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Small drops to put out large fires

12.02.2007
Russian scientists are developing a mathematical model of both the fire itself and the technology needed to fight it that uses disperses water.

As a result it will be possible not only to gain a thorough understanding of the processes that occur with microscopic water droplets in the zone of the fire, but also to select optimal, that is the most effective and economical, means to put out a fire. Information support for the project comes from the International Science and Technology Centre, whose specialists found the project to have immense potential.

At the heart of the project, which involves the work of specialists from the All-Russia Research Institute of Experimental Physics (Sarov) and their colleagues from the St. Petersburg State Polytechnic University, lies the use of sprayed water where the individual droplets are about 100 microns in size. While it may seem strange to the layperson that such small droplets could put out a blazing fire, the specialists are in no doubt at all. Its advantages are in the following.

An aero-suspension (aerosols or droplets in air) has an immense specific surface, so the burning zone rapidly cools. The volume of the dispersed water cloud, too, is an order greater than from a jet of water of the same mass. Therefore the efficiency coefficient or, more specifically, the coefficient of water use, is tens of times higher as compared with a usual jet, even if such a jet is both powerful and precisely directed. Furthermore, in the event of a forest fire, for example, when the area of coverage is very large, it is practically impossible to cope with the fire with individual jets or with a localized strike per se; while pouring on water in one place, the fire flares up in another. In addition, the damage caused to a building by putting out a fire is comparable to the damage from the fire itself, while a mist would be very unlikely to even affect electronic equipment.

In theory, there is no doubt that the method has considerable promise. However, in practice things do not always turn out so simply. To ensure the method really is effective, a multitude of factors have to be borne in mind. These factors include the correlation between the specific power of the focal point of the burning and the sizes of the space to be protected, the size, concentration and rate of movement of the droplets, the intensity and duration of the process of extinguishing the fire for different fire fighting systems that use fundamentally different means of obtaining the active material and many others.

Furthermore, serious theoretical studies are needed, relative to the formation and the behaviour of water droplets at high temperature, and to the parameters of gas flows in the area of the fire and in its direct vicinity. The fact is that the high efficiency of this method is conditioned by the fact that dispersed water rapidly cools the zone of the burning, while water vapour forces out the oxygen from it. This is the so-called phlegmatization method, in part resembling how fire is covered with a thick blanket, only here a cloud of mist covers the entire zone of the fire at once, depriving the fire of heat, air and, finally, its very life. However, this is of course if the dispersed water reaches the zone of the fire, and in the sufficient quantity. Therefore, conditions have to be created so that its convection flows were as if drawn into the area of the fire before the minute droplets can evaporate from the heat or before they are carried away by a flow of hot gases.

Mathematical modelling helps the scientists to cope with such complicated and multiple-factor research. They have already achieved serious success in developing the mathematical model of certain processes that occur during the burning and extinguishing of fires with dispersed water. However, virtual experiments alone are not enough for a complete solution of the set tasks; what are needed are genuine, full-scale experiments, as it is these that will enable the more precise definition of the numerical model and the final determination of the optimal means for putting out a fire with dispersed water.

VNIIEF has the required conditions, that is, a testing ground, where such experiments may be conducted. However, unfortunately it is not yet sufficiently equipped with the requisite equipment, such as IR sensors that would enable rapid measurement of temperature in any point, both in the very focal point of the fire and in the space around it. Yet the scientists know for sure what equipment they need and they are confident that, with the required financing, they could develop the technology to help put out a fire rapidly, effectively and as safely as it could possibly be done. The guarantee of success lies in the colossal experience and knowledge that is available.

Andrew Vakhliaev | alfa
Further information:
http://www.tech-db.ru

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