Papers by Sara McAllister
Frontiers in Mechanical Engineering
The vast majority of wildland fires occur in windy conditions. However, most operational wildland... more The vast majority of wildland fires occur in windy conditions. However, most operational wildland fire models do not account for changes in burning rate or duration due to wind as no simple model exists. To gain some understanding of how wind and fuel bed properties interact to influence the burning rate and duration of wildland fuels, a relatively simple fuel bed, wood cribs, was first considered. The burning rate of 23 crib designs was measured in a wind tunnel under a range of windspeeds from 0 to 0.7 m/s. Fuel element thickness varied from 0.32 to 1.27 cm and fuel bed width from 12.7 to 60.96 cm. A range of crib porosities was tested as well covering the loosely-packed to densely-packed regime. A clear threshold behavior of the burning rate was seen depending on fuel bed geometry. For fuel beds with element length to thickness ratio (l/b) <30, the burning rate increased with wind. However, for fuel beds with element length to thickness ratio larger than 30, the burning rate actually decreased with wind. This change in burning rate was linked to a visual change in burning behavior. When the burning rate increased, the wind and flames were observed to penetrate the internal portions of the fuel bed and the crib would burn uniformly. When the burning rate decreased, the wind and flames did not penetrate the entire fuel bed and the burning front would most often propagate from the upwind edge to the downwind edge. It appeared that for these fuel bed geometries the wind was forced around the fuel, preventing any horizontal or, more importantly, vertical flow through the bed. These results are likely most applicable to isolated, small clumps of elevated fuel where the wind has the opportunity to divert around the fuel bed. Future work will include experiments that force the airflow through the fuel bed.
Frontiers in Mechanical Engineering
Recent mega wildfires have become one of the most dangerous and devastating hazards, with a wide ... more Recent mega wildfires have become one of the most dangerous and devastating hazards, with a wide range of negative impacts on the economy, society, and environment. As cylindrical shrubs and twigs are typical fuel loads in wildfires, it is important to understand how the diameter and arrangement of cylindrical fuels affect their ignition behaviors. In this work, the piloted ignition of cylindrical wood rods with different diameters (3.2 ∼15.9 mm) are conducted under the irradiation up to 50 kW/m 2. Three fuel groups are tested: (I) single vertical rod, (II) single horizontal rod, and (III) horizontal rod bed attached to the ground. For a single vertical rod, the measured ignition time decreases as the diameter is decreased from 15.9 to 6.4 mm, showing a thermally-thin behavior. However, the ignition of the 3.2-mm rod is more difficult than the 9.5-mm rod, because of the enhanced convective cooling by the larger curvature. Nevertheless, when the rod fuels are placed horizontally on the ground, the curvature-enhanced convective cooling becomes limited. For a single rod, when both the fuel diameter and the irradiation are small, only smoldering ignition occurs, and eventually the sample collapses. For the rod bed, flaming ignition always occurs, and it is easier to ignite because of a smaller convective cooling. For both horizontal configurations, the fuel ignition temperature increases almost linearly with the diameter from 270 • C (3.2 mm) to 330 • C (15.9 mm) but is insensitive to the irradiation level. This research quantifies the effect of fuel diameter and arrangement on the piloted ignition and reveals that the traditional classification of thermally thin and thick fuel for flat materials may not be suitable for cylindrical wildland fuels.
Frontiers in Mechanical Engineering
An experimental study was conducted to understand the intermittent heating behavior downstream of... more An experimental study was conducted to understand the intermittent heating behavior downstream of a gaseous line burner under forced flow conditions. While previous studies have addressed time-averaged properties, here measurements of the flame location and intermittent heat flux profile help to give a time-dependent picture of downstream heating from the flame, useful for understanding wind-driven flame spread. Two frequencies are extracted from experiments, the maximum flame forward pulsation frequency in the direction of the wind, which helps describe the motion of the flame, and the local flame-fuel contact frequency in the flame region, which is useful in calculating the actual heat flux that can be received by the unburnt fuel via direct flame contact. The forward pulsation frequency is obtained through video analysis using a variable interval time average (VITA) method. Scaling analysis indicates that the flame forward pulsation frequency varies as a power-law function of the Froude number and fire heat-release rate, f F˜ Fr/Q * 1/2 0.4. For the local flame-fuel contact frequency, it is found that the non-dimensional flame-fuel contact frequency f + C remains approximately constant before the local Ri x reaches 1, e.g., attached flames. When Ri x > 1, f + C decreases with local as Ri x flames lift up. A piece-wise function was proposed to predict the local flame-fuel contact frequency including the two Ri x scenarios. Information from this study helps to shed light on the intermittent behavior of flames under wind, which may be a critical factor in explaining the mechanisms of forward flame spread in wildland and other similar wind-driven fires.
The kickoff workshop of the new permanent working group, sponsored by the International Associati... more The kickoff workshop of the new permanent working group, sponsored by the International Association for Fire Safety Science (IAFSS), entitled Large Outdoor Fires and the Built Environment was held from 3:00 pm to 4:30 pm on Sunday October 21, 2018. The workshop was held as a part of the 11 th Asia-Oceania Symposium on Fire Science and Technology (AOSFST) in Taipei, Taiwan. The working group is co-led by Sara McAllister of the U.S. Forest Service (unable to come to Taiwan), Sayaka Suzuki of National Research Institute of Fire and Disaster, and Samuel L. Manzello of NIST's Engineering Laboratory. The IAFSS permanent working group consists of three subgroups, with subleaders appointed by Manzello, McAllister, and Suzuki, and these are prioritized into the following topics: Ignition Resistant Communities (IRCled by Elsa Pastor, UPC, unable to come to Taiwan), Emergency Management and Evacuation (EME, led by Enrico Ronchi, Lund University, unable to come to Taiwan), and Large Outdoor Firefighting (LOFF, led by Raphaele Blanchi, CSIRO). The IRC subgroup is focused on developing the scientific basis for new standard testing methodologies indicative of large outdoor fire exposures, including the development of necessary testing methodologies to characterize wildland fuel treatments adjacent to communities. The EME subgroup is focused on developing the scientific basis for effective emergency management strategies for communities exposed to large outdoor fires. The LOFF subgroup is providing a review of various tactics that are used, as well as the various personal protective equipment (PPE), and suggest pathways for research community engagement, including environmental issues in suppressing these fires. The overall objectives are to bring the full depth of knowledge of the IAFSS community to work on these priority topics. At the kickoff workshop, detailed ideas were presented regarding the planned activities of the working group, especially the large workshop to be held at IAFSS 2020.
Combustion Science and Technology
Large outdoor fires present a risk to the built environment. Wildfires that spread into communiti... more Large outdoor fires present a risk to the built environment. Wildfires that spread into communities, referred to as Wildland-Urban Interface (WUI) fires, have destroyed communities throughout the world, and are an emerging problem in fire safety science. Other examples are large urban fires including those that have occurred after earthquakes. Research into large outdoor fires, and how to potentially mitigate the loss of structures in such fires, lags other areas of fire safety science research. At the same time, common characteristics between fire spread in WUI fires and urban fires have not been fully exploited. In this paper, an overview of the large outdoor fire risk to the built environment from each region is presented. Critical research needs for this problem in the context of fire safety science are provided. The present paper seeks to develop the foundation for an international research needs roadmap to reduce the risk of large outdoor fires to the built environment.
Combustion and Flame, 2010
There are a number of situations when fires may occur at low pressures and oxygen concentrations ... more There are a number of situations when fires may occur at low pressures and oxygen concentrations that are different than standard atmospheric conditions, such as in buildings at high elevation, airplanes, and spacecraft. The flammability of materials may be affected by these environmental conditions. Since ignition delay is a measure of material flammability and directly influences whether a fire will occur, experiments were conducted to assess the variation of the ignition delay of PMMA in sub-atmospheric pressures and elevated oxygen concentrations. Three sets of experiments were performed at different pressures and in air, in an atmosphere having 30% oxygen/70% nitrogen by volume, and in a ''normoxic" atmosphere (constant oxygen partial pressure). It was observed that as the pressure is reduced, the ignition time decreased, reached a minimum, and then increased until ignition did not occur. Several mechanisms were considered to explain the ''U-shaped" dependence of ignition time on pressure, and three regimes were identified each having a different controlling mechanism: the transport regime where the ignition delay is controlled by changes in convection heat losses and critical mass flux for ignition; the chemical kinetic regime where the ignition delay is controlled by gas-phase chemical kinetics; and an overlap region where both the transport and chemistry effects are seen. The results provide further insight about the effect of the environmental conditions on the flammability of materials, and guidance about fire safety in low pressure environments. Published by Elsevier Inc. on behalf of The Combustion Institute.
... Sara McAllister USDA Forest Service Missoula Fire Sciences Laboratory, 5775 W US Highway 10... more ... Sara McAllister USDA Forest Service Missoula Fire Sciences Laboratory, 5775 W US Highway 10, Missoula, MT 59808 USA, [email protected] Mark Finney USDA Forest Service Missoula Fire Sciences Laboratory, 5775 W US Highway 10, Missoula, MT ... [4] and Rich et al ...
The burning rate of unconfined cribs has long been identified to occur in two regimes: the densel... more The burning rate of unconfined cribs has long been identified to occur in two regimes: the denselypacked regime where the burning rate is proportional to the crib porosity and the loosely-packed regime where the burning rate is independent of porosity. Though the cribs used to define these burning regimes were primarily cubic in dimension, there are seemingly endless possible ways to build a crib with a given porosity. This work explores the burning rate of cribs with a wide variety of geometries to determine whether the porosity-burning rate relation in the literature holds. One set of experiments was performed to validate the testing apparatus against the known data in the literature. A second set of experiments explored the effect of crib layout in the loosely-packed crib regime. The porosity was kept approximately constant while the number of sticks per layer, number of layers and the length to thickness ratios (l/b) were varied. For l/b less than 36, the burning rate of all cribs matched the porosity-burning rate relation from the literature. For larger l/b, the burning rate was considerably reduced, implying that the crib porosity is a function of l/b above some critical threshold. A third set of experiments was performed to examine the effect of the spacing distance between the crib and the support platform. The effect of spacing distance is strongly dependent on l/b, with no difference seen for l/b = 10 and a 60% change for l/b =96. Future work will focus on exploring the burning rate and the effect of the crib-platform spacing for cribs with large l/b.
Advances in forest fire research, 2014
ABSTRACT Experiments were performed using stationary gas burners and liquid fuel-soaked wicks to ... more ABSTRACT Experiments were performed using stationary gas burners and liquid fuel-soaked wicks to study fundamental wildland fire behaviour, including unsteady flame heating. These experiments were motivated by observations of instabilities in spreading fire experiments that suggest they play a critical role in fire spread. Stationary fire experiments in forced flow and on inclined surfaces exhibited instabilities similar to those observed in spreading fires but allowed for more detailed analysis of the mechanisms responsible. Large scale inclined experiments were performed using an ethylene gas-fed burner at angles from 10 to 60 degrees. Forced flow experiments were performed on liquid-soaked wicks and small scale gas burners at wind speeds from 0.2 to 3 m/s. Results presented include observations of the general flame structure, including streamwise streak spacing and flame fluctuation frequencies which relate to instabilities observed in large spreading experiments. A description and correlations of flame geometry, useful for predictions of wildland fire spread are also presented.
In order to reduce the risk of decompression sickness associated with extravehicular activity, NA... more In order to reduce the risk of decompression sickness associated with extravehicular activity, NASA is designing the next generation of exploration vehicles with a different cabin pressure and oxygen concentration than used previously. This work explores how the flammability of solid materials changes in this new environment. One method to evaluate material flammability is by its ease of ignition. To
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Papers by Sara McAllister