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The study reports the experimental results of warming-up and evaporation of homogeneous and heterogeneous water droplets at high temperatures, 100–600 °C. The heterogeneous water droplet is formed by covering a solid opaque graphite particle by a thin water layer. Differences between the warming-up mechanisms of the homogeneous and heterogeneous water droplets with an initial liquid volume from 10 μl to 20 μl at the convective heating are shown. The contactless method Planar Laser-Induced Fluorescence allows the analysis of the temperature distributions inside the homogeneous droplets and inside a liquid phase of the heterogeneous droplets, laying emphasis on the novelty of the research. A maximum temperature in the central part of the liquid phase of the heterogeneous droplets is approx. 90 °C; a maximum temperature in the homogeneous droplets is approx. 50 °C. The heterogeneous droplet heats up by 20 ± 3% longer than the homogeneous one. Moreover, a warming-up rate of the liquid phase of the heterogeneous droplet is less than that of the homogeneous water droplet by 85%. However, the size decreasing rate of the heterogeneous droplets is sometimes higher than that of the homogeneous droplets by 80%. The mean and instant evaporation rate of homogeneous and heterogeneous water droplets are compared. The study discusses an influence of convection in the liquid phase of the heterogeneous droplets on evaporation characteristics. The time of the complete water evaporation decreases by 40% due to a solid inclusion presence. A dimensionless criterion of the convective heat transfer enhancement in a water droplet containing the solid opaque inclusion is used to generalize the experimental data. The findings obtained are critically important to improve the existing high-temperature technologies and methods of water purification and to develop innovative ones.

Salt solutions are widely used in many industrial technologies, including those associated with high heat fluxes. The main constraint to the introduction of many promising high-temperature gas-vapor technologies with using salt solutions is that the mechanisms of interrelated processes of heat and mass transfer and phase transformations of such solutions under intense heating have not been sufficiently studied yet. In most modern devices, blocks and setups, intensive convective heating is realized both through gas and liquid media. As a result, the study of these processes under high-temperature convective heating is topical. The most significant results may be obtained by using the low-inertia non-contact measurement methods. This article presents the results of experimental studies of the processes of heating of drops of typical salt solutions (LiBr, CaCl2, LiCl, NaCl) placed in the flow of heated air on the holder. The temperature range was chosen from 20 °C to 600 °C to cover a large group of gas–vapor technologies, primarily evaporation (boilout) and burning (burnout) of impurities from water. Thus, the main attention was paid to the study of convective heat exchange of drops of salt solutions with a heated gas medium. The optical method Planar Laser Induced Fluorescence (PLIF) was used to control the temperature field of the droplet. The convection velocity in the droplet was registered using the optical method of Micro Particle Image Velocimetry (Micro PIV). The main attention was paid to the effects of crystallization, processes of heating and evaporation of droplets, taking into account different properties and component composition of salt solutions. The influence of these factors and processes is shown at different heat-up temperatures. The differences in the rates of heating and evaporation of salt solution drops under intense convection heating have been established. Recommendations on the application of research results for the development of promising gas–vapor technologies have been formulated.

The heat and mass transfer of evaporating heterogeneous water drops was numerically studied to determine the effects of internal solid non-transparent inclusions with different shapes on evaporation characteristics under intensive heating. Explosive breakup times of heterogeneous water drops were determined for various temperature of the gaseous environment with different solid inclusion shapes. Predicted results show that at 550 K, the minimum fragmentation time (~5.5 s) corresponded to the water drops with spherical inclusion as well as the maximum one (~9 s) was the water drops with ellipsoidal inclusion. At higher temperature (850 K), the minimum fragmentation time (~1.5 s) corresponded to the water drops with ellipsoid inclusion while the maximum time was for the water drops with sphere inclusion. Research results could be used for the development such technologies as fire extinguishing, surface treatment by heterogeneous flows, defrosting of granular media by high-temperature gas-vapor-drop flows, and thermal and flame water purification.

High-temperature (over 500 K) evaporation of water, emulsions, slurries and solutions has been extensively studied over the recent years due to a number of prospective applications. Some of these are, for example, the evaporation or burnout of impurities in the course of for thermal wastewater treatment, gasification, as well as flue gas waste heat recovery by intermixing them with water and water vapor to create a new generation of heat transfer agents. Further development of these technologies is constrained by the fact that the approaches used so far are mostly empirical. Obtaining reliable experimental data on evaporation characteristics (vaporization rate and lifetime) of water emulsion droplets in the course of thermal evaporation and burnout of impurities is a pressing task. It is also desirable to experimentally cover all the temperature ranges, droplet sizes, and component concentrations, typical of advanced water treatment. In this research, we use high-speed hardware-and-software system to track the free surface of a droplet. We determine the dependence of heating and complete evaporation time as well as heating and vaporization rate on the gas environment temperature, droplet size, and emulsion component concentration. We then derive the approximating equations for the dependences determined. The difference in evaporation conditions and characteristics is also defined for droplets of water and emulsions with different additive types and concentrations. Finally, we outline different modes of evaporation as well as the required conditions for monotonous evaporation or explosive breakup (pulverization).

The enhancement of the evaporation rate of a heterogeneous water drop is experimentally investigated. A water drop encapsulating a solid graphite inclusion is instantaneously placed in the very hot air. Under action of the high ambient temperature and due to different thermal properties between water and solid inclusion, the initial drop explodes and creates numerous very small droplets. The vaporization rate of the water drop is strongly increased leading to a short time life of the drop. High speed video recording is used to detail the explosive breakup mechanism of a drop immersed in hot air when its temperature varies in the range between 1073 K and 1373 K. The target of the study is to determine the possible conditions for the increase of the water/air surface of the drop after breakup, i.e., the ratio of the final to initial surface, Sout/Sin, and to verify that this enlargement is responsible of the evaporation rate enhancement. For this purpose, we investigate drops of different initial volumes (5–15 µl) with graphite solid inclusions of different shapes and volumes: 2 × 2 × 1 mm, 2 × 2 × 2 mm, or 2 × 2 × 3 mm. The water surface ratio Sout/Sin is given as function of Vw/Vinc, which is the ratio of initial volume of water drop Vw to the volume of solid graphite inclusion Vinc. The study has shown that the maximum value of the ratio Sout/Sin ≈ 15 can be reached when Vw ∼ Vinc. When, the ratio of Sout/Sin varies in the range 1 to 15. The increase of the evaporating surface is of the highest interest to improve the heat transport and to be used and developed in heat technologies.

This paper examines the evaporation features of water droplets moving through high-temperature combustion products of typical flammable liquids (on an example of gasoline, kerosene, ethanol, and acetone). Using Shadow Photography and Interferometric Particle Imaging methods contributed to perform the given experimental research. Such approach enabled to establish the integral characteristics of droplet motion (sizes and velocities) and evaporation process (evaporation rate). The initial droplet sizes (radii) and the temperatures of combustion products varied in a wide range: 0.05–4 mm and 400–1900 K, respectively. The choice of these ranges allowed us to summarize the research results, which can be applied in a group of promising high-temperature (usually more than 1000 K) gas-vapor-droplet technologies (thermal or flame methods of liquid waste treatment; heat carriers based on flue gases, steam, and water droplets; polydisperse firefighting). Conditions of almost the complete and insignificant (with minimum change of liquid mass) droplet evaporation were determined. In addition, the integral characteristics of water droplet evaporation under non-stationary heat transfer (at a temperature of combustion products from 1900 K to 400 K) were compared with the same characteristics under nearly stationary heat transfer (at a temperature of 1100 ± 30 K). During short-time heating (less than 0.5 s), values of these parameters correlated well (deviations did not exceed 8–12%).

This paper investigates experimentally the evaporation rate of water droplets with typical subsoil impurities (clay, silt, soil, sand) during the motion through high-temperature gas environment. The experiments were conducted at Tomsk Polytechnic University using high-speed video recording and modern video processing methods. Here, we analyze the influence of the impurity concentration on the intensity of heating and phase changes. The present study defines, how the preheating of impurities affects the completeness of the evaporation of water droplets with impurities. As a result, of studies, we draw the conclusion that it is possible to use water from natural reservoirs without prior preparation for wildfires extinguishing.

Experimental investigation of integral characteristics of typical extinguishing liquid (water) droplet evaporation in flaming combustion zone was carried out with varying of their basic initial parameters (sizes, relative concentration in a flow or current, temperature, screenings content, structure homogeneity, motion velocities into a flow). Optical methods of two-phase and heterogeneous gas–vapor–droplet mixtures diagnostics (“Particle Image Velocimetry” and “Interferometric Particle Imaging”) were employed. The significant influence of temperature, sizes, structure and droplet concentration in the atomized water flow on evaporation characteristics was determined. The sufficiently moderate influence of salt admixtures on water evaporation conditions at its finely divided injection in the flame was established. Some modes of water droplet motion in the flame area were indicated according to their moving velocity and sizes, and combustion product velocities too. Values of water droplet basic parameters were calculated. These parameters provide the maximum evaporation in the flame zone with intended sizes.