Modeling Wildland Fire Spot Ignition by Metal Sparks: Fluid Mechanics Aspects

  • Fernandez-Pello, Carlos (Univ. California Berkeley)
  • Urban, James (WPI)

Please login to view abstract download link

Wildland, and Wildland Urban Interface (WUI), fires are a problem in many areas of the world, and may have major consequences in terms of safety, air quality, damage to the infrastructure and the ecosystem. It is expected that with climate change the wildland fire and WUI fire problem will only intensify. The spot fire ignition of a wildland fire by hot metal particles/sparks is an important fire ignition pathway by which wildfires, WUI fires, are started, and may propagate. There are numerous cases reported of wildland fires started by hot metal particles from clashing power-lines, or generated by machines. Spot ignition of a wildland fire by hot metal particles can be separated in three major processes: particle generation and ejection characteristics; coupled transport and thermochemical change during the particle flight to the ground; and potential ignition of target fuel bed. The particle characteristics at generation are normally obtained from experiments. Once the metal particles are ejected from their source, the particles experience both drag by the wind and gravitational forces during their flight. The model used to calculate the trajectory and temperature history uses conventional fluid mechanics calculations and is basically a ballistic trajectory simulation with a hot body cooling to the ambient air. The resulting particle trajectory is roughly parabolic, and the landing location depends on the particle generation location, ejection velocity (magnitude and direction), the wind velocity (magnitude and direction) and particle characteristics (diameter, temperature). During the trajectory the metal particles cool down by convection and radiation reaching the ground at a temperature that depends of the particle initial characteristics and ambient air conditions. After landing the particles may have the potential to ignite a bed of vegetation depending on their size and temperature at landing, and the characteristics of the fuel bed. The fuel bed ignition process has a strong thermo-fluids component, including: the heating of the fuel by the spark in a porous media convective environment; the fuel gasification; the mixing of the gaseous fuel and air; and finally the ignition of the flammable mixture by the metal particle. All of these processes involved several complex thermofluidic mechanisms. In this study we present a methodology to analyze the problem and an application to a specific wildland fire case that occurred in the estate of Washington,