مجله مهندسی مکانیک امیرکبیر
سال پنجاه و سوم شماره 1 (فروردین 1400)

Studying the dynamic behavior of droplets is very important in electrowetting phenomena. Due to the widespread application of non-Newtonian fluids, especially in bio applications, in the present study, the dynamics of non-Newtonian Carreau droplets under the electrowetting phenomenon has been investigated. The effects of the viscosity, the size and the applied voltage on the oscillations and the change in the height of the droplets have been inspected. The simulations have been conducted using the Finite Element Method (FEM) and in order to validate the method, the results have been compared with the available experimental and numerical results. The results indicate that by increasing the viscosity the amplitude of the oscillations increases but the frequency remains constant. These are similar to those of the Newtonian fluids with this difference that in Newtonian fluids the amplitude is larger but the frequency is smaller. Also, for a Carreau fluids when the power index is smaller than one the results are similar to the Newtonian fluids but when the power index is larger than one the droplet reaches to its final height faster and without any fluctuation. Increasing the height in the non-Newtonian fluid leads to an increase in the amplitude of the oscillations and decreases the amount of frequency in the fluid, which is similar to the Newtonian fluid, with the difference that in the non-Newtonian fluid the amplitude is less but the frequency is higher.

Injection of foam into oil reservoir, is a new method for enhanced oil recovery (EOR). One of the advantages of using foam is the stable displacement of porous submersible oil due to its relative low mobility. The challenge for foam application in the oil recovery is to maintain its stability when it comes to contact with the oil phase. Therefore, before using foam in the EOR process, its properties and interactions with the oil must be characterized. in this research, by creating two different laboratory setups at the bubble and bulk scale, the foam stability contributing factors and the viscosity of different types of foam were measured. Changes of different type of foam’s height in the vertical column were studied at the bulk scale. Ate the bubble scale, by building a transparent hele-shaw cell that is equipped with pressure sensors, the evolution of the foam were examined quantitatively and qualitatively. The Results showed that the surfactant type had a significant effect on the stability of foam bubbles; so that the combination of 1:1 SDS and CAPB with the highest stability, increased foam half life time by 124% at the bubble scale and by %33 at the bulk scale. in the presence of the oil phase its destructive effect on foam stability, increased with the reduction of the viscosity and the density of the oil agent. Also, the results showed that the foam quality would directly affect the viscosity of the foam, while its flow rate had an inverse effect.

Efficient drilling cuttings transport is one of the most important parameters affecting drilling rate of wells. Foam has a great potential to reduce drilling problems compared to conventional well drilling fluids due to its unique properties such as low density and high viscosity. Proper description of foam behavior enables us to improve the performance of foam for hole- cleaning. In this paper, cuttings transport using foam was studied using a computational fluid dynamics approach. In this study the Eulerian multi-phase model was used to describe cuttings-fluid mixture flow. Foam rheology was expressed by the non-Newtonian power law model. Effects of foam quality and injection velocity, cuttings size, pipe eccentricity and rotational speed of drillpipes were studied. Modeling results were also compared with experimental data. Results showed that increase of foam quality improved hole-cleaning operation mainly due to the enhanced foam viscosity. Increase of foam injection velocity led to a reduction in in-situ cuttings concentration. This was due to the foam capability to destruct stationary cuttings bed. Accordingly, as foam injection velocity increased from 3 to 5 ft/s, cuttings concentration decreased by 1.3 times. Also, an increase in the size of the cuttings caused a poor well cleaning. It was found that pipe eccentricity resulted in the accumulation of cuttings in the wellbore annulus. But, increase of the drillpipe rotational speed provided a better hole-cleaning, so that by increasing the rotational speed to 40 RPM, cuttings concentration decreased by 1.8 and 1.4 times in concentric and eccentric pipe, respectively.

The inner walls reflection of ultraviolet radiation is one of the effective components on the ultraviolet reactor efficiency. In this study, the effect of inner walls reflection of the reactor on the performance of the multi-lamp ultraviolet reactor has been evaluated using of UV dose distribution and log inactivation values of MS2 and Bacillus subtilis microorganisms. The simulation of the flow field is performed using the SSTk-ω model and discrete ordinate model (DOM) for radiation is considered. The wall reflectivity are in the range of zero (no reflection) to 100 (ideal reflection) percent. In this range of reflections, for the reactor walls with aluminum cover, reflectivity of 80.5% and 26.1% for the steel wall and UV transmittance (UVT) of 87.7 and 78.5% was calculated. For UV transmittance (UVT) of 78.5%, the received dose and log inactivation change with increasing reflectivity was very low but for UVT of 87.7%, there is a tangible increase in these values. The results variation procedure for two different flow rate was similar in UVT of 78.5%. The performance of the reactor was investigated in two different lamp powers and UVT of 78.5%, which in higher power, the reflectivity effect has become more apparent.

The feed channel spacers cause the membrane plates to be separated. These mesh spacers increase the pressure drop in the channel and, in contrast, improve the mass transfer process. In this study, investigate hydrodynamics and mass transfer in the spacer-filled channel in the Reverse osmosis module by using the simulation of computational fluid dynamics coupled with response surface method. Input parameters include the average inlet velocity, the attack angle, the mesh angle, and the output parameters include the pressure drop over the computational domain and the water flux across the membrane walls. The Latin Hypercube Sampling Design method was used to sample the input parameters and the Kriging model has been used for response surface model. Also, genetic algorithm and Screening was used to determine the optimal output parameters. The sensitivity analysis of the input parameters on the output parameters indicates that the average inlet velocity and the attack angle are the most and the least influential parameters respectively. The optimum configuration taking all the operational parameters into account was stood up at the attack angle of 72.74 degrees, the mesh angle of 85.19 degrees, and the inlet velocity of 0.13 m/s.

The placement of a pier causes a three-dimensional complex flow pattern around the pier, which lead to scouring hole around the pier. In this research, modeling of scouring around piers was performed both experimentally and numerically using pimpleLPTdensFoam solver in OpenFOAM. In the first section, two different scenarios were used to simulate the scouring process. In the two numerical models, the effect of phase coupling and drag models on the scoured bed was investigated. One-way coupling with sphere drag model was implemented in the first scenario and four-way coupling with nonsphere drag model in the second one. According to the results, the first model was not satisfying, but the result of the second model was in good agreement with experimental results. The maximum depth of scour at the cross-section in numerical and experimental results was equal (6 percent error). All the simulations were done using the Euler-Lagrange approach and k-w turbulence model to simulate turbulent flow. In the second section of this research, to investigate effective parameters on the reduction of scour, three numerical models including a simple cylindrical pier and two more models using pier with collar were simulated. In order to study the effect of a collar on the reduction of scouring, the simple cylinder without a collar and then with the presence of collar at two different levels were performed. The results show that using collar in level 3.5 and level 4 may respectively lead to 49.2 and 29.7 percent reduction in maximum depth of scour.

Study of vortex shedding and flow downstream of a triangular bluff body can be used to design a device for measuring the flow angle, a vortex flowmeter or to calibrate the hot-wire anemometer at low velocities. In this paper, flow velocity, turbulence intensity and vortex shedding from a 10 mm triangular bluff body have been investigated experimentally using hot-wire anemometer. Results show that flow angle has little effect on flow velocity distribution and turbulence intensity. However, variations of Strouhal number (St) with respect to the flow angle is large, so that Strouhal number at flow angle of 20 °has the maximum value of 0.23 and at angle of 62 ° , it has the minimum value of 0.133. To design the flow angle measuring device or calibration of hot-wire anemometer, it is necessary to measure flow velocity in addition to the measurement of vortex shedding. Under this condition, if the probe is placed in the region: x/a=2.5 and 2.5

In the present study, one of the most important mechanisms of aerodynamic noise generation is investigated numerically. Large-eddy simulation approach used to solve the unsteady flow equations of turbulent boundary layer with Mach number 0.06 over a flat plate of length 30-cm. Lund's inflow boundary model used to reduce the computational cost. To calculate the parameters affecting trailing edge noise (include surface pressure spectra, spanwise length scale of the surface pressure fluctuations and eddy convection velocity), data acquisition of SPFs values in different points over the flat plate surface done using the probe tool in OpenFOAM software. Finally, the farfield noise predicted using analytical Amiet-Roger model. The results showed that the numerical solution method used in this study, while having reasonable computational cost, has a good ability to predict the effective parameters on the trailing edge noise. In addition, studying the spectral parameters affecting the turbulent boundary layer trailing edge noise showed that prediction and direct estimation of these parameters, while are using in far-field noise prediction models, provide accurate and proper information on physics of the flow and dimensions and life time of turbulent boundary layer vortex structures.

Providing thermal comfort conditions in sleeping environment can significantly affect the occupants’ health. So, the parameters of the air conditioning system must be properly set to satisfy thermal comfort criteria for persons with different physiological characteristics. In the present study, a task/ambient air conditioning (TAC) system is modeled and the effect of gender and body mass index on thermal sensation has been analysed by individualized three-node model. Results show that although Gagge standard model predict pleasant thermal comfort condition, thermal sensation index varies from -0.63 to 0.66 based on individualized three-node model, and in some cases exceed the thermal comfort region. Moreover, healthy and fat people have a warm sensation in clothed parts of the body and thin people, especially women, have a cold sensation in bare parts of the body. Also, based on the results women compared with men, and thin people compared with obese people feel colder. By changing BMI, thermal sensation index can change up to 0.36, which is very significant in the assessment of thermal comfort.

The solution of the heat diffusion equation in most practical applications involving complex geometry, thermophysical properties, and boundary conditions is not simply possible and there are some limitations for available numerical solutions. In this research, the finite volume Monte Carlo method was proposed for the solution of the isotropic heat diffusion equation due to two intrinsic capability of the finite volume method; first, each cell is energy conserved and second, the grid transformation is not necessary for complex geometries. The Monte Carlo method as a statistical approach based on physical simulation of the problem capable to solve heat diffusion equation with any degree of complexity was utilized in three different problems. First, a simple problem was investigated for validation of the method by comparing results with the analytical solution. Second, the prediction performance of the proposed method was evaluated in a problem with complex geometry, varying properties, and boundary conditions. Finally, the performance of the finite volume Monte Carlo method was investigated in estimating the temperature distribution of a three-layer body with different thermal conductivities and convection boundary condition. In all of the considered test cases, the predicted results were in good agreement with analytical and CFD solutions. It was also indicated that for a relatively small number of particles, it is possible to achieve acceptable accuracy with a low computational cost.

Tomography is a non-invasive method that is used to visualize internal structure of an object. Electrical tomography is a method that recognizes the geometry of an object by applying voltage or current and using an image reconstruction algorithm. In spite of some advantages such as simplicity and low cost, reconstructed images by this method have low resolution and low quality. In addition to the system hardware, the reconstruction algorithms also reduces the quality of images. In this research, Levenberg – Marquardt Algorithm which is used to solve inverse heat transfer problems, is employed for image reconstruction in electrical tomography. In this technique, image reconstruction is perfomed by changing the thermal conductivity of material. Three different test cases are selected to investigate the capability of proposed algorithm in estimation of the unknown geometry and the results show that the Levenberg – Marquardt algorithm has ability to estimate unknown shapes. Sensitivity analysis is also performed in order to examine the effect of noise in detection of the unknown geometry. The results showe that with increasing the noise, the error created in the estimation of shape increases, but the estimated shape has good agreement with original geometry. Also, results of choosing different combinations of active sensors for applying thermal flux show that in addition to effectiveness of active sensors in increasing the accuracy of shape estimation, the unknown geometry can be estimated with two measurements.

Determination of multiphase flow dynamics and thermal behavior of falling flow in a channel and around a cylinder are of importance due to its wide application in various industries. The small-scale surface tension effect plays a major role in multiphase flow behavior. So, based on simulation efficiency, precision, and stability, the mesoscopic Lattice Boltzmann method superiority has caused its broadening application. In the current study, the thermal-hydraulic behavior of subcooled falling flow in a vertical channel and around a single horizontal tube is simulated by using Lee method and Phase-Filed model, and thermal Passive Scalar model. The modified Boundary conditions for curved tube and two different boundary conditions for side boundaries are investigated. Simulations have been done for density ratio 20 and relevant viscosity and conductivity ratios of water. The film of liquid falls on a tube with a temperature of 110˚C and the outside diameter of 28.9mm. The results including contours and streamlines of the flow field, temperature, and pressure contours are presented and detailed understanding about the movement of the three-phase contact line, circulating flow and local and average Nusselt number are determined. The film thickness, thermal boundary layer variation by the film thickness, Reynold number effect on Nusselt number and mass conservation are investigated as verification. The results have shown good consistency and high effectiveness in the simulation of multiphase gas-liquid flows in the presence of a circular obstacle, and for viscosity and thermal diffusivity ratios of water.

In this numerical three dimensional study, the effects of uniform magnetic field on thermal and thermo-hydraulic performance and entropy generation of water flow through a trapezoidal silicon microchannel heat sink, with four different inlet/outlet configurations, have been investigated in three dimensions. An electronic chip embedded on the base plate of heat sink generates uniform heat flux of 50kW/m2. Simulations have been performed for mass flow rates of 0.02, 0.03, 0.04 and 0.05g/sec and Hartmann numbers of 0, 2, 4, 8 and 16. The results show that, in overall the best configuration is the A-type arrangement, in which the flow enters to the center of the distributing chamber and exits from the center of collecting chamber. For this arrangment and a constant mass flow rate, with increasing Hartmann number from 0 to 16, thermal resistance reduces between 4.39% and 9.15%, theta between 1.81% and 7.91% and performance evaluation criterion between 81.61% and 87.15%, but total entropy generation increases between 10.13% and 77.07%. Also for a constant Hartmann number, with increasing the mass flow rate, average Nusselt number, frictional entropy generation and magnetic entropy generation increase and thermal resistance, theta, performance evaluation criterion, solid and fluid thermal entropy generations and total entropy generation decrease. In the best arrangement, the best thermal performance occurs in the mass flow rate of 0.05g/sec and Hartmann number of 16 and the best thermo-hydraulic and entropy generation performances occur in the mass flow rate of 0.02g/sec and Hartmann number of zero.

In the current study, the problem of two-dimensional heat conduction in a truncated hollow cone made of functionally graded materials is referred and an exact analytical solution is presented. Functionally graded materials are materials with special production processes in which different thermophysical properties can be gradually changed. In the present study, the properties of a material are modified in accordance with a power function. The thermal boundary conditions are also assumed to be non-homogeneous. The separation of variable (SOV) method is implemented to acquire the exact steady-state temperature distribution. The obtained solution is adequately verified using numerical data. To further demonstrate the ability of the solution, an illustrative case which is exposed to a combination of boundary conditions is studied. In particular, the influences of effective parameters on the temperature distribution are investigated for the current geometry. It is shown that by using functionally graded material more flexible attitudes in terms of temperature distribution are seen. The outcome of this study would be helpful to shed light on the process of designing and optimizing relatively complex geometries. Also, considering the analyticity of the present solution, the results of this study can be useful for a better understanding of the heat transfer mechanisms of functionally graded materials. In the present case, increasing the amount of m and κ, the thermal conductivity increased by about 8 and 2 percent respectively, which would increase the distribution of cone temperature.

In this article, the transient heat transfer problem with both convection and radiation boundary conditions is studied. The meshless Radial Point Interpolation Method (RPIM) is implemented in this numerical study. Also, two integration methods, the Cartesian Transformation Method (CTM) and the Gaussian Quadratuer (GQ) method which uses background cells, are employed for domain integral calculations. First the problem of transient heat transfer in a homogenouse domain with both convection and radiation boundary conditions is considered. The temperature distribution results of the meshless domain using the proposed method, are compared with the analytical temperature results in this problem and excellent agreement is obtained. Then, to verify the better accuracy of results obtained from CTM, compared to those obtained from conventional background cell method, a number of example problems in a layered composite and a Functionally Graded (FG) sample with both convection and radiation boundary conditions are solved and the temperature results are compared with those of ABAQUS software. Consequently, using CTM in compare with using background cell method in convection boundary conditions reduce error to half and in radiation boundary conditions reduce error to one quarter. This numerical method is a meshless method which doesn’t need any background mesh. Moreover, the amount of error using the background cell method in problems with radiation boundary conditions is more than thoes with convection boundary conditions. This shows the advantage of using CTM in problems with radiation boundary conditions which have a higher degree of nonlinearity, due to the temperature dependent boundary conditions.

Advanced oxidation processes (AOPs) for wastewater treatment have received a great deal of attention in recent years. Photocatalytic oxidation processes decompose water pollutants using nano-structured photocatalyst materials, titanium dioxide (TiO2) and ultraviolet irradiation. Although there is extensive experimental research in this field, designing a photoreactor for water treatment is still a challenge. An effectual approach to this issue is the application of computational fluid dynamics. Performance of the catalyst, which is activated by UV irradiation, is one of the important factors affecting photoreactor efficiency. In case of poor UV radiation distribution inside the reactor, the reactor performance decreases due to catalyst inactivity. In this study, a computational fluid dynamics model for the simulation of radiation distribution inside a photoreactor was developed and evaluated against experimental data. Simulations were then carried on different catalyst loading, lamp power and wall reflectivity. The performed analysis showed that at low concentration of catalyst (0.4 g/L), the reaction rate increases up to 50% by increasing the wall reflectivity to 98%, this is due to small absorption coefficient of the medium. At high catalyst concentration (0.6 g/L), the increase in reaction rate is only 5% because the radiation amount that reaches the reactor walls is negligible. At the lamp power of 2P and P, the reaction rate increases up to 12.2 % and 11% respectively which means only 1% increase in reaction rate while increasing lamp power.

Water is one of the products of the interaction between hydrogen and oxygen in a fuel cell. The presence of this product can reduce the fuel cell performance and cause problems in its operation. The purpose of this study is to introduce a water level control system preventing the loss of reactant gases, such as hydrogen and oxygen, by improving the process of water separation from these gases. Therefore, unused gases are returned to the fuel cell, resulting in a reduction in the cost of using reactant gases to generate electricity. In this paper focuses on the performance of the water level control system in terms of fuel cell specifications and construction constraints. The system consists of a mechanical unit and a control unit and is based on various reactive gases. Effective parameters for controlling the discharge valve, such as the length of the connection path, the sensor angle, the time lags, have been investigated. The path length check showed that if the path length being too high, the pressure changes oscillate, failing to establish a pressure condition for time, and the system loses its performance. The sensor angle only affects on , somehow reaches its maximum by placing the sensor at a 90-degree angle. In addition, according to the tests performed, time between 0.3 and 0.5 seconds is recommended for . Finally, in order to control the water level control system automatically, relations are proposed based on the fuel cell pressure. Overall, the control algorithm showed the correct performance.

Due to the capability of using exhausted gases of power plants directly in reforming process and producing high quality syngas, the tri-reforming process is a considerable kind of reforming process. In the present study, a power generation system based on solid oxide fuel cell with specific capacity and equipped to the external reforming is proposed and investigated form the viewpoint of thermodynamics. In order to conduct the external reforming process, exhausted gases from the system including the steam, carbon dioxide and oxygen are recycled and utilized as the reforming agents in the reactor. In order to model the proposed system thermodynamically, the principals of mass and energy balance are applied in Engineering Equation Software (EES). Effects of such important parameters as the current density and solid oxide fuel cell operating temperature on the system performance indicators including the various voltages, fuel mass flow rate, power generation and the energy as well as the exergy efficiencies of the system are investigated. The parametric study results shows that for the lower values of the current density and higher values of the SOFC operating temperature as well as the fuel utilization factor, the system energy and exergy efficiencies enhances.

Clean energy sources such as hydrogen are developing because of environmental issues. Partial oxidation of methane is important among different methods of hydrogen production due to the reduction in carbon deposition, do the process in a lower temperature range, the high conversion of methane and low energy consumption. In the present work, the numerical simulation of partial oxidation of methane with Rh/Al2O3 catalyst is conducted in a fixed bed flow reactor. The effect of different percentage of product gas recirculation on the hydrogen, carbon monoxide and carbon dioxide production is calculated for various temperatures (500-900°C) and oxygen to methane ratios equal 0.4, 0.5, 0.6 and 0.7, respectively. The results show that in the temperature range, increase in the production gas recirculation cause to increase in Hydrogen and CO production and decrease in CO2 production. Moreover, the calculated data shows that the inlet oxygen to methane ratio equal to 0.5 and 600°C reaction temperature are suitable to enhance Hydrogen production performance by production gas recirculation. Also, it is demonstrated that 50% volumetric product gas recirculation in that temperature and O2/CH4 ratio, cause to increase in H2 production and decrease in CO2 production about 30%.

In this study, the thermodynamic and thermoeconomic analysis of an Absorption Heat Transformer (AHT), Organic Rankine Cycle (ORC) and Reverse Osmosis (RO) desalination combined system was performed. The performance of the system was examined with the objective of generating electricity and fresh water from low temperature heat sources. All analyses are carried out based on the thermodynamic and thermoeconomic laws by programming in the EES software. The results have shown that the AHT with increase temperature to 105°C, is produced 494.7 kW thermal energy in absorber, having the Coefficient of Performance (COP) of 0.4372. By applying the AHT produced thermal energy; it is possible to produce 63.15 kW of electricity in the ORC. By using this amount of electricity in the reverse osmosis system, 216.2 m3/day of freshwater are produced where the cost of this amount of water produced achieved 2.374 $/m3. Also, in thermoeconomic analysis, the unit cost of the exergy for all points of the system and the unit cost of the electricity and fresh water were calculated. The levelized cost of electricity at different heat rates was determined, according to the results, the levelized cost of electricity is reduced when the heat rate increases. Also the effects of interest and inflation rate changes on the unit cost of the fresh water were studied.

In the present study, a numerical simulation was conducted to investigate the separation of blood cells using an integrated dielectrophoretic-photophoretic method in a new microfluidic device. In this simulation, the migration behavior of human blood cells under laser radiation with a wavelength of 522 nm and in the presence of fluid flow has been investigated. Studies show that the photophoretic migration of red cells under the irradiation of laser beam is higher than platelets and other blood cells, so that the magnitude of the applied photoelectric force on the red blood cells has been calculated about 9 times that of the white blood cells under the irradiation of laser beam of 50 μm. In this separation using photophoretic forces, red cells were first separated from the platelets and white cells. Subsequently, using the hydrodynamic forces induced by the fluid on the particles and the dielectrophoretic forces, the separation of the platelets from the white blood cells was carried out in different branches of the microchannel. In this study, the dielectrophoretic forces were created using electrode arrays, located on the one side of the microchannel of the microfluidic device. The combination of dielectrophoretic and photophoretic methods has led to a reduction in the magnitude of the required peak to peak voltage for electrodes to a remarkable magnitude of 3v. The proposed design, in addition to high separation efficiency, has a negligible cell loss, so that it can be used as an effective method in many diagnostic processes and medical applications.