Giovanni Manzini, Ph.D., engineer Luca Iannantuoni and Professor Pierangelo Andreini of the Energetics Department of Politecnico di Milano in Milan, Italy, are investigating the development of fire with flames characterized by horizontal plane rotational motions (whirling flames or, in high intensity cases, fire tornadoes).
Fire tornadoes are an infrequent but potentially catastrophic event. These phenomena, with flame vortices with diameter values more than 300 meters and height up to 1200 meters, were observed in the Chicago and Peshtigo fires in October 1871, and well as the fire bombings of Hamburg in 1943 and Dresden in 1945.
The enhancement of the heat release rate (HRR), the large sizes (especially with regard to height), and the strong drag actions on the surrounding areas can determine high-magnitude damages. Fires of this kind can develop either outdoors or in confined environments, and the main development causes are the great values of HRR and the ventilation conditions. Because of the complexity of these phenomena, the means necessary to do an effective analysis are physics and mathematical analysis, experimental activity. and computational thermo-fluid -dynamics (CFD) modeling and codes. In this way it’s possible to clarify the phenomenology of these fires and contribute to optimization of prevention and protection activities.
The formation of whirling flames can be undertaken in the controlled or accidental combustion fields. In accidental cases the rotational motion is not directly imposed to reagents mixture, but it is present inside the air surrounding the flames (“the circulation is externally imposed”). This rotational motion may be because of forced or natural ventilation (outdoor winds, room doors or windows). In such cases, a mechanism to boost the initial and slow rotational motion of reagents mixture is needed. This mechanism is the accelerated vertical motion imposed to fluids by heat release of combustion reaction.
This type of phenomena is characterized, many times, by a greater axial development compared to no rotational motion cases. This means, usually, greater flame height, greater flame volume, and greater HRR value. Moreover, in general, this flame has also relevant development and sustainment difficulties, because many times it is unstable (instability leading to vortex breakdown).
Overall, the whirling flames formation, development and sustainment (of the accidental type) needs a vorticity source, which puts an initial rotation into the flame (a slow rotation is sufficient), and a mechanism to amplify this motion, which increases the speed of flame rotation and some conditions improving flame rotational motion stability. These “whirling flames primary mechanisms” are the subject of the Politecnico di Milano team’s study, done in cooperation with Marioff HI-FOG water mist protection systems.