Mehdi ashjaee
University of Tehran, Mechanical Engineering, Faculty Member
- I am a professor at the Mechanical Engineering Department of University of Tehran.edit
Abstract In this paper, the effects of rotational forces such as centrifugal buoyancy and Coriolis forces caused by parallel mode channel rotation on turbulent air flow through tubes with different aspect ratios are investigated... more
Abstract In this paper, the effects of rotational forces such as centrifugal buoyancy and Coriolis forces caused by parallel mode channel rotation on turbulent air flow through tubes with different aspect ratios are investigated numerically. However, rotational effects does not necessarily increase heat transfer and pressure loss since a number of parameters such as main flow condition and channel aspect ratio must be regarded. This paper discusses rotational effects and establishes novel criteria that illustrate how the aspect ratio affects the formation of secondary vortices and thus convective heat transfer coefficient on different rotational conditions. The numerical modeling results are validated by the available experimental and numerical data in literature and it is shown that the normalized Nusselt number (Nu/Nu0) result points for each aspect ratio collapses on a single curve if it is plotted against GrΩα/Re which it is revealed that α depends on aspect ratio only. However, it is demonstrated that once the GrΩα/ Re
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In this study, the thermal conductivity and viscosity of SiO2/multiwalled carbon nanotube (MWCNTs) hybrid nanofluids are investigated. The volume fraction of the nanofluids varied in the range of 0.5% to 2%, while the SiO2 to MWCNTs... more
In this study, the thermal conductivity and viscosity of SiO2/multiwalled carbon nanotube (MWCNTs) hybrid nanofluids are investigated. The volume fraction of the nanofluids varied in the range of 0.5% to 2%, while the SiO2 to MWCNTs volume proportion is either 95-5 or 90-10. The nanofluids are synthesized using a wet chemical method and a two-step technique is used to disperse nanoparticles in glycerol (base fluid). The thermal conductivities and viscosities of the nanofluids are measured using a modified transient hot-wire method and falling ball viscometer, respectively. The colloidal stability of the dispersion was investigated visually. Effective application of an ultrasonic disruptor and a suitable surfactant (gum arabic) enhance the dispersion behavior. When the effects of temperature and volume fraction on the thermal conductivity and viscosity of SiO2/multiwalled carbon nanotube (MWCNTs) hybrid nanofluids are studied, the results showed that the thermal conductivity of nanofluids increased with an increase in the volume fraction and temperature. Further, their viscosities increased with an increase in the volume fraction but decreased when the temperature increased. The thermal conductivity and viscosity of the hybrid nanofluids increased by 16.7% and 105.4%, respectively, at a volume fraction of 2% and volume proportion of 90-10. The experimental results are compared with those predicted by classical theoretical models. Two correlations for thermal conductivity and viscosity of hybrid nanofluids are proposed on the basis of the experimental results.
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Abstract This paper performing both experimental and numerical approaches underlines the aiding and opposing effects of secondary vortices resulting from parallel mode rotation on convective heat transfer coefficient in a rectangular... more
Abstract This paper performing both experimental and numerical approaches underlines the aiding and opposing effects of secondary vortices resulting from parallel mode rotation on convective heat transfer coefficient in a rectangular channel in smooth and rib roughened channels. The Nusselt number is obtained by measuring the wall and fluid temperature at the Reynolds number ranging from 5118 to 10,677 and 0.07 Δ ρ / ρ 0.42 . In addition, the Nusselt number is investigated in ribbed channels with one wall being perpendicularly ribbed at pitch ratio ( P / e ) and blockage ratio ( e / D h ) equal to 15 and 0.088, respectively. Numerical simulations also allow to consider the effects of rotational induced secondary flows caused by centrifugal and the Coriolis forces in high rotation numbers ( 0 Ro 0.56 ) and Buoyancy numbers ( 0 Buo 1 ). The numerical results with non Boussinesq approximation for density variation show good agreement with experimental results, while it is revealed that the Nusselt number is underestimated if Boussinesq approximation is used. As the Reynolds number increases, the larger Buoyancy number is required to affect the Nusselt number. That is, under certain conditions which is expressed in terms of Gr Ω α / Re the rotational buoyancy effects may be negligible and the Nusselt number is equal to stationary case. However, in rib roughened channels the rotational buoyancy causes the reattachment point to occur closer to upstream rib while physical presence of ribs reduce the enhancing effects of rotation. Thus, for practical use in rotating cooling systems such as turbo-generators or electrical machinery, rib roughened channels are not recommended.
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Calculation of the temperature distribution around axisymmetric geometries such as cylinders and cones and the local heat transfer coefficient on such geometries is often encountered. This problem occurs for example in studies of flow... more
Calculation of the temperature distribution around axisymmetric geometries such as cylinders and cones and the local heat transfer coefficient on such geometries is often encountered. This problem occurs for example in studies of flow performance in cylindrical channel, temperature distribution around a burner flame and heat transfer from cylindrical tanks. Using a Mach-Zehnder interferometer, interferograms around an isothermal vertical cylinder at three different surface temperatures have been obtained. In this paper, in order to calculate the temperature distributions and the local heat transfer coefficients from interferograms, four methods of interferogram analysis including three classical methods and one transform method have been presented. In order to investigate the accuracy of the methods, results have been compared with the results of the analytical solution and relative accuracy of each method has been obtained. Results show that the transform method, while being less time consuming, is the most accurate method.
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The effect of the lateral aspect ratio on the natural convective heat transfer across rectangular enclosures at different orientations is reported in this paper. Experimental observations indicated that the heat transfer may be suppressed... more
The effect of the lateral aspect ratio on the natural convective heat transfer across rectangular enclosures at different orientations is reported in this paper. Experimental observations indicated that the heat transfer may be suppressed by placing lateral partitions within the enclosure. These partitions promote laminar behavior rather than turbulent behavior. For horizontal enclosures the partitions have little effect, whereas when the enclosure is tilted, the heat transfer is reduced up to 50%. Comparisons with other schemes for partitioning the enclosure reveal that the lateral partition is more effective for the suppression of the convective heat transfer.
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The heat transfer characteristics of a circular cylinder exposed to a slot jet impingement of air has been studied experimentally. The study focused on low Reynolds numbers ranging from 120 to 1210 and slot-to-cylinder spacing from 2 to 8... more
The heat transfer characteristics of a circular cylinder exposed to a slot jet impingement of air has been studied experimentally. The study focused on low Reynolds numbers ranging from 120 to 1210 and slot-to-cylinder spacing from 2 to 8 of the slot width. A Mach-Zehnder interferometer was used in the experimental study. Local Nusselt numbers at cylinder surface were obtained
Research Interests: Mechanics, Chemistry, Heat Transfer, Natural Convection, Data Reduction, and 12 moreHeat transfer coefficient, Experimental Study, Stagnation Point, Free Convection, Mach-zehnder Interferometer, Nusselt Number, Minimum Distance Inference, Cylinder, Circular Cylinder, Low Reynolds Number, Reynolds Number, and Nozzles
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In this paper, heat transfer characteristics of a miniature heat sink cooled by Al2O3–water nanofluids were investigated experimentally. Based on plate fin heat sinks, a new type of plate pinfinnned heat sink is developed which is... more
In this paper, heat transfer characteristics of a miniature heat sink cooled by Al2O3–water nanofluids were investigated experimentally. Based on plate fin heat sinks, a new type of plate pinfinnned heat sink is developed which is composed of a plate fin heat sink and columnar pins between the plate fins. The heat sink was fabricated from aluminum and insulated by plexiglass cover plates and consisted of five pin-finned rectangular channels with a length of 42 mm. The volume fraction of the Al2O3–water nanofluid particles was in the range from 0.5 to 2%. Mixtures were prepared without a dispersion agent. Tests were performed while supplying a 180 W/cm 2 heat flux to the bottom of heat sink. Experimental results showed that dispersion of Al2O3 nanoparticles in water increased significantly the overall heat transfer coefficient, while the thermal resistance of heat sink decreased. Also, the plate pin-finned heat sink showed an increase in the heat transfer coefficient up to 20% in com...
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Detecting bubble in two-phase flow has been a basic issue in two-phase flow systems. A new method for measuring the frequency of bubble formation is presented in this paper. For this purpose, an electronic device was designed and... more
Detecting bubble in two-phase flow has been a basic issue in two-phase flow systems. A new method for measuring the frequency of bubble formation is presented in this paper. For this purpose, an electronic device was designed and constructed which works based on a change in intensity of laser beam. For this purpose, continues light beam is embedded just above the needle, which is received by a phototransistor. When bubbles go through this light beam, make a deviation on that and change the intensity of light. So, the electrical resistance between two bases of phototransistor changes and this variation sensed by an electronic board. According to the number of interruption and duration time, the frequency of bubble formation can be calculated. Liquid and gas phases of present work are water and air respectively. Tests are performed in constant liquid height (60 mm above the needle), constant needle diameter (1.6 mm), and gas flow rates between 50 to 1200 ml/hr. Also, three other metho...
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In this study, an off-grid PV system is optimized to supply a Conex electricity demand in the top ten earthquake-prone cities using mixed-integer linear programming techniques. The stand-alone photovoltaic system is designed by a... more
In this study, an off-grid PV system is optimized to supply a Conex electricity demand in the top ten earthquake-prone cities using mixed-integer linear programming techniques. The stand-alone photovoltaic system is designed by a photovoltaic array, a cooling/heating system, battery banks, an inverter, and a charge controller. For determining the optimum size and specifications of the system components such as PV panel, HVAC coefficient of performance, by considering two objectives of the study, a mixed-integer linear programming method is used. These conflicting objectives are the probability of lack of power and total cost of the system. The weighted factor method is utilized, and final optimized systems are achieved using MATLAB 2019b. Using the weighted factor method, several optimum solutions, in which the importance of objectives are different from each other, are obtained for each case concerning objectives. The suggested model is optimized for ten earthquake-prone cities glo...
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In this paper, a novel integrated system is proposed to improve the performance of a conventional low-grade geothermal-based organic Rankine cycle (ORC). The main idea is to utilize two TEG units to recover the waste heat of the condenser... more
In this paper, a novel integrated system is proposed to improve the performance of a conventional low-grade geothermal-based organic Rankine cycle (ORC). The main idea is to utilize two TEG units to recover the waste heat of the condenser and geothermal brine. The proposed model is investigated and compared with simple ORC from the energy, exergy, and exergoeconomic viewpoints through the parametric study. Furthermore, the payback period of the systems is calculated to investigate the economic aspects of the model in more details. Results show that the exergy efficiency of the proposed system would be 56.81% at the base case (4.67% higher than the simple geothermal-based ORC system) and the total product cost of the proposed integrated system is 24.55 $/GJ at the base case (5.5% lower than simple ORC), while the payback period of the suggested system is 2.422 years (15 days lower than the simple ORC cycle). Furthermore, the net power output of the novel proposed system is 75.24 kW (...
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In this study, the effect of different nanofluids such as SiO2, Fe3O4, and Al2O3 on bubble characteristics is studied experimentally. The nanoparticles concentration for the SiO2 nanofluid is 0.05 wt% and for other nanofluids is... more
In this study, the effect of different nanofluids such as SiO2, Fe3O4, and Al2O3 on bubble characteristics is studied experimentally. The nanoparticles concentration for the SiO2 nanofluid is 0.05 wt% and for other nanofluids is 0.005 wt%. Bubbles are formed by injection of air at a constant gas flow rate (between 600‐1200 mL/h) into a stagnant isothermal liquid column. Experimental data of formation, growth, and detachment of the air bubbles were recorded by a high‐speed digital camera (1200 fps), and the image processing method was used to analyze the bubble characteristics. In the present study, bubble characteristics such as diameter, size, aspect ratio, and detachment frequency were studied in four different liquids. The results show that a bubble has the biggest size in the pure water and adding nanoparticles to the pure water decreases bubble size. Also, the variation of detachment frequency has an inverse relation with the bubble size behaviour. Between different nanoparticl...
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Utilizing nanofluids in heat exchangers can lead to improved thermal performance. Nanofluids with suspended carbon nanotubes are specifically desirable in thermal systems because of their unique capabilities. In this study, convective... more
Utilizing nanofluids in heat exchangers can lead to improved thermal performance. Nanofluids with suspended carbon nanotubes are specifically desirable in thermal systems because of their unique capabilities. In this study, convective heat transfer and required pumping power are studied simultaneously for a helical coiled heat exchanger with laminar water flow while incorporating 0.1 and 0.3 percent volume fraction of the hybrid nanofluid MWCNT + Fe3O4/water. Two different geometries of bare and ribbed tubes are used for the heat exchanger part. The ribs are chosen to be orthogonal, i.e., 90° with respect to the inclined ones. Three different Reynolds numbers are selected for investigation, all in laminar flow regime based on the non-dimensional M number defined in coiled tubes. Computational fluid dynamics is used to study thermal and fluid behavior of the problem. The convective heat transfer coefficient can serve as a criterion to measure the effectiveness of utilizing nanofluids...
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Abstract Hydrogen/methane mixtures have been considered as viable alternative fuels to gasoline, due to overall lower emissions and much better lean burn capabilities. Nevertheless, due to different combustion characteristics of hydrogen... more
Abstract Hydrogen/methane mixtures have been considered as viable alternative fuels to gasoline, due to overall lower emissions and much better lean burn capabilities. Nevertheless, due to different combustion characteristics of hydrogen compared to conventional fuels, its utilization in SI engines requires further investigations. The current study investigates the effect of hydrogen addition to methane SI engines using an open cycle Computational Fluid Dynamics (CFD) simulation coupled with chemical kinetics. The Eddy dissipation concept (EDC) model is used in combination with a two-equation turbulence model (k-e) and GRI-Mech 3.0 kinetic mechanism. The effect of various volume fractions of hydrogen from 0 to 0.5 on methane combustion is investigated and results are validated with experimental data in literature. A good insight of combustion, turbulence flow and species consumption inside combustion chamber is obtained. A sensitivity analyses is also performed in order to get profound understanding of combustion phenomena. The results showed that hydrogen addition reduces combustion duration, improves flammability range, and reduces carbon monoxide and carbon dioxide amounts. Hydrogen also has a great effect on initiation of combustion process due to its lower ignition delay and higher flame propagation speed. Hydrogen addition does not affect NOx levels considerably, which is on the other hand much affected by combustion air and maximizes at 20–30 percent excessive air. The results also indicate that at Maximum Brake Torque (MBT) condition, all fuel is consumed up to 10 degrees after Top Dead Center (TDC), while carbon monoxide which is maximum at this point will transmute to carbon dioxide afterwards.
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In this paper, an optimization was performed to achieve uniform distribution of convective heat transfer coefficient over a target plate using two impinging slot (air) jets. The objective function is the root mean square error (Erms) of... more
In this paper, an optimization was performed to achieve uniform distribution of convective heat transfer coefficient over a target plate using two impinging slot (air) jets. The objective function is the root mean square error (Erms) of the local Nusselt distribution computed by computational fluid dynamic (CFD) simulations from desired Nusselt numbers. This pattern search minimized this objective function. Design variables are nozzle widths, jet-to-jet distance, jet-to-target plate distance, frequency of pulsations (for pulsating jets), and the flow rate. First, an inverse design is performed for two steady jets for simplicity and the obtained errors for three different desired Nusselt numbers, NuD = 7, 10, and 13, were 20.73%, 20.08%, and 22.92%, respectively. Uniform distribution of heat transfer coefficient for two steady jets was not achieved. Thus, two pulsating jets are considered. The range of design variables for pulsating state is as same as steady-state and heat transfer ...
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This paper focuses on the amount of diesel consumption as one of the non-renewable energy sources consumed by the agricultural machinery and water pumps in wheat and corn farms in Dire County, Kermanshah Province, Iran. The population of... more
This paper focuses on the amount of diesel consumption as one of the non-renewable energy sources consumed by the agricultural machinery and water pumps in wheat and corn farms in Dire County, Kermanshah Province, Iran. The population of the study was determined by Bartlett et al . (2001) 'stable. Stratified random sampling method and census was used for the selection of respondents. The sample was composed of 247 wheat growers, 235 corn growers, and 57 tractor drivers selected by Bartlett et al .(2001) 'stable. Combine drivers as well as farm irrigation diesel owners were selected by census (n=15, n=48, respectively). The data collection tool was a questionnaire and the data were analyzed by SPSS software package. The findings revealed that in wheat farms, soil preparation had the highest consumption of diesel by 49% and in corn farms inter culturing had the highest diesel consumption by 38%. Moreover, the amount of diesel consumed in 3600 ha wheat and a corn farm in Dire C...
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A numerical study has been carried out for slot air jet impingement cooling of horizontal concentric circular cylinders. The slot air jet is situated at the symmetry line of a horizontal cylinder along the gravity vector and impinges on... more
A numerical study has been carried out for slot air jet impingement cooling of horizontal concentric circular cylinders. The slot air jet is situated at the symmetry line of a horizontal cylinder along the gravity vector and impinges on the bottom of the outer cylinder which is designated as θ=0°. The outer cylinder is partially opened at the top with a width of W=30mm and is kept at constant temperature T= 62°C. The inner cylinder which is a part of the slot jet structure is chosen to be insulated. The effects of jet Reynolds number in the range of 100≤ Rej ≤1000 and the ratio of spacing between nozzle and outer cylinder surface to the jet width for H=4.2 and H=12.5 on the local and average Nusselt numbers are examined. In the numerical study, FLUENT CFD package is used and validated by comparing the results with the experimental data at the same Reynolds number. It is observed that the maximum Nusselt number occurs at the stagnation point at (θ=0°) and the local heat transfer coef...
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In present study, the thermal conductivity of magnetic nanofluids (MNFs) containing MFe2O4 (M = Fe and Co) nanoparticles suspended in deionized water are investigated in the absence and the presence of uniform magnetic field. Fe3O4 and... more
In present study, the thermal conductivity of magnetic nanofluids (MNFs) containing MFe2O4 (M = Fe and Co) nanoparticles suspended in deionized water are investigated in the absence and the presence of uniform magnetic field. Fe3O4 and CoFe2O4 nanoparticles are synthesized using the co-precipitation method. The X-ray diffraction, transmission electronic microscopy and vibration sample magnetometer are used to characterize the structure, size and magnetic properties of nanoparticles. The thermal conductivity of MNFs are measured at different volume fractions between 0% and 4.8% and the magnetic field intensity range of 0–500 G. The experimental results show that the thermal conductivity of MNFs increases with increase in volume fraction and magnetic field intensity before reaching its saturation point. Finally, new correlations are presented based on the experimental results to predict thermal conductivity of MNFs in both the absence and the presence of magnetic field.
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The thermal management of electronic devices has become of great concern in the last decade. Recent research has focused on heat transfer enhancement methods for heat removal from such devices. In this study, we investigated the... more
The thermal management of electronic devices has become of great concern in the last decade. Recent research has focused on heat transfer enhancement methods for heat removal from such devices. In this study, we investigated the convective heat transfer and pressure drop of alumina–water nano-fluids in a miniature heat sink occupied with a copper porous medium under a 62.5 heat flux. Nano-fluid volume fractions of 0.1, 0.3, and 0.5% were synthesized by a two-step method, and the thermal conductivity and kinematic viscosity of the nano-fluids were measured experimentally. Two copper metal foam heat sinks, with different pore densities of 15 pores per inch (PPI) and 30 PPI, were manufactured and inserted into a channel. The results indicated an enhancement in the convective heat transfer coefficients and Nusselt numbers with increase in the Reynolds number. The enhancement in the heat transfer coefficients using different nano-fluid volume fractions in higher- and lower-porosity media...
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Article history: Received: 1 May 2020 Accepted: 30 May 2020 Published: 1 June 2020 This study uses real driving cycles of a city bus and a standard driving cycle “WLTP” to implement a full comparison for energy demand and fuel consumption... more
Article history: Received: 1 May 2020 Accepted: 30 May 2020 Published: 1 June 2020 This study uses real driving cycles of a city bus and a standard driving cycle “WLTP” to implement a full comparison for energy demand and fuel consumption for different propulsion systems (i.e., Diesel ICE, Fuel cell and Electric engines). These results were obtained by simulating each propulsion system in MATLAB, SIMULINK and EES. To better understand the comparison, a life cycle assessment is conducted using “GREET” and “GHGenius” software, which represents a clear demonstration of side effects and emissions of each engine on the environment. The results show that for “WLTP” cycle the bus needs 2423kJ energy for traveling each kilometer while the averaged amount of energy for traveling one kilometer of real driving cycle reaches to 1708kJ, the difference is due to speed range difference and number of stop/start points. In both cycles inertia force consumes the most used energy portion of the bus. B...
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Abstract In this paper, the effects of rotational forces such as centrifugal buoyancy and Coriolis forces caused by parallel mode channel rotation on turbulent air flow through tubes with different aspect ratios are investigated... more
Abstract In this paper, the effects of rotational forces such as centrifugal buoyancy and Coriolis forces caused by parallel mode channel rotation on turbulent air flow through tubes with different aspect ratios are investigated numerically. However, rotational effects does not necessarily increase heat transfer and pressure loss since a number of parameters such as main flow condition and channel aspect ratio must be regarded. This paper discusses rotational effects and establishes novel criteria that illustrate how the aspect ratio affects the formation of secondary vortices and thus convective heat transfer coefficient on different rotational conditions. The numerical modeling results are validated by the available experimental and numerical data in literature and it is shown that the normalized Nusselt number (Nu/Nu0) result points for each aspect ratio collapses on a single curve if it is plotted against GrΩα/Re which it is revealed that α depends on aspect ratio only. However, it is demonstrated that once the GrΩα/ Re
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Abstract The efficient cooling of turbo-generators especially in the end-windings region directly affects the power output rating. Modeling the coolant flow passing through the stator end-winding of a high-powered generator, however,... more
Abstract The efficient cooling of turbo-generators especially in the end-windings region directly affects the power output rating. Modeling the coolant flow passing through the stator end-winding of a high-powered generator, however, remains a challenge due to a large number of parts with irregular geometries as well as the intricacy of cooling flow paths. This paper focuses on a numerical investigation of flow and heat transfer in stator end-winding aiming at replacing the geometry of winding with a porous medium model. Using computational fluid dynamics analysis to examine the viscous and inertial contributions to the pressure drop of airflow in the stator end-winding region, a generalized porous medium model is developed for the real physical geometry of winding together with a correlation for interfacial Nusselt number. The results indicate that the inertial effect considerably prevails over the viscous effect, that is, viscous contribution becomes small and almost negligible. It is shown that not only the pressure drop characteristics match closely within both real and porous models, but also the heat transfer results agree as well which means that the replaced porous model can amply describe the macroscopic behavior of the winding.
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Abstract Toxic effects of drugs on healthy tissues in the traditional way of chemotherapy are considered as the serious challenges of cancer treatment. Magnetic drug targeting is a method that can be used for topical treatment of... more
Abstract Toxic effects of drugs on healthy tissues in the traditional way of chemotherapy are considered as the serious challenges of cancer treatment. Magnetic drug targeting is a method that can be used for topical treatment of cancerous tumors as well as to reduce the side effects of drugs. In this paper, a finite element simulation is carried out to calculate the trajectories and capture of drug carrying Fe3O4 magnetic nanoparticles under the influence of an external magnetic field. Two different cases of uniform distribution of the nanoparticles in the blood flow, and a separate injection in the vein are considered and the effects of different parameters such as blood velocity, magnetic field intensity, and particle diameter on the capture efficiency are investigated. Results show that injection of magnetic nanoparticles in an appropriate site can be very effective in blood vessels with high blood flow velocities. The capture efficiency of the particles in a separate injection mode is shown to be improved up to 85% in comparison with the case of uniform flow of the particles in the blood vessel. However, it is indicated that the drug targeting is not successful when the injection site is located opposite to the permanent magnet. Moreover, it is observed that the capture efficiency of the drug carrying particles depends on the magnitude of the magnetic and drag forces. The capture efficiency increases with increase of the magnetic field intensity and particles diameter and decrease with the blood flow velocity.
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Abstract This paper investigates the potential of renewable energies utilization in detail through three in-house developed strategies to increase the renewable power generation share until the year 2050 assuming either an optimistic 100%... more
Abstract This paper investigates the potential of renewable energies utilization in detail through three in-house developed strategies to increase the renewable power generation share until the year 2050 assuming either an optimistic 100% or a practical 50% based on the national policies. Solar, wind, and waste energy are the most feasible alternative energy resources in Iran. In the first strategy, power plants are phased out according to their lifetime and replaced by renewable resources in 5-year time steps. The second strategy employs a 3% replacement rate to reach a 100% renewable power generation in 2050. In the third strategy, the national plan of the power ministry is utilized to adopt a more practical pathway for increasing the renewable power generation share up to 50%. Pollution and water shortage crises are also considered within the framework of this study. The following work can lay the foundation for future studies of its kind in Iran as well as other countries all over the world. Developing the presented results can lead to the practical implementation of the study, which will vastly benefit the residents. Moreover, it will provide a great investment opportunity for foreign and domestic companies working on the field.
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The effect of orifice shape on the mechanism of bubble formation in gas–liquid two-phase flow is investigated experimentally with three different orifice geometries regarding a circle, a square, and a triangle with same cross-sectional... more
The effect of orifice shape on the mechanism of bubble formation in gas–liquid two-phase flow is investigated experimentally with three different orifice geometries regarding a circle, a square, and a triangle with same cross-sectional areas. The liquid and gas phases are purified water at 20 °C and air at room temperature, respectively. Gas is injected at the rate of 50–1200 mlph into a stagnant pool of liquid in distances of 5, 10, and 15 cm below the liquid surface. The position, velocity, and acceleration of bubbles are measured at bubbles’ centers of mass (CM) and the effects of these parameters on the bubble volume are investigated. Moreover, the forces acting on a bubble are balanced and the effects of geometry and gas flow rate on each force are presented. In addition, the changes of the acting forces versus time are plotted and discussed for a specific condition. Results show the bubbles formed with the square and circular orifice cross-sectional areas have the most and least volumes at detachment, respectively.
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Abstract A complete renewable energy source based on a solar and a geothermal system is proposed to produce desired electricity and cooling. The proposed system consists of a concentrated PVT, a double-effect LiBr-H2O absorption chiller... more
Abstract A complete renewable energy source based on a solar and a geothermal system is proposed to produce desired electricity and cooling. The proposed system consists of a concentrated PVT, a double-effect LiBr-H2O absorption chiller and a geothermal unit. To better understand the system performance, energy, exergy and exergoeconomic analyses are investigated. The results show that recovering the waste heat of the geothermal unit increases the coefficient operation factor by about 15%. Second law analysis exhibits that one of the main parts of irreversibility occurs in the PVT with 29.6 kW. Results of exergoeconomic analysis show that in the second condenser and the cooling set, the exergy destruction cost has major effect on the component cost rate. In addition, the parametric study of major parameters (i.e., geothermal temperature, high pressure and low pressure turbine inlet pressure, PVT module’s temperature and area) is performed thermodynamically and thermoeconomically. In addition, by considering the overall exergy efficiency and the total product unit cost as objective functions, a multi-objective optimization is implemented based on genetic algorithm. From the Pareto frontier diagram, the value of an optimal point for single and multi-objective optimization are determined. Obtained results show that at the optimal point where the overall exergy efficiency and total product unit cost are optimized, the corresponding values are 12.31% and 35 $/GJ, respectively. Scattered distribution of the major parameters reveals that the geothermal temperature is a very sensitive parameter which should be kept at its highest value (i.e., 245 °C).
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Performance of transcritical CO2 Rankine cycles with and without reheat process is investigated thermo-economically in low-grade waste heat recovery applications. Two reheating scenarios are proposed to evaluate the effect of bounded and... more
Performance of transcritical CO2 Rankine cycles with and without reheat process is investigated thermo-economically in low-grade waste heat recovery applications. Two reheating scenarios are proposed to evaluate the effect of bounded and unbounded reheats on the cycle. In the first method, the constant available heat flow is distributed between the evaporator and the reheater via an optimized ratio, while in the second, the required energy for the reheat process is provided with optimized additional fuel consumption. The proposed cycles are modeled and optimized for source temperatures ranging from 150 to 300 °C at fixed flow rate of 1000 kg/s. The results obtained from thermodynamic optimization indicate that reheat cycle with burning additional fuel leads to the largest power generations ranging from 14 to 57 MW depending on the source temperature, while the reheat cycle with heat stream division shows the weakest performance by producing 8–37 MW. In the thermo-economic optimization, the ratio of power output to the cycle total bare module cost has been maximized. Under these conditions, the reheat cycle with burners still shows the highest rate of power production, while economic indicators limit the power generation and introduce the simple Rankine cycle as the best option.