Nikolay B. Miskiv

This study experimentally investigates how surface wettability and adhesion influence the deformation and stability of water droplets in a uniform external electric field. Special focus is placed on highly hydrophobic surfaces with strong adhesion (“rose petal effect”), which may arise on functional coatings during operation. Hydrophilic silicon, fluoropolymer, a natural rose petal, and a laser-textured biphilic surface with a controlled seating spot were used as model substrates. Experiments were performed with droplets of different volumes, enabling variation of the balance between capillary and electrostatic forces. The results show that droplet deformation strongly depends on adhesion and surface microstructure. On superhydrophobic and biphilic substrates, droplets exhibit significant elongation along the field direction, leading to Taylor-cone-like deformation and the onset of morphological instability. A threshold-type response is observed at fields around 4 kV/cm. Surfaces with high adhesion can retain larger droplets, which experience possible local field enhancement near the cone-like apex under high electric fields. These findings are relevant for designing hydrophobic coatings for high-voltage insulators and for assessing the risk of discharge initiation on degraded or defective surfaces.

The relevance of the work is due to the need to improve the efficiency and uniformity of spray cooling of large surfaces in heat-loaded systems. An experimental study of heat transfer during spray cooling with water of unmodified and laser-textured titanium surfaces measuring 70 × 70 mm² was carried out under heating conditions below the saturation temperature. Laser treatment led to the formation of a developed micro- and sub-microrelief and a significant increase in surface hydrophilicity: the water contact angle decreased from 60° for unmodified titanium to 4.8° for the modified surface.,It has been shown that under the same heating and spray cooling conditions, the laser-textured surface provides a lower average temperature and more uniform wetting of the working area, including peripheral regions. At a heat flux density of 14.3 W/cm2, the average temperature of the modified surface was 1.4–1.6 °C lower than that of the unmodified surface. The maximum achieved heat flux density increased from 16 W/cm2 for the original surface to 20 W/cm2 for the laser-modified surface. It was found that under identical spray cooling and heating conditions, the heat transfer coefficient of the modified surface exceeded that of the unmodified surface by 14–42%, with the relative increase growing as the coolant flow rate increased. The obtained results indicate that increased hydrophilicity promotes more uniform liquid distribution, efficient liquid film renewal, and intensification of single-phase convective heat transfer. The practical significance of this work lies in substantiating the use of laser texturing of titanium surfaces as a method to enhance the intensity and spatial uniformity of spray cooling over large surfaces.

The relevance of this work is related to the need to improve the efficiency of spray cooling of heat surfaces under high thermal load conditions. An experimental study of heat transfer during spray cooling with water of unmodified and laser-textured titanium surfaces measuring 70 × 70 mm² was carried out under heating conditions below the saturation temperature. Laser treatment led to the formation of a developed micro- and sub-microrelief and a significant increase in surface hydrophilicity: the water contact angle decreased from 35.7° for unmodified titanium to 4.8° for the modified surface.It has been shown that under the same heating and spray cooling conditions, the laser-textured surface provides a lower average temperature and more uniform wetting of the working area, including peripheral regions. At a heat flux density of 14.3 W/cm2, the average temperature of the modified surface was 1.4–1.6 °C lower than that of the unmodified surface. The maximum achieved heat flux density increased from 16 W/cm2 for the original surface to 20 W/cm2 for the laser-modified surface. It was found that under identical spray cooling and heating conditions, the heat transfer coefficient of the modified surface exceeded that of the unmodified surface by 14–42%, with the relative increase growing as the coolant flow rate increased. The obtained results indicate that increased hydrophilicity promotes more uniform liquid distribution, efficient liquid film renewal, and intensification of single-phase convective heat transfer. The practical significance of this work lies in substantiating the use of laser texturing of titanium surfaces as a method to enhance the intensity and spatial uniformity of spray cooling over large surfaces.

A new model for heating and evaporation of a multi-component liquid film that takes into account the contribution of the heat flux from/to the wall (the Neumann boundary condition) and supply/removal of liquid to/from the film is described. The Robin boundary condition at the outer surface of the film is used. The film is assumed to be sufficiently thin to allow for the description of the heat transfer and component diffusion processes using one-dimensional heat transfer and component diffusion equations. These equations are used at each time step of the calculation. The solutions to these equations at the end of the time step are used as initial conditions for the next time step with updated values of the input parameters. The predictions of the numerical code are validated using in-house experimental data that refers to water spray impact on a vertical wall. The verification of the model is performed by comparing the predictions of two in-house numerical codes, one based on the new model and the other on the numerical solutions of the heat transfer and component diffusion equations, for a water–ethanol mixture with initial equal mass fractions of the components.

The article presents an approach and tools for estimating the sizes of vortex structures generated in the annular space of the Couette–Taylor flow. T

New experimental and modeling results, referring to heating and evaporation of sessile and pendant water droplet on a biphilic surface, are presented. Two modeling approaches are used: one based on the previously developed variable density model in which the droplet shape in the presence of gravity is described by the Bashforth–Adams equation, and the other based on ANSYS Fluent. It is shown that the results predicted by both approaches almost coincide in the absence of gravity, which can be considered as verification of both approaches. The predictions of both approaches are shown to be close to experimental results for pendant droplets. For sessile droplets, however, both approaches tend to under-predict experimental data. The difference in model predictions, taking and not taking into account the effect of natural convection, is shown not to exceed 3% for the experimental conditions under consideration.