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Russian researchers work to boost fire-fighting potential with water droplet explosions

Researchers from the Tomsk Polytechnic University (TPU) have come up with a new approach to extinguish wildfires that effectively uses water.
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The approach is based on the explosive splintering of liquid drops, which include solid non-transparent particles.

«In a small non-homogeneous drop, on the boundary between a liquid and a solid particle, the high-temperature heating causes an almost simultaneous vaporization which, in turn, sparks a small explosion. Scores of tiny droplets are formed, the evaporation surface area is consequently increased, which means that more thermal energy can be converted into steam. As a result, the time needed to extinguish a fire and the volume of used water are reduced,» explained study’s coauthor, a research assistant at the Department of Heat and Power Process Automation at TPU, Maxim Piskunov Ph.D. in Physical and Mathematical Sciences.

Procedures to put out forest fires are customarily carried out using aircraft. A plane takes the water and drops it down on the target, which leads to flooding an area while the fire-fighting effect remains rather minimal. The descending flow is composed of large drops, which fail to transform a blaze’s thermal energy into steam, up to 90% of delivered water goes into the ground and the fire-fighting effect of the volume dumped on it, as a rule, does not justify the expenses needed to deliver water to the emergency site.

The result could be improved by supplying water in the form of smaller drops. The larger the evaporation area is, the faster the steam forms. However, the small drops are disappear with the flow of heated air, and they do not reach the fire source.

«It is essential that a large enough drop, upon striking the flame, can break down to the necessary size and then evaporate. The solution to this problem is to apply explosive fragmentation of a liquid which envelops solid particles inside the drop,» the scientist explained.

During the course of the experiments, the researchers added graphite particles ranging in size from 1 to 3 mm to the drops, which occupy about 50−70% of a drop’s volume. Under high-temperature heating (from 500 to 900 degrees Centigrade which corresponds to normal fire conditions), the mineral is heated very fast because of its thermal, physical, and optical properties. This results in steam forming on the boundary between the water and the particle.

With this, the water’s surface area triples and can even expand up to 15-fold, and, consequently, the evaporation area is also increased. According to the scientists, this approach makes it possible to reduce the amount time needed to extinguish a fire and the volume of water by one-third when compared to the conventional approach where water without any admixtures is applied.

«Now, we continue our investigation by studying the influence of the composition, shape, and size of particles on the behavior of drops. According to our data, the explosive fragmentation and intensive steam formation on the boundary between a particle and a liquid take place only with graphite of natural origin. However, we also believe that limestone is prospective material, since it shows thermal and physical properties similar to graphite» Piskunov noted.

The study’s key task is to find a way to conduct the fragmentation of non-homogeneous drops during a flight within a split second. Now, the minimal time required for fragmentation in stationary binding at high-temperature conditions is 2−3 seconds, while the formed drops of micron size evaporate simultaneously. This value is already notably smaller than the time (more than 10 seconds) needed for evaporation of a similar-sized (5−15 microliters) pure drop. For a drop under wildfire conditions, this is enough time to penetrate the flames and absorb into the ground.

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