Maxim V. Piskunov

Mechanism of heat transfer in heterogeneous droplets of water under intense radiant heating

Thermocouple measurements of temperature have been performed at three main points of heterogeneous water droplet–high-temperature gases system: on the surface and in the depth of a solid inclusion, as well as on the free surface of the water droplet. Investigations have been carried out for water droplets of an initial volume of 5–15 μl with single inclusions of cubic graphite particles of a typical size of 1 mm. The gas temperature varied from 700 K to 1200 K, which corresponds to the main practical applications: thermal purification of water from solid and liquid impurities, fire extinguishing, treatment of heat-loaded surfaces of power equipment, etc. A hypothesis about the dominant role of radiant heat transfer in vaporization within heterogeneous water droplets has been grounded. It has been shown that in a short period (a few seconds), the surface temperature of an opaque solid inclusion within a droplet can reach the boiling point of water. A significant change in the optical properties of water with increasing temperature has been revealed, i.e., water became partially transparent to the infrared radiation. Presence of an opaque heterogeneous inclusion enhances this effect due to intensification of the heating of the water film. The heat and mass transfer characteristics obtained in the experiments were used for designing a model that takes into account the radiative properties of water film and adequately reproduces the results of thermocouplemeasurements. Based on the findings of the investigations, a conclusion has been formulated that models of high-temperature evaporation of water droplets should be developed with due account of changes in the optical properties of water and formation of a vapor buffer layer around inclusions.

Modeling the Water Droplet Evaporation Processes with Regard to Convection, Conduction and Thermal Radiation

A predictive model was developed for investigation of high-temperature heating and evaporation of water droplets. The model takes into account the basic interrelated processes of heat transfer and phase transitions. Typical velocity and temperature profiles were found in the high-temperature gas–water droplet system with external gas medium temperature varied from 100 to 800°C. Various formulations of the problem, significantly different in the type of considered processes and factors, are considered.We analyzed temperature conditions of heating and evaporation of water droplets, which allow the use of simplified models and which need consideration of all complex interrelated processes of heat and mass transfer (including convection, conduction and radiant heat transfer in droplets, and also in the surface vapor–gas layer).

Enhancing Boiling and Explosive Breakup of Evaporating Heterogeneous Water Drops in High Temperature Gaseous Media

The influence of the passive method of heat transfer enhancement by increasing of the surface area of solid inclusions in water drops, on the conditions and characteristics of their boiling and breakup in conditions of intensive vaporization, was experimentally studied. Moreover, the main characteristics of these processes were defined: heating time of a drop till breakup, number and total surface area of formed droplets in comparison with the initial one (before breakup), limiting (critical) temperatures of sustainable (controlled) breakup of water drops. The substantial influence of artificial surface irregularities, porous surface geometry before and after water vaporization and inclusion shape (using the example of an irregular shape polyhedron and a cube) on the enhancement of heat-exchange processes in the water drop – solid inclusion system and the initiation of the explosive breakup of the liquid film of a heterogeneous drop was also demonstrated. Horizons of the explosive breakup application were distinguished for systems of thermal and flame purification of water from impurities, manufacturing a syngas from water solutions, emulsions and suspensions.

Development of the mathematical model of heat and mass transfer for researching the processes of evaporation of inhomogeneous water droplets

On the basis of experimental date the heat and mass transfer models are developed in ANSYS Fluent software package for researching the processes of evaporation of inhomogeneous water droplets. The influence of the temperature of the gases (550-850 K) on the evaporation of inhomogeneous water droplets is estimated. Times of complete liquid evaporation from free surface of inhomogeneous water droplets and explosive vaporization of water droplets at different gas temperatures are established.