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.