# Rates of High-Temperature Evaporation of Promising Fire-Extinguishing Liquid Droplets
## 1 Introduction  
Technologies of large-scale fire extinguishing are being developed toward simultaneous intensification of different mechanisms of localization and suppression of combustion and thermal decomposition [1]. These are reducing the temperature in the combustion and pyrolysis zone, displacing the oxidant from the combustion zone, and blocking the mixing of pyrolysis and combustion products with the oxidant. Experimental and theoretical studies [2; 3; 4] have shown that the largest influence on the characteristics of combustion suppression is exerted by an endothermic phase transition: evaporation of extinguishing compositions due to a high heat of vaporization (in particular, those based on water--about 2 MJ/kg). Comparing the influence of heat flux due to high heat capacity and energy of vaporization, the authors of Reference [4] justify that it is expedient to spray the compositions in the flame combustion zone. Introducing liquid in large monolithic volumes to this zone is not sufficient. In this case, the liquid volume variation has little effect on combustion characteristics of a fire bed. Therefore, it is important to intensify its evaporation. In recent years, quite a few experimental results have been published for the suppression of pyrolysis and combustion of substances and materials of different dispersion [5] and component composition [6] by aerosol flows. In the interpretations of such research results, it is often concluded that a reliable determination of the values of droplet evaporation rates is rather difficult. It is especially complex at high temperatures (from 300 to 1100 \({}^{\circ}\)C) and at anessentially inhomogeneous component composition, i.e., when using suspensions, emulsions, and solutions. For forest fires, the average temperature of the flame burning of tree crowns is 1100 \({}^{\circ}\)C [2; 3; 4]. At the tree base, the temperature can reach 200 to 500 \({}^{\circ}\)C [2; 3; 4]. Therefore, the most relevant range of studied temperatures of intensive evaporation of fire extinguishing agents is 200-1100 \({}^{\circ}\)C. At temperatures above 600 \({}^{\circ}\)C, inhomogeneous liquid droplets (especially water-containing emulsions and suspensions) are characterized by the effects of intensive dispersion (in puffing or micro-explosion modes) [7; 8; 9; 10; 11]. Therefore, it is difficult to assess the differences in their evaporation rates under such conditions. Thus, as the first approximation, it is advisable to study the evaporation characteristics of fire-extinguishing water-containing compositions in the temperature range up to 600 \({}^{\circ}\)C.  
Papers [12; 13] presented the determined rates (and, accordingly, the typical durations) of high-temperature heating and evaporation of water droplets at intensification of one of the three mechanisms of energy supply (conductive, convective, and radiation). Heating systems with substrate, air flow, and muffle furnace were used. With each of these systems, the total heat flux included components due to conductive, convective, and radiative heat transfer. When certain temperature values were reached, the pronounced dominance of one of them was recorded. Since droplet heating in the experiments [12; 13] was essentially unsteady and was accompanied by intensive evaporation, the use of even miniature low-inertia thermocouples was complicated. Therefore, the non-contact optical method of Planar Laser Induced Fluorescence (PLIF) was used [12; 13]. Three stands were developed for heating water droplets on a substrate (conductive mechanism) in a muffle furnace (radiation) and in a heated air flow (convective). Heat flux densities corresponded to promising high-temperature gas-vapor-droplet technologies in the field of fire and thermal purification of liquids from non-regulatory impurities, and corresponding to polydisperse fire extinguished with rational water consumption, etc.