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Wenquan Tao - One of the best experts on this subject based on the ideXlab platform.
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an numerical investigation on the cooling capacity of needle ring type electrostatic fluid accelerators for round plate with Uniform and non Uniform Heat Flux
International Journal of Heat and Mass Transfer, 2017Co-Authors: Shuang Wang, Jianfei Zhang, Lingjian Kong, Wenquan TaoAbstract:Abstract In this paper, a 3-D model of an electrostatic fluid accelerator was simulated using the finite element method (as implemented in COMSOL Multiphysics). To investigate the Heat transfer performance of the accelerator, the impinging flow of the accelerator was used to cool down a Heating round plate. The Heat transfer coefficient and temperature distribution of the plate were numerically investigated with respect to the Heat Flux and radius of the round plate, and a comparison was made between the impinging flow of the accelerator and a Uniform jet flow. To explore this in greater depth, a non-Uniform Heat Flux was applied on the round plate to study the cooling effect of the accelerator on the surface with a hot spot. The effects of the hot spot Heat Flux and the hot spot size were then investigated. This study provides a reference for the practical cooling process of commercial chips, with and without hot spots, by adopting electrostatic fluid accelerators.
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numerical investigations on fully developed mixed turbulent convection in dimpled parabolic trough receiver tubes
Applied Thermal Engineering, 2017Co-Authors: Zhen Huang, Wenquan TaoAbstract:Abstract The fully-developed mixed turbulent convective Heat transfer characteristics in dimpled tubes of parabolic trough receiver are numerically studied at a certain Reynolds number of 2 × 104 and different Grashof numbers ranged from 0 to 3.2 × 1010 to produce substantial surface Heat transfer augmentations with relatively small pressure drop penalties. The Boussinesq approximation is applied, in which variations in fluid properties other than density are ignored. The Realizable k-e two-equation turbulence model with enhancement wall treatment is adopted. The influences of outer wall Heat Flux distributions and dimple depth on flow resistance and Heat transfer rate are illustrated and analyzed. The results indicate that the average friction factor and Nusselt number in dimpled receiver tubes under non-Uniform Heat Flux (NUHF) are larger than those under Uniform Heat Flux (UHF). In most cases, the comprehensive performance of dimpled receiver tube under NUHF is also better than that under UHF. The deep dimples (d/Di = 0.875) are far superior to the shallow dimples (d/Di = 0.125) at a same Grashof number.
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numerical study on combined natural and forced convection in the fully developed turbulent region for a horizontal circular tube Heated by non Uniform Heat Flux
Applied Energy, 2017Co-Authors: Zhen Huang, Wenquan TaoAbstract:Abstract The present work focuses on the fully developed mixed turbulent flow and Heat transfer in receiver tube Heated by non-Uniform Heat Flux, especially the effect of local buoyancy force induced by the non-Uniform Heat Flux at Reynolds number of 2 × 104–105, Prandtl number of 1.5 and Grashof number of 0–1012. The friction factor and Nusselt number between forced convection and mixed convection under Uniform Heat Flux and non-Uniform Heat Flux are analyzed quantitatively. The effect of solar elevation angle on the fluid flow and Heat transfer is also investigated. It is concluded that the mixed fluid flow and Heat transfer under non-Uniform Heat Flux is different from that under Uniform Heat Flux. The solar elevation angle has strong influence on the mixed fluid flow and Heat transfer characteristics. A criterion for the buoyancy free is proposed. It is not feasible to perform the Heat transfer design and prediction for parabolic trough solar collector based on the experimental correlations for forced convection or conventional mixed convention.
Oronzio Manca - One of the best experts on this subject based on the ideXlab platform.
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transient natural convection in a vertical microchannel Heated at Uniform Heat Flux
International Journal of Thermal Sciences, 2012Co-Authors: Bernardo Buonomo, Oronzio MancaAbstract:Abstract Miniaturization of devices has received a rapid expansion in the recent years and a great volume of research activities have been dedicated to micro-flow due to its new applications of micro-fluidic systems and components. In the present paper, an investigation of transient natural convection in parallel-plate vertical microchannels is carried out numerically. The vertical microchannel is considered asymmetrically or symmetrically Heated at Uniform Heat Flux. The first-order model for slip velocity and jump temperature is assumed in micro-scale conditions. The analysis is performed under laminar boundary layer assumption for different values of Knudsen number, Rayleigh number and the ratio of wall Heat Flux in order to evaluate their effects on wall temperatures, mass flow rate, velocity profiles and Nusselt number. Wall temperature overshoots are detected for the different conditions. These values increase with the increasing Knudsen number, Kn, at high Rayleigh number, Ra, whereas for lower Ra the lowest wall temperature is obtained at Kn = 0.05. Mass flow rate increases with increasing Kn whereas Nusselt number decreases with increasing Kn. A composite correlation to evaluate the average Nusselt number in terms of Rayleigh and Knudsen numbers is proposed in the ranges 0 ≤ Kn ≤ 0.1 and 10−3
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natural convection slip flow in a vertical microchannel Heated at Uniform Heat Flux
International Journal of Thermal Sciences, 2010Co-Authors: Bernardo Buonomo, Oronzio MancaAbstract:Abstract Numerical solutions for steady state developing natural convection flow in air, in vertical parallel-plate microchannels are accomplished. An asymmetric Heating is considered and the walls are assumed to be at Uniform Heat Flux. A first-order model is used for slip and jump boundary conditions and an analytical solution for the fully developed flow is also given. Results are performed for air, for the Heat Flux ratio in the 0.0–1.0 range, for Rayleigh, Ra, and Knudsen, Kn, numbers from 10−1 to 8 × 103 and from 0.0 to 0.10, respectively. The maximum mass flow rate is always obtained for the highest considered Kn value, whereas the average Nusselt number, Nu, increases for lower Ra ( 100. Wall temperature profiles have the lowest values for highest considered Kn value at lower Ra, whereas for the developing flow, they present opposite trends. For developing flow, velocity profiles for asymmetric and symmetric Heating are completely different. In developing flow velocity profiles along the wall present the highest increases for asymmetric Heating and the highest considered Kn value.
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transient natural convection in vertical channels symmetrically Heated at Uniform Heat Flux
Numerical Heat Transfer Part A-applications, 2009Co-Authors: Assunta Andreozzi, Bernardo Buonomo, Oronzio MancaAbstract:A numerical transient analysis of natural convection in air between two vertical parallel plates, Heated at Uniform Heat Flux, is carried out. The problem is two-dimensional and laminar and the full Navier-Stokes and energy equations are employed. The control volume method is used to discretize the equations on a Uniform grid. Results are given at different aspect ratio values and Rayleigh number values. The simulation allows detection of complex structures of the flow inside and outside the channel. Temperature profiles as a function of time show an overshoot and undershoot increase at the lowest aspect ratio and highest Rayleigh number. Inside the channel conductive and convective regimes as well as an inverse fluid motion are observed. Transient average Nusselt number presents oscillations before the steady-state.
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experimental investigation on natural convection in horizontal channels with the upper wall at Uniform Heat Flux
International Journal of Heat and Mass Transfer, 2007Co-Authors: Oronzio Manca, Sergio NardiniAbstract:Abstract An experimental investigation on natural convection of air in horizontal channels with well-insulated lower wall and a Heated upper wall is carried out. Flow visualization and air temperature measurements are employed to obtain a phenomenological description of air natural convection inside a horizontal, open-ended cavity with a Heated upper plate and an unHeated lower one. A laminar flow, with a C-loop shape, is observed inside the open-ended cavity. The penetration length is dependent upon the Grashof number. The penetration length increases particularly by increasing either the distance between the walls or the Heat Flux. The temperature measurements confirm the flow visualization observations. The air temperature profiles inside the open ended cavity indicate that the temperature gradients along the gap cavity are weak for low Heat Fluxes values. Scale analysis is carried out and shows that penetration length depends on Ra1/2, in accordance with other authors. Furthermore, the estimation of the penetration length depends on the values of the thickness of the boundary layer, evaluated in terms of distance between the walls. Monomial correlations for average Nusselt numbers and dimensionless wall temperatures are proposed in a range of Rayleigh number from 2.78 × 103 to 3.90 × 105 and for an aspect ratio between 2L/b = 10 and 2L/b = 20. The equations agree highly with the experimental results.
Zhen Huang - One of the best experts on this subject based on the ideXlab platform.
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numerical investigations on fully developed mixed turbulent convection in dimpled parabolic trough receiver tubes
Applied Thermal Engineering, 2017Co-Authors: Zhen Huang, Wenquan TaoAbstract:Abstract The fully-developed mixed turbulent convective Heat transfer characteristics in dimpled tubes of parabolic trough receiver are numerically studied at a certain Reynolds number of 2 × 104 and different Grashof numbers ranged from 0 to 3.2 × 1010 to produce substantial surface Heat transfer augmentations with relatively small pressure drop penalties. The Boussinesq approximation is applied, in which variations in fluid properties other than density are ignored. The Realizable k-e two-equation turbulence model with enhancement wall treatment is adopted. The influences of outer wall Heat Flux distributions and dimple depth on flow resistance and Heat transfer rate are illustrated and analyzed. The results indicate that the average friction factor and Nusselt number in dimpled receiver tubes under non-Uniform Heat Flux (NUHF) are larger than those under Uniform Heat Flux (UHF). In most cases, the comprehensive performance of dimpled receiver tube under NUHF is also better than that under UHF. The deep dimples (d/Di = 0.875) are far superior to the shallow dimples (d/Di = 0.125) at a same Grashof number.
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numerical study on combined natural and forced convection in the fully developed turbulent region for a horizontal circular tube Heated by non Uniform Heat Flux
Applied Energy, 2017Co-Authors: Zhen Huang, Wenquan TaoAbstract:Abstract The present work focuses on the fully developed mixed turbulent flow and Heat transfer in receiver tube Heated by non-Uniform Heat Flux, especially the effect of local buoyancy force induced by the non-Uniform Heat Flux at Reynolds number of 2 × 104–105, Prandtl number of 1.5 and Grashof number of 0–1012. The friction factor and Nusselt number between forced convection and mixed convection under Uniform Heat Flux and non-Uniform Heat Flux are analyzed quantitatively. The effect of solar elevation angle on the fluid flow and Heat transfer is also investigated. It is concluded that the mixed fluid flow and Heat transfer under non-Uniform Heat Flux is different from that under Uniform Heat Flux. The solar elevation angle has strong influence on the mixed fluid flow and Heat transfer characteristics. A criterion for the buoyancy free is proposed. It is not feasible to perform the Heat transfer design and prediction for parabolic trough solar collector based on the experimental correlations for forced convection or conventional mixed convention.
Josua P Meyer - One of the best experts on this subject based on the ideXlab platform.
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influence of non Uniform Heat Flux distributions on the secondary flow convective Heat transfer and friction factors for a parabolic trough solar collector type absorber tube
Renewable Energy, 2017Co-Authors: Izuchukwu F Okafor, Jaco Dirker, Josua P MeyerAbstract:Non-Uniform Heat Flux profiles on circular tubes are found in a number of Heat transfer applications, including solar Heating. In this numerical study the influence of the circumferential angle spans of non-Uniform Heat Flux distributions are considered on the secondary buoyancy-driven flow, internal fluid Heat transfer coefficients, and friction factors in horizontal absorber tubes in parabolic trough solar collector applications for water Heating in the laminar flow regime. Inlet Reynolds numbers ranging from 130 to 2200 for 10 m long tubes with different inner diameters were considered. Sinusoidal type incident Heat Flux distributions, tube-wall Heat conduction and Heat losses were taken into account. It was found that due to buoyancy-driven secondary flow, overall and local internal Heat transfer coefficients were increased significantly due to the non-Uniformity of the incident Heat Flux. Average internal Heat transfer coefficient increased with the Heat Flux intensity, the incident Heat Flux angle span and the inlet fluid temperature. The effective friction factor decreased with an increase in the absorber tube inlet fluid temperature. It was found that improved thermal efficiencies can be achieved for low mass flow rate water Heating applications by employing parabolic trough collector systems compared to flat plate systems.
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influence of circumferential solar Heat Flux distribution on the Heat transfer coefficients of linear fresnel collector absorber tubes
Solar Energy, 2014Co-Authors: Izuchukwu F Okafor, Jaco Dirker, Josua P MeyerAbstract:Abstract The absorber tubes of solar thermal collectors have enormous influence on the performance of the solar collector systems. In this numerical study, the influence of circumferential Uniform and non-Uniform solar Heat Flux distributions on the internal and overall Heat transfer coefficients of the absorber tubes of a linear Fresnel solar collector was investigated. A 3D steady-state numerical simulation was implemented based on ANSYS Fluent code version 14. The non-Uniform solar Heat Flux distribution was modelled as a sinusoidal function of the concentrated solar Heat Flux incident on the circumference of the absorber tube. The k – e model was employed to simulate the turbulent flow of the Heat transfer fluid through the absorber tube. The tube-wall Heat conduction and the convective and irradiative Heat losses to the surroundings were also considered in the model. The average internal and overall Heat transfer coefficients were determined for the sinusoidal circumferential non-Uniform Heat Flux distribution span of 160°, 180°, 200° and 240°, and the 360° span of circumferential Uniform Heat Flux for 10 m long absorber tubes of different inner diameters and wall thicknesses with thermal conductivity of 16.27 W/mK between the Reynolds number range of 4000 and 210,000 based on the inlet temperature. The results showed that the average internal Heat transfer coefficients for the 360° span of circumferential Uniform Heat Flux with different concentration ratios on absorber tubes of the same inner diameters, wall thicknesses and thermal conductivity were approximately the same, but the average overall Heat transfer coefficient increased with the increase in the concentration ratios of the Uniform Heat Flux incident on the tubes. Also, the average internal Heat transfer coefficient for the absorber tube with a 360° span of Uniform Heat Flux was approximately the same as that of the absorber tubes with the sinusoidal circumferential non-Uniform Heat Flux span of 160°, 180°, 200° and 240° for the Heat Flux of the same concentration ratio, but the average overall Heat transfer coefficient for the Uniform Heat Flux case was higher than that of the non-Uniform Flux distributions. The average axial local internal Heat transfer coefficient for the 360° span of Uniform Heat Flux distribution on a 10 m long absorber tube was slightly higher than that of the 160°, 200° and 240° span of non-Uniform Flux distributions at the Reynolds number of 4 000. The average internal and overall Heat transfer coefficients for four absorber tubes of different inner diameters and wall thicknesses and thermal conductivity of 16.27 W/mK with 200° span of circumferential non-Uniform Flux were found to increase with the decrease in the inner-wall diameter of the absorber tubes. The numerical results showed good agreement with the Nusselt number experimental correlations for fully developed turbulent flow available in the literature.
Paul H Roberts - One of the best experts on this subject based on the ideXlab platform.
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generation of a strong magnetic field using Uniform Heat Flux at the surface of the core
Nature Geoscience, 2009Co-Authors: Ataru Sakuraba, Paul H RobertsAbstract:Numerical simulations that assume realistic core-fluid viscosities have been unsuccessful in fully reproducing the unique characteristics of the Earth’s geomagnetic field. An evaluation of boundary conditions suggests that the prescription of a Uniform Heat Flux at the core’s surface could generate a more Earth-like magnetic field.
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Generation of a strong magnetic field using Uniform Heat Flux at the surface of the core
Nature Geoscience, 2009Co-Authors: Ataru Sakuraba, Paul H RobertsAbstract:The Earth’s main magnetic field is thought to be generated by motions in the planet’s fluid outer core, which lead to an effect similar to that of a dynamo^ 1 , 2 , 3 . Recent high-resolution numerical simulations produce only a non-dipolar^ 4 or a dipolar but comparatively weak magnetic field^ 5 , 6 unlike that of the Earth. Older models that did generate a strong, Earth-like field needed to use unrealistically high viscosities for the core fluid^ 7 , 8 , 9 , 10 . Common to most of the models is the assumption of a laterally Uniform core-surface temperature. Here we use a low-viscosity geodynamo model to evaluate the effect of a different and more realistic boundary condition—a Uniform Heat Flux at the surface of the core—on the simulation of an Earth-like magnetic field. Our results show that when the surface temperature is laterally Uniform, only a weak magnetic field is generated because planetary-scale fluid circulations are suppressed. In contrast, a laterally Uniform Heat Flux at the core’s surface leads to large-scale convective flows, and a comparatively strong dipole-type magnetic field. Contrary to previous work^ 11 , 12 , we suggest that thermal conditions at the core surface have a strong effect on low-viscosity geodynamo models. Numerical simulations that assume realistic core-fluid viscosities have been unsuccessful in fully reproducing the unique characteristics of the Earth’s geomagnetic field. An evaluation of boundary conditions suggests that the prescription of a Uniform Heat Flux at the core’s surface could generate a more Earth-like magnetic field.
