مجله مهندسی مکانیک امیرکبیر
سال پنجاه و پنجم شماره 1 (فروردین 1402)

Renewable energy is vital for the future of the energy supply because of its properties, which include sustainability, affordability, and environmental friendliness. The low dependability of these sources is a disadvantage due to their nondeterministic and unpredictable production patterns. Utilizing several energy production sources in conjunction with energy storage and backup sources could relieve the problem of system dependability. This study recommends a hybrid microgrid that is independent of the grid and consists of primary sources of wind and solar energy production in addition to a battery and generator backup system. This is achieved through the use of dynamic decision-making algorithms, modeling, construction, and assessment, as well as thirteen distinct strategies for the electrical supply of residential units. Under the scenario to use 24% renewable energies, the consumption of fossil fuel is 1,1 liters per day, and the yearly production energy of the total renewable energy conversion system is comparable to 1,697 kWh with a net present value of $553.68. By increasing the renewable energy factor to 54 percent, the consumption of fossil fuel is reduced to 0.69 liters, and the annual production energy is increased to 1,652 kWh. Consumer energy management with a renewable energy factor of 100 percent, an annual energy usage of 1,933 kWh, and a net present value of -$379.

Providing fresh water has been one of the most important concerns of mankind and various methods have been used to solve this concern. In this regard, one of the simplest devices is the stepped solar still. In this research, the effect of using fabric as an absorbent material and its related parameters on the amount of freshwater production has been investigated experimentally. In this regard, stepped solar still has been built and the parameters related to the fabric and their interactions have been tested. These parameters were: type, color, number of layers, and the angle of the fabric relative to the surface of the steps, as well as the height of the water weir in each step. The experiments were designed using the Taguchi method. Among the examined parameters, the color and the material of the fabric have been identified as the most effective factors in increasing the volume of produced fresh water. The results showed that the maximum water production during the day was 2790 ml/m2 using black cotton fabric in three layers with an angle of 90 degrees relative to the steps' surface and a water weir height of 1 cm.

Among the different methods of water supply, extracting water from atmospheric fog has recently received more attention from scientific communities due to its advantages and unique features. In this method, water droplets are trapped and separated from the air using metal meshes. The main challenge facing this technology is the low efficiency of water harvesting from humid air. To overcome this issue, the idea of using a metal porous collector has been tested in the presence of an electrostatic field. Parameters such as the porosity percentage of the collector, airflow speed, electric field intensity, and the distance between the emitter and the collector (field distance) on the water harvesting efficiency were studied experimentally. In the range of investigated porosities, the efficiency of the fog collector has its maximum value with a porosity of 95.6%. Also, reducing the field distance increases the efficiency. Investigating the effect of electric field voltage on the efficiency of the device showed that with the increase in voltage from 15 to 24 kV, the efficiency increases from 21% to more than 42%. At this voltage, the phenomenon of voltage saturation has occurred, so that a further increase in voltage does not have much effect on the efficiency. Investigating the effect of flow speed showed that the maximum efficiency of the device was achieved at a speed of 1.1 m/s.

In recent years, with the continuous growth of natural gas consumption in Iran, the number of pressure drop stations has increased significantly. In throttling valves of these stations, the temperature drop due to the Joule-Thomson effect causes the gas to hydrate, freeze the valves, and block the transmission path. Hence, about 14,000 indirect-fired water-bath heaters have a duty for preheating high-pressure gas before entering them. Unfortunately, the 30% average efficiency of indirectly fired water-bath heaters wastes nearly one billion cubic meters of processed natural gas every year, equivalent to a 400 MW power plant capacity. In this article, intending to optimize, indirect-fired water-bath heaters were modeled thermodynamically and thermo-economically, and three objective functions including thermal efficiency, entropy generation number, and wasted cost number are defined and the mathematical model was proposed in two scenarios. Then the model was solved based on the multi-objective genetic algorithm, using the entropy generation minimization method, and the Pareto optimal fronts of the scenarios were determined. The model implementation results with a deviation of less than ±10% compared to the results of a real sample indicate its acceptable performance. Based on the techno-economic justified results, it is possible to improve the efficiency of indirectly fired water-bath heaters between 48 and 55% depending on the gas volume flow rate. The relations, curves, and dimensionless groups obtained, can be used as a reference for the optimal design of indirect-fired water-bath heaters.

The important feature of viral respiratory diseases is the rapid transmission and spread of these viruses through respiratory processes. In this study, computational fluid dynamics and heat transfer have been used for the airflow inside the human Airway and the surrounding environment, as well as the penetration and spread of droplets from sneezing for a 65-year-old non-smoking man. In the current study, about one million drops from sneezing with an initial temperature of 35 degrees Celsius were injected inside the mouth, and the majority of these drops are small drops with a diameter of 4 to 16 microns. In this study, the k-ω SST turbulence model was used to investigate the flow, and the Euler-Lagrange approach with a one-way view was used to investigate the forces acting on the droplets and also the phase change of the droplets. For a human sneeze with a maximum flow rate of 553 liters per minute through the respiratory system with a temperature of 35 degrees Celsius and a relative humidity of 95% in an environment with an air pressure of one atmosphere, a temperature of 24 degrees Celsius and a relative humidity of 65%, it was determined that the maximum amount of penetration belongs to large drops equal to 3 meters. While the highest amount of spread belongs to small drops equal to 1 meter and the results of droplet evaporation indicated that more than 95% of the droplets injected during sneezing evaporated at the end of 5 seconds.

The present research numerically studies the effect of geometric parameters on the performance of two-phase solid-liquid ejectors. The equations governing the flow inside the ejector include continuity and momentum equations from an Eulerian perspective using the control volume method. The geometric parameters under study were the convergence angle, divergence angle, area ratio (nozzle to the throat), and nozzle position (distance between the nozzle outlet and the start of the throat) in the ejector. In this study, significant design parameters, including the entrainment ratio, critical pressure, and ejector efficiency were introduced and calculated for all the geometric parameters. The homogeneous and the two-phase mixture models were employed to simulate the secondary flow. The results indicate that the data from the two models were in good agreement at low volume fractions (5%), such that the largest error occurred at an area ratio of 0.26 and was equal to 2.3%. The results also indicate that the ejector efficiency increases with an increase in the convergence angle up to 20°, after which it decreases. Moreover, an increase in the area ratio up to 0.22 improves the efficiency of the ejector, after which this efficiency is reduced. Decreasing the divergence angle and increasing the nozzle-to-throat distance also enhance the ejector efficiency. In addition, optimal values were obtained for the design parameters by varying the geometric parameters. These values can be employed according to the application for which the ejector is being used.

Pre-swirl stators can operate as a device to improve the hydrodynamic performance of the propeller and reduce the excess propeller torque on the underwater vehicle. This excess torque on the underwater vehicle with a circular cross-section can cause harmful rolling motion. The most important and influential parameters in the design of these stators are the chord length, distance from the propeller, and angle of attack. In this paper, these parameters are investigated using the Taguchi method and stator design to simultaneously reduce the underwater vehicle roll motion (reduction of excess propeller torque) and increase propeller efficiency using computational fluid dynamics with the help of commercial software STAR-CCM+. To validate the calculations, the numerical simulation results of a B-series propeller are compared with the existing experimental test, the numerical results are obtained with less than ten error percentages compared to the experimental results.  The Grid Convergence Index has also been used to ensure the independence of the results obtained from the mesh. The final stator designed in this paper reduces the total propulsion torque by 44.47% compared to the non-stator mode and improves efficiency by 2.29%. Also, the designed stator reduces the vortex of the blade tip and the propeller hub.

Engineering problems are generally mathematical models of physical phenomena. The solution to the physical problem can be found using engineering and simulation approaches such as numerical modeling. Augmented reality technology adds virtual content created by a computer or mobile phone with a camera to the physical environment of users. Using an augmented reality-based system, after analysis, the simulation results can be placed directly on real-world objects in a two or three-dimensional model corresponding to the location and dimensions of the physical body in order to better understand the results. In this research, using a combination of augmented reality and computational fluid dynamics techniques, a simulation model in an indoor environment with different pollutants has been developed. For this purpose, first, the basic concepts of augmented reality are expressed and its differences with virtual reality are examined. After examining the implementation method of the proposed model, the results for carbon dioxide and carbon monoxide pollutants have been obtained. To evaluate the proposed model, the data obtained from the mobile software are compared with the related benchmark results and show relatively good accuracy and computational capability.