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Fire Whirls and Building AerodynamicsRobert N. MeroneyWind Engineering and Fluids LaboratoryColorado State University, Fort Collins, CO
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Introduction: Fire whirls are a typically rare but potentially catastrophic form of fire. They are observed during urban and forest fires, where fire “tornadoes” are characterized by large-scale whirling flames which rise in 2 to 360 m diameter vortices from 10 to 1200 m high. These fire whirls accelerate combustion, produce significant suction pressures and lifting forces, and can carry burning debris, logs and even buildings thousands of meters from the main fire.During the Great Chicago Fire in 1871 fire whirls carried burning debris up to 3/8 mile beyond fire lines rapidly extending the fire out of control. In the same year in Peshtigo, Wisconsin, the largest and deadliest fire in US history fire tornadoes were reported to lift roofs off buildings and resulted in the death of 1,500 people. In the 1923 Kanto Earthquake fire in Tokyo 38,000 people were killed in 15 minutes when a huge fire whirl descended upon refugees sheltering in an open area adjacent to the urban fire. Recently, CFD calculations performed by the author of atria fires inside a proposed building produced very energetic fire whirls 5m to 20 m diameter and 40 m tall which roared from one end of the 45 m open space to the other and back in less than a minute (Banks, Chang & Meroney, 2002). Unfortunately, as building atria get larger, attempts to control ventilation during fires in atria may introduce vorticity, which can thus generate “internal” fire whirls.
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