# Rates of High-Temperature Evaporation of Promising Fire-Extinguishing Liquid Droplets
## 4 Results and Discussion
If the radiation was absorbed only in the near-surface layers, the evaporation rates of the droplets of all five liquid compositions would be very close. During the experiments, the surface temperature of the droplets \(T_{\mathrm{d}}\) was measured. Similar to the experiments with water droplets in the study [12], we have found out the tendency to a pronounced time interval during which the droplet warms up intensively. However, after its completion, the droplet temperature does not increase, i.e., it passes to a quasi-stationary state. Similar to the study [12], we can conclude that there are asymptotic values of \(T_{\mathrm{d}}\). For each of the heating temperatures \(T_{\mathrm{a}}\), the maximum values of droplet heating \(T_{\mathrm{d}}\) differed such as in the experiments with water [12]. For example, in experiments with water at \(T_{\mathrm{a}}\) of about 100 \({}^{\circ}\)C, the droplet warmed up to 40 \({}^{\circ}\)C. Differences in \(T_{\mathrm{d}}\) registering from experiments of Reference [12] were about 2 \({}^{\circ}\)C, which is due to a slightly different layout of droplets on the holder and inertia of the measurement system. In Reference [12], a system based on the optical PLIF method was used, and the present work employed a low-inertia thermocouple. In this paper, it was not possible to apply the PLIF method for all the studied compositions since the luminosity of the fluorophores (dyes) Rhodamine B and Rhodamine 6G significantly depended on impurities in water. In the comparative analysis of the time for warming up droplets of the studied compositions to some practically constant (asymptotic values) temperatures, it is possible to distinguish key features of the influence of thermophysical and rheological characteristics of liquids. In particular, Figures 10 and 11 demonstrate the results of registration of the heating times (\(t_{\mathrm{h}}\)) of droplets to almost constant values \(T_{\mathrm{d}}\) and temperature drop \(\Delta T_{\mathrm{d}}\) (the difference between the final and initial value of \(T_{\mathrm{d}}\)). It is clearly seen that the slowest heating was characteristic of droplets of EA-5 solution. This is due to the above highlighted property, associated with the formation of a thermal insulation buffer film. Therefore, the properties characteristic of EA-5 are simultaneous extinguishing capability and fire-protective ability, blocking the phase flaming combustion, and smoldering of combustible forest materials. Foaming agent emulsion and water without impurities warmed up faster. Bentonite suspension and bischofite salt solution warmed up a little longer, but to large asymptotic values of \(T_{\mathrm{d}}\). When comparing the data of Figures 10 and 11, the relation of maximum values of \(T_{\mathrm{d}}\) and warm-up times \(t_{\mathrm{h}}\) was observed for almost all compositions. The higher the \(T_{\mathrm{d}}\) value was reached, the longer the heating took place, but the evaporation process was generally faster (i.e., the total evaporation times were smaller). Analyzing the features of the \(T_{\mathrm{d}}\) changes for the EA-5 solution, we can note the maximum temperatures in contrast to other droplets, i.e., the droplets of this solution intensively accumulated the heat supplied. For other compositions, the \(T_{\mathrm{d}}\) values were generally quite close to each other. The previously mentioned information explains the peculiarities of the formation of two buffer zones in the droplets of EA-5, which leads to the maintenance of the maximum temperature on the drop surface, and leads to significantly lesser energy passing into the deeper layers. As a consequence, heat accumulation occurred in the near-surface layer of EA-5 droplets (thus, the \(T_{\rm d}\) values on the surface are maximum), unlike in other compositions. The results of the studies have shown that the integral characteristics of the processes of heating and evaporation of fire extinguishing agent droplets significantly depend on a set of factors considered in this paper.