# Rates of High-Temperature Evaporation of Promising Fire-Extinguishing Liquid Droplets
## 1 Introduction
In such applications, operating temperatures are usually above 500 \({}^{\circ}\)C and sometimes exceed 1000 \({}^{\circ}\)C. The initial dimensions (radii) of water droplets ranged from 1 to 2 mm [12; 13]. A high-speed (up to \(10^{5}\) frames per second) video recording system was used to determine the times of complete evaporation of water droplets (their existence). Quantitative differences in the heating rates (the ratio of heating to complete evaporation times) of water droplets on the three stands used were established using the PLIF method. The maximum temperature values of droplet heating at different mechanisms of energy supply were determined. The criterion dependences relating the main characteristics of formation of the temperature field of the evaporating water droplet to the heating conditions were obtained. The prospects of application of research results and development of the created experimental approach were highlighted. However, in References [12; 13], the experimental data are given only for water droplets. In the case of inhomogeneous component compositions of droplets, the PLIF method will show a significant error in registering their temperature (due to the influence of additives on the optical properties of a drop with a fluorophore dye for the implementation of PLIF measurements). Therefore, the most rational method is to study the evaporation rates of inhomogeneous extinguishing agents using high-speed video recording, i.e., the rate of reduction of droplet size during heating [14]. In recent years, a lot of promising water additives has been developed for effective extinguishing and localization of forest fires. Among them, there is a typical group of powders and liquid additives [15; 16; 17; 18; 19; 20]: bentonite, bischofite, foaming agent, and extinguishing agent EA-5. With their application of aqueous solutions, suspensions and emulsions are prepared. The rates of heating and evaporation of water droplets with the addition of such promising fire-extinguishing impurities have not been established yet. Since their rheological and thermophysical properties significantly differ from water, noticeable discrepancies in the rates of heating and evaporation are also foreseeable. Furthermore, these experimental data will serve to predict the rates of heating and evaporation not only of fire extinguishing agents, but also of various suspensions, emulsions, and solutions, in which the evaporation has been actively studied in recent years. The approach based on a significant decrease in the temperature of liquid droplets due to endothermic phase transformations is used in many technological processes, especially in microelectronics [21], chemical industry [22], spray cooling [22; 23], spray combustion [24], and more. Aqueous salt solutions are used in heat pumps, which are widely applied in the energy sector [25]. Therefore, the results of experimental studies of heating and evaporation of droplets of suspensions, solutions, and emulsions are important for developing not only firefighting but other technologies as well. As a result, in the well-known research works, the ranges of heating temperatures and concentrations of components of suspensions, solutions, and emulsions are very wide. For example, some of the studies provided data for evaporation of a drop of sulfur-purified wastewater (electrolyte) with different concentrations of solute, obtained by suspending it on a glass fiber in a heated chamber at varying ambient temperatures (90-170 \({}^{\circ}\)C) [26]. The radius of the droplet during evaporation was recorded. When a drop of distilled water evaporated, the temperature was measured (a thermocouple junction was placed in the drop). A mathematical model was developed to take into account the influence of concentration of the water activity and density changes. The solution concentration in the experiment was significantly higher than that obtained in the simulation.