International Communications in Heat and Mass Transfer, Feb 1, 2021
Abstract In this paper, the effects of rotational forces such as centrifugal buoyancy and Corioli... 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
Journal of Nanoscience and Nanotechnology, Jun 1, 2019
In this study, the thermal conductivity and viscosity of SiO2/multiwalled carbon nanotube (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.
Thermal science and engineering progress, Dec 1, 2021
Abstract This paper performing both experimental and numerical approaches underlines the aiding a... 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.
Calculation of the temperature distribution around axisymmetric geometries such as cylinders and ... 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.
The effect of the lateral aspect ratio on the natural convective heat transfer across rectangular... 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.
The heat transfer characteristics of a circular cylinder exposed to a slot jet impingement of air... 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
In this paper, heat transfer characteristics of a miniature heat sink cooled by Al2O3–water nanof... 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...
Detecting bubble in two-phase flow has been a basic issue in two-phase flow systems. A new method... 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...
In this study, an off-grid PV system is optimized to supply a Conex electricity demand in the top... 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...
In this paper, a novel integrated system is proposed to improve the performance of a conventional... 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 (...
International Communications in Heat and Mass Transfer, Feb 1, 2021
Abstract In this paper, the effects of rotational forces such as centrifugal buoyancy and Corioli... 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
Journal of Nanoscience and Nanotechnology, Jun 1, 2019
In this study, the thermal conductivity and viscosity of SiO2/multiwalled carbon nanotube (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.
Thermal science and engineering progress, Dec 1, 2021
Abstract This paper performing both experimental and numerical approaches underlines the aiding a... 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.
Calculation of the temperature distribution around axisymmetric geometries such as cylinders and ... 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.
The effect of the lateral aspect ratio on the natural convective heat transfer across rectangular... 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.
The heat transfer characteristics of a circular cylinder exposed to a slot jet impingement of air... 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
In this paper, heat transfer characteristics of a miniature heat sink cooled by Al2O3–water nanof... 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...
Detecting bubble in two-phase flow has been a basic issue in two-phase flow systems. A new method... 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...
In this study, an off-grid PV system is optimized to supply a Conex electricity demand in the top... 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...
In this paper, a novel integrated system is proposed to improve the performance of a conventional... 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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