Live Fuels: Identification of Key Processes Controlling Ignition and Fuel Consumption
Dr. David Blunck | Oregon State University
Predicting and managing burns of live fuels is critical to effectively supporting Department of Defense (DoD) missions. Most fires, whether prescribed or wildland, propagate through living fuels or mixtures of live and dead fuels. Ironically, the majority of fire studies and models have focused on burning of dead fuels. This discrepancy is significant because the fire behavior of live and dead fuels can vastly differ. Moreover, the (limited) prior studies of live fuels have typically assumed that natural fuels are thermally thin (i.e., isothermal). This is now recognized as inaccurate and has significant implications to how live fuels burn. Thus, there is a critical need to gain knowledge and develop modeling tools which are applicable to live fuels of varying thermal thicknesses. With this background the overall goal of this effort is to strengthen the fire community’s ability to more accurately model fire spread and fuel consumption of live fuels.
The following specific objectives will be accomplished to enable achieving the overall goal:
- Identify the role of pyrolysis rates and pyrolyzates in causing observed differences in ignition and fuel consumption behavior between different live fuels, and live and dead fuels of the same species.
- Ascertain changes in processes controlling ignition and fuel consumption rates of live fuels for varying thermal thicknesses (e.g., needles, branches).
- Quantify the heat release and heat transfer characteristics of burning live fuels at field-scales.
- Develop and evaluate a fuel consumption model which is applicable to a variety of live and dead fuels with varying thermal thicknesses.
Coupled laboratory, modeling, and field studies will be used to address the objectives of this effort and to help achieve the overall goal. Thrust 1 will focus on identifying key physical and chemical processes that cause ignition and fire spread behavior to differ between live and dead fuels using laboratory and field studies. This will be accomplished using a suite of laboratory (e.g., infrared camera, laser diagnostic, fourier-transform infrared [FTIR]) and detailed computational studies that will be deployed to systematically identify key parameters controlling ignition and burning behavior. Field studies will be used to augment the laboratory experiments and provide data for relevant conditions. Thrust 2 will improve models to predict fuel consumption in live fuels (in contrast to models which were designed for dead fuels). A semi-empirical fire consumption model will be created (and evaluated) that can be used by fire managers.
Anticipated benefits to the DoD are as follows.
- A fuel consumption model for live and dead fuels with varying thermal thicknesses will be developed and made available for fire managers. This model will be significant because it will allow fire managers to more accurately predict fire behavior for mixtures of live and dead fuels to more effectively plan prescribed burning activities and wildfire responses.
- Burning and ignition behavior for live fuels significant for DoD fire management behavior (e.g., sagebrush, Douglas-fire, cheat grass, chaparral) will be measured.
- A pioneering methodology for coupling a laser diagnostic with infrared imaging and FTIR to study ignition and burning behavior will be established. The research team expects that this will help enable new measurements and insights which are critical for developing physics-based models.
- Results from this study will also provide valuable information needed to help predict smoke plumes and convective structures because burning rate and fuel consumption are direct drivers of wildland fire emissions rates and concentrations.
- Finally, this work will improve the community’s ability to model crown fire intensity because data and knowledge will be obtained for thermally thick and thin live fuels. Existing models are based on untested assumptions that only a small fraction of the living branchwood burns in the flaming front of crown fires.