Understanding the physical processes of fire spread
Despite the many operational models now used in fire management, the physical processes responsible for fire spread are not well understood. Advancements are hindered by a variety of factors, including a lack of a basic understanding of crown fire spread and spread thresholds, the implications of mountain pine beetle attack on crown fires, and fire and fuel dynamics of crown fire ecosystems. To address this, researchers are working to develop new approaches and theoretical understanding of fire spread under various conditions.
Supported by the National Fire Decision Support Center, studies at the Rocky Mountain Research Station address the following questions: 1) Why is radiation insufficient to heat fine fuel particles to ignition? 2) What is the structure of turbulent flames at their edge, and how well do simple turbulence models characterize it? 3) What is the critical mass loss rate for ignition of live and dead wildland fuels? and 4) What are the properties of live fuels that allow them to ignite, lose moisture, and burn?
Laboratory experiments suggest that the radiant intensities found in wildland fires are not sufficient to ignite fine fuel particles such as needles and grasses, but flame convection provides the critical heating of particles to ignition. Moreover, other experiments show that the edges of turbulent flames are responsible for igniting fuel particles after subjecting them to intermittent heating and cooling over time scales of about 1/10th of a second.
Researchers also discovered that the ignition of wood depends on a critical rate of converting solid mass to combustible gas similar to other substances (such as plastic), and that ignition depends on the heat flux and wind flow, providing an improved definition of ignition and flammability limits. And finally, research shows that live fuels such as conifer foliage can burn at moisture contents many times higher than dead fuels because they release moisture explosively compared to slow diffusion in dead fuels, and they contain large amounts of non-structural carbohydrates.This research suggests a completely new approach to understanding and modeling fire spread that is based on an experimentally supported theory, creating new opportunities for developing models that can be used for fire management applications.For more information, see: Cohen J.D. and M.A. Finney. 2010. An examination of particle heating during fire spread. Proceedings of the 6th International Conference on Forest Fire Research.
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