Vol 28, No 2 (2019)

A superhydrophobic surface coating of a nanotubes array grown directly on a titanium substrate was prepared by using the method of electrochemical anodic oxidation under ultrasonic assistance and fluoroalkyl silane modification. Experimental investigations of nucleate pool boiling and fouling behaviors were carried out to assess the heat transfer enhancement and fouling inhibition on a superhydrophobic titanium-based nanoporous coating. The superhydrophilic titanium-based nanoporous surface and the bare titanium substrate surface were also investigated as contrasts. The results indicate that the heat transfer performance of the superhydrophobic titanium-based nanoporous coating is superior to the bare titanium substrate and the superhydrophilic titanium-based nanoporous surface due to higher density of nucleation sites in the range of low heat fluxes. Besides, the superhydrophobic nanoporous coating also has lower fouling resistance and better antifouling performance at pool boiling of CaCO₃ solution in comparison with bare and superhydrophilic surfaces.

The article presents the results of an experimental study of the effect of static pressure on reduction of turbulent friction on a plate in gas saturation of the boundary layer. The experiments were carried out on the hydrodynamic stand of the Research Institute of Mechanics of the Moscow State University in the flow rate range U = 4–11 m/s and the static pressure range P = 10–200 kPa, the Reynolds number varied in the range of (3–8)×10⁶. The local friction was measured with a “floating wall” sensor on a strain gage balance, and the gas was blown through a special permeable coating. It was shown that increase in the gas flow rate through a porous wall of flat plate test model could make the reduction of the local friction as high as 80–90%. However, increase in the static pressure in the flow significantly lowered the efficiency of control of the turbulent shear stresses. The friction reduction due to gas saturation of boundary layer depends on the gas flow rate (ceteris paribus) and the static pressure in the flow, which one should take into account when performing integral estimate of the efficiency of the method and assessing the energy loss for gas inlet into the flow.

The present paper deepens the analysis on the dynamics and thermal-fluid-dynamics of a sprinkler nozzle-emitted water droplet traveling in air before reaching the ground. The following analysis variables were considered, as main causes of the phenomenon assessed: initial droplet diameter, initial droplet velocity, initial droplet temperature, variable droplet temperature, air temperature, diffusion coefficient of water in air, air relative humidity, inlet droplet throw inclination, thermal radiation, and wind velocity. These variables proved their effect on the droplet travel distance, the droplet time of flight, and the final droplet temperature. The study was carried out thanks to an analytical-numerical model, recently proposed by the same authors of this paper, equipped with a few assumptions that did not alter the realistic description provided. The results proved the effect of each analysis variable and clarify the picture of a complicate yet extremely important phenomenon.

In the present work, heat and mass transfer from different foods by impinging a slot hot air jet at low Reynolds numbers is numerically researched. Banana and apples are selected as foods due to their trading importance. Low Reynolds numbers are operated as Re = 100, 200 and 300. The diameters of the foods and initial jet height are taken as fixed in all examined situations and the impinging jet is laminar and two-dimensional. The shape of foods is assumed as a cylinder. The distance (D/H) between the cylinder and the slot is taken as another effective parameter of drying process and it is used with the three different distances (D/H = 0.22, 0.25, and 0.33) to search jet effectiveness on heat and mass transfer. A finite volume method is employed to solve governing equations of mass, momentum and energy by means of ANSYS Fluent 17.0 program. There is a good agreement with the numerical and experimental data available in the literature. The heat transfer increments and temperature variations for different Reynolds number values and for varying distance, D/H, and also temperature and mass distributions are researched for both inside the foods. It is obtained that heat and mass transfer enhanced with reducing the distance, D/H, between the jet and the cylinder. In addition, locally, the most effective jet drying is achieved close to the stagnation point on the front face of the foods.

Density measurement was performed on S,N-GQDs nanoparticles dispersed in base fluids such as water, ethylene glycol and 60:40 water/ethylene glycol. Firstly, we synthesized S,N-GQDs nanoparticles by the chemical vapor deposition method. UV-Vis absorption spectrometry, XRD, FT-IR, EDX and SEM analysis were done to elucidate the structure and morphology of the as prepared samples. The measurements were performed with different weight percentages from 0.09 to 0.9%. The temperature range of the measurements ranged from 20° to 60° C. Also, artificial neural network method and improved Tao-Mason equation of state (TM EOS) were used to calculate the density of nanofluids. These models are in good agreement with experimental results.