The purpose of this collaborative research project is to develop advanced modeling technology for describing fire initiation and propagation in vegetation with low canopy bulk density. The ability to predict the spread of wildland fires is paramount in protecting life, property, and natural resources. Current operational models predict overall fire behavior well for the conditions for which the model was correlated (e.g., dead fuel beds), but they do not perform as well for live bushes or trees with high moisture content. Detailed physical models, at either laboratory or landscape scale, require improved sub-grid scale models of combustion, especially to describe fire behavior in vegetation that does not act like a dense fuel bed due to the relative sparseness of the vegetation. This technology will be based on fundamental combustion measurements of live fuels, but it will apply to models of landscape-scale fires. The research objective will be achieved via four inter-related tasks: (1) flame propagation measurements in live leaves and small branches, (2) fire spread measurements in shrubs for varying bulk densities, (3) flame propagation models of bushes and trees, and (4) multi-bush fire behavior models.
The research will provide a cohesive picture of the phenomenon of fire spread starting from ignition of a single fuel element, such as a leaf, to a self-sustaining fire spreading through a larger fuel array such as a forest. The fundamental physical and chemical processes investigated are also relevant to the problem of surface fire propagation leading to ignition of crown fires. An improved fundamental understanding of fire behavior in sparse vegetation will be beneficial in promoting better predictive capability in other areas such as fire safety or arson investigation pertaining to identifying ignition sources. Gaining an understanding of the conditions under which a fire may or may not propagate within a sparse vegetative fuel bed will provide understanding of how to control fire spread more effectively in these fuel types through improved fuel management.
This research program focused on fire behavior in sparse vegetation will stimulate interest in graduate studies among ethnic minorities and women in engineering at both at UCR and BYU by providing a new and innovative means of exploring ignition and combustion from single fuel elements to fire spread through multiple fuel elements. Graduate and undergraduate students trained at UCR and BYU will have the unique opportunity to interact closely with scientists at the USDA Forest Service laboratories in Riverside and Montana. This research has the potential to be used by international organizations concerned with fire spread in shrub lands which includes Australia, countries surrounding the Mediterranean Sea, South Africa, and Chile.
The project was motivated by our desire to develop an advanced, physics-based modeling technology to describe fire initiation and propagation in vegetation with varying density of fuel distribution. A secondary attribute that dictates fire behavior is fuel moisture content. Thus, we were motivated to study the burning behavior of individual leaves, and conglomerate of leaves in a bush representative of different seasons during a year to simulate varying moisture content. The ignition and burning characteristics of individual leaves is described by the efforts of our collaboration with BYU and not repeated here. The distribution of bulk density of fuel in the vertical direction was obtained via experiments carried out at the USDA Riverside Fire laboratory. This bulk density variation was obtained for chamise and manzanita shrubs harvested from the San Bernardino mountains in southern California. Following these measurements, individual chamise shrubs were placed vertically in a wind tunnel and ignited through a surface fire over a bed of excelsior distributed under the shrub. Fire behavior was studied using measurements of mass loss rate, burning time, time to reach maximum mass loss rate, radiative and convective heat flux, flame height and flame angle. These experiments served as a point of reference to validate our computational simulations. Empirical relationships to model measured distribution of bulk density were utilized in computational simulations of burning of an isolated shrub. Our results showed that the propagation rate of the fire through the shrub in the vertical direction decreased continuously as the moisture content increased from 20% to 100%. In the case of two shrubs, it is found that the vertical fire spread rate and convective heat transfer from the gaseous products to unburnt fuel were enhanced when the separation distance between the shrubs decreased. As part of our investigations, we developed a novel method of generating a statistically stationary fire for the purpose of studying its turbulence characteristics. Our findings were presented in several technical conferences and one manuscript has been published in a peer reviewed journal. Additional manuscripts are under preparation at the time of submission of this report. In terms of broad impacts, one undergraduate student (in year 1) and three graduate students gained significant knowledge in fire science and engineering principles. After completing a Masters thesis at the University of California, Riverside (under the PI's supervision), one graduate student is pursuing his PhD degree in the fire protection engineering field at the University of Maryland. A second graduate student at the University of Alabama in Huntsville is expected to complete his PhD requirements in 2013; he has already secured a job offer with a private company to advance their combustion modeling capability.