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

In this study, for the first time, a nanofluid's natural convection heat transfer in a two-dimensional square cavity has been numerically investigated by use of the lattice Boltzmann method with the constant heat flux boundary condition. The horizontal walls of the cavity are insulated, and the vertical walls are kept at a constant heat flux. The diameters of the nanoparticles inside the cavity are the same and have a homogeneous distribution, and there is no chemical reaction between the particles. The flow is also assumed to be the steady state and two-dimensional. Constant temperature, streamlines, velocity, and average Nusselt have been investigated for different nanoparticle volume fractions and Rayleigh numbers. The results showed that the lattice Boltzmann method efficiently analyzes the natural heat transfer of nanofluids; moreover, by use of nanofluid in the cavity increases the heat transfer rate. With the increase in the nanoparticle volume fraction, the average Nusselt number on the right wall of the cavity increased. For a volume fraction of 20% with Grashof number 10⁵, the average Nusselt number increased by almost 50% compared to the base fluid at the same Grashof number. It has been observed that as the volume fraction of nanoparticles in the fluid increases, the fluid’s viscosity also increases; consequently, the velocity of the fluid is found to decrease.

Abstract

The noise induced by cavitation in a centrifugal pump is investigated by collecting the noise of the centrifugal pump under different available net positive suction heads (NPSHa) in the rated flow through experiments. Experimental results are combined with numerical calculations to establish the relationship between cavitation degrees and noise. Firstly, the collected noise signal is denoised using the independent component analysis (ICA) method, and combined with time domain, Fast Fourier transform (FFT), wavelet transform (WT), and spectral proper orthogonal decomposition (SPOD) methods to analyse the characteristics of cavitation noise signal after noise reduction. After being denoised by ICA, the noise signal can effectively reflect the inception and development of cavitation. In the frequency domain, the typical frequency band of noise induced by cavitation is 2 ~ 8 kHz. During severe cavitation, the amplitude of the shaft and blade frequency in the low-frequency band (0 ~ 600 Hz) gradually decreases until they become low-frequency broadband signals. In the time–frequency domain, when cavitation develops to an unstable cavitation state, the 0 ~ 1 kHz noise amplitude fluctuates irregularly. Finally, the coherent structure of cavitation noise feature signals is established using the SPOD method. Higher-order modes 3 and 4 can capture the characteristic changes of the centrifugal pump cavitation noise at different NPSHa.

Numerous irreversibilities exist in the solar subsection of solar power tower (SPT) plants, as was previously recognized, and cannot be prevented. Therefore, it is necessary to develop a new and efficient power generation unit to enhance the performance of the SPT plant. The unique combined cycle for SPT plant was developed in the current study. Working fluid helium was employed in the helium Brayton cycle (HBC), and the medium temperature organic Rankine cycle (ORC) was utilized for waste heat recovery. Using engineering equation solver software, the suggested system’s exergy and energy analysis was carried out. Additionally, a parametric study was performed to look into how important characteristics affected plant performance. Simultaneously, working fluid selection study has been performed for ORC. It was concluded that energy efficiency and network output were enhanced by 19.11% and 19.09%, respectively, by implementing ORC to the basic HBC system. The network output, exergy and energy efficiency of the plant (SPT-HBC-ORC) were obtained as 19,135 kW, 39.74% and 37.11%, respectively. The fluid R1233zd(E) was recommended as the thermodynamically best fluid. The current system performs better than supercritical CO₂ and the Rankine cycles based systems, according to a comparison with previous studies. Also, present developed solar power system is more efficient and easier to configure compared to previous research to generate the carbon free power.

In computational fluid dynamics, RANS expressions are solved numerically, as a cheap replacement for experimental work with an acceptable forecast accuracy compromise. Recently, use of machine learning techniques has increased significantly and has been useful in many sectors including aerodynamics. This paper examines the application of three distinct machine learning approaches to compute and predict aerodynamic coefficients of airfoil. We employ back-propagation neural networks, regression trees, and support vector machines to model the complex relationship between airfoil geometry, flow conditions, and the resulting aerodynamic coefficients. Our study investigates the applicability of these machine learning models and compares their performance to identify the most effective model for predicting airfoil coefficients. Overall, among all the different machine learning models examined, back-propagation neural networks demonstrated the best performance in terms of mean squared error and correlation coefficient values. Notably, for predicting coefficient of drag, the fine tree model achieved the lowest mean squared error of 3.1704 (times) 10^–7, while for the prediction of coefficient of lift, the lowest mean squared error of 4.9766 (times) 10^–7 was obtained by the back-propagation neural networks. This research not only offers deeper understanding of how machine learning techniques could play a pivotal role in enhancing airfoil coefficients predictions but also provides a practical application for improving aerodynamic designs in various engineering fields.

This paper aims to study the effect of waves from gas channels on the interaction of liquid droplets growing from two micropores in a wavy gas channel of PEMFC. The computational domain consists of a wavy gas channel in which liquid water is injected from two micropores with different diameters from the bottom of the computational domain. Also, the airflow entering the gas channel is fully developed with Poiseuille velocity. A multi-component multiphase pseudopotential Lattice Boltzmann method with a multi-relaxation time collision operator is present to simulate it. The forcing term in the collision operator has been improved to reach the real conditions of liquid water and air component density ratio and thermodynamic consistency. The different parameters such as Capillary number, temperature effect, wave amplitude, micropore diameter, and distance between two micropores on growth, detaching, and movement of liquid in the gas channel are studied. The simulation results indicate that by enhancing the Capillary number, the drag shear force rises, and the droplet detaches faster and improves its movement in the gas channel. Also, it is found that when the micropore diameter increases, the flow pattern changes from dripping flow to a continuous jet regime and raises the water removal time. The simulation is performed for a higher amplitude wavelength ratio to increase the maximum velocity, thus facilitating the droplet exit from the gas channel.

The symmetry of the switching forces of the fully compliant bistable mechanism is an important factor affecting its keeping stability in the equilibrium state. However, existing research on the three-segment fully compliant bistable mechanism (TSFCBM) does not reveal the influence law of its structural geometric parameters on the symmetry of the switching forces. Hence, this paper introduces the bistable characteristic parameter, establishes the optimization design model of the switching forces symmetry of the TSFCBM, and investigates the influence mechanism of the structural geometric parameters on the switching forces symmetry. In addition, this study obtains the relationships between the structural geometric parameters of the TSFCBM with optimal switching forces symmetry and verifies the optimization result by simulation. Through extensive experiments on the switching forces symmetry of the TSFCBM, the relative error of the bistable characteristic parameter obtained by theoretical calculations, finite element simulations, and experimental measurements is less than 15.5%, verifying the correctness and effectiveness of the optimization model and the influence law. The established optimization model and the revealed influence law provide the theoretical basis and practical method for designing a TSFCBM with switching forces symmetry that achieves the parametric design of a TSFCBM.

This paper investigates the accuracy of several meshless methods to solve elasticity problems. The methods include the well-known smoothed particle hydrodynamics (SPH) (called model 1 in this study), and Voronoi-based SPH (model 2), moving particle semi-implicit (MPS) (model 3), and Voronoi-based MPS (model 4) methods and three different proposed least squares models based on Taylor series expansion (TSE). The accuracy of both the SPH and MPS methods is improved by employing the Voronoi diagram as an alternative approach to estimate the computational node volume parameter. One of the least squares methods (model 5) uses TSE truncated to the second-order to solve the standard quadratic differential equations of elasticity problems, considering displacements as unknown variables. The two last methods employ the first-order (model 6) and second-order (model 7) TSE to approximate the function in the mixed formulation, where the governing equations can be written as a system of the first-order differential equations with unknown variables of stresses and displacements. The mixed formulation improves the prediction accuracy of unknown parameters, especially stress, by eliminating the second derivative calculations. The results indicate that the least squares methods, particularly model 5, can achieve higher accuracy and computational efficiency than SPH and MPS methods, especially in stress calculations. Noteworthy, the second-order mixed model exhibits considerable superiority over the first-order model while requiring approximately the same computational effort.

In this paper, a combination of unidirectional fiberglass mat and chopped strand mat layers in isophthalic resin is used to fabricate hybrid composites. The vacuum infusion process is applied for composite fabrication. The effects of the fiber orientation of the unidirectional plies and the addition of the chopped strand mat plies on the physical and mechanical properties of the developed hybrid composites are investigated. Three different laminates are developed with seven and fifteen laminae. Physical properties including fiber density and weight fraction are evaluated. A modified rule of mixture is proposed to evaluate the theoretical densities of the hybrid laminates, which has an excellent agreement with experimental densities. The hybrid laminates exhibit higher densities and lower void content than the unidirectional laminates. The mechanical properties are determined using tensile and three-point bending tests. The hybrid laminate with seven plies has the best properties in all fiber orientations and is the optimum laminate. Increasing the fiber orientation has drastically reduced the tensile and flexural strengths of the unidirectional composite. However, the variation of the strengths with the fiber orientation is not significant in the hybrid laminates. By increasing the laminate thickness, the tensile and flexural strengths have not changed significantly. However, the maximum flexural strain has increased with laminate thickness. The hybrid laminate with fifteen plies has considerable strain under bending and is suitable for applications where high ductility is required.

This paper focuses on the free vibration analysis of sandwich plates with variable stiffness composite laminated (VSCL) face sheets and a functionally graded (FG) porous core. The problem is solved using the hierarchical finite element method (FEM) based on the three-dimensional (3-D) elasticity theory. The use of an FG material core in a VSCL sandwich plate offers many advantages in terms of lightweight properties, high stiffness, as well as high strength and toughness. The sandwich plate is modeled by an assembly of 3-D p-elements, where each element or layer has an independent thickness and material properties. The layers of the sandwich plate are assumed to be perfectly bonded between the interfaces. The present 3-D solutions are validated through convergence and comparison studies with the published results of various sandwich plates that employ different theories and methods. A parametric study is performed to investigate the effects of several factors, including the volume fraction function index, porosity, core-to-face sheet thickness ratio, plate thickness, fiber orientation angles and boundary conditions on the vibrational frequencies. The results show that the incorporation of composite curvilinear fibers in the face sheets, combined with a porous FG core, significantly enhances the stiffness of the sandwich plate. These results can be used to establish benchmarks for future comparisons.

Controlling turbulent flow to improve wind turbine airfoils' aerodynamic characteristics is a desirable task. The current study evaluated the potential of adding a wedge flap (WF) at the trailing edge of the NACA0021 airfoil. The effect of different WF heights and lengths on optimum height (L/H) on the aerodynamic performance and flow over the airfoil has been studied numerically using two-dimensional computational fluid dynamics simulation. The simulation solves the Reynolds-Averaged-Navier–Stokes with shear stress transport k–ω turbulent model. The results indicate that adding WF can effectively suppress flow separation and improve aerodynamic efficiency in all studied cases compared to clean airfoil. The aerodynamic performance is influenced significantly by the height of WF compared to the slight influence by the length at L/H < 1. Inclined WF achieves the highest lift and lift-to-drag values with total maximum increments of 71.67% and 45.79%, respectively, at optimum height and length with 6%c and 1%c, respectively, in comparison with the clean airfoil case. The results observed that WFs have advantages over the Gurney flaps discussed in this study. WF appears to be an effective passive flow control device that can be used in wind turbines if its dimensions are properly chosen.