ИСТИНА

Evaporation at the free surface of liquid results in cooling of the surface and intensive heat exchange between bulk liquid, thin surface layer and atmosphere. The surface layer has thickness about 1 mm, and temperature drop is typically about 0.5 K. Depending on the conditions, different structures can be observed at the surface and inside the surface layer, including Marangoni convection vortices, cold liquid filaments, motionless “cool skin” and tops of Rayleigh vortices. Despite its small thickness, it is this surface layer which determines the intensity of heat exchange between liquid and atmosphere and evaporation rate. In the present study temperature fields in evaporating liquids are measured by simultaneous use of Background Oriented Schlieren (BOS) technique for the side view and IR thermal imaging for the surface distribution. Good agreement between the two methods is obtained with typical measurement error less than 0.1 K. Two configurations of surface layer are observed: thermocapillary convection state with moving liquid surface and small thermal cells, associated with Marangoni convection, and “cool skin” with negligible velocity at the surface, larger cells and dramatic increase of velocity within 0.1 mm layer beneath the surface. These configurations are shown to be formed in various liquids (water with various degrees of purification, ethanol, butanol, decane, kerosene, glycerine) depending rather on initial conditions and ambient parameters than on the liquid. Water, which has been considered as the liquid without observable Marangoni convection, actually can exhibit both kinds of behavior during the same experimental run. Evaporation is also studied by means of numerical simulations. Separate problems in air and liquid are considered, with thermal imaging data of surface temperature making the separation possible. It is shown that evaporation rate can be predicted by numerical simulation of the air side with appropriate boundary conditions. Comparison is made with known empirical correlations for Sherwood-Rayleigh relationship. Numerical simulations of water-side problem reveal the issue of velocity boundary conditions at the free surface, determining the structure of surface layer. Flow field similar to observed in the experiments is obtained with special boundary conditions of third kind, presenting a combination of no-slip and surface tension boundary conditions. This implies extremely viscous character of the surface film, which has not been explained yet.