نشریه یافته های نوین کاربردی و محاسباتی در سیستم های مکانیکی
سال دوم شماره 4 (زمستان 1401)

As an eternal and widespread energy source, solar energy has a low density while its intensity is changing continuously. Unavailability of the solar energy in nights and the gap between the time of radiation and its consumption are concerned as the main drawbacks of this type of energy. In applications such as domestic hot water (DHW), phase change materials (PCMs) can successfully remove this shortcoming due to their high thermal capacity and constant temperature during the phase change process. However, thermal conductivity of water is relatively low which reduces the performance of vacuum tube solar collectors. This properties can be improved substantially with the utilization of nanofluids. This paper presents a numerical study on nanofluid natural convection in a vacuum tube solar collector with phase change materials. The studied nanoparticles include copper oxide, titanium oxide, iron oxide, aluminum oxide, and graphene oxide. The obtained results show that for all of the current nanoparticles, rise in the nanoparticles volume fraction is accompanied by a decrease in the exit temperature of the collector. It is found that the highest temperature belongs to the graphene oxide nanoparticles.

In this research, two types of composite coatings based on epoxy paint reinforced by TiO2 nano and micro particles and TiO2 particles hydrophobicized by polysiloxane were prepared separately with concentrations of 1, 2, 3, 4 and 5% by weight to improve the properties of epoxy paint. and in order to investigate their behavior, they were applied on API 5L X52 steel and then their properties were investigated by conducting corrosion and wear tests. The thickness of the coatings was checked by a digital thickness gauge. The corrosion resistance behavior of the coatings was evaluated using the polarization method in 3.5% NaCl solution. The results of the corrosion test showed that by adding and increasing the concentration of reinforcing particles in the coatings, the corrosion resistance was improved, and also the composite coating of epoxy paint with 5% by weight of the hydrophobic TiO2 reinforcement showed better corrosion resistance than other samples. A scanning electron microscope (SEM) equipped with EDX was used to examine the morphological properties of the surface. In order to check the dispersion of micro particles in the field of coating, Map analysis was taken from the samples. The results obtained from Map analysis have shown the appropriate and uniform distribution of hydrophobic particles in the field of epoxy coating. The wear resistance of the coatings was performed by the bullet test machine on the disc. The results of the abrasion test showed that by adding and increasing the concentration of reinforcing particles in the coating, the abrasion resistance of the coating increased, but by comparing the epoxy composite samples, it was found that the hydrophobicity of the reinforcing particles had no effect on the abrasion resistance.

Statistical methods are used in the analysis of all types of data. In this paper, using multivariate analysis of variance and multivariate regression, the data related to the thermal conductivity of nanofluids have been analyzed and the results of the two methods have been compared. A two-step method was used to prepare ethylene glycol-magnesium oxide nanofluid. In order to prepare the nanofluid by suspending the nanoparticles in the base fluid, an ultrasonic homogenizer was used. For this purpose, nanoparticles with diameters of 20, 50, and 100 nm in volume fractions of 0.25, 0.5, 0.75, 1, and 1.25% have been used at temperatures of 25, 30, 35, 40, 45, and 50 degrees Celsius. Transient hot wire method was used to measure thermal conductivity in different volume fractions. Then the obtained experimental values were analyzed using SPSS.26 software. The coefficient of determination and the graphs of the errors obtained in the two methods showed that when the independent variables are defined as grouped, the use of multivariate analysis of variance can better describe the dispersion of the thermal conductivity.

Cooling panels using the half pipe method is one of the most important methods used to increase the efficiency of solar resources. In this article, a practical project for cooling a solar panel based on the half-pipe method has been carried out with the aim of increasing the efficiency of solar panels. In the proposed design, the half-pipes are installed directly on the bottom surface of the panel, and the available fluid, which is water in this technique, is inside the half-pipes and in direct contact with the panel, and performs the cooling process in a spiral on the surface of the panel. Two panels have been used to compare the performance of a panel equipped with a half pipe and a panel without this system, the panels have dimensions of 150 x 180 and power of 200 watts. Three sensors have been used to measure the temperature of the panels and the environment. The results show that using the half-pipe method can reduce the temperature of the panels by about 10 degrees Celsius. In addition, the results show a 36% increase in solar panel efficiency by using the half-pipe method.

In this research, the thermal and hydraulic characteristics of the shell side of the shell and tube heat exchanger with perforated quatrefoil plate are optimized by a gray wolf and genetic algorithms in a single-objective multi-objective manner. The objective functions are heat transfer capacity for the maximum value and pressure drop for the minimum value. Shell and tube heat exchanger variables for optimization are: the diameter and number of tubes, the Reynolds number, the distance between baffles, and the height of the quatrefoil hole. The results show that for the maximum heat transfer of the quatrefoil baffle, the tube diameter is 0.03 m, the number of tubes is 30, The Reynolds number is 20000, the height of the perforated hole is 0.0018 m, and the distance between the baffles is 0.15 m. For the lowest pressure drop value, the diameter of the tubes is 0.03 m for the square arrangement and 0.01 m for the triangle arrangement; the Reynolds number is 5000, the height of the perforated hole is 0.003 m, and the distance between the baffles is 0.25 m. The optimization by the gray wolf and genetic algorithms has the same results for the shell and tube heat exchanger with a quatrefoil baffle.

Stress cracking is an important threat to steel industries. When certain conditions are created in the environment of steel, the loading of this defect leads to intergranular cracks in the radial-axial plane. These cracks can grow under continuous adverse loading and eventually lead to rupture. A lot of research and experiments have been done in this regard. This article is a review of the combination of conditions governing steels that lead to the growth of stress cracking. Failures caused by stress cracking occur in a long period during several stages such as germination, crack growth and final failure. On the other hand, we know that stress cracking depends on environmental, metallurgical and mechanical conditions. Therefore, in this article, the purpose of investigating the loading conditions at each stage of stress cracking and the effects of repetitive stress on steels will be discussed. Also, the synergistic effects of different parameters during the entire life cycle of steels will also be discussed. The main focus of this review is to relate the loading conditions at each stage of crack propagation, which plays an important role in estimating the remaining life of steels susceptible to stress cracking. This discussion will include how cyclic loading conditions can change steel properties and contribute to the initiation of intergranular cracking. Finally, the modeling of stress crack growth will be discussed and suggestions such as failure analysis due to stress crack growth on sheets were given.