S.m. Hosseini Baghdad Abadi, S. Zirak , M. Rajabi Zargar Abadi ,
Volume 19, Issue 1 (1-2019)
Abstract
In this paper, the effect of the angle of injection on the film cooling effectiveness with sinusoidal wave pulsation is investigated at various frequencies. Four angles of injection are selected at 20, 25, 30, and 35 degrees. The pulsed flow is investigated at 3 frequencies of 2, 50, and 500 Hz. Geometry was simulated in Gambit and numerical analysis was done by Fluent software. The SST k-ω model was used for modeling turbulence. The results showed that the injection angle between 20 and 25 degrees in the frequencies studied had the most film cooling effectiveness of the central and lateral line, especially in the areas far from the edge of the hole. Higher frequencies (500 Hz) increase the effectiveness of the film cooling at the lower initial distances of the hole. At far distances, the lower frequency (2 Hz) is the most effectiveness. As the frequency increases, the difference in the cooling efficiency of the central and lateral lines decreases at different angles. As the frequency increases, the interruptions of the flow-off and the flow-on are reduced, and as a result, the instantaneous effectiveness also has a slower variation than the lower frequencies. The blowing ratio of 0.5 had the most value in comparison with the blowing ratio of 0.75 and 1 in all angles and frequencies.
M. Khatibi , M. Mohammadzadeh Kowsari, H Niazmand,
Volume 19, Issue 3 (3-2019)
Abstract
In this study, the thermo-physical properties effects of the heat exchanger body on the adsorption chillers performance have been investigated. For this purpose, an adsorbent bed with a rectangular finned flat-tube heat exchanger is simulated by employing a three-dimensional control volume scheme. Furthermore, silica gel SWS-1L-water has been used as a working pair. In order to investigate the effects of thermo-physical properties of the heat exchanger body material, two main parameters including the thermal conductivity coefficient and the volumetric thermal capacity are examined. Also, the effects of these parameters along with variations of the fin height and fin pitch on the specific cooling power (SCP) and the system coefficient of performance (COP) are investigated. The results indicated that the SCP increases with the increase in thermal conductivity coefficient up to a certain value, which increases and decreases with the increase in fin height and fin pitch, respectively. The results also showed that the effects of the volumetric thermal capacity on the SCP are negligible such that it can be considered independent of the heat exchanger body material volumetric thermal capacity. Unlike the SCP, the COP is strongly influenced by the volumetric thermal capacity. The increase in volumetric thermal capacity results in decreasing the COP. The slope of the decrease in the COP decreases with increasing the fin height and pitch. Also, by increasing the thermal conductivity coefficient, the COP slightly decreases.
V. Dabirpour, O.r. Mohammadipour,
Volume 19, Issue 3 (3-2019)
Abstract
In this study, convective heat transfer around a heated circular cylinder covered with an annular porous medium in a flat channel was numerically investigated. To enhance the heat transfer, the porous medium is chosen to have a high thermal conductivity, whereas it is equipped with two different dispersions to reduce the pressure drop through the channel. To create two different dispersions (bi-disperse porous medium), the cylinder is covered uniformly by multiple porous fins with a porosity of 0.9. In this regard, the fin porosity will be the first levels of porosity (microscopic porosity) and the arrangement of fins will be referred to as the second levels (macroscopic porosity) of the porous medium. The main goal of this research is to investigate and optimize flow conditions to achieve the highest outlet temperature and the highest heat transfer rate, where the pressure drop is reduced to a minimum value. This optimization is carried out for flow Reynolds number of 60 to 120, the Darcy number of 10-3 to 10-5, macroscopic porosity of 0.25 to 0.75, and outer to inner fin ratios of 1.5 to 2. Numerical simulations are conducted, using the lattice Boltzmann method and the validity of simulations is assessed by the use of numerical and experimental data available in the literature. To optimize, the response surface methodology (RSM) with a central composite design is used and numerical results indicate that predictions obtained by RSM are in good agreement with actual flow condition in the optimum configuration. This research can provide new insight into the optimization process in heat exchanger designs.
S. Khalili Sarbangholi, Y. Aghdoud Chaboki,
Volume 19, Issue 3 (3-2019)
Abstract
Waste heat recovery systems, which make use of waste sources for their input energy, have considerable importance in industry since they utilize streams, which will be disposed to nature if not employed. Ship’s engines are one of the places, where a large amount of energy is wasted in different forms. In the present article, the idea of making use of these loss streams and consequently producing useful power in the outlet is proposed in the form of two systems. In the first system, the only stream of exhaust gases is utilized, while in the second system, the jacket cooling water is used together with the engine exhaust gases. Screening in the working fluids is conducted in order to select appropriate fluids, which have suitable characteristics in the physical, safety, and environmental aspects. The analyses indicate that using R600a presents the highest net power output, which reaches to the value of about 575 kW at the most. Comparison of the two introduced systems shows that preheating the working fluid by the jacket cooling water makes the better operation of the system and the power output is increased up to about 31-58% in different fluids. The lowest payback period in the systems is achieved through the use of R600a as the working fluid, which is about 3.48 year in the second system.
M. Abouali Shamshiri, M. Asgari ,
Volume 19, Issue 3 (3-2019)
Abstract
In this paper, a nonlinear theoretical solution is proposed to simulate thermoelectric generators. A thermoelectric generator (TEG) setup was designed and constructed to measure the thermoelectric properties of a specified TEG, and, then, to validate the simulation results. The setup is composed of four bismuth telluride based TEGs, which are placed between an electrical heater and water cooled heatsinks to generate power as the result of the temperature difference. In the first section, the thermoelectricity phenomenon is introduced and governing equations are presented in order to develop the finite element solution by weighted residual Galerkin method. The FEM code is written in MATLAB software. In the second section, the designed and fabricated setup is explained and it is investigated how to perform the experiments. The TEG properties including the Seebeck coefficient and internal electrical resistance were measured, which are, then, used for setup simulation. First, the thermal-fluidic parameters including temperature and velocity distribution are obtained by simulation in Ansys-Fluent software. Then, the thermoelectricity simulation is performed by means of both the proposed finite element solution, and Ansys-Thermal electric software; so, the output voltage, power, and efficiency are calculated. The results indicate the accuracy of the modeling. Also, using the proposed finite element solution, the impact of the geometrical dimensions and temperature conditions on the TEG performance is investigated.
F. Karami, M. Sabzpooshani,
Volume 19, Issue 3 (3-2019)
Abstract
The aim of this research is an analytical investigation of heat and mass transfer for the MHD nanofluid flow passed between non-parallel stretchable/shrinkable walls. In order to model nanofluid flow, effects of Thermophoresis, Brownian diffusion, and Joule heating are considered. The governing mass, momentum, and energy equations are solved analytically by applying Duan-Rach method, which caused to get a solution for the undetermined coefficients from conjectured profiles of variables without using numerical methods. Comparison between the current results with the numerical results of other references shows good agreement. The effects of the Reynolds number, opening angle parameter, and the Hartman number on the temperature, velocity, and concentration profiles have been investigated in the case of both convergent and divergent plates, either stretched or shrunk. Also, the effects of the Thermophoretic and Brownian parameters on the Nusselt number are obtained. This study indicates that increasing the Hartman number decreases the concentration profile and increasing in the temperature profile for divergent channels. In this case, as the opening angle parameter rises, the thickness of the thermal boundary layer increases. Also, for convergent and divergent channels, the increase in the thermophoretic parameter causes increases the Nusselt number. By applying an identical magnetic field to two divergent stretching and shrinking channels, the concentration profile in the stretching channel is more than the shrinking one. For convergent channels, this treatment of concentration profile is completely vice versa.
A.r. Ghaedamini Harouni, S.h. Hashemi Mehne,
Volume 19, Issue 3 (3-2019)
Abstract
Multidisciplinary shape optimization of a re-entry capsule with aero-thermodynamic, trajectory, stability and the geometry considerations are presented in this research. The method is based on decomposition of the underlying problem into disciplinary routines performing separated analysis for each goal.The current research is separated into four main components: shape parameterization of re-entry capsule, aero-thermodynamic analysis, re-entry trajectory analysis and optimization.The re-entry capsule that is studied here belongs to the family of the Orion-like capsule and its shape composed of three analytic surfaces: a spherical nose, a ring section and a rear conical part. The objectives of the optimization are maximizing volumetric efficiency, minimizing longitudinal stability derivative, and minimizing the ballistic coefficient, subject to constraints on geometry, heating load, and deceleration. Utilizing a multi-objective genetic algorithm will result in a collection of non-dominated Pareto optimal solutions. Then, the multi-disciplinary multi-objective optimization process allows finding a Pareto front of the best shapes. Resulting optimal solutions obviously show the compromises among volumetric efficiency, longitudinal stability and ballistic coefficient. In the end, the results containing dimension’s characteristics of the re-entry capsule is presented.
S. Ghorbanzadeh , M. Nazari , M.m. Shahmardan , A. Hasannia, M. Nazari ,
Volume 19, Issue 4 (4-2019)
Abstract
In this paper, heat transfer and magnetic fields in a vacuum induction melting furnace have been studied numerically. To solve the coupled equations of thermal and magnetic induction heating, the finite element method has been used. An induction furnace model is simulated using an industrial geometry. The studies indicate that the effect of the geometry of the crucible and the coil on the melting time has not been thoroughly investigated and requires more in-depth studies. It is attempted to improve the shape of the induction furnace, so that in less time aluminum is melted in a small scale furnace. The effect of the diameter-to-height ratio of the crucible on the duration of melting has been investigated. By decreasing the diameter-to-height ratio, the temperature reaches melting temperature in a shorter time. The results show that for the diameter-to-height ratio of less than 0.4, there will not be a significant change at the average temperature. 10% reduction in the distance between the coils leads to an increase in the average temperature of the working material inside the furnace. With considering the constant density of the coil current and the constant induced current in the heated material, the effects of the number of coil turns on the temperature distribution and magnetic flux are investigated. In this way, the accuracy of the model is also checked by induction heating concepts. The effect of frequency on temperature has been investigated in different coil lengths. The results show that an increase of 4 times in the frequency caused an increase of 1.7 times in the average temperature.
S. Mirzaparikhany , M.r. Ansari,
Volume 19, Issue 4 (4-2019)
Abstract
In this paper, a theoretical model is proposed for Leidenfrost droplet evaporation by solving the mass, momentum, and energy conservation equations. This model involves a set of four equations, of which the values of vapor layer thickness, evaporation rate on the lower surface of the drop, the volume of evaporating droplet, and temperature distribution in vapor layer are obtained. This set of equation is solved with Fortran code by the predictor-corrector method. The main unknown value in these equations is the vapor layer thickness, which is predicted in every step of simulation and corrected by the balance of forces that act on the drop. In this study, the upper surface of the drop, where contacts with air and the lower surface of droplet, where contacts with the vapor layer are predicted with high accuracy by solving the Young- Laplace equation. The vapor layer thickness obtained from the proposed model is compared with experimental data and encouraging agreement is observed.
S.a. Fanaee , M. Rezapour ,
Volume 19, Issue 4 (4-2019)
Abstract
In this paper, heat transfer and fluid flow characteristics in a porous coil have been investigated. The characteristic of the boundary layer, distribution of velocity, pressure, and thermal field effects into a porous coil as high heat transfer resource have been analyzed. The developed Brinkman method in fluid flow and power law model of conduction heat transfer coefficient considering porosity and permeability factor is calculated for constant solar heat flux. In order to solve the problem, the COMSOL software based on finite element method with porous medium algorithm is used, using the MUMPS solver. The comparison between variation of normalized temperature at the presented model and experimental data at similar conditions shows an acceptable agreement with an error up to 3%. At constant permeability, decreasing the porosity coefficient, velocity profile is extended due to presence of pores into coil with an accelerated flow, so that the maximum velocity is equal to 2.5m/s at porosity coefficient of 0.2. In porous coil, Nusselt number increased, where the greatest difference between porous and the nonporous coil occurs at the beginning of the coil, with a value of 32%, and the smallest difference is 27%. In the porous coil, absorbing solar energy is higher and the heat transfer is improved. However, the amount of pressure drop also increases.
S.sh. Hosseini Dehshiri, Sh. Talebi,
Volume 19, Issue 4 (4-2019)
Abstract
New passive double L-shaped micromixers have been investigated based on the split and recombination flow. Numerical study on micromixers was performed in the Reynolds number range of 50 to 200. The three-dimensional Navier-Stokes equations have been used to analyze flow and mixing behavior. Two different configurations from the positioning of L units have been investigated and two solutions have been proposed to improve the mixing index. If two L units are same shaped, aligned on one plate (design 1), the mixing index is low due to inappropriate split and recombination. The placement of two L units of the same shape on a two-plane parallel and non-aligned (design 2) improve the mixing index and increase to over 95% in Reynolds numbers of 100, 150, and 200. The orthogonal solution to the inputs did not affect the pressure drop and only in design 1, the mixing index could exceed 95% in all Reynolds numbers. Unbalanced micromixer solution improves mixing index by increasing pressure drop. The effect of geometric parameter of asymetric width ratio in both designs was studied and design 1 in asymetric width ratio 2.5 and design 2 in asymetric width ratio 2 and 2.5 have been completely mixed in all Reynolds numbers. Also, the performance of proposed micromixers was better than L-shaped micromixer due to the split and recombination mechanism. In addition, the mixing index was higher in porposed micromixers compared to the split and recombined micromixers of previous researchers due to the use of L-shaped units.
H. Hajabdollahi, B. Masoumpour,
Volume 19, Issue 6 (6-2019)
Abstract
The present study investigated modeling and optimization of a multi-tube heat exchanger (MTHE) network considering the effect of different on the tube side. After thermal modeling in ε-NTU method, optimization was performed from the perspective of increasing effectiveness and decreasing total annual cost as 2 objective functions, using 8 parameters, including number of MTHE and concentration. In addition, was performed at 3 various cold mass flow rates and different including AL2O3, and ZrO2 (water). The results show that the Pareto front was improved in case, and the rate of improvement in CuO case, especially in higher effectiveness and lower mass flow rates is more significant compared with the other studied cases. In addition, because of the improved thermal performance of MTHE network in the case, the heat transfer surface area and consequently the volume of MTHE network for fixed values of effectiveness are significantly reduced. Finally, after display of the results of the design parameters versus effectiveness, sensitive analysis of particle concentration on the objective functions was performed for typical and the results were discussed.
H. Azarkish,
Volume 19, Issue 6 (6-2019)
Abstract
In the present work, a novel configuration is proposed to improve the cooling performance of a capillary-driven system. In this approach, the possibility of meniscus formation inside the is increased for a wide range of operating temperature by controlling the capillary and viscous forces. The proposed consists of three sections. The first section is a narrow part of to control the pressure drop. The second section of is an evaporator. The meniscus is formed in this section due to of the capillary and viscous forces. It can move along the The third section is a wide part of The meniscus cannot move further in this section due to decreasing the capillary pressure. The evaporation rate from meniscus is estimated by using the thin film evaporation theory. Results show that the heat flux up to 30-100 W/cm2 70-100⁰C) can be dissipated by the evaporation mechanism from a hydrophilic membrane.
M. Akbari Paydar, B. Mohammad Kari, M. Maerefat, M. Abravesh,
Volume 19, Issue 6 (6-2019)
Abstract
The optimal insulation thickness is a function of the insulation initial cost and the cost of energy carriers for the internal space heating and cooling due to heat transfer from the wall. In Iran, by allocating subsidies to the energy sector, tariffs for energy carriers are sensibly lower than global prices. In order to determine the insulation optimal thickness, energy carrier tariffs were considered variable according to consumption. Electricity and gas costs were divided into 4 ascending tariffs for low, moderate, high, and very high consumption cases. In addition, the case of energy carriers without subsidies was also examined the 5 . The outer wall consists of a typical hollow with 20cm thickness, insulated with an expanded polystyrene layer, placed the outside. Heat load due to heat transfer from the external wall was calculated by using EnergyPlus simulation software in different geographical directions and different thermal insulation thicknesses in Tehran climate. The optimum insulation thickness was determined based on the total cost over the lifetime of 30 years. According to the results, in the first tariff, which refers to low-cost subscribers, the use of thermal insulation in some geographic directions does not allow the payback period over a lifetime. In other directions, economic savings are low and . For higher tariffs, the optimum insulation thickness increases. In the 2 5 , the thermal insulation thickness from 6 to 18 cm. Also, the calculated payback periods of these configurations are between 6 and 28 years.
Y. Maaref, H.a. Pakravan, Kh. Jafarpur,
Volume 19, Issue 7 (7-2019)
Abstract
During the last 3 decades, different therapeutic methods have been used for cancer treatment. Hyperthermia is one of these methods, which destroys the tumor cells with applying temperatures about 41-46°C. Thermal ablations of hepatic tumors near large blood vessels are affected by the heat sink effect of blood vessels. In this study, the heat sink effect of blood vessels on hepatic mono-polar radiofrequency and microwave ablation was investigated. The simulation is performed by numerical solution of bio-heat transfer equation with equations of electrical current or electromagnetic waves. To analyze the heat sink effect of blood vessels, the tissue is modeled with and without blood vessel. The fraction of necrotic tissue is determined for 3 different diameters of blood vessels including 5, 10, and 15 mm. The results show that when the applicator distance to the blood vessel is less than or equal to 8 mm, the necrotic value significantly decreases and the heat sink effect becomes important; however, for distances larger than 30 mm, the necrotic value does not change and the heat sink effect is diminished. The heat sink effect increases with blood vessel diameter due to the blood flow increase. In addition, the results indicated that
the microwave ablation is less affected by the heat sink effect in comparison with the mono-polar radiofrequency.
K. Taghizadeh Azari, M.r. Matini , M. Zare ,
Volume 19, Issue 8 (8-2019)
Abstract
The development of built environment and increase of energy source utilization have led to paying attention to different procedures to optimized energy consumption in buildings. Designing different sort of double skin façade provides opportunities to keep building in more balanced environment and use less energy to provide comfort condition. As a natural process that optimizes energy consumption by balancing between different solutions, homeostasis is used as a pattern in designing this sort of homeostatic façade. Nowadays, different sorts of smart façade have been used on the boundary of building and environment. A sort of smart façade, which is designed based on homeostatic process, is able to create a sustainable balance between different solutions, adapting to environmental changes, and define the hierarchy of their use in different conditions, so as to provide thermal comfort conditions inside the building with higher efficiency than conventional smart façades. In this study, temperature fluctuation limits in homeostatic façade is determined and solutions are derived from a natural homeostasis system, and used in the design of the desired façade. The aim of this research is to compare the efficiency of temperature reduction solutions in different conditions and specified optimal one. For this end, a modulus of homeostatic façade is built and the operation under laboratory condition is evaluated, and also its behavioral relationship is examined with temperature fluctuations.
S. Omiddezyani , I. Khazaee, S. Gharehkhani , M. Ashjaee, F. Shemirani, V. Zandian,
Volume 19, Issue 8 (8-2019)
Abstract
Today, nanofluid is attracting intense research due to its potential to augment the heat transfer rate and the cooling rate in many systems. On the other hand, new research progresses indicate that graphene nanofluids even in very low concentrations could provide higher convective heat transfer coefficient in comparison to the conventional nanofluids. For this reason, we used nanofluid containing the CoFe2O4/GO nanoparticles as working fluid to perform experimental investigation of its effect on laminar forced convective heat transfer in the flow passing through a copper tube, which is under a uniform heat flux. It should be noted that utilizing magnetic field on nanoparticles is one of the active methods for improving the heat transfer rate. To achieve this objective, the effect of external magnetic field intensity and also the effect of applying different frequencies on the improvement of heat transfer in Reynolds number and different concentration is also investigated and the optimum frequency were obtained. The results showed that the heat transfer of the studied hybrid nanofluid has been improved in the presence of constant and alternating magnetic fields and the amount of heat transfer increment, due to an alternating magnetic field, is more significant compared with a constant magnetic field. The results also show that in the absence of magnetic field, using ferrofluid with concentration of φ=0.6%, improves the average enhancement in convective heat transfer up to 15.2% relative to the DI-water at Re=571, while this value is increased up to 19.7% and 31% by using constant and alternating magnetic field, respectively.
H. Ghaderi , A. Ghasemi , S. Rouhi , E. Mahdavi ,
Volume 19, Issue 9 (9-2019)
Abstract
In this paper, the thermal conductivity coefficient of multi-walled boron nitride nanotubes has been investigated, using molecular dynamics simulation based on the Tersoff and Lenard Jones potential functions. The effects of diameter, length, and temperature on the thermal conductivity of double-walled boron nitride nanotubes have been studied. Also, by considering the 2, 3, 4, and 5-wall nanotubes, the effect of number of walls on the thermal conductivity of boron nitride nanotubes were studied. Finally, by considering of zigzag and armchair nanotubes, the effect of chirality has been investigated. The results showed that the thermal conductivity coefficient of double-walled boron nitride nanotubes increases by increasing the diameter of nanotubes and decreases by increasing temperature. It had been demonstrated that with 73% and 82% increase in the outer diameter of nanotubes, the thermal conductivity increases 93% and 98%, respectively. Furthermore, regarding to the chirality, the armchair nanotubes have a higher thermal conductivity than the zigzag ones. Also, the simulation results showed that thermal conductivity coefficient increases by increasing the length of boron nitride nanotubes and 50% increase of effective nanotube length increases the thermal conductivity by 25% approximately. Finally, by studying the effect of the number of walls, it is concluded that in the same length and temperature, nanotubes with higher number of walls have higher thermal conductivity coefficient in comparison.
M. Nikmehr, V. Kalantar,
Volume 19, Issue 10 (10-2019)
Abstract
Nowadays with the increase of the power of electronic components, their heat generation rates have also increased therefore, therefore it is necessary to use new methods to cooling different parts. One of the solutions to cool the high-power components is the use of vapor chambers. The vapor chamber consists of three sections, the evaporation, the middle and the condensation section, which are flattened and can transfer a significant amount of heat without the need for external power and only by using a fluid phase change. In this study, two vapor chambers with a length and width of 120 mm and a height of 15 mm were made to cool the high-power printed circuit board, where the evaporation section of one of them was roughened and the condensation section is cooled down by the fin and through the air. In this research, the effect of roughening the evaporation section, the angle of the vapor chamber relative to the horizon, different heat input and the geometric deformation of the heat source in the fixed area, as well as changing the location of the heat source in the evaporation section, on the thermal performance of the vapor chamber, is experimentally reviewed and compared. The results of the experiments show that increasing the heat input and roughing the evaporation section improves the performance of the vapor chamber and the thermal resistance of the vapor chamber is also the function of changing its angle relative to the horizon, deformation, and location of the thermal source.
A. Dadgar Fard, M. Rajabi,
Volume 19, Issue 11 (11-2019)
Abstract
In this paper, a simple, practical and versatile model has been developed for a self-activated acoustic driven spherical swimmer that its surface may oscillate partially at dipole state (first mode of vibration). Regard to the nonlinear acoustic effects, the net acoustic radiation force exerted on the device is analytically derived and the non-zero states are approved. Considering hydrodynamics effects assuming low Reynolds number operating condition, the effects of active section angle and frequency of operation on the force, velocity and requirement power of swimmer are discussed. It is shown that comparing with many types of artificial and natural living matter swimmers, the swimming velocity of the developed model is satisfactory. The challenge of the random walk due to host medium fluctuations is discussed, and it is shown that the developed model can overcome the ubiquity of the Brownian motion, as well. Due to the simplicity of the developed model which leads to computing the swimmer features (such as force, velocity, etc.) analytically, this study can be considered for development of contact-free precise handling, drug distribution and delivery systems, entrapment technology of active carriers and the self-propulsive controllable devices which are essential in many engineering and medicine applications.
