Studying wildfire spread using stationary burners

Abstract

A method for performed experiments using stationary gas burners and liquid fuel-soaked wicks to study flame geometry and buoyant instabilities important to fundamental wildland fire behaviour has been developed. This thesis focuses on experiments performed with stationary fires to carefully study instabilities observed in spreading fires that suggest they play a critical role in fire spread. Two types of flow conditions were used to perform experiments similar to wildfire spread conditions: sloped fuel surface and forced-flow (wind aided). Small- scale inclined experiments for performed at the University of Maryland with liquid- fuel soaked wicks and large-scale experiments at the USDA Forest Service Missoula Fire Sciences Laboratory with a gas-burner. These experiments were performed with over a range of heat-release-rates and burner sizes for angles from 0 to 60 degrees from the horizontal. Forced-flow experiments were performed in a large-scale wind tunnel at the Missoula Fire Laboratory and at the University of Maryland with a well characterized wind blower with gas-burners. These experiments were performed for a range of heat-release-rates and burner sizes in wind speeds from 0.2 to 3.0 ms−1 The flame geometry was determined using high-speed videography. Important two-dimensional flame geometry parameters such as centerline flame length and flame tilt angle were measured from these images. Flame intermittency and pulsation close to the surface was measured using high-speed videography and micro-thermocouples. A method was developed to track the extension of the flame close to the surface which would come in direct contact with unburnt fuels ahead of the fire. These methods showed that the pul- sation frequency is complicated suggesting large scale structures in the flow. Using these frequency the stationary experiments follow similar Strouhal-Froude scaling for flame pulsations in spreading fires. Stream-wise streaks in the flow were observed and measured using high-speed videography. Streak spacing has been observed to be associated with possible Gotler votice structures in the fire. The spacing for streaks at the base of the flame for these startionary experiments appear to be dependent on the boundary- layer conditions and could possibly scale with the centerline flame length similar to flame towers observed in spreading fires.