Iranian Journal of Science and Technology-Transactions of Mechanical Engineering最新文献

Abstract

Cross-section analysis is an important tool used to recover stresses and strains in a structure at specific cross-sections of arbitrary geometries, without the need for a full 3D model. This is particularly essential for large-scale structures such as aircrafts, wind turbine blades, etc. where making a full model can be computationally very expensive or impractical. The majority of currently available cross-section analysis frameworks are based on stepwise prismatic assumptions, which are hardly suited for the analysis of tapered beams. In fact, high-fidelity stress analysis obtained from analytical and full 3D models shows that predictions of stepwise prismatic approximations can significantly deviate from the correct solution of tapered beams. In this work, a prismatic 3D cross-section analysis method is extended to analyze a symmetrically tapered finite cross-section slice. In this study, the cross-section slice is discretized with 8-node and 20-node solid elements. The boundary conditions are applied as six constraint equations via the Lagrange multiplier method. The external nodal forces acting on the cross-section faces are obtained from the equivalent tractions induced by the cross-section forces. The developed numerical model is validated against the exact analytical solutions of a wedge as well as commercial finite element (FE) software COMSOL and it is shown that the numerically predicted displacement and stress fields agree well with those provided by the wedge’s analytical solution and the FE COMSOL results. This work contributes to the advancement of high-fidelity numerical tapered cross-section analysis methods with an application for many engineering structures.

In this paper, the nonlinear vibration and dynamic bifurcation of axially moving plates under subsonic airflow in a narrow space concerning the background of the mining industry are investigated. The nonlinear dynamic equations interacting with narrow space airflow are established using Hamilton’s principle and linear potential flow theory. The dynamic bifurcation of vibration characteristics of axially moving plates caused by airflow is studied. The displacement–time diagrams, phase diagrams, and Poincare maps are plotted to distinguish the motion behaviors. The incremental harmonic balance method is used to study nonlinear vibration. The effects of airflow velocity, axial velocity and the narrow gap height on stability and nonlinear vibration characteristics are discussed. With the increase of axial velocity and air velocity and the decrease of narrow gap height, the resonance frequency of the plate decreases and the vibration peak increases. A smaller narrow gap height magnifies the effect of airflow on stability and nonlinear vibration, and a larger narrow gap height makes the magnification disappear. The findings in this paper provide valuable insights into the nonlinear vibration of axially moving thin plates interacting with subsonic airflow in a narrow space, and improve the understanding of the stability, controllability, and predictability of this system in future design works.

In this study, stresses and deformations of a rotating functionally graded magneto-electro-elastic (FGMEE) disk with non-uniform thickness considering internal heat generation, convection, and radiation heat transfer were investigated. The power-law function of the radial coordinate was considered for the properties of the material. Also, the heat conduction and convection coefficients are functions of temperature and radius. The heat transfer equation was derived considering thermal gradient, convection thermal boundary, heat source, and solar radiation. The differential transformation method (DTM) was used for solving the obtained nonlinear differential equation of heat transfer. Then, the equilibrium equation of the disk was derived and solved analytically. So, the radial stress, hoop stress, radial deformation, electric and magnetic potential, electric displacement, and magnetic induction can be obtained. Finally, some numerical examples were presented to examine the effects of the heat source, convection heat transfer, temperature dependency, solar radiation, inhomogeneity index, and angular velocity on the stress, deformation, electric displacement, and magnetic induction of the disk. The results show the tensile radial stress, deformation, electric displacement, and magnetic induction decrease for higher values of source power and solar flux intensity, while changes for the higher values of convection coefficient and thermal conductivity are opposite. Also, using a non-uniform thickness disk with an outer thickness smaller than the inner thickness can reduce the displacement and electromagnetic potentials.

Aerators are extensively utilized in wastewater treatment applications; however, they encounter challenges, such as high energy consumption, which has a direct impact on their environmental and economic benefits. Among these challenges, the leakage occurring at the top gap of the aerator impeller is a significant factor leading to impeller loss. Installing leaf tip winglet on the impeller can partially inhibit the flow through the top gap and enhance the performance of the impeller. This research focuses on optimizing the meridian and rotary surface parameters of the impeller using the Taguchi design methodology. The optimized model is then chosen, and CFD software is employed to simulate the impact of various leaf tip winglet widths on the internal flow and performance of the aerator impeller at the suction surface. The findings reveal that increasing the width of the leaf tip winglet can diminish the flow through the top gap and delay the formation and shedding of the leakage vortex at the top. Furthermore, it alters the location of the vortex, shifting it away from the suction surface and reducing separation loss. At the highest efficiency point, a 15 mm leaf tip winglet width results in a 0.61% increase in full-pressure efficiency. This study offers valuable insights for the development of energy-efficient aerators.

In this study, the flow and heat transfer components of convection are numerically investigated in a hybrid nanofluid-filled, porous-medium enclosure with wavy walls. The flow is considered to be buoyancy-driven under a constant inclined magnetic field and heat radiation (Rd). The cavity is partially heated from its left wall and is cooled by its wave-like right wall while the other walls are adiabatic. To express the results, streamlines, isothermal, and the Nu are used. Analysis is done to determine how heat transfer is affected by thermal radiation (Rd), the Hartmann number Ha, the inclined magnetic field, the left heater’s dimensionless location (D), the heat source’s dimensionless length (B), and the hybrid nanofluid’s volume fraction. The average Nusselt number is increased when the volume friction of hybrid nanofluids increases. Additionally, as the dimensionless heat source length B rises, the rate of heat generation rises as well, enhancing the buoyancy force while reducing the impact of shear-driven force. The left heater’s dimensionless position, D = 0.7, exhibits the largest local Nu in contrast to other occurrences. It was found that the minimum Nu occurred at the heat generation/absorption coefficient Q = − 8 at the lowest wall of the enclosure because the intensity of the isothermal formed at the upper wall of the enclosure was greater than that at the bottom of the enclosure in comparison to other cases. The results also showed that, due to the irreversibility of magnetic force, which is one of the main processes for heat transmission, isentropic lines diffuse toward the interior of the enclosure as porosity decreases. On the surface of the enclosure’s vertical left wall (Y-axis at X = 0), the Nu shows as symmetrical profiles, and it can be seen that the Nu increases as the wave length of the wavy walls diminishes. The effects of the Hartmann number and Darcy number on streamlines and isothermal temperature are also investigated.

Exergoeconomic assessment of an energy conversion system based on energy-exergy analysis and appropriate economic principles, is essential to identify the costs of the inefficiencies both for the whole integrated system and for individual energy components. The current study contributes to an exergoeconomic analysis focusing on the steady-state performance of a biomass-fed combined heat and power (CHP) system including a two-stage auto-thermal biomass gasifier, a direct internal reforming planar solid oxide fuel cell (DIR-PSOFC) and a micro-gas turbine (mGT). A one-dimensional model of the DIR-PSOFC is used to investigate the temperature gradient within the solid structure of the fuel cell under different operating conditions. In order to assess the effect of the main system input parameters on the performance of the cogeneration system, a comprehensive parametric analysis is carried out. The results show that the highest rate of exergy destruction takes place in the gasifier with an amount of 39.23%, followed by the afterburner and the SOFC due to the highly irreversible nature of the process of these components. The system input exergy supplied by biomass is 525.7 kW, of which 53.2% is wasted in the system components and the exergy efficiency of the total CHP system is determined to be 49.72%. Furthermore, the results indicate that the highest exergy destruction cost rate is related to the afterburner with 2.39 ($⁄h). Based on the results of the sensitivity analysis, the trends of the performance parameters demonstrate some conflicts with the variation of the operating parameters, which implies the necessity of an optimization procedure. In all the operating conditions considered, the temperature difference along the cell length is kept below the maximum allowable temperature gradient, which is 150 K. Two-step multi-objective optimization has been conducted by use of non-dominated sorting genetic algorithm technique. Significant and newsworthy relationships between the optimal operating parameters and the considered design variables have been unveiled using the Pareto-based multi-objective optimization procedure.

In the industrial sphere, mixed-flow centrifugal pumps are widely used to convey acids, alkalis, brines, petroleum and other media. Their performance has a significant impact on the production efficiency of industrial systems. This paper presents an analysis of the influence of positive pre-rotation on the hydraulic performance and energy conversion characteristics of mixed flow pumps under off-rated flow conditions. The accuracy of the numerical methods adopted in this study was validated by experiments, with less than 5% error observed in head and efficiency between the experimental and numerical results at rated flow condition. The influence of positive pre-rotation on the pump’s performance was found to be contingent upon the prevailing flow conditions. Under part-load flow conditions, positive pre-rotation improved the match between the medium inlet angle and blade inlet angle, leading to a reduction of the high vorticity area near the blade inlet, and consequently an improvement of hydraulic performance. However, under overload flow conditions, positive pre-rotation causes a further increment in the difference between the media inlet angle and the blade inlet angle. An increase in the secondary flow intensity within the impeller is induced, which leads to flow instability and ultimately to significant energy losses. These findings contribute to the theoretical basis for the prediction of mixed flow pump performance and operational stability improvement under off-rated flow conditions.

In the present research, a novel cycle for simultaneous, distributed, micro-scale production of electricity, water, and hydrogen is proposed based on solid oxide fuel cell (SOFC), micro-gas turbine (MGT), thermal vapor compression single-effect desalination (TVC-SED), and metal hydride (MH). The exhaust of the SOFC is first used in an external steam reformer; then, it turns an MGT and finally produces steam for a TVC-SED unit. Excess hydrogen is stored in a metal hydride. The cycle is modeled thermodynamically and then coded in the Engineering Equation Solver. The models of the SOFC and TVC-SED are validated. The parametric analyses show that the TVC-SED performance is most sensitive to the boiling temperature and compression ratio of ejector and least sensitive to the seawater salinity, motive steam pressure and seawater temperature. The SOFC and the MGT are most sensitive to the reformer temperature. The cycle is applied to a case study in the warm climate in Iran and revealed that the cycle produces the electricity demand of a five-family residential building and water demand of 32 people. The performance ratio of TVC-SED, electrical and overall efficiencies is 1.55, 65.01 and 72.99%. The cycle stores 623.7 kg of hydrogen and sells 1.32 MW of electricity annually.

Pitch error and tooth surface error are inevitable in the machining of helical gears, which will directly impact the time-varying meshing stiffness (TVMS) and transmission error (TE), thereby altering the vibration response of the gear. However, these errors are often ignored or replaced by simplified meshing errors in previous research, which cannot accurately reflect the specific effects of different errors. In this work, the distribution of various errors on the meshing surface of helical gears is fully considered, and a nonlinear contact model for helical gears with errors is established. The influence mechanisms of different errors on meshing excitation are elucidated through quasi-static meshing analysis. By incorporating the deformation coordination relationship of the meshing unit at each time sub-step, the corresponding TVMS and TE are coupled to the gear transmission in real-time, and a dynamic model influenced by tooth surface deviation is established. Finally, the time–frequency domain and dynamic load characteristics of the system under different errors are explored and the influence of mixed modification on the dynamic response of helical gears is analyzed. The findings reveal that the modification has a notable suppression effect on load fluctuation, underscoring the generality of the model for unconventional gears.

Abstract

The study delves into the issue of anti-plane mode cracking in dissimilar structures, commonly encountered in welds, composites and functionally graded materials. Achieving an accurate representation of these structures involves acknowledging a gradual variation of elastic properties across interfaces, achieved by incorporating a non-homogeneous layer characterized by finite width and bounded variable elastic properties. The investigation builds upon a model previously developed employing a numerical solution to a singular integral equation using the Dugdale cohesive law. In this paper, a comparable model based on the finite element method, incorporating an implemented cohesive model is introduced. The primary focus is on calculating the fracture load, allowing for a subsequent comparative analysis of results. The ensuing discussion revolves around the calculated relative sizes of cohesive zones, considering the corresponding implications of small/large-scale yielding conditions. While both approaches yield sufficiently similar fracture load values for small cohesive zone sizes, noticeable scatter is observed in instances of larger cohesive zone sizes.