UNDERSTANDING THE INFLUENCE OF WIND AND SLOPE ON FLAMES IN WILDLAND FIRES

Abstract

Wildland fire spread is typically described as a function of fuel, weather, and topography. An understanding of how these parameters are interrelated can help to close the gap on our understanding of the flame spread process. Fire spread over steep slopes is unique because the flame dramatically transitions from a detached, “plume” mode to an attached “boundary layer” mode at what appears to be a critical angle. The change in flame shape significantly increases the rate of spread by increasing the length and magnitude of heating ahead of the burning region. This attachment behavior has been observed in the literature, however the correlation between fire intensity, slope, wind, and flame shape is not yet well described or in a form which allows for prediction of fire behavior. A series of experiments using stationary gas burners have been undertaken to describe the behavior of a steady flame at multiple slopes under both wind and non-wind conditions. A stationary gas burner has been used to emulate flames under various fire intensities, burner aspect ratios, slopes, and wind conditions. A small-scale apparatus was first used to image hot gases using a shadowgraph technique coupled with downstream temperature measurements. Later a larger-scale apparatus was used with downstream temperature measurements to determine instantaneous downstream heating lengths. Both steady and time-dependent analysis of the attachment process is presented, along with its relation to fire spread. Based on the observed trends from these relationships non-dimensional parameters are introduced to relate the effects of inclination, wind, fire size and aspect ratio to the length of the heating region ahead of the burner. It is proposed that this value may be useful as a simple way to incorporate these effects in a wildland fire spread model.