A Study Of Intermittent Convective Heating Effects On Fine Fuel Ignition

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Recent studies have suggested the potential importance of intermittent convective heating on the ignition of fine fuels during wildland fire spread. In this study, a novel pulsed-gas line burner similar to a Rubens' tube, driven by acoustic oscillations, is used to re-create the pulsations observed in wildland fires in a controlled environment. After acoustically stimulating a long tube with perforations at the top, creating a pulsed linear flame, thin fuels with different densities and diameters are quickly placed in the center of the flame. The temperature of these fuels is measured using an infrared camera, distinguishing the temperature at which the fuel starts to pyrolyze. As expected, smaller-diameter fuels ignite faster when exposed to flames; however, they also are least affected by intermittent heating. Larger-diameter fuels are more dramatically affected by intermittent heating frequencies, in large part due to cooling effects between pulses and the larger thermal mass of the fuels. The results are discussed and compared with a simple numerical model incorporating measured velocities and temperatures present in the burner and their effect on a thermally-thin fuel element over time.