مجله مهندسی مکانیک امیرکبیر
سال پنجاه و چهارم شماره 9 (آذر 1401)

Cyclones are normally used to separate relatively larger particles from the aerosol. In this article, the feasibility of using cyclone to classify particles in specific mass range by applying an electric field between the outer cylinder and the vortex finder is studied. Moreover, the effect of cyclone geometry and the magnitude of the electric field on the cyclone efficiency and the classified particle diameter is quantified. FEM was used for the simulations of 3-D, steady and two-phase flow. It should be noted that the Reynolds number of inlet flow ranged between 4,000-10,000. The results reveal that the diameters of inner and outer cylinders have negligible effects on the cyclone efficiency. However, an increase in the length of the cyclone specifically the length of the vortex finder can significantly affect the cyclone performance which can be attributed to the higher particle residence time within the cyclone. For cyclones with twice larger cylinders, the classification efficiency is 6% - 17% higher based on the geometric standard deviation of the particle size distribution. It was also shown that different particle masses can be classified by adjusting the flow rate of the inlet aerosol or the magnitude of the electric field applied to the charged particles.

Prediction of flow behavior in the transition region is the key issue in many scientific problems. Many attempts have been made by researchers to propose and modify the models estimating the flow behavior in this region. In these flows, the governing equations, including the continuity, the Navier-Stokes, and the transmittancy along with the SST models are solved simultaneously to predict the flow behavior. There are several coefficients in the governing equations which affect the flow simulation. In this study, the transitional SST model is modified by altering two coefficients in the intermittency equation. A combination of these coefficients are implemented, and the effects are studied. In order to assess the suitability of the proposed models, they are applied to three individual internal flows, including a smooth axisymmetric pipe, two parallel plates, and a backward-facing step. Different variables such as the friction factor coefficient, fully developed friction factor, and the reattachment length are explored. A comparison between the results and both analytical and experimental data confirms a good accuracy in the predictions. Furthermore, the presented models the entrance length is well predicted in turbulent and transitional flows.

One of the most important issues facing today's society is the issue of energy production and the challenges surrounding it. For this reason, it is very important to address the issue of energy harvesting from various methods. One of these methods is energy harvesting from vibrations caused by fluid flow. Vibrations generated by the fluid flow around three parallel piezoelectric blades behind a circular cylinder at different longitudinal distances can be one of the best options for examining and evaluating the amount of electrical voltage generated by piezoelectric blade vibrations. According to this study, a situation in which the middle piezoelectric blade is shifted by half the length of the blade to the right and the direction of the clamp is opposite to the direction of the clamp of the up and down blades is the optimal structure for voltage output and reducing collision probability. Due to the reduced probability of the blades colliding with each other in this optimal state, the maximum Reynolds number without the blades colliding increased from 2400 in non-optimal structures to 2600 in the optimal structure, which increased the voltage output in the middle blade by 12% and about 14% for up and down blades.

When a shock wave propagates through a flow field with nonlinear thermodynamic properties, different processes occur simultaneously. wave compression, wave refraction, and vortex generation are examples of these processes that cause the waveform and thermodynamic properties of the fluid to change. The interaction of a shock wave with a cylindrical bubble is a simple example of a wave-bubble collision problem in which, the above processes are observed. Due to the high computational cost of density-based algorithms in solving compressible interfacial flows problems such as wave interaction with the two-phase flow, using a fully coupled pressure-based algorithm is a good solution that will solve the problem with proper accuracy while reducing computation time. In this paper, using this algorithm, the interaction of the shock wave with the bubble is investigated; while validating the results, the effect of the computational grid size and the method of discretization of the terms in the governing equations are determined on the results. It was observed that by increasing the number of computational grids according to the first-order upwind method, the simulation results become more accurate, and the numerical diffusion decreases. Also, by changing the discretization method to second-order upwind, the instabilities on the interface of the two phases increase due to spurious fluctuations, and the shape of the interface obtained from the numerical solution moves away from the experimental results.

The reflection of acoustic loads from the rocket engine exhaust gases on the launch pad has become a major challenge in the aerospace industry. The effect of reflection of acoustic loads on the launch vehicle depends on the parameters of flow turbulence, generated vortices, nozzle geometry and launch platform geometry. The purpose of the present study is to simulate the reflection of sound waves from the deflector surface by the combined CFD / BEM method. The characteristics of the rocket output jet are Mach 2, pressure 90 kPa and temperature 1000 K. Also, to study the effect of sound wave reflection from the launch pad, the results are compared in two cases, considering the jet deflector and without it. Numerical simulation is considered to be 3D, unsteady, compressible and turbulent. The boundary element method was used as an efficient method to calculate the diffusion and reflection of sound waves. The results show that the amount of noise generated increases significantly by considering the reflection of supersonic jet sound loads from the jet deflector. The noise generated by the presence of the jet deflector is 8 to 10 db more than the noise generated without the deflector. The results also show that the presence of a jet deflector causes the sound waves to be uniform on the vehicle surface.

Temperature of Li-ion battery significantly affects its performance and keeping the temperature in the suitable operational temperature is important. Even more, it guarantees the safety and life time. In this study, performance of a battery temperature control management system for a 6-cell Li-ion battery pack is investigated for cold climate and the initial temperature of -20°C. Two main parameters of maximum temperature difference and average temperature of the batteries are considered as the heating system performance criterions. Effects of the heating fluid mass flow rates, number of the plates and the fluid flow arrangement including three methods of simple, counter flow and zigzag counter flow, on the performance criterions and the heating times are investigated. Results shows that generally the mass flow rate the batteries reach the 20°C sooner and the temperature difference decreases. For similar mass flow rates, the heating time decrease by using more heating plates at high mass flow rates. The zigzag flow arrangement has the best performance among the other investigated flow arrangements and reduces the temperature difference up to 8 times with maximum value of 2.1 degrees. But the all counter flow arrangements increase the heating time in comparison with the simple flow arrangement.

Because saltwater covers the majority of the Earth's surface and solar energy is readily available, water treatment using solar energy has piqued academics' interest. A stepped solar still was built and tested experimentally in Arak as part of this study. For this purpose, the effect of five input parameters on the amount of freshwater production per unit area of absorber plate as the output variable was investigated using the Taguchi design of experiment method, including saline water flow rate, device angle, absorber plate color, number, and spacing of distilled water-conducting threads in each row. This research is unique in that it uses plastic threads to create channels on the cover glass surface that lead distilled water to the freshwater tank. In addition, the simultaneous study of the effect of input parameters is one of the innovations of this research. According to the findings, the highest freshwater output of 1975 ml/m2, was obtained when the input saline water flow rate was 50 ml/min, the device angle was 40 °, the absorber plate was black, the number of water-conducting threads in each row was 2, and the row spacing was 8 mm. Also, the research showed that using two water-conducting threads in each row and spacing them 8 mm apart increased the amount of water produced per unit of surface area.

In this reserach, adsorbed natural gas (ANG) methods have been studied. The adsorbents used in this thesis are carbon-based nano-sorbents (activated carbon, pure and functionalized carbon nanotubes, and porous graphene) which were synthesized by the chemical vapor deposition (CVD) method. The accuracy of synthesized results was examined using SEM, TEM, FTIR, XRD, and BET analyzes. The adsorption capacity of adsorbents for methane gas adsorption at three temperatures of 28, 45, and 60 ° C was calculated and matched with three isotherm equations of Langmuir, Freundlich, and Temkin. The R of the Langmuir isotherm for pure and functional nanotube adsorbents were 0.9963 and 0.9997, respectively, and for activated carbon was 0.9995, which is the closest isothermal equation for these adsorbents, while for the graphene adsorbent the closest prediction is Temkin isotherm with calculated R of 0.9986. It can be concluded that with increasing temperature, the amount of adsorbed gas decreases, and with increasing pressure, the amount of adsorbed gas increases. Therefore, the maximum adsorption for all adsorbents occurred at a temperature of 28 oC and a pressure of 40 bar. Among the used adsorbents, porous graphene showed the best performance at a temperature of 28 oC, and a pressure of 40 bar, which according to its high specific surface area, BET analysis (1200 m2/g), and significant pore size, such an outcome was predictable.

In the present study, direct simulation Monte Carlo method is utilized to investigate the effect of obstacles number, location and also injection of a flow on the mixing of flow in a channel with 16 μm length and 1 μm height. A mixing length is defined which is the length at which two species are mixed completely. Eight cases with different blockage ratio are considered to study obstacle effect on the mixing. Blockage ratio shows the reduced flow cross section due to addition of obstacles. In All cases CO2 and N2 gases enters the domain and separated by a splitter plate which extends up to 1/3 of channel. Blockage ratio increasing decreases mixing length up to 10%. Whereas mass flow rate decrease significantly. Flow injection into channel is also studied. Four cases are considered: the first case is simple channel without injection, second case has cross injection, third case has inverse injection, and flow is injected vertically through an obstacle in forth case. Mixing length is increased 17% and 5% for case 2 and case 3, respectively. In case 4, mixing length is decreased 2% due to the obstacle. In case 2 and 3,

To survey the radiative heat transfer in a participating medium, the radiative transfer equation must be solved. Except in particular cases, there is no analytical solution to it. Solving the RTE with numerical methods is also time consuming. In combined conductive-radiative or convective-radiative heat transfer, and inverse heat transfer problems, the RTE must be solved several times; Therefore, the computation time will be momentous. In this work, a fast method based on proper orthogonal decomposition to solve the RTE is presented. A number of properties (such as: emissivity, absorption and scattering coefficients) are selected as independent parameters. The RTE for specified modes of these parameters is solved using the discrete ordinates method, and the system responses form the snapshot matrix. Using the singular value decomposotion, it is decomposed. According to the singular values, only certain columns of the matrices are selected. As a result, the degrees of freedom of the system are reduced, and a reduced-order model is created. Employing the radial basis functions, the system response can be predicted rapidly for any desired input vector (including independent parameters). The results show that the ROM has a high accuracy compared to the DOM results. The complexities of the system have no effect on the CPU Time, and regardless of the value of the independent parameters, the computation time is of the order of 0.02 seconds.

In the present paper, heat transfer in a cavity containing a mixture of water + phase change materials surrounded by nanoparticles is investigated. The left and right walls are fixed at hot and cold temperatures, respectively, and horizontal walls are assumed to be adiabatic. There is a circular rotating cylinder in the center of the hole that can rotate clockwise or counterclockwise. The problem is considered two dimensional and fundamental governing equations such as continuity, momentum, and energy are solved in a coupled manner utilizing the finite element method (FEM). To check the accuracy of the numerical results, a comparison with the outputs of others is provided, which indicates a very good agreement of the results. The parameters studied in this study are: dimensionless radius of the cylinder (R), Rayleigh number (Ra), dimensionless melting temperature of the phase change material (θfu), Stephan number (St) and dimensionless angular velocity of the rotating cylinder (Ω). By increasing the dimensionless radius of the cylinder from R = 0.1 to R = 0.4 Ω = -300, the heat transfer rate enhances by 23.37%. On the other hand, with R = 0.4 and considering no-rotation case, the heat transfer rate will decrease by about 59.7% compared to the cavity without the cylinder. Which indicates the importance of rotation of the cylinder inside the cavity in the heat transfer rate enhancement.

Thermoelectric generators are a sustainable and environmentally friendly technology that can recover wasted heat energy and convert it to electricity. Meanwhile, integrated thermoelectric generators have been able to significantly increase the performance of thermoelectric generators. In this paper, the effect of flow channel cross-sections on integrated thermoelectric power generator performance is investigated numerically. In this regard, various flow channel configurations including circles, trapezoids, squares and rectangles have been taken into account and the effect of cross-sectional area ratio (𝛷), semiconductor length (d) and Reynolds number (Re) on the performance of the device have been evaluated. In this study, the top and bottom of conductor surfaces are exposed to a cold temperature and a hot fluid with a constant velocity and temperature enters the channel. The results show that the power output, voltage and thermal efficiency of 36 rectangular configurations are higher than other flow channels. Also, the heat input, power output and thermal efficiency at 𝛷=0.28 are respectively found to be 1.68, 1.77 and 1.52 times higher than at 𝛷 =0.68. In addition, an optimal length for a semiconductor is determined, in which the maximum output power is achieved.