Understanding the impact of intermittent convective heating and flame contact on the ignition of small diameter woody fuels.

Spreading wildfires experience pulsating, turbulent flow that exposes unburnt fuels to intermittent convective heating ahead of the flame front. Currently understudied, this intermittent heating can play an important role in forward fire spread. We developed an apparatus and an experimental technique to produce intermittent flaming conditions that can then be used to study their impact on the heating and ignition of fine woody fuels. Similar to a Ruben's Tube, the experimental setup uses an aluminum cylindrical tube filled with gaseous propane that exits through a series of small holes on the top, fueling a constant flame. A speaker is attached to one end of the tube, so when it is energized, propane is forced out through the top of the tube by the sound waves, causing the existing flame to pulse upward while the sound is on and return to a lower height when the sound is off. Wooden dowels are then held above the flame to characterize the impact of intermittent heating on the ignition of these small-diameter fuels. Bursting frequencies between 0.5 Hz and 3 Hz were tested for Birch wood with diameters 8 mm, 5 mm, 3 mm, and 2 mm, and Basswood of 1.6 mm and 2 mm. This test array enabled a comparison of the impact on diameter across a single type of wood, as well as some comparisons across different materials. For the Birch dowels, there was also a large enough variation in diameter to see the transition between thermally thin and thermally thick fuels. Across diameters, increasing pulsing frequency decreased the ignition time and variation between tests. Further testing will allow for additional characterization of the impact of intermittent convective heating on small-diameter fuels.