Abstract. The process of spotting occurs in wildland fires when fire-lofted embers or hot particles land downwind, leading to ignition of new, discrete fires. This common mechanism of wildland fire propagation can result in rapid spread of the fire, potentially causing property damage and increased risk to life safety of both fire fighters and civilians. Despite the increasing frequency and losses in wildland fires, there has been relatively little research on ignition of fuel beds by embers and hot particles. In this work, an experimental and theoretical study of ignition of homogeneous cellulose fuel beds by hot metal particles is undertaken. This type of well-characterized laboratory fuel provides a more controllable fuel bed than natural fuels, and the use of hot metal particles simplifies interpretation of the experiments by reducing uncertainty due to unknown effects of the ember combustion reaction. Spherical steel particles with diameters in the range from 0.8 mm to 19.1 mm heated to temperatures between 500°C and 1300°C are used in the experiments. A relationship between the size of the particle and temperature required for flaming or smoldering ignition is found. These results are used to assess a simplified analysis based on hot-spot ignition theory to determine the particle size-temperature relationship required for ignition of a cellulose fuel bed.

Wildland and wildland/urban interface fires are a serious problem in many areas of the world. It is expected that with global warming the wildfire and wildland/urban interface fire problem will only intensify. The ignition of natural combustible material by hot metal particles or embers is an important fire ignition pathway by which wildland and urban spot fires are started. There are numerous cases reported of wild fires started by hot metal particles from clashing power lines, or from sparks generated by machines or engines. Similarly there are many cases reported of industrial fires caused by grinding and welding sparks. Despite the importance of the subject, the topic remains relatively unstudied. The senior author of this article and his collaborators have been working for the past few years on this problem. In this article, we provide a comprehensive summary of that work to date. The work includes experimental and theoretical modeling of the ability of hot metal particles and embers to cause the ignition of cellulosic fuel beds. The metal particles studied are representative of clashing conductors (aluminum and copper) and those produced by machine friction and hot work such as welding (stainless steel and brass). In addition glowing and flaming wood embers are considered, as they represent an important source of fire spotting in wildfires. The overall results show a hyperbolic relationship between particle size and temperature, with the larger particles requiring lower temperature to ignite the fuel bed than the smaller particles. An important finding is that although particle energy is important in the capability of the particle to ignite the fuel, both energy and temperature are determining factors of the particle ignition capabilities. The thermal properties of the metal play a lesser role with the exception of the energy of melting if it occurs. It also appears that the controlling ignition mechanisms by large particles are different than those from the small particles. The former appear to be determined primarily by the particle surface temperature while the latter by the particle energy and surface temperature. Sparks are a specific type of particles with very small sizes and very high temperatures. Because of the small sizes, their energy is small and it is postulated that the sparks must accumulate for ignition of a fuel bed to occur. The results with embers indicate that the smoldering is the easier form of ignition, although Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/ gcst. 269Downloaded by [New York University] at 02:40 14 July 2015 270 A. C. FERNANDEZ-PELLO ET AL.flaming ignition can occur if the ember is flaming and the air velocities are moderate. To provide further information about the fire spot ignition process, both analytical and numerical modeling are used and compared with the experimental results. Although the models provide qualitative predictions further development is necessary to reach quantitative predictive capabilities.

This paper details a methodology for quantification of errors and uncertainties of a finite element heat transfer model applied to a Ruggedized Instrumentation Package (RIP). The proposed verification process includes solution verification, which examines the errors associated with the code's solution techniques. The model was subjected to mesh resolution and numerical parameters sensitivity studies to determine reasonable parameter values and to understand how they change the overall model response and performance criteria. To facilitate quantification of the uncertainty associated with the mesh, automatic meshing and mesh refining/coarsening algorithms were created and implemented on the complex geometry of the RIP. Similarly, highly automated software to vary model inputs was also developed for the purpose of assessing the solution's sensitivity to numerical parameters. The model was subjected to mesh resolution and numerical parameters sensitivity studies. This process not only tests the robustness of the numerical parameters, but also allows for the optimization of robustness and numerical error with computation time. Agglomeration of these studies provides a bound for the uncertainty due to numerical error for the model. An emphasis is placed on the automation of solution verification to allow a rigorous look at uncertainty to be performed even within a tight design and development schedule.

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