The Influence of Season, Heating Mode and Slope Angle on Wildland Fire Behavior.

摘要

Wildland fire behavior research in the last 100 years has largely focused on understanding the physical phenomena behind fire spread and on developing models that can predict fire behavior. Research advances in the areas of live-fuel combustion and combustion modeling have highlighted several weaknesses in the current approach to fire research. Some of those areas include poor characterization of solid fuels in combustion modeling, a lack of understanding of the dominant heat transfer mechanisms in fire spread, a lack of understanding regarding the theory of live-fuel combustion, and a lack of understanding regarding the behavior of flames near slopes.;In this work, the physical properties, chemical properties and burning behavior of the foliage from ten live shrub and conifer fuels were measured throughout a one-year period. Burn experiments were performed using different heating modes, namely convection-only, radiation-only and combined convection and radiation. Models to predict the physical properties and burning behavior were developed and reported. The flame behavior and associated heat flux from fires near slopes were also measured. Several important conclusions are evident from analysis of the data, namely (1) seasonal variability of the measured physical properties was found to be adequately explained without the use of a seasonal parameter. (2) ignition and burning behavior cannot be described using single-parameter correlations similar to those used for dead fuels, (3) moisture content, sample mass, apparent density (broad-leaf species), surface area (broad-leaf), sample width (needle species) and stem diameter (needle) were identified as the most important predictors of fire behavior in live fuels, (4) volatiles content, ether extractives, and ash content were not significant predictors of fire behavior under the conditions studied, (5) broadleaf species experienced a significant increase in burning rate when convection and radiation were used together compared to convection alone while needle species showed no significant difference between convection-only and convection combined with radiation, (6) there is no practical difference between heating modes from the perspective of the solid---it is only the amount of energy absorbed and the resulting solid temperature that matter, and (7) a radiant flux of 50 kW m-2 alone was not sufficient to ignite the fuel sample under experimental conditions used in this research, (8) the average flame tilt angle at which the behavior of a flame near a slope deviated from the behavior of a flame on flat ground was between 20° and 40°, depending on the criteria used, and (9) the traditional view of safe separation distance for a safety zone as the distance from the flame base is inadequate for fires near slopes.;Keywords: physical properties, live fuels, fuel growth patterns, ignition, fire behavior, seasonal burning behavior, radiation, convection, Coanda effect, fire attachment on slopes, safe separation distance, firefighter safety zone.