نشریه مهندسی مکانیک مدرس
سال بیست و چهارم شماره 12 (آذر 1403)

This study investigates the effects of water-methanol mixture injection on the performance and emissions of the EF7 TC engine. Using GT Power software, the engine was first simulated and validated with gasoline fuel. Subsequently, a nozzle was used to introduce the water-methanol mixture, simulated in three different ratios: 50% water-50% methanol, 25% water-75% methanol, and 75% water-25% methanol. The novelty of this research lies in the simulation of this injection process to enhance combustion quality. Results indicate significant temperature reductions at various points, alongside notable changes in knock characteristics and emissions, including nitrogen oxides (NOx), carbon monoxide (CO), carbon dioxide (CO₂), and nitrogen monoxide (NO).

Biogas is a low-calorie fuel comprises 50-70% methane and 30-50% carbon dioxide, with small amounts of other particles. Combustion of low-calorie fuels often involves significant challenges related to flame stability in most burners. Combustion of porous media is an effective method of directing flame heat to the input mixture, which can increase flame stability. In most studies, biogas has been used in experimentaly work or numerical simulation with simple geometry. In this paper, researchers simulate a two-layer porous burner with biogas fuel, based on an experimental design, in two dimensions. They evaluate the effect of the burner geometry, which was not investigated in previous researches, on the temperature distribution and the radiation efficiency. The results show that reducing the amount of carbon dioxide increases the burner surface temperature. Additionally, changes in the interface of the porous layers, simulated in two conical and spherical forms in two converging and diverging states, cause changes in the place of flame, the maximum combustion temperature, the temperature of the burner surface, and the radiation efficiency. The maximum combustion temperature and the maximum burner surface temperature occur for the conical geometry in convergent mode. Increasing 10% of carbon dioxide in the biogas fuel reduces the radiation efficiency by 25% on average. The radiation efficiency of the divergent burner is more than the convergent mode, about 37% for conical geometry and about 25% for spherical geometry. The maximum radiation efficiency is achieved when the burner is divergent and the amount of carbon dioxide is 30%.

Nowadays, fresh water scarcity is one of the major concerns of the global community. To tackle the freshwater scarcity situation, several solutions have been suggested, including the extraction of water from fog-laden flow. Fog harvesting is known as a sustainable and effective approach to supplying freshwater. Various types of fog collecting elements (FCEs) have been implemented in studies to collect water from fog-laden flow. Woven meshes with square-shaped holes are among the most frequently employed FCEs in studies. One of the major drawbacks of these types of FCEs is their low water collection efficiency, particularly at low wind velocities. In this study, two alternative mesh hole geometries, triangular and hexagonal, were proposed to enhance the collection efficiency and compared with an equivalent square mesh in terms of the shading coefficient (SC). The evaluations were conducted experimentally using an experimental setup capable of mimicking atmospheric fog-laden flow at two different air velocities. The results indicate that the water harvesting rate is highly affected by mesh hole geometry. Using triangular and hexagonal meshes, compared to square mesh, can improve the water collection rate by up to 12.6% and 29%, respectively

The accurate prediction of crack initiation and growth in manufacturing processes is crucial for minimizing production costs and enhancing the reliability of components. This study focuses on integrated experimental investigation and fracture modeling approach for ductile metals, particularly addressing the mechanisms of ductile fracture and shear localization. The importance of establishing robust damage criteria for accurate reliable numerical simulations cannot be denied. Current literature reveals a significant lack of data on shear and ductile fracture criteria for materials like stainless steel alloy 304. To address this gap, a series of experimental tests was conducted to extract the necessary coefficients for these criteria. Various sample geometries were analyzed to investigate the effects of different triaxiality stress states and loading rates on fracture initiation. The triaxiality stress states were chosen within a range of 0.2 to 2 and strain rates were applied at values of 0.02 s-1, 4.5 s-1, and 30 s-1. A set of coefficients for modeling ductile and shear fracture was derived, taking into account the effects of loading rate and orientation. This research not only provides critical coefficients for fracture modeling but also supports the optimization of manufacturing processes in the automotive industry and other sectors, ultimately contributing to improved material performance and component reliability

In thermal barrier coatings (TBC), surface cracks, debonding, and thickness degradation may occur during the manufacturing process or life cycle, leading to poor performance and ultimately a dangerous system failure. The main goal of non-destructive testing of thermal barrier coatings is to detect these defects and determine the health of the coating. Various non-destructive inspection methods have been proposed to evaluate thermal barrier coatings, and due to the numerous advantages of thermography, including high speed, low cost, safety, no need for direct contact, automation capability, and inspection of a large area of ​​the part, this method has received special attention from researchers. This study will present a method for manufacturing samples with different diameters of artificial separation defects. The following is the equipment's arrangement and the sample's thermography process. It was concluded that blackening the surface of the sample by increasing the amount of thermal energy absorption increased the ability to identify separation defects and increased the signal-to-noise ratio by 257%. Finally, by implementing different filters on the recorded raw thermal images, it has been shown that in both cases the best filter in terms of SNR is the median filter and then the Gaussian filter. The background removal filter also had no noticeable effect on increasing the signal-to-noise ratio and acted as a complement to the median and Gaussian filters by reducing the fixed error.

This study investigates the combustion of hydrogen-methane mixtures in the annular combustion chamber of a C30 microturbine. The primary objective is to evaluate the impact of premixed methane-hydrogen combustion on pollutant emissions and outlet temperature in an annular combustion chamber. Simulations were performed using a partially premixed combustion model and the k-ε turbulence model, employing the Probability Density Function (PDF) approach for chemical reaction modeling. To ensure a detailed analysis of pollutant emissions, comparisons were conducted at a constant turbine inlet temperature. The results indicate that adding hydrogen to methane increases NOx emissions due to the higher flame temperature compared to pure methane, even at constant turbine inlet temperatures. However, this blend can reduce fuel consumption by up to 35%. Additionally, a fuel mixture of 60% methane and 40% hydrogen results in a 61% reduction in CO2 emissions. The study further revealed that, owing to the premixed nature of the fuel-air mixture, the annular geometry, and the swirling flow pattern within the combustion chamber, a fuel blend containing 30% hydrogen can lower NOx emissions to 16.1 ppm—significantly less than the 46 ppm reported in previous studies. Moreover, increasing the hydrogen fraction in the fuel reduced CO emissions by 16%. These findings demonstrate that annular combustion chambers with premixed flows and hydrogen-methane fuel blends have considerable potential for reducing pollutant emissions and optimizing fuel consumption.