Vaporization of water droplets with non-metallic inclusions of different sizes in a high-temperature gas

Abstract

Experimental research on mechanisms of vapor production by boiling and evaporation was carried out on heterogeneous droplets of water containing graphite particles of different sizes and concentrations, heated by high-temperature environment (up to about 1373 K). Different steps have been observed, such as free surface evaporation of the droplet, bubbles boiling at the solid inclusion/liquid interface, the explosive disintegration of a drop into a cluster of small droplets and efficient evaporation of the produced smaller droplets. We proposed the conditions for the appearance of this explosive breakup of droplets. The present paper reports that the addition of small graphite particles into heterogeneous droplets can reduce their lifetimes by about 50% in this type of high-temperature environment. Moreover, this facilitated the explosive breakup of droplets, allowing intense vaporization. This behavior led us to make the hypothesis that an insulating layer of vapor appears at the outside droplet surface and at the interface between the solid inclusions and the droplet. Measurements on the explosive breakup of heterogeneous water droplets have allowed establishing an increase of the evaporation surface area by almost fifteen times, as compared to the initial surface of droplets. However, when using graphite suspension, the measured increase of the evaporation surface was not larger than threefold. The evaporation surface area during the explosive breakup of water-graphite suspensions was 5.4 smaller as compared to water without impurities. We reported on the reasons for the above differences. The results are useful for developing technologies of fire extinguishing by using water sprays containing non-metallic solids. Practical implementation of the explosive breakup of heterogeneous droplets can eventually permit extinguishing fire on larger areas with an identical water load.