Iranian Journal of Mechanical Engineering Transactions of the ISME, Mar 1, 2011
In this paper, a modified two-fluid model has been adopted to simulate the process of upward vert... more In this paper, a modified two-fluid model has been adopted to simulate the process of upward vertical subcooled flow boiling of refrigerant R-113 in a vertical annular channel at low pressure. The modified model considers temperature dependent properties and saturation temperature variation and was validated against a number of published low-pressure subcooled boiling experiments. The results show good agreement with experimental data including radial profiles of void fraction, phase velocities and liquid temperature. A sinusoidal axial distribution of wall heat flux was applied as well as constant wall heat flux. Results show that by increasing the wall heat flux, the bubble boundary layer will become thicker and the profiles of axial liquid velocity will gradually depart from those of singlephase flow.
ABSTRACT In this paper, convection heat transfer and laminar flow of nanofluids with non-Newtonia... more ABSTRACT In this paper, convection heat transfer and laminar flow of nanofluids with non-Newtonian base fluid in a rectangular microchannel have been investigated numerically using two-phase mixture model. This research investigates the advantages of using nanoparticles in non-Newtonian fluids with particles size equal to 30 nm. The factor that makes nanoparticles feasible is the significant increase in rate of heat transfer within the fluids that are common in today's industry. The power law model is used both Newtonian and non-Newtonian fluids. The flow behavior and rate of heat transfer performance of microchannel heat sink have been taken into account by looking into the effects of Al2O3 nanoparticles concentrations, Peclet number and flow behavior index. Our results demonstrate significant enhancement of heat transfer of non-Newtonian fluids using nanoparticles particularly in the entrance region. By increasing the volume fraction, higher heat transfer enhancement can be observed. The thermal resistance with Peclet number of 700 and 4% volume fraction reduces approximately 27.2% with shear thinning non-Newtonian base fluid and pressure drop will increase approximately 50.7%. Further analysis on particles type effect is also implemented with Al2O3 and CuO nanoparticles.
A fuel element in nuclear reactors generally consists of fuel pellets, gaseous gap and cladding. ... more A fuel element in nuclear reactors generally consists of fuel pellets, gaseous gap and cladding. Normally the pellets and cladding are coaxial. In this case; heat conduction equation in the fuel element is two dimensional, independent on angle (θ). Certainly elimination of gaseous gap in one direction (eccentricity of fuel pellets), will increase cladding and coolant temperature at the same direction. In this work, three dimensional analytical and numerical steady state heat transfer behavior in eccentric fuel element were studied to determine maximum temperature of fuel pellets, cladding, and coolant. Good agreement between analytical and numerical results was obtained. It is concluded that because of eccentricity of fuel element, melting of cladding and boiling in coolant will not occur.
In this study the 2D laminar and steady water-based Al2O3 nanofluid flow over a cylinder with cir... more In this study the 2D laminar and steady water-based Al2O3 nanofluid flow over a cylinder with circular, horizontal and vertical elliptical cross section by constant surface temperature boundary condition has been studied. The main goal of this research is to investigate the effects of different natural and mixed convection heat transfer mechanisms on the convective heat transfer coefficient, and the entropy generation due to the thermal and frictional origination. Conservation equations of the mass, momentum and energy under the assumption of incompressible, Newtonian nanofluid, by using the homogeneous single phase method have been solved. The impact of considered parameters in this study (alteration in cross section, convective flow direction and volume fraction of nano particles) in enhancing the heat transfer rate is studied in association with the entropy generated value in each case. Based on the results, the vertical elliptical cross section, in comparison with others, shows the highest entropy generation value and the heat transfer coefficient in all considered mechanisms. Moreover, mixed convection heat transfer type 2, in which the force flow is perpendicular to the buoyant flow direction, has the highest entropy generation and heat transfer rate for all cross sections. In addition, in all cases in the presence of the nanoparticles, the heat transfer rate and entropy generation increases.
In this study the 2D laminar and steady water-based Al2O3 nanofluid flow over a cylinder with cir... more In this study the 2D laminar and steady water-based Al2O3 nanofluid flow over a cylinder with circular, horizontal and vertical elliptical cross section by constant surface temperature boundary condition has been studied. The main goal of this research is to investigate the effects of different natural and mixed convection heat transfer mechanisms on the convective heat transfer coefficient, and the entropy generation due to the thermal and frictional origination. Conservation equations of the mass, momentum and energy under the assumption of incompressible, Newtonian nanofluid, by using the homogeneous single phase method have been solved. The impact of considered parameters in this study (alteration in cross section, convective flow direction and volume fraction of nano particles) in enhancing the heat transfer rate is studied in association with the entropy generated value in each case. Based on the results, the vertical elliptical cross section, in comparison with others, shows the highest entropy generation value and the heat transfer coefficient in all considered mechanisms. Moreover, mixed convection heat transfer type 2, in which the force flow is perpendicular to the buoyant flow direction, has the highest entropy generation and heat transfer rate for all cross sections. In addition, in all cases in the presence of the nanoparticles, the heat transfer rate and entropy generation increases.
Journal of Magnetism and Magnetic Materials, Mar 1, 2018
Multiphase flow is one of the most complicated problems, considering the multiplicity of the rela... more Multiphase flow is one of the most complicated problems, considering the multiplicity of the related parameters, especially the external factors influences. Thus, despite the recent developments more investigations are still required. The effect of a uniform magnetic field on the hydrodynamics behavior of a two-phase flow with different magnetic permeability is presented in this article. A single water vapor bubble which is rising inside a channel filled with ferrofluid has been simulated numerically. To capture the phases interface, the Volume of Fluid (VOF) model, and to solve the governing equations, the finite volume method has been employed. Contrary to the prior anticipations, while the consisting fluids of the flow are dielectric, uniform magnetic field causes a force acting normal to the interface toward to the inside of the bubble. With respect to the applied magnetic field direction, the bubble deformation due to the magnetic force increases the bubble rising velocity. Moreover, the higher values of applied magnetic field strength and magnetic permeability ratio resulted in the further increase of the bubble rising velocity. Also it is indicated that the flow mixing and the heat transfer rate is increased by a bubble injection and applying a magnetic field. The obtained results have been concluded that the presented phenomenon with applying a magnetic field can be used to control the related characteristics of the multiphase flows. Compared to the previous studies, implementing the applicable cases using the common and actual materials and a significant reduction of the CPU time are the most remarkable advantages of the current study.
ABSTRACT The effects of input parameters such as pressure, mass flux, and inlet subcooling on bub... more ABSTRACT The effects of input parameters such as pressure, mass flux, and inlet subcooling on bubble departure diameter, frequencies, and quadrupole magnetic field have been investigated. It was observed that process of changes in diameter and bubble departure frequency was similar with and without magnetic field, and the bubbles proved a regulated growth in the presence of a magnetic field. Compared to alike working conditions without a magnetic field, with subcooled, mass flux and pressure increase, the bubble departure diameters decreased by an average of 9.55%, 15.18% and 15%, and the bubble departure frequencies increased by 45.45%, 30.38% and 21.71%, respectively.
Magnetic treatment is a widely used physical treatment technique in oilfields. The influencing me... more Magnetic treatment is a widely used physical treatment technique in oilfields. The influencing mechanism of magnetic field on waxy crude oil has drew much attention in recent years. Molecular dynamics simulation was employed in this paper to study the influence of magnetic field on waxy crude oil. The waxy crude oil model consists of light oil and 24.9% by weight of paraffin. The simulation results showed that the density of waxy crude oil model firstly decreased and then increased with the strengthening of magnetic field intensity, while the diffusion coefficient increased in low magnetic field intensity and decreased in high intensity. Viscosity was calculated by Rouse Model. The reduction of viscosity only appeared within a specific range of magnetic field intensities, mainly concentrated on the low intensities. The viscosity increases when the range is overstepped, which is supposed to be the reason of the unstable effects of magnetic treatment. The aggregation of paraffin molecules in magnetic field was proved by analysing the radial distribution function. The analysis of energy represented that the magnetic field had greater influence on molecular morphology than distances between molecules. The simulation results provide micro-perspective of waxy crude oil in the presence of magnetic field, which help to offer the mechanism explanation of magnetic treatment effects.
This paper investigates numerically the characteristics of subcooled flow boiling of a magnetic n... more This paper investigates numerically the characteristics of subcooled flow boiling of a magnetic nanofluid (refrigerant-113 and 4 vol % Fe 3 O 4) in a vertical annulus, which is exposed to a nonuniform transverse magnetic field generated by the quadrupole magnet. A control volume technique and SIMPLEC algorithm have been used for discretizing the governing equations and pressure-velocity coupling, respectively. The two-fluid model has been used to simulate subcooled flow boiling of the refrigerant-113. The results indicate that subcooled flow boiling characteristics change not only by using nanofluid as the working fluid, but also by applying the nonuniform transverse magnetic field. In the presence of the aforementioned magnetic field due to the Kelvin force, the fluid attracted to the outer wall. This leads to higher bubble detachment frequency so that the heat pumping is increased and the void fraction on the heated wall is decreased. Thus, the critical heat flux as one of the most important parameters in boiling processes will be increased.
International Journal of Thermal Sciences, Feb 1, 2021
In this study, the 3D numerical modeling of the condensation of a bubble inside subcooled liquid ... more In this study, the 3D numerical modeling of the condensation of a bubble inside subcooled liquid has been carried out under a uniform magnetic field and the effects of vertical and horizontal magnetic fields on the condensation behavior of the bubbles with initial diameters of 1.008 mm and 4 mm have been studied. The Tanasawa's mass transfer model was used to simulate the condensation process. The terms related to this model and energy equation have been implemented in OpenFOAM solver. The Maxwell equations, scalar magnetic potential equation, and the magnetic boundary conditions were also applied to the solver by coding. The volume of fluid (VOF) method has been used for capturing the interface of the phases. The obtained results show that the magnetic field stretches the bubble along the magnetic field lines. Besides, the magnetic field increases the pressure inside the bubble, which accelerates the condensation of the bubble in comparison with no magnetic field case. For the bubble with a smaller diameter, the vertical magnetic field contributes to faster condensation of the bubble, while for the bubble with a larger diameter, the bubble condenses and disappears faster in the presence of the magnetic field.
International Communications in Heat and Mass Transfer, Jul 1, 2016
In this paper, swirling flow boiling of a dilute nanofluid (water and 0.1 vol%Fe 3 O 4) in an ann... more In this paper, swirling flow boiling of a dilute nanofluid (water and 0.1 vol%Fe 3 O 4) in an annulus with a twisted fin on the outside of the inner wall in the presence of transverse magnetic gradient has been numerically investigated, using a two fluid model and a control volume technique. The results indicate that, in the boiling of swirling flow, the rate of the heat transfer increases. This phenomenon can be attributed to the effect of centrifugal force on the liquid phase flow and also reduction of the conductive sub-layer thickness that exists on the heated wall. The effects of improved surface wettability induced by nanoparticle deposition during the boiling process are accounted. The results demonstrate that the modified liquid property due to the existence of nanoparticles in the liquid has a negligible effect on the boiling heat transfer performance with dilute nanofluids while the improved surface wettability plays an important role and leads to reduction of the void fraction and consequently, an increase of critical heat flux. Applying a transverse magnetic field causes augmentation of the centrifugal force and results in increased flow turbulence. Furthermore, in the presence of the magnetic field due to magnetic force, the bubble departure diameter is reduced and bubble detachment occurs faster. Therefore, the critical heat flux will be increased. Swirling flow boiling in the presence of magnetic field is strongly suggested in devices requiring high heat transfer rates.
Experimental Thermal and Fluid Science, Oct 1, 2014
ABSTRACT In the present work, effects of using magnetic nanofluid and also applying an external m... more ABSTRACT In the present work, effects of using magnetic nanofluid and also applying an external magnetic field on the critical heat flux (CHF) of subcooled flow boiling has been studied experimentally. The experiments have been applied in upward flow direction in a 12 mm I.D., 19 mm O.D. and 0.75 m length annular test section. Inlet subcooling was kept constant and the mass flux was varied in the range of 0-150 kg/m2.s while the exit was at atmospheric pressure. Ferrofluids with water as a base fluid and 0.01 and 0.1% volume fractions of Fe3O4 nanoparticles were utilized. The results indicates that the CHF of subcooled flow boiling was increased by using nanofluid as a working fluid, which was mainly due to the deposition of the nanoparticles on the surface of inner tube, and consequently, increasing the surface wettability. Furthermore, an external magnetic field by utilizing quadrupole magnet was applied on the subcooled boiling flow at the near exit of the test section. The obtained results indicated that applying magnetic field caused an enhancement in CHF values of both pure water and ferrofluids. The main reasons for such effect of magnetic field can be justified to changing water properties under action of the magnetic field, single-phase convection heat transfer enhancement, suppression of nucleate boiling, and stabilization of boiling flow.
In this paper, the effects of the variations of the coil pitch and coil diameter on ferrofluid fl... more In this paper, the effects of the variations of the coil pitch and coil diameter on ferrofluid flow boiling characteristics inside helical tubes have been investigated using two-fluid model and control volume technique. The effects of the non-uniform magnetic field on the ferrofluid flow boiling inside helical tubes have also been studied. The results indicated showed that decreasing the coil diameter enhances the heat transfer coefficient. This is due to the effect of centrifugal force which is increased by decreasing the coil diameter. Also, the results confirmed the dual role of the centrifugal force on heat transfer (increase) and vapor volume fraction (decrease) and at the same time remarkable increasing of locally void fraction on heated wall and raising possibility of occurrence of CHF. In addition, the improvement of surface wettability which is induced by nanoparticles sedimentation during the boiling process has been considered. Results showed the reduction of the void fraction. The application of a magnetic field at critical regions resulted in the bubble departure diameter and vapor volume fraction generation reduction. Consequently, the critical heat flux is increased.
Iranian Journal of Mechanical Engineering Transactions of the ISME, Mar 1, 2011
In this paper, a modified two-fluid model has been adopted to simulate the process of upward vert... more In this paper, a modified two-fluid model has been adopted to simulate the process of upward vertical subcooled flow boiling of refrigerant R-113 in a vertical annular channel at low pressure. The modified model considers temperature dependent properties and saturation temperature variation and was validated against a number of published low-pressure subcooled boiling experiments. The results show good agreement with experimental data including radial profiles of void fraction, phase velocities and liquid temperature. A sinusoidal axial distribution of wall heat flux was applied as well as constant wall heat flux. Results show that by increasing the wall heat flux, the bubble boundary layer will become thicker and the profiles of axial liquid velocity will gradually depart from those of singlephase flow.
ABSTRACT In this paper, convection heat transfer and laminar flow of nanofluids with non-Newtonia... more ABSTRACT In this paper, convection heat transfer and laminar flow of nanofluids with non-Newtonian base fluid in a rectangular microchannel have been investigated numerically using two-phase mixture model. This research investigates the advantages of using nanoparticles in non-Newtonian fluids with particles size equal to 30 nm. The factor that makes nanoparticles feasible is the significant increase in rate of heat transfer within the fluids that are common in today's industry. The power law model is used both Newtonian and non-Newtonian fluids. The flow behavior and rate of heat transfer performance of microchannel heat sink have been taken into account by looking into the effects of Al2O3 nanoparticles concentrations, Peclet number and flow behavior index. Our results demonstrate significant enhancement of heat transfer of non-Newtonian fluids using nanoparticles particularly in the entrance region. By increasing the volume fraction, higher heat transfer enhancement can be observed. The thermal resistance with Peclet number of 700 and 4% volume fraction reduces approximately 27.2% with shear thinning non-Newtonian base fluid and pressure drop will increase approximately 50.7%. Further analysis on particles type effect is also implemented with Al2O3 and CuO nanoparticles.
A fuel element in nuclear reactors generally consists of fuel pellets, gaseous gap and cladding. ... more A fuel element in nuclear reactors generally consists of fuel pellets, gaseous gap and cladding. Normally the pellets and cladding are coaxial. In this case; heat conduction equation in the fuel element is two dimensional, independent on angle (θ). Certainly elimination of gaseous gap in one direction (eccentricity of fuel pellets), will increase cladding and coolant temperature at the same direction. In this work, three dimensional analytical and numerical steady state heat transfer behavior in eccentric fuel element were studied to determine maximum temperature of fuel pellets, cladding, and coolant. Good agreement between analytical and numerical results was obtained. It is concluded that because of eccentricity of fuel element, melting of cladding and boiling in coolant will not occur.
In this study the 2D laminar and steady water-based Al2O3 nanofluid flow over a cylinder with cir... more In this study the 2D laminar and steady water-based Al2O3 nanofluid flow over a cylinder with circular, horizontal and vertical elliptical cross section by constant surface temperature boundary condition has been studied. The main goal of this research is to investigate the effects of different natural and mixed convection heat transfer mechanisms on the convective heat transfer coefficient, and the entropy generation due to the thermal and frictional origination. Conservation equations of the mass, momentum and energy under the assumption of incompressible, Newtonian nanofluid, by using the homogeneous single phase method have been solved. The impact of considered parameters in this study (alteration in cross section, convective flow direction and volume fraction of nano particles) in enhancing the heat transfer rate is studied in association with the entropy generated value in each case. Based on the results, the vertical elliptical cross section, in comparison with others, shows the highest entropy generation value and the heat transfer coefficient in all considered mechanisms. Moreover, mixed convection heat transfer type 2, in which the force flow is perpendicular to the buoyant flow direction, has the highest entropy generation and heat transfer rate for all cross sections. In addition, in all cases in the presence of the nanoparticles, the heat transfer rate and entropy generation increases.
In this study the 2D laminar and steady water-based Al2O3 nanofluid flow over a cylinder with cir... more In this study the 2D laminar and steady water-based Al2O3 nanofluid flow over a cylinder with circular, horizontal and vertical elliptical cross section by constant surface temperature boundary condition has been studied. The main goal of this research is to investigate the effects of different natural and mixed convection heat transfer mechanisms on the convective heat transfer coefficient, and the entropy generation due to the thermal and frictional origination. Conservation equations of the mass, momentum and energy under the assumption of incompressible, Newtonian nanofluid, by using the homogeneous single phase method have been solved. The impact of considered parameters in this study (alteration in cross section, convective flow direction and volume fraction of nano particles) in enhancing the heat transfer rate is studied in association with the entropy generated value in each case. Based on the results, the vertical elliptical cross section, in comparison with others, shows the highest entropy generation value and the heat transfer coefficient in all considered mechanisms. Moreover, mixed convection heat transfer type 2, in which the force flow is perpendicular to the buoyant flow direction, has the highest entropy generation and heat transfer rate for all cross sections. In addition, in all cases in the presence of the nanoparticles, the heat transfer rate and entropy generation increases.
Journal of Magnetism and Magnetic Materials, Mar 1, 2018
Multiphase flow is one of the most complicated problems, considering the multiplicity of the rela... more Multiphase flow is one of the most complicated problems, considering the multiplicity of the related parameters, especially the external factors influences. Thus, despite the recent developments more investigations are still required. The effect of a uniform magnetic field on the hydrodynamics behavior of a two-phase flow with different magnetic permeability is presented in this article. A single water vapor bubble which is rising inside a channel filled with ferrofluid has been simulated numerically. To capture the phases interface, the Volume of Fluid (VOF) model, and to solve the governing equations, the finite volume method has been employed. Contrary to the prior anticipations, while the consisting fluids of the flow are dielectric, uniform magnetic field causes a force acting normal to the interface toward to the inside of the bubble. With respect to the applied magnetic field direction, the bubble deformation due to the magnetic force increases the bubble rising velocity. Moreover, the higher values of applied magnetic field strength and magnetic permeability ratio resulted in the further increase of the bubble rising velocity. Also it is indicated that the flow mixing and the heat transfer rate is increased by a bubble injection and applying a magnetic field. The obtained results have been concluded that the presented phenomenon with applying a magnetic field can be used to control the related characteristics of the multiphase flows. Compared to the previous studies, implementing the applicable cases using the common and actual materials and a significant reduction of the CPU time are the most remarkable advantages of the current study.
ABSTRACT The effects of input parameters such as pressure, mass flux, and inlet subcooling on bub... more ABSTRACT The effects of input parameters such as pressure, mass flux, and inlet subcooling on bubble departure diameter, frequencies, and quadrupole magnetic field have been investigated. It was observed that process of changes in diameter and bubble departure frequency was similar with and without magnetic field, and the bubbles proved a regulated growth in the presence of a magnetic field. Compared to alike working conditions without a magnetic field, with subcooled, mass flux and pressure increase, the bubble departure diameters decreased by an average of 9.55%, 15.18% and 15%, and the bubble departure frequencies increased by 45.45%, 30.38% and 21.71%, respectively.
Magnetic treatment is a widely used physical treatment technique in oilfields. The influencing me... more Magnetic treatment is a widely used physical treatment technique in oilfields. The influencing mechanism of magnetic field on waxy crude oil has drew much attention in recent years. Molecular dynamics simulation was employed in this paper to study the influence of magnetic field on waxy crude oil. The waxy crude oil model consists of light oil and 24.9% by weight of paraffin. The simulation results showed that the density of waxy crude oil model firstly decreased and then increased with the strengthening of magnetic field intensity, while the diffusion coefficient increased in low magnetic field intensity and decreased in high intensity. Viscosity was calculated by Rouse Model. The reduction of viscosity only appeared within a specific range of magnetic field intensities, mainly concentrated on the low intensities. The viscosity increases when the range is overstepped, which is supposed to be the reason of the unstable effects of magnetic treatment. The aggregation of paraffin molecules in magnetic field was proved by analysing the radial distribution function. The analysis of energy represented that the magnetic field had greater influence on molecular morphology than distances between molecules. The simulation results provide micro-perspective of waxy crude oil in the presence of magnetic field, which help to offer the mechanism explanation of magnetic treatment effects.
This paper investigates numerically the characteristics of subcooled flow boiling of a magnetic n... more This paper investigates numerically the characteristics of subcooled flow boiling of a magnetic nanofluid (refrigerant-113 and 4 vol % Fe 3 O 4) in a vertical annulus, which is exposed to a nonuniform transverse magnetic field generated by the quadrupole magnet. A control volume technique and SIMPLEC algorithm have been used for discretizing the governing equations and pressure-velocity coupling, respectively. The two-fluid model has been used to simulate subcooled flow boiling of the refrigerant-113. The results indicate that subcooled flow boiling characteristics change not only by using nanofluid as the working fluid, but also by applying the nonuniform transverse magnetic field. In the presence of the aforementioned magnetic field due to the Kelvin force, the fluid attracted to the outer wall. This leads to higher bubble detachment frequency so that the heat pumping is increased and the void fraction on the heated wall is decreased. Thus, the critical heat flux as one of the most important parameters in boiling processes will be increased.
International Journal of Thermal Sciences, Feb 1, 2021
In this study, the 3D numerical modeling of the condensation of a bubble inside subcooled liquid ... more In this study, the 3D numerical modeling of the condensation of a bubble inside subcooled liquid has been carried out under a uniform magnetic field and the effects of vertical and horizontal magnetic fields on the condensation behavior of the bubbles with initial diameters of 1.008 mm and 4 mm have been studied. The Tanasawa's mass transfer model was used to simulate the condensation process. The terms related to this model and energy equation have been implemented in OpenFOAM solver. The Maxwell equations, scalar magnetic potential equation, and the magnetic boundary conditions were also applied to the solver by coding. The volume of fluid (VOF) method has been used for capturing the interface of the phases. The obtained results show that the magnetic field stretches the bubble along the magnetic field lines. Besides, the magnetic field increases the pressure inside the bubble, which accelerates the condensation of the bubble in comparison with no magnetic field case. For the bubble with a smaller diameter, the vertical magnetic field contributes to faster condensation of the bubble, while for the bubble with a larger diameter, the bubble condenses and disappears faster in the presence of the magnetic field.
International Communications in Heat and Mass Transfer, Jul 1, 2016
In this paper, swirling flow boiling of a dilute nanofluid (water and 0.1 vol%Fe 3 O 4) in an ann... more In this paper, swirling flow boiling of a dilute nanofluid (water and 0.1 vol%Fe 3 O 4) in an annulus with a twisted fin on the outside of the inner wall in the presence of transverse magnetic gradient has been numerically investigated, using a two fluid model and a control volume technique. The results indicate that, in the boiling of swirling flow, the rate of the heat transfer increases. This phenomenon can be attributed to the effect of centrifugal force on the liquid phase flow and also reduction of the conductive sub-layer thickness that exists on the heated wall. The effects of improved surface wettability induced by nanoparticle deposition during the boiling process are accounted. The results demonstrate that the modified liquid property due to the existence of nanoparticles in the liquid has a negligible effect on the boiling heat transfer performance with dilute nanofluids while the improved surface wettability plays an important role and leads to reduction of the void fraction and consequently, an increase of critical heat flux. Applying a transverse magnetic field causes augmentation of the centrifugal force and results in increased flow turbulence. Furthermore, in the presence of the magnetic field due to magnetic force, the bubble departure diameter is reduced and bubble detachment occurs faster. Therefore, the critical heat flux will be increased. Swirling flow boiling in the presence of magnetic field is strongly suggested in devices requiring high heat transfer rates.
Experimental Thermal and Fluid Science, Oct 1, 2014
ABSTRACT In the present work, effects of using magnetic nanofluid and also applying an external m... more ABSTRACT In the present work, effects of using magnetic nanofluid and also applying an external magnetic field on the critical heat flux (CHF) of subcooled flow boiling has been studied experimentally. The experiments have been applied in upward flow direction in a 12 mm I.D., 19 mm O.D. and 0.75 m length annular test section. Inlet subcooling was kept constant and the mass flux was varied in the range of 0-150 kg/m2.s while the exit was at atmospheric pressure. Ferrofluids with water as a base fluid and 0.01 and 0.1% volume fractions of Fe3O4 nanoparticles were utilized. The results indicates that the CHF of subcooled flow boiling was increased by using nanofluid as a working fluid, which was mainly due to the deposition of the nanoparticles on the surface of inner tube, and consequently, increasing the surface wettability. Furthermore, an external magnetic field by utilizing quadrupole magnet was applied on the subcooled boiling flow at the near exit of the test section. The obtained results indicated that applying magnetic field caused an enhancement in CHF values of both pure water and ferrofluids. The main reasons for such effect of magnetic field can be justified to changing water properties under action of the magnetic field, single-phase convection heat transfer enhancement, suppression of nucleate boiling, and stabilization of boiling flow.
In this paper, the effects of the variations of the coil pitch and coil diameter on ferrofluid fl... more In this paper, the effects of the variations of the coil pitch and coil diameter on ferrofluid flow boiling characteristics inside helical tubes have been investigated using two-fluid model and control volume technique. The effects of the non-uniform magnetic field on the ferrofluid flow boiling inside helical tubes have also been studied. The results indicated showed that decreasing the coil diameter enhances the heat transfer coefficient. This is due to the effect of centrifugal force which is increased by decreasing the coil diameter. Also, the results confirmed the dual role of the centrifugal force on heat transfer (increase) and vapor volume fraction (decrease) and at the same time remarkable increasing of locally void fraction on heated wall and raising possibility of occurrence of CHF. In addition, the improvement of surface wettability which is induced by nanoparticles sedimentation during the boiling process has been considered. Results showed the reduction of the void fraction. The application of a magnetic field at critical regions resulted in the bubble departure diameter and vapor volume fraction generation reduction. Consequently, the critical heat flux is increased.
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