Effect of heat treatment on corrosion of laser-textured aluminum alloy surfaces

The kinetics of electrochemical corrosion of aluminum alloy (AlMg6) surfaces with different wettability was analyzed. The surfaces were processed by three different methods, in particular, polishing, laser texturing, the combination of laser texturing and low-temperature heating. After laser processing, the dimple-like texture was formed on the surface, and the wettability significantly enhanced. Low-temperature heating of laser-textured AlMg6 alloy surfaces led to the wettability inversion from strongly hydrophilicity to superhydrophobicity. Microscopic and profilometric methods were used to estimate the surface degradation due to corrosion when aggressive solution droplets (a mixture of NaCl and hydrogen peroxide aqueous solutions) evaporated. The potentiodynamic polarization measurements of AlMg6 alloy surfaces were conducted. The typical modes of corrosion and evaporation of aggressive solution droplets were detected. The kinetics of corrosion was estimated by the behavior of the corrosion area evolution. In addition, when immersing laser-textured sample with strongly hydrophilic properties into aggressive solution, the higher corrosion rate was found in the liquid meniscus region (aggressive mixture / alloy / air interface) compared to the textured site immersed in the mixture. This was explained by convection increasing the rate of reaction products removal and promoting a stronger deviation from the equilibrium state in the aggressive mixture. Experimental results of the potentiodynamic polarization measurements revealed that laser-textured samples exhibit enhanced corrosion protective properties compared to polished samples.

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The behavior of water droplets on the heated surface

The effect of the volume of distillate droplets and temperature of the heated wall on evaporation process was investigated. The strong influence of thermal-physical and geometrical parameters of the wall on droplet evaporation is shown. A change in the ratio of droplet diameter to the wall thickness can lead to a change in evaporation regimes. Distribution of interface temperature along the droplet length is measured. At evaporation the wall temperature under the droplet changes significantly. A new method for measurement of the droplet mass on the heated wall is presented. The regimes of droplet boiling differ significantly from pool boiling. The droplet mass is directly changed in time; evaporation is separated into several stages.     

The dependence of evaporation and crystallization kinetics on dynamic and thermal background

In technical problems there is often the need to simultaneously describe both non-stationary evaporation and non-stationary crystallization of droplets and films. Poor wettability of the wall and high heat fluxes lead to the film rupture and the formation of pools, dry spots, droplets. To date, there are no well-developed methods for calculating crystallization and evaporation in the presence of the factors. Evaporation and crystallization of a drop are shown to fundamentally differ from those of a thin layer. The rate of evaporation prior to crystallization controls the crystallization kinetics. The evaporation and crystallization rates for the film (under identical conditions) are significantly higher than for the drop. The difference in the thickness of the diffusion and dynamic boundary layers affects the evaporation rate of an aquatic salt solution. For correct simulation of evaporation, it is necessary to take into account the thicknesses of the thermal, diffusion and concentration boundary layers.     

Minimization of stresses in stiffening ribs of the cover of a water turbine

The procedure for optimum design of the contour of an opening in the stiffening ribs of the cover of a water turbine is proposed. The problem of nonlinear programming with the purpose of minimization of maximum stresses in the given structure is solved with the use of the gradient method.     

Graviatom of Superheavy Dark Matter as a Source of Gravitational Radiation

The nature of dark matter (DM) in modern physics is an important problem. We consider a quantum system consisting of two identical particles which are candidates for the role of DM.We call them graviatoms, and they could be formed in the early Universe. We find the energy spectrum of the gravitatom from DM particles, the Bohr radius, the intensity and frequency of gravitational radiation, and the lifetime of a state. This lifetime is large for superheavy particles. Estimates of these values for different masses of DM particles are given. The amplitude of a gravitational wave from a cloud of dark matter in the Milky Way is also estimated.     

Free Solution Convection at Non-Isothermal Evaporation of Aqueous Salt Solution on a Micro-Structured Wall

Evaporation and heat transfer of layers of aqueous salt solutions have been studied. The behavior of salt solutions is compared for a smooth and micro-structured wall with a rectangular profile. The evaporation rate of the salt solution on the structured wall is 20–30% higher than on the smooth one at high salt concentration. Previously, it was thought that the heat transfer for solutions can be calculated for thin layers and films without taking into account the natural convection in liquid. In this paper, the liquid free convection is shown to play a key role. A simple model linking the solutal and the thermal Marangoni numbers and the Peclet number with free convection of the liquid on a hot structured wall is considered. For correct simulation of the non-isothermal heat and mass transfer, it is necessary to take into account local characteristics of thermal and velocity fields inside a layer of the salt solution, as well as to determine the average characteristic scales of circulation into the liquid. To simplify the analysis it is possible to effectively consider four types of characteristic convective scales, the role of which depends on the thickness and diameter of the solution layer, as well as on the wall temperature. The strong influence of free convection in a thin layer of the solution is extremely important for accurate modeling of a wide range of modern technologies. Intensification of heat transfer and evaporation due to the use of a structured wall can be applied in heat exchangers, to improve efficiency in desalination of water, in energy technologies (e.g., in heat absorption pumps), as well as in chemical technologies.