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Individual Highlight

Role of Buoyant Flame Dynamics in Wildfire Spread

Photo of Top-view of the flame zone of a spreading fire in the laboratory showing pocket structures resulting from buoyant-flow instabilities. USDA Forest ServiceTop-view of the flame zone of a spreading fire in the laboratory showing pocket structures resulting from buoyant-flow instabilities. USDA Forest ServiceSnapshot : The phrase “spreads like wildfire” is well-known but until recent discoveries through experiments, it wasn’t well-known how wildfires actually spread.

Principal Investigators(s) :
Finney, Mark A.  
Research Location : Missoula Fire Sciences Lab, Missoula, Montana; University of Maryland; Camp Swift, Texas
Research Station : Rocky Mountain Research Station (RMRS)
Year : 2015
Highlight ID : 816

Summary

Predictions of wildfire spread and behavior have relied for decades on empirical correlations. Empirical models provide useful predictions but not physical explanations. They also can’t be extended to address the many fire behaviors that lie beyond their restrictive data sets and assumptions and which represent critical safety problems and limitations to managing wildland fires. Attempts to develop physical models have conceded a diversity of proposed formulations, rather than a foundational theory, because the exact physics of wildfire spread has not yet been discovered. In particular, the organization of radiation and convection heat transfer processes that lead to fuel particle ignition (and thus spread) has remained unexplained. This new research clearly revealed the heat transfer and ignition processes occurring in wildland fires and why they have remained mysterious for so long: because the physics is counterintuitive. First, small fuel particles, like grasses and pine needles, which are ubiquitous as wildland fuel and thought to be the most easily ignited, were actually shown to be the most difficult to heat and ignite by thermal radiation alone. Laboratory studies showed that fine particles cool efficiently by convection of the surrounding cool air when exposed to thermal radiation. Strangely, larger particles (the size of a pencil) ignited while the thin particles did not. This means that fine particles ignite only after heating from flame contact (convection heating by hot gasses). Second, the study discovered how convection from flame contact occurs; specifically, how very hot flames (greater than 1800 degrees Fahrenheit or 982 degrees Celsius) flow downward to ground levels and make contact with fuel particles. Previously, it was reasoned that hot flames should rise like a hot air balloon because the low flame density at high temperature means they would float. But the new studies discovered that the floating or buoyancy of the flame gasses produces two kinds of “instabilities” by interacting with the cooler and denser air around the fire. One kind of instability causes air at the flame front to spin in paired vortices that force flames upward and downward resulting in visible flame peaks and troughs. Without these vortices, flames would not contact the fuel near the ground. The second instability causes waves to form near the back of the flame zone. A series of waves appear like pockets of flame that flow forward and pulse or burst intermittently to contact and ignite the fuel particles ahead of the fire. Flame structure related to both of these instabilities was observed and measured on fires in the laboratory as well as in the field. This study opens the door into the little known world of flame dynamics by bringing scientists closer to understanding the complexities of radiative and convective heat and how they govern wildfire spread. The information obtained through this research is significant with the potential to improve firefighter safety by providing better training to recognize and anticipate wildfire behavior; simplify the physical principles of wildfire spread that can lead to the development of improved prediction models; and, improve the ability to mitigate fuel hazards by accurately modeling and describing fuel contribution to wildfires.

Additional Resources

Role of buoyant flame dynamics in wildfire spread(publication)

Forest Service Partners

External Partners

  • Jason M. Forthofer
  • Missoula Fire Sciences Laboratory, US Forest Service: Jack D. Cohen
  • Sara S. McAllister
  • Brittany A. Adam
  • Daniel J. Gorham. Department of Mechanical Engineering, University of Kentucky: Kozo Saito
  • Department of Fire Protection Engineering, University of Maryland: Michael J. Gollner
  • Justin D. English
  • Nelson K. Akafuah

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