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The characteristics of water droplet heating and evaporation on structured hydrophobic and hydrophilic surfaces in the range of static contact angles from 73° to 155° were studied experimentally using high-speed video recording. Two fundamentally different technologies for applying coatings on a metal surface were used in comparison with the results on a polished surface. Microscopic studies were conducted to identify the features of the formed coatings. The wetting properties were characterized by means of the static contact angle and the contact angle hysteresis: on polished surface No. 1 (contact angle—73°, hysteresis—11°), on structured surface No. 2 (contact angle—125°, hysteresis—9°), and on structured surface No 3 (contact angle—155°, hysteresis—7°). The experimental dependences of the droplet evaporation rate on the different surfaces under normal conditions (ambient air temperature—293 K, atmospheric pressure, humidity—35%) were obtained. The evaporation regimes of droplets on the surfaces under study were identified. Water droplets evaporated in the pinning mode on surfaces No. 1 and No. 2. When a water droplet evaporated on surface No 3, the droplet was in the constant contact angle regime for ≈90% of its lifetime. Based on the experimental data obtained, a two-dimensional model of conjugate heat and mass transfer was developed, which describes the heating and evaporation of a liquid droplet on structured hydrophobic and hydrophilic surfaces at a wide range of contact angles. Satisfactory agreement was obtained between the numerical simulation results and experimental data. Using the model, the fields of temperature, concentration and other key characteristics were established at different points in time. Recommendations for its application in the development of gas–vapor–droplet applications were formulated. [ABSTRACT FROM AUTHOR]

The main characteristics of ignition and combustion of fuel droplets (organic coal-water fuel, oil emulsion, and oil in the usual state) placed on the heated surfaces of structural steel (DIN standard grade X16CrNi25-20) were experimentally determined under conditions corresponding to the start-up and nominal operation of power-generating equipment. It was shown that due to the application of texture on the steel surfaces, it is possible to change the ignition and combustion characteristics of fuel droplets. A graphic-analytical method was developed to predict the dimensions of textures in the form of microchannels using laser technology for processing metal surfaces. It was found that the texture configuration in the form of microchannels formed by nanosecond laser radiation on steel surfaces makes it possible to significantly increase the surface resistance to adhesion of combustion products of liquid and slurry fuels. [ABSTRACT FROM AUTHOR]

The contact line movement, advancing and receding, of the distilled water droplet was studied on the polished and laser patterned aluminum surfaces. The obtained dependences of the dynamic contact angles on the contact line speed were fitted by the combined theory consisting of molecular-kinetic and hydrodynamic theories. Results obtained on laser patterned surface are found to fit better to the theory in comparison with polished surface. The assumption was made that this result is connected with the low contact angle hysteresis on the patterned surface compared to the polished one. It means that combined theory does not consider the contact angle hysteresis.

The influence of drop irrigation of the condensation part on the thermosyphon operational mode was studied experimentally. The temperature and pressure fall was found in the internal space of the upper chamber after applying the liquid on the upper wall of the thermosyphon. The temperature difference along the height of vapor channel increases and the effective resistance decreases. When complete evaporation of liquid droplets from thermosyphon, it returns to the quasi-stationary mode of operation.

Paper presents the experimental results of mass evaporation rate from ethanol layer surface into a moving in parallel way air flow. Dependences of the mass evaporation rate on the gas velocity, temperature difference between gas and liquid layer, and liquid layer thickness were obtained. The assumption about the formation of the concentration boundary layer at the interface was made. It is found that with increasing the gas flow velocity the gradient of vapor concentration in the gas phase increases; as a result the evaporation rate rises. When changing the liquid layer thickness, the local maximum of evaporation rate was observed, but for 2 mm thickness layer two maxima were found. Probably, it is due the appearance of unstable structures on the liquid surface.

A model of a closed two-phase thermosyphon was developed to study thermo-physical processes in a thermosyphon thermal condition systems of energy-saturated equipment. This model differs from the known by reasoned choice of thermocouples installation providing sealing the device during operation. In addition, the program VI (Virtual Instrument) was created in graphical environment Labview for registration of the temperatures and pressure in the thermosyphon.

Results of experimental studies of heat transfer in a thermosyphon illustrating the influence of the filling ratio and the heat load on the temperature distribution in the vapor channel, evaporation and condensation zones are presented. The thermosyphon was made of copper and was 161 mm high with side walls 1.5 mm thick, bottom cover 2 mm thick, an internal dimmer of the evaporation part of 54 mm and an internal diameter of the vapor channel of 39.2 mm. Based on the results of experimental studies, temperature dependences were established in the characteristic cross sections of the thermosyphon on the heat flux value supplied to the bottom cover. In addition, a well-appearing thermosyphon self-regulation property has been found – the growth of the heat load in the evaporation zone in the range from 1940 to 7685 W/m2 does not lead to a decrease in the heat removal intensity from the heat-release region.

We present the results of the experimental study of temperature fields in a closed two-phase thermosyphon. Operational modes of a thermosyphon with different heat supply conditions are studied experimentally using setup consisting of the copper case, systems of heat supply and removal in evaporation and condensation zones, and temperature recording facilities. The height of the heat exchanger is 161 mm, thickness of the side walls and bottom wall are 1.5 mm and 2 mm, respectively, inner diameter is 39 mm. Heat is supplied to the bottom wall by heating element. The heat carrier is distilled water. We obtained thermograms when heat fluxes to the bottom wall of the thermosyphon are 695 – 2136 W/m2.

The paper presents an analysis of known dependencies for determining the critical heat flux density in diphasic thermosyphons. The critical heat flux density for the created experimental model of thermosyphon were calculated on the basis of the theoretical contributions of 1) the occurrence of a “flooding” regime in a thermosyphon characterized by a disturbance of the hydrodynamic stability of the phase interface and the entrainment of the liquid phase by the gas flow; 2) the mutual influence of gravitational forces and surface tension; 3) S.S. Kutateladze hydrodynamic theory of the heat transfer crisis during boiling. It is found that the existing theoretical contributions which can be used to calculate the critical heat flux density and subsequently determine the minimum filling ratio of a thermosyphon are conditionally applicable.

Change of geometrical characteristics of distilled water and aqueous ethanol solution droplets was studied under their evaporation on aluminum surface. According to change in the contact diameter three evaporation modes of distilled water droplet on polished aluminum surface were detected: increase in the contact area, pinning of a droplet (constant contact area), and droplet depinning (decrease in the contact diameter). During evaporation of aqueous ethanol solution droplets, two evaporation modes were detected: increase in the contact area, and droplet depinning.

Experimental results analyses of macroscopic contact angle change from the drop growth rate of distilled water on stainless steel substrates is presented. The value of contact angle is found to depend on the drop growth rate (m): contact angle decreases at m up to 0.02ml/s and increases at m more than 0.02ml/s. At the final stage of spreading the contact line pinning occurs, depending on the drop growth rate and surface roughness.

The paper presents experimental data on the contact angle hysteresis of the distilled water drop spreading over the surfaces of non-ferrous metals. The measurements of the advancing and receding contact angles were carried out by method of sitting drop on the horizontal surface during increasing and decreasing drop volume with a syringe pump. It was found that the contact line speed has a great influence on the hysteresis of the polished non-elastic substrates. The mechanism of spreading was described using the balance of the forces from the physical point of view.

The paper presents the experimental study of the effect of the liquid volume on the static contact angle under the condition of the stationary contact line on the rough and polished surfaces of non-ferrous metals. The experiments were carried out using the shadow optical system. Comparison between two methods of drop profile processing was conducted. It was found that in addition to the friction and gravity forces, the structure of the non-ferrous metals significantly influence on the static contact angle during changing the drop volume.

This work presents the experimental results of the studying the effect of surface roughness, microstructure and liquid flow rate on the dynamic contact angle during spreading of distilled nondeaerated water drop on a solid horizontal substrate. Copper and steel substrates with different roughness have been investigated. Three spreading modes were conventionally indicated. It was found that the spreading of drops on substrates made of different materials occurs in similar modes. However, the duration of each mode for substrates made of copper and steel are different. Spreading of a liquid above the asperities of a surface micro relief was observed to be dominant for large volumetric flow rates of drops (0.01 ml/s). Liquid was spreading inside the grooves of a rough substrate at low rates (0.005 ml/s).

This paper presents the experimental dependence of the evaporation rate of a nondeaerated distilled water drop from the heat flux density on the surfaces of non-ferrous metals (copper and brass). A drop was placed on a heated substrate by electronic dosing device. To obtain drop profile we use a shadow optical system; drop symmetry was controlled by a high-speed video camera. It was found that the evaporation rate of a drop on a copper substrate is greater than on a brass. The evaporation rate increases intensively with raising volume of a drop. Calculated values of the heat flux density and the corresponding evaporation rates are presented in this work. The evaporation rate is found to increase intensively on the brass substrate with raising the heat flux density.

The concept of conducting the experiments for studying thin liquid film boiling and evaporation was developed. Implementing this conception on developed experimental setup, we will obtain the data on the change of liquid film thickness in thermosiphon and temperature distribution in the liquid film, also the evaporation rate of liquid film and heat transfer coefficient change will be calculated using the measured values in the experiment. Three series of preliminary experiment were conducted. As a result, the main influencing factors and their values were defined.

The investigation of dynamic contact angle between distilled water droplet and superhydrophobic surface was conducted experimentally. To obtain the values of contact angle during spreading, shadow method was implemented. We used Schlieren method to control the droplet symmetry. The droplet was formed on the surface by syringe pump using nontraditional bottom-up methodology. The drop growth rate was varied by the syringe pump from 0.005 ml/s to 0.32 ml/s. DCA versus drop volume was obtained for different values of the drop growth rate. Some features of spreading on superhydrophobic surface were pointed out. Interestingly, that the dynamic contact angle increases during all stages previously selected for copper substrates (1 – droplet formation; 2 – spreading; 3 – formation of the equilibrium contact angle). However, at the droplet growth rate 0.005ml/s we found decrease in the contact angle. The droplet was found to take a shape close to a spherical cap in the range of the drop growth rate 0.005 ml/s – 0.16 ml/s. At higher rates (0.32 ml/s and higher) liquid splashing was observed.

We studied experimentally the evaporation process of distilled water drops on the surfaces of non-ferrous metals. Investigations were conducted on the experimental setup using a shadow optical system. The main elements of this system are the source of plane-parallel light and photographic camera. According to the contact diameter change during the evaporation, three stages have been determined (spreading, pinning, depinning). It has been found, that the dependence of evaporation rate on drop volume at low temperatures appear to be well fit by a power function.