When a large-scale fire, such as a town area fire by an earthquake disaster and forest fire, happens, there can be a fire whirlwind, which is a strong flow including strong flame and sparks. It is sometimes called a firestorm. Fire whirlwind is exposed to high heat, and possesses high heat itself. Therefore, the fire whirlwind is very dangerous. The whirlwind moves and promotes spread of a fire and may enlarge the damage rapidly. Various studies are performed about fire whirlwind, but the property and outbreak mechanism of the whirlwind are not elucidated enough till now. Therefore, in this study, we pay our attention to the flow that is a basic phenomenon of fire whirlwind, and examine the influence that a natural convection gives to outbreak and behavior of fire whirlwind by numerical computation. The numerical analysis performed three-dimensional calculation with analysis software FLUENT6.1. Firstly, a model of the fire domain is constructed. The model is referred from an example of large-scale fire at the Great Kanto Earthquake (1923 in Japan), and natural convection is observed for different heat flux. Then, it is analyzed in a similar model whether a whirlwind occurs or not, after natural convection is fully-developed with changing velocity of the air flow from the side. Furthermore, it is analyzed in a model in which a fire occurs at random whether a whirlwind occurs or not after natural convection is fully-developed with changing velocity of the air flow from the side. As a result of analysis, a whirlwind occurs. The whirlwind sometimes moves and extinct. Then, the influence that a natural convection gives outbreak of the whirlwind is evaluated with changing heat flux and velocity of wind.

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