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.