احمدرضا رحمتی

In recent years, many studies have been conducted on the field of microchannels' sinks. The microchannel sink absorbs heat from other equipments and transfers it to the fluid by maximizing the contact area. In the present research, three-dimensional water flow, nanofluid and hybrid nanofluid in heat sink with constant heat flux, have been studied. The simulation process have been done in the commercial software ANSYS fluent. In the simulation process, the constant and uniform heat flux boundary condition has been applied to the bottom of the heat sink's wall. The influence of variables such as the nanoparticles volume fraction, nanoparticles types and Reynolds values on the hydrodynamic performance and heat sink will be investigated. After this stage, two types of nanofluid and one type of hybrid nanofluid in 0.04, 0.02 and 0.01 volume fractions will be considered as well. In this paper, first the pressure drops and heat transfer will be examined, and then the best nanofluid will be defined by considering the coefficient of thermal performance. The simulations show that the heat transfer decreases with increasing the volume fraction in the water-aluminium oxide nanofluid. On the other hand, the heat transfer increases with raising the volume fraction in the water-copper nanofluid. Also in the water-aluminium oxide-copper hybrid nanofluid, the heat transfer rises up in the 0.04 volume fraction in the range of 400 and above values for the Reynolds number.

Most of the large-scale gas turbines installed in Iranian power plants are V94.2 type and domestically made. Considering the position of these units in the electricity and energy industry of the country, in this research, the importance of regular maintenance and repair of these turbines, repairs to life time extension and their upgrading process have been explained. The priority and necessity of converting these power plants to combined cycle and the financial costs of this process have been described. Then it has been calculated that if these power plants are converted to combined cycle, at least equal to 200 million cubic meters of natural gas, annually will be saved for each block. If all the large gas units of the country are combined, it is possible to save about 6 billion cubic meters of natural gas annually with a current value of nearly one billion dollars. Also, considering the special characteristics of carbon dioxide at the critical point, including low pressure and temperature values, high density and low viscosity, the advantages of using the cycle with this working fluid and using it instead of the steam cycle in combined cycles have been investigated. Until now, power plants up to 25MW have been built with this working fluid in the world, but they have not yet reached the commercialization stage.

In the current study, transportation of the microparticles deposition through a channel has been investigated where elliptical obstacle with constant cross sectional area but different shape factors was assumed in the channel. Numerical simulation was conducted using lattice Boltzmann method, and Lagrange method was used for particle tracking. A two-dimensional and nine-velocity model was used as the network model. A curved boundary condition was applied for the obstacle boundaries. In the designed model, particles at standard condition were injected at the inlet of the channel. Gravity force, drag force, Brownian force and Soffman lift force were applied in the motion equation of the particles. The effect of shape factor as a geometrical parameter, which was defined as the ratio of the diameters of elliptical obstacle, and the flow parameters such as Reynolds’ number was examined on the particle deposition and particle scattering. Results were examined at eight different shape factors and five different Reynolds numbers .Results revealed that the change in the shape factor varies the effect of the obstacle in the flowing stream, and also changes the flow regime. This variation was obtained at different Reynolds numbers. Furthermore, changes of the shape factor associated with variations in the flow regime and deposition mechanisms, changes the forces exerted on the particles. Generally, the effect of the mentioned parameters can be interpreted based on the number of the precipitated particles.

The most obvious feature of heat sinks is their ability to transfer heat and their cooling properties. In this research, a vertical single-channel active heat sink with Galinstan liquid metal fluid was used and the discretization of Navier Stokes equations was done using the second-order upwind finite volume method. Investigation of Mixed Convection heat transfer with Richardson numbers 0.45, 1 and 10 has been done in both directions of flow from top to bottom and flow direction from bottom to top and the effects of external magnetic field in two directions perpendicular to the flow axis have been investigated. The results showed that the flow direction from bottom to top with a Richardson number of 10 without the presence of a magnetic field improved the Nusselt number by 11.30% compared to the flow direction from top to bottom. With the Richardson number of 1 and the flow direction from bottom to top, the effect of applying the magnetic field in the Z direction (perpendicular to the current axis) with the Hartmann number of 129, 164.5, and 194, respectively, is 11.29, 13.63, and 15.88 percent of the Nusselt number has been improved. With the Richardson number of 1 and the flow direction from the bottom to the top, the effect of applying the magnetic field in the X direction (perpendicular to the flow axis) with the Hartmann number of 64.6, 129 and 194, respectively, is 7.08, 8.28 and 8.76% of the Nusselt number has improved.

the shell and tube heat exchanger with the percentage of baffle cutting and the number of different tube passes in four different modes (use of water fluid on the shell side and tube containing supercritical carbon dioxide gas without the presence of turbulator, use of water fluid on the shell side and tube containing supercritical carbon dioxide gas with the presence of turbulator, the use of water-aluminum oxide nanofluid on the side of the shell and tube containing supercritical carbon dioxide gas without the presence of turbulator and also the use of water-aluminum oxide nanofluid on the side of the shell and tube containing supercritical carbon dioxide gas with the presence of an turbulator) are studied using HTRI software. The results show that the highest value of the heat transfer coefficient on the shell side and, as a result, the appropriate thermal efficiency is related to the case where the baffle cut is 30%. On the other hand, with the increase in nanofluid concentration, the pressure drop on the shell side increases from 4.48 to 5.66%. the results show that the use of a microfin turbulator is more suitable than a twisted tape turbulator and the use of a microfin turbulator increases the heat transfer coefficient on the shell side by 5.76 to 12.77% compared to the use of a twisted tape turbulator. Also, it increases the heat transfer coefficient of the pipe side by 62 and 78% on average, respectively, compared to the case without the turbulator.

A strong magnetic field provides a new method of heat transfer with high heat flux. A numerical simulation for a heat sink with high heat flux under an external uniform magnetic field in three different directions is used to investigate the flow field and displacement heat transfer between liquid metal and hot surfaces. Due to its high density and large thermal and electrical conductivity coefficients, gallinsten liquid metal has been used as a working fluid. Discretization of the Navier-Stokes equations is performed by the upstream second-order finite volume method. The results show that the effect of applying a magnetic field in the Y and Z directions (perpendicular to the flow axis) on the heat sink with a Hartmann number of 88, improves the displacement heat transfer coefficient by 15% and 8%, respectively. The best efficiency in increasing the heat transfer was obtained by applying the magnetic field in the Y direction. By applying the magnetic field in the Y direction to the heat sink, the displacement heat transfer coefficient was increased by 11.9% for Hartman number of 44, 15% for Hartman number of 88, and 17.7% for Hartman number of 132, compared to zero Hartman number. By applying the magnetic field in Z direction to the heat sink, the displacement heat transfer coefficient was increased by 4.3% for Hartmann number of 44, 8% for Hartmann number of 88, 11.4% for Hartmann number of 132 and 22.1% for Hartmann number of 330, compared to Hartmann number of zero. Also, the results show that the effect of applying a magnetic field perpendicular to the flow axis has increased the velocity gradient. As a result, the pressure drop and friction coefficient of the heat sink have increased.

Increasing the heat transfer rate in various industries in order to improve the efficiency of equipment, prevent damage to parts and reduce costs is one of the essential discussions in the industry. One of the solutions to increase heat transfer is the use of active heat sink. In this research, an active heat sink with Galinsten liquid metal fluid was used and the discretization of Navier-stokes equations was done using the second order upstream finite volume method. The effect of applying the magnetic field in the Y direction (perpendicular to the flow axis) to the heat sink has caused the creation of a force against the flow direction called the Lorentz force, which has caused the M-shaped velocity distribution. According to the constant flux boundary condition, increasing the flow velocity in the vicinity of the walls has caused the surface temperature to decrease and the heat transfer to improve. The results showed that the effect of applying a uniform external magnetic field in both Y and X directions with a Hartmann number of 517 improved the Nusselt number by 38% and 13%, respectively, compared to a Hartmann number of zero. The effect of applying a magnetic field in the Y direction to the heat well with a Hartmann number of 517, 38%, Hartmann number of 258, 22% and Hartmann number of 129, 13% has improved the heat transfer.

Anaerobic Digestion (AD) is an alternative process for the treatment of organic waste that enables the energy production in biogas. In addition, the total volume of waste transported to landfills decreases, thus increasing the lifespan of landfills. Considering the different sources of organic waste, food waste (FW) is of great importance because large quantities are produced and buried. For this purpose, co-digestion of FW and green waste (GW) with five mixing ratios of raw materials was performed to evaluate biogas production. Increasing the percentage of GW in the substrate led to increased methane yield. The highest biogas production was determined in the ratio of FW and GW 50:50. About 90% of methane yield was obtained after 21 days of digestion with a total yield of 425 ml/g VS. Based on the preferred ratio, the effect of total solids (TS) on co-digestion of FW and GW ranged from 5 to 25% TS. The results showed that the methane yield from digestion of high-solids AD (HS-AD) (15 to 20% TS) was higher than that of liquid AD (L-AD) (5-10% TS), while further increasing the TS content to 25%, disrupted the function of methanogenesis. This inhibition may be due to overloading of the organic charge and the formation of excess ammonia.

A two-phase lattice Boltzmann model considering the interaction forces of nanofluid has been developed in this paper. It is applied to investigate the flow and natural convection heat transfer of Al2O3–H2O nanofluid in an enclosure containing internal heat generation. To understand the heat transfer enhancement mechanism of the nanofluid flow from the particle level, the lattice Boltzmann method is used because of its mesoscopic feature and numerical advantages. By using a two-component lattice Boltzmann model, the heat transfer enhancement of the nanofluid is analyzed through incorporating the different forces acting on the nanoparticles and the base fluid . The effects of interaction forces, nanoparticle volume fractions (0.0-0.05), and internal and external Rayleigh numbers (103-106) on the nanoparticle distributions and heat transfer characteristics are investigated. The average Nusselt number increases with the increase of nanoparticle volume fraction and Rayleigh number. We also compared and analyzed adding internal heat generation on the nanoparticles and the base fluid separately, and it was found that by considering heat generation on the base fluid, it mostly affects the temperature field, and by considering that on nanoparticles, it mostly affects the stream field.

In order to simulate multiphase flow in the presence of dielectric current using the Lattice Boltzmann Method (LBM), three distribution functions are used, two of which for using the He-Chen-Zhang phase field model and one for solving the potential field. Initially, the ability of the code to apply surface tension was tested using the Laplace law and the drop release test. The results show that the present numerical program is capable of modeling well the regulated surface tension force. Then, the Rayleigh–Taylor instability simulation is used to evaluate the codechr('39')s ability in applying volume forces. The results by the developed numerical program are in good agreement with the numerical results in the references. In this study, for the first time, the effect of electric field on a droplet immersed in another fluid and the presence of droplet in a porous medium is investigated by LBM. For this purpose, first the droplet motion due to the potential difference in the porous and non-porous media is investigated. After modeling the droplet motion due to the potential difference, two electric fields areapplied to the droplet to reverse the droplet deformation. Through various tests, it is shown that at a given potential difference, the droplet breaks down after much deformation and is divided into smaller droplets. The decomposition of droplets in a pre-mixed emulsion is a common technique in the production of monodisperse droplets. The presence of monodisperse droplets in an emulsion improves the physical properties of polymer science experts.

In the present work, for the first time, using the finite - difference based lattice Boltzmann method, the convection heat transfer of the nanofluid in a cavity and channel is simulated. And also the effects of various factors such as Rayleigh number, nanofluid volume fraction and Reynolds number have been investigated. The Large Eddy Simulation (LES) method applied for modeling the turbulent flow. The natural convection heat transfer in the cavity for the Rayleigh range of 103 to 1010 and the volume fraction range of 0 to 1% has been evaluated. The forced convection heat transfer convection in the channel for the Reynolds number range of 50 to 3000 and the volume fraction range of 0 to 1% has been evaluated. The results show that the finite - difference based lattice Boltzmann method is able to simulate turbulent flows in different geometries. The results also show that natural convection heat transfer in the cavity, by enhancing Rayleigh number and the nanofluid volume fraction, the heat transfer rate increases. The forced convection heat transfer in the channel, enhancing the Reynolds number and enhancing the volume fraction of nanoparticles increases the heat transfer rate.

In this paper, the effect of changing position of heat source on nanofluid heat transfer under the influence of magnetic field in the wavy channel with variable amplitude and number of oscillations is investigated by Lattice Boltzmann Method. A uniform magnetic field is applied perpendicular to the channel. The first half of the upper channel wall, wavy form with the amplitude and number of variable oscillations at constant cold temperature, and the half of the bottom channel with variable position, are at constant hot temperature. Other walls are insulated for heat and mass. In this study, the effect of parameters such as Reynolds number, nanoparticle volume fraction, Hartmann number, hot wall position and amplitude and number of wavy wall oscillations were evaluated. The results show that at a specific position location of the hot wall, the average Nusselt number increases with the increase of other parameters. The highest heat transfer also occurs when the hot wall is closer to the channel inlet that results in an average 20% increase in the Nusselt number. The effect of increasing the Hartmann number on heat transfer is greater when the hot wall is closer to the channel outlet. Increasing the percentage of nanoparticles increases heat transfer and this effect increases with decreasing Reynolds number.

In this article, the magnetic field effect on the natural convection heat transfer is simulated via LBM. The vertical wall of the left side of the cavity is at a constant hot temperature, while the vertical wall of the right side of the cavity has three different temperature boundary conditions, 1) constant cold temperature, 2) linear temperature and 3) constant hot temperature. A lozenge-shaped obstacle located in the center of the cavity is examined in four different modes, 1) cold, 2) conducting, 3) adiabatic, and 4) hot. The bottom wall of the cavity is also evaluated in three different slopes. The results show that increasing the slope of the wall and the Rayleigh number by unchanged all the parameters leads to an increase in heat transfer. Also, changing the boundary temperature of the walls and the obstacle can affect the amount of heat transfer. In addition, increasing the strength of the magnetic field reduces the average Nusselt number, which differs in different conditions.

In this study, the comparison of the heat transfer of heat exchanger of the spiral tube and the direct pipe using the hybrid nanofluid in the turbulent flow has been empirically investigated.The nano fluid used in this study consists of a mixture of titanium oxide, silicon oxide and magnesium oxide with a medium diameter (20-30 nm) in a base water fluid at a temperature range of 60 to 30 ° C. Nanoparticles have been used in the volume fractions range of 0.25, 0.5, 0.75 and 1%.The results showed that the Nusselt number increased by increasing the Reynolds number and the concentration in the spiral tube compared to the direct tube by 6.6% on average. Also, the results are reported as temperature difference, the use of nanoparticles in the spiral tube is higher than the direct tube in the volume fraction of 0.25% and the temperature of 60 ° C, 37.6%. The percentage increase in the optimal temperature difference of the hybrid nano fluid in the spiral tube in the volume fraction is 0.25% and 60 ° C, which is 10% higher than the base fluid and in the direct tube in the volume fraction of 0.75% and 60 ° C Compared to the base fluid, the temperature difference is increased by 6%.

In present work, for the first time, gas flow with considering slip velocity and temperature jump boundary condition is studied in a heat sink consisting rectangular fins and microchannels with calculating conjugated heat transfer.In this paper, helium gas flow with Knudsen number between 0.048 to 0.06 has been studied. Heat flux applied to the bottom of aluminum heat sink is 500W/m2. The governing equation for fluid flow has been discretized using second order upwind method and solved with using Coupled algorithm in Ansys-Fluent commercial software.Results show that inlet and local Knudsen numbers decrease with increasing pressure ratio and also local Poiseuille number decreases with increasing inlet Knudsen number. Also with increasing inlet Knudsen number (reduction of pressure ratio), first the average Nusselt number decreases and then increases.In this case, the average Nuselt number decreases about 54.4% with increasing Knudsen number from 0.006 to 0.024 and the average Nusselt number increases with increasing Knudsen number from 0.024 to 0.048.With increasing Knudsen number, thermal resistance increases continuously. The results show that with increasing inlet Knudsen number, slip effects are pronounced and slip and jump temperature coefficients increase.

Atherosclerosis is responsible for almost 35% of annual deaths in developed countries. The disease could be due to an artery blockage by the interaction of an externally second phase (air bubbles, medicine carrying capsules) with a particle which is entered to the bloodstream. The effect of some most important affecting parameters on the blockage time of a microchannel due to the impact of a particle and a second moving second phase is investigated using lattice Boltzmann method and with programming Fortran90. The authors tried to mimic the physic of the flow of a small artery by generating the same geometry and changing geometrical and physical parameters. Lee and Lin Lattice Boltzmann multi-phase model is used beside the immersed boundary method. It is investigated the small changes in Capillary flow has no meaningful effect on the interaction of second phase and particle. But, the ratio of particle size to the channel width affects the blockage time in the microchannel. In fact, the blockage time will increase by an increase in the size of the particle. The initial size of the second phase to particle size ratio has the highest effect on the blockage time.

In this paper natural convection of nanofluid around a hot obstacle simulates in a square cavity with east and west cool walls and an adiabatic wall in north and a hot wall in south by Lattice Boltzmann Method. Flow is quiet and non-compressible and nanofluid is water-Tio2. We use D2Q9 LBM for velocity and fluid temperature. The purpose of this study is investigation of heat transfer around a hot obstacle in a square cavity and the effect of Rayleigh number, obstacle dimension, volume fraction of nanofluid, cavity dimensions, surface ratio and various models of computing heat transfer conductivity coefficient and viscosity coefficient on Nusselt number. This investigat is done for the first time. The results show that, by increasing of Rayleigh number and volume fraction, average of Nusselt number will increase. The average of Nusselt number will increase when obstacle dimensions increase to 0.5L but it will decrease when the obstacle dimensions increase to 0.7L. Vortexes will create in 0.8L and it causes to increase of heat transfer. By reduplicating the obstacle width heat transfer is better than reduplicating the obstacle length. The average of Nusselt number increases by increasing of cavity’s length and it will decrease by increasing of cavity’s wide. All results are equal in Hamilton-crosser and Maxwell- Garnett model when the surface ratio is one. But heat transfer will increase by decreasing surface ratio. The average of Nusselt Number in Wang model is less than Nusselt Number in Brinkman model.

In this paper, for the first time, natural convection heat transfer of a nanofluid in the presence of a uniform magnetic field inside a parallelogram shaped cavity with two triangular obstacles with different boundary conditions is simulated by using lattice Boltzmann method. The right vertical wall of the cavity is assumed to be adiabatic and the inclined walls are kept at constant cold temperature, while the left vertical walls are kept at constant hot temperature. The flow and temperature field is calculated by solving lattice Boltzmann equations for velocity and temperature distribution functions simultaneously. D2Q9 lattice arrangement for each distribution function is used. The results have been validated with available results in the literature. The effects of different parameters such as Rayleigh number (103-105), Hartmann number (0-90), nanoparticle volume fraction (0-0.05) and different boundary conditions at triangular obstacles on natural heat convective heat transfer are investigated. The results show that, at a constant Rayleigh and Hartmann number, the average Nusselt number takes its maximum and minimum value when the triangular obstacles are kept at constant cold and hot temperatures, respectively. For all cases, it is found that the average Nusselt number increases with enhancement of Rayleigh number. Also, increasing of Hartman number decreases the flow velocity and heat transfer rate. Furthermore, increase of volume fraction of nano particles enhances heat transfer rate, however its changes for different Rayleigh and Hartman numbers are not the same.