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Title: D2.5-4-36 Fire safety analysis around targets using FDS: Final achievements - Transport of firebrands and attack on buildings
Subject: [FIRE PARADOX Deliverables, Fire Paradox Modules, Module 2 - Physical mechanisms, WP2.5 - Fire effects on buildings and people]
Description: Spotting is an important mechanism of wild land fire spread. Burning particles such as twigs and leaves lofted by the buoyant plume from forest fires can be carried by ambient winds anywhere from few meters up to even a kilometre from their source. Firebrands can then start new fires far from the original fire front. This makes it difficult to predict fire-fronts movements and can cause surprising and life threatening situations for fire fighters. Firebrands also pose a significant fire hazard at Wildland-Urban Interfaces (WUI). It is possible that firebrands are even the main fire hazard at WUI locations. This study addresses the effect of firebrand attack from a forest fire on an isolated building. The effect of the firebrand attack on a house is studied at Wildland Urban Interface (WUI) conditions. Penetrations of firebrands into the house under firebrand attack as well as firebrands landing distances are studied. Fire Dynamics Simulator (FDS) [19] is used to simulate WUI fire scenarios. The results show that firebrands of all studied sizes can reach the house even across distances as large as 50 meters. The overwhelming majority of the particles reaching the house land on the roof. Few firebrands can be observed hitting the facade of the building on the side facing the fire front. Only relatively small particles were found to penetrate into the house through small openings and vents. This is attributable to the fact that smaller particles tend to follow the surrounding gas flow whereas larger particles tend to fly in parabolic trajectories and thus most probably landing on the roof or in front of the house. Compared to earlier studies on firebrand propagation, somewhat shorter propagation distances were observed. Most of the earlier studies have used some form of simple plume model or otherwise simulated a steady state flow. Results in this study suggest that the turbulence of the plume has a significant effect on the motion of firebrands. The results also suggest that for larger firebrands the propagation occurs in two distinct phases: A firebrand is lifted to high altitude by a strong updraft and is then carried further by ambient winds. A firebrand model was added to the Fire Dynamics Simulator and was used to simulate a WUI fire. While considerable uncertainties remain in modelling the fire front and the firebrand material properties, it was shown that the model can be used to investigate firebrand attacks in WUI locations.
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Language: English
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Creator: VTT Technical Research Centre of Finland
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Rights restrictions: restricted
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Audience: public
Download: D2.5-4-36 Fire safety analysis around targets using FDS: Final achievements - Transport of firebrands and attack on buildings