Сapillatry effects manifested in the process of draining a liquid film that evaporated into a gas cross flow
Authors: Kostiuchenko Yevhen , Butar Vitalii , Romanko Serhii , Lukashov Volodymyr 
Published: March 2022

The aim of the work is to determine the regularities of changes in the width of the flowing liquid film evaporating into a gas cross flow resulting from thermocapillary and concentration-capillary effects (Marangoni effect).

The research was carried out experimentally employing an installation with a heated (working) surface, on which a liquid film (water or aqueous glycerol solution) was flowing. A flow of air was fed into the space between the working surface and the enclosing wall located at a distance from it, evenly in height and perpendicularly to the film movement. The film width was measured along the height of the working surface and the obtained dependencies were presented as graphs for different parameters of the flowing process.

In all experiments, the graphs have an initial section with a sharp decrease in the film width, below which its change occurs smoothly. The degree of reduction in the film width depending on the parameters of the flowing process can reach 80%. In laminar mode, the reduction of the film width is more uniform than in turbulent mode, when some rise of the curves beyond the initial section is observed and can be explained by intensive mixing of the liquid. As the initial liquid flow increases, the degree of reduction in the film width decreases. Significant influence on the change in the film width is exerted by the temperature of the working surface during the flow of water: with increasing temperature, the film width decreases. At values of this temperature greater than 100℃, the rise of the curves is observed, possibly associated with the transition to volumetric evaporation.

When the glycerol solution is flowing, the temperature of the working surface less influences the change in film width, although its decrease along the height of the working surface is greater than for water. The process parameters such as initial liquid temperature, initial solution concentration, and velocity and air temperature have relatively little effect on the change in film width. To correct the area of the film evaporation surface, determined by technological calculation, the width of the average evaporation surface is introduced under capillary effects. Its value increases with the initial liquid flow. For water, this dependence is characterized by a jump-like decrease in the film width in the transition from laminar to turbulent flow mode. This effect is absent on the dependence for glycerol solution, which can be explained by its higher viscosity. As the temperature of the working surface increases, the width of the average evaporation surface of the film decreases, some decrease being also observed with an increase in air velocity. Empirical equations for estimation of the correction factor were obtained. Capillary effects in this case are expressed by decrease in the width of the film at the height of its runoff, which leads to the need to correct the area of the film evaporation surface determined by technological calculation.