Critical Ignition Conditions of Structural Materials by Cylindrical Firebrands

Abstract

Smoldering firebrands (embers) are a major cause of ignition and eventual structural damage during wildfires at the wildland-urban interface (WUI). These small pieces of wood can loft several kilometers ahead of the main flame front and ignitespot fires directly on structural elements such as decks. In this research, dense structural materials such as wood and engineered wood will be studied with a focus on capturing the critical thermal conditions necessary for ignition. Unique to this study will be a configuration where whole piles of firebrands are placed on the recipient material, emulating observations from WUI fires. In order to design appropriate fire safety standards at the WUI and, someday, to model the propagation of these fires, the conditions leading to ignition of common WUI materials by piles of lofted firebrands must be quantified. Firebrands were modeled using small cylindrical wooden dowels which were ignited and placed in a small-scale wind tunnel. Two tests were performed for each loading condition of firebrands, one studying ignition of wooden structural elements such as decking and marine-grade plywood and another measuring temperatures and heat fluxes over an inert piece of ceramic insulation. A single-point water-cooled heat flux gauge was used for time-resolved measurements of heat flux at the center of the inert setup surrounded by thin-skin calorimeters and K-type thermocouples which allowed for a spatial characterization of heating. The wind speed was the main quantity of interest changed during the test to determine the effects of wind speed on the heat flux released from the glowing dowels to recipient fuels. The results showed a drastic increase in heating from piles of firebrands vs. individual brands. The piles also produced higher heat fluxes under increasing winds. This is due, for the most part, to higher surface temperatures resulting from increased surface oxidation

under higher wind speeds. Both smoldering and flaming ignition of wood was found to be similarly dependent on wind speed. Larger piles also produced higher peak heat fluxes at the center of the pile, highlighting the role of re-radiation within the pile influencing heat fluxes to recipient fuels. Critical heat flux and firebrand loading conditions required to achieve smoldering and

flaming ignition of structural materialsused in the WUI are determined by comparing tests with inert and flammable fuels.

These critical conditions can be used to model the propagation of WUI fires over structural elements to design appropriate fire safety standards at the WUI. A non-dimensional relationship incorporating fuel type, geometry, and ambient conditions is also proposed to describe the results.