once these improved models for fire spread are developed, they can be used for predicting fire suppression, evacuations, and fire weather alerts. Solving the wildfire problem will take more than just advanced research. A workforce trained to understand both the practical and scientific fire problems in wildland fire will also be necessary. In conjunction with this project, two outreach programs will be implemented. First, an educational program for middle and high school girls will use fire science as a unique avenue to introduce underrepresented students to STEM careers. Second, researchers will work with the first responders - wildland firefighters, to inform them of hazards and safety techniques. The research objective of this proposal is to understand the previously-unexplored role of intermittent heating that is driven by reacting-flow instabilities on wildland fire spread, in the hope that this knowledge may lead to a testable, physical theory of wildland fire spread. Current models either rely on empirical results or assume that radiation is the dominant heat transport mechanism. Recent observations by the PI and his collaborators have instead shown that intermittent heating from flames is, in many regimes, an equally important process driving flame spread and cannot be neglected. Buoyant instabilities originating in the reacting flow of wildland fires eject flames far ahead into unignited fuel, driving the spread. During the most devastating wildfires, these regimes tend to be dominant and, hence, current flame spread models fail. This study of intermittent fuel heating will contribute to these important numerical tools which will address critical failures during extreme fire conditions. Experimental work will include the use of stationary burners under forced-flow, inclined, and combined configurations. In addition, a pulsed-gas burner will be used to couple the effects of flame instabilities on fine fuel ignition which drive flame spread. Theoretical work will include developing theoretical models and scaling relationships for the formation of coherent structures in the flow and their effects on intermittent heating of fine fuels. The development of new models and collaboration with the U.S. Forest Service will propel these results directly into practice and couple the disparate fields of wildland fire and combustion science.