The Experts below are selected from a list of 38661 Experts worldwide ranked by ideXlab platform
Th Wetzel - One of the best experts on this subject based on the ideXlab platform.
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numerical analysis of a solar tower receiver tube operated with Liquid Metals
International Journal of Thermal Sciences, 2016Co-Authors: G Cammi, Luca Marocco, J Flesch, Th WetzelAbstract:Abstract Computational fluid dynamics is used in the present work to analyze the conjugate heat transfer in the receiver tube of a solar thermal tower operated with a Liquid metal. A circumferentially and longitudinally non-uniform heat flux, due to solar irradiation, is applied on half the external surface while the other one is considered as insulated. The heat transfer mechanism of Liquid Metals differs from that of ordinary fluids. As a consequence, the Reynolds analogy, which assumes a constant turbulent Prandtl number close to unity, cannot be applied to these fluid flows. Therefore two additional equations, namely one for the temperature variance and one for its dissipation rate are additionally solved, in order to determine the turbulent thermal diffusivity. The effects of the wall thickness ratio, the solid-to-fluid thermal conductivity ratio, the Peclet number and the diameter-to-length ratio have been analyzed. The calculated average Nusselt numbers closely agree with those evaluated with appropriate correlations for Liquid Metals, valid for uniformly distributed heat flux. Nonetheless these are not suited to evaluate the local Nusselt number and wall temperature distribution.
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Review of data and correlations for turbulent forced convective heat transfer of Liquid Metals in pipes
Heat and Mass Transfer, 2015Co-Authors: J Pacio, L. Marocco, Th WetzelAbstract:Within the present work the dataset of experimental points and the heat transfer correlations available in literature for Liquid-metal fully-developed, forced-convective heat transfer in pipes are reviewed and critically analyzed. Over 1,100 data points from 21 different sources are considered for constant heat flux, covering a wide range of operating conditions (velocity, heat flux, diameter, among others). Among 15 evaluated correlations, four appropriate ones are recommended for forced turbulent convection: one covering all the data points and the other three respectively related to alkali Liquid Metals, lead alloys and mercury. Moreover, a new correlation has been derived as a best fit of the limited number of available data points for constant wall temperature, while an alternative evaluation method is also described for this boundary condition.
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thermodynamic evaluation of Liquid Metals as heat transfer fluids in concentrated solar power plants
Applied Thermal Engineering, 2013Co-Authors: J Pacio, Cs Singer, Th Wetzel, Ralf UhligAbstract:Abstract Concentrated solar power, and in particular central receiver systems, can play a major role as a renewable energy source with the inherent possibility of including a thermal energy storage subsystem for improving the plant dispatchability. While current commercial projects are dominated by direct steam generation and molten nitrate salt concepts, next-generation systems will require higher operating temperature and larger heat-flux densities in order to increase the efficiency and reduce costs. In that context, Liquid Metals are proposed in this work as advanced heat transfer fluids that can face those challenges. The main advantages, regarding higher temperature and improved heat transfer performance, are discussed and quantified using simplified models. Indirect thermal storage solutions are proposed for compensating their relatively small heat capacity. Overall, provided that some practical challenges can be overcome, Liquid Metals present large potential as efficient heat transfer fluids.
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assessment of Liquid metal technology status and research paths for their use as efficient heat transfer fluids in solar central receiver systems
Solar Energy, 2013Co-Authors: J Pacio, Th WetzelAbstract:Abstract Concentrated solar power, and in particular central receiver systems, can play a major role as a renewable energy source with the inherent possibility of including a thermal energy storage subsystem for improving the plant dispatchability. Next-generation systems, in an effort for increasing the overall efficiency and reducing specific costs, will require higher operating temperatures and larger heat flux densities. In that context, Liquid Metals as advanced heat transfer fluids can face those challenges and largely contribute to the economics of future systems. Liquid Metals have been proposed in recent publications from a thermodynamic perspective. The present article focuses in a complementary way on the current state of Liquid metal technology. Based on the main requirements and previous experiences, three main candidate Liquid Metals are considered: sodium (Na), lead–bismuth eutectic alloy (LBE or PbBi) and molten tin (Sn), each of them with relative advantages and limitations. The state-of-the-art is reviewed, indicating that the readiness of Liquid metal technology is quite advanced, mainly for the two first candidates. Recommended research and development activities are outlined, mainly in two directions: compatibility with structural materials at high temperature and indirect thermal storage solutions. Overall, provided that some challenges can be overcome, significant advantages can be obtained from the use of Liquid Metals as heat transfer fluids in central receivers systems.
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application of Liquid Metals for solar energy systems
EPJ Web of Conferences, 2012Co-Authors: W Hering, Robert Stieglitz, Th WetzelAbstract:Liquid Metals have excellent properties to be used as heat transport fluids due to the high thermal conductivity and their wide applicable temperature range. The latter opens the gate utilizing more efficient power conversion options beyond the limitations of current thermal solar energy systems. By utilizing them as thermal storage medium, an improved coupling of state-of-the art power conversion systems (PCS) to solar plants seems promising. To avoid adverse effects of highly reactive fluid a compact design is envisaged, which fits best to concentrating solar power (CSP). In this context a solar storage system is proposed and will be qualified for fast reaction to compensate solar flux fluctuations and to optimize PCS. The paper will provide an outlook based on a concept study for a 100 MWth plant with a comparison to existing thermal solar plants. The vision also includes the capability for base load applications, including a day-wide storage system. Combining both systems, a fast response of the plant to compensate short term fluctuations from other renewable sources or from energy consumption is feasible.
J Pacio - One of the best experts on this subject based on the ideXlab platform.
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heat transfer to Liquid Metals in a hexagonal rod bundle with grid spacers experimental and simulation results
Nuclear Engineering and Design, 2015Co-Authors: J Pacio, F Menghini, K Litfin, A Batta, M Viellieber, Andreas G Class, H Doolaard, F Roelofs, Sandro Manservisi, Michael BottcherAbstract:Abstract Thermal-hydraulics is a key scientific subject to be investigated for the development of innovative reactor systems. For applications using Liquid Metals as coolants this task is particularly challenging due to their very low Prandtl number (Pr), preventing the application of common analogies between the turbulent transport of momentum and heat. Thus specific models and validation data with low-Pr fluids are required for representing safety-related thermal-hydraulic scenarios, such as heat transfer in fuel assemblies. Aiming to achieve a better understanding of these flow scenarios, in the European FP7 cooperation project THINS (2010–2014) this subject is investigated at three complementary levels. An experimental campaign consisting of an electrically heated 19-pin hexagonal rod bundle cooled by lead-bismuth eutectic (LBE) is carried out at typical reactor conditions in terms of operating temperature, power density and velocity. Both pre- and post-test analyses using existing numerical tools are performed for evaluating the differential pressure and heat transfer characteristics of the test section. Moreover, advanced turbulence models and numerical techniques are developed and applied to this geometry. Overall, the goals of this project are achieved. The experiments show good degree of repeatability and provide reliable validation data. For intermediate flow rates a good agreement is observed with the results of the heat transfer simulations, based on a constant turbulent Prandtl number. Two advanced approaches for representing the turbulent heat transport considering look-up tables and a four-equation model are successfully tested and overcome the limitations of using a constant turbulent Prandtl number. Using a coarse-grid CFD approach the turbulent momentum transport along two bundles is studied, yielding a good accuracy with a 1000-fold mesh reduction.
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Review of data and correlations for turbulent forced convective heat transfer of Liquid Metals in pipes
Heat and Mass Transfer, 2015Co-Authors: J Pacio, L. Marocco, Th WetzelAbstract:Within the present work the dataset of experimental points and the heat transfer correlations available in literature for Liquid-metal fully-developed, forced-convective heat transfer in pipes are reviewed and critically analyzed. Over 1,100 data points from 21 different sources are considered for constant heat flux, covering a wide range of operating conditions (velocity, heat flux, diameter, among others). Among 15 evaluated correlations, four appropriate ones are recommended for forced turbulent convection: one covering all the data points and the other three respectively related to alkali Liquid Metals, lead alloys and mercury. Moreover, a new correlation has been derived as a best fit of the limited number of available data points for constant wall temperature, while an alternative evaluation method is also described for this boundary condition.
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Liquid Metals as efficient coolants for high intensity point focus receivers implications to the design and performance of next generation csp systems
Energy Procedia, 2014Co-Authors: J Pacio, Andreas Fritsch, Csaba Singer, Ralf UhligAbstract:Abstract To a large extent, the economic viability of CSP projects is influenced by the main parameters characterizing the receiver, such as concentration ratio, heat-flux-intensity and distribution, fluid outlet temperature and, naturally, the receiver efficiency. The design ranges for these parameters are in turn limited by the selection of the heat transfer fluid (HTF). In this work, two candidate Liquid Metals (LMs), namely sodium (Na) and lead-bismuth eutectic (LBE), are proposed as efficient HTFs that shall allow extending these design ranges beyond the current state-of-the-art, and thus contribution to the development of next-generation point-focus central receiver systems (CRSs). LMs offer two significant advantages compared to other HTFs used in CRSs. First, very high heat transfer coefficients can be achieved; roughly one order of magnitudes higher than with other Liquids like molten nitrate salts, and many times the typical values of pressurized air; allowing to operate at higher heat flux intensities. Second, high fluid outlet temperatures can be achieved within stable Liquids up to their boiling point (at 1 bar, Na: 883 °C, LBE: 1533 °C). Design considerations for implementing advanced concepts based on LM-cooling are analyzed in this work, evaluating their advantages and limitations. Previous experiences and theoretical evaluations indicate that a superior performance can be achieved with LMs, and the current state of technology reached a satisfactory maturity level for operating large-scale facilities
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thermodynamic evaluation of Liquid Metals as heat transfer fluids in concentrated solar power plants
Applied Thermal Engineering, 2013Co-Authors: J Pacio, Cs Singer, Th Wetzel, Ralf UhligAbstract:Abstract Concentrated solar power, and in particular central receiver systems, can play a major role as a renewable energy source with the inherent possibility of including a thermal energy storage subsystem for improving the plant dispatchability. While current commercial projects are dominated by direct steam generation and molten nitrate salt concepts, next-generation systems will require higher operating temperature and larger heat-flux densities in order to increase the efficiency and reduce costs. In that context, Liquid Metals are proposed in this work as advanced heat transfer fluids that can face those challenges. The main advantages, regarding higher temperature and improved heat transfer performance, are discussed and quantified using simplified models. Indirect thermal storage solutions are proposed for compensating their relatively small heat capacity. Overall, provided that some practical challenges can be overcome, Liquid Metals present large potential as efficient heat transfer fluids.
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assessment of Liquid metal technology status and research paths for their use as efficient heat transfer fluids in solar central receiver systems
Solar Energy, 2013Co-Authors: J Pacio, Th WetzelAbstract:Abstract Concentrated solar power, and in particular central receiver systems, can play a major role as a renewable energy source with the inherent possibility of including a thermal energy storage subsystem for improving the plant dispatchability. Next-generation systems, in an effort for increasing the overall efficiency and reducing specific costs, will require higher operating temperatures and larger heat flux densities. In that context, Liquid Metals as advanced heat transfer fluids can face those challenges and largely contribute to the economics of future systems. Liquid Metals have been proposed in recent publications from a thermodynamic perspective. The present article focuses in a complementary way on the current state of Liquid metal technology. Based on the main requirements and previous experiences, three main candidate Liquid Metals are considered: sodium (Na), lead–bismuth eutectic alloy (LBE or PbBi) and molten tin (Sn), each of them with relative advantages and limitations. The state-of-the-art is reviewed, indicating that the readiness of Liquid metal technology is quite advanced, mainly for the two first candidates. Recommended research and development activities are outlined, mainly in two directions: compatibility with structural materials at high temperature and indirect thermal storage solutions. Overall, provided that some challenges can be overcome, significant advantages can be obtained from the use of Liquid Metals as heat transfer fluids in central receivers systems.
F Menghini - One of the best experts on this subject based on the ideXlab platform.
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heat transfer to Liquid Metals in a hexagonal rod bundle with grid spacers experimental and simulation results
Nuclear Engineering and Design, 2015Co-Authors: J Pacio, F Menghini, K Litfin, A Batta, M Viellieber, Andreas G Class, H Doolaard, F Roelofs, Sandro Manservisi, Michael BottcherAbstract:Abstract Thermal-hydraulics is a key scientific subject to be investigated for the development of innovative reactor systems. For applications using Liquid Metals as coolants this task is particularly challenging due to their very low Prandtl number (Pr), preventing the application of common analogies between the turbulent transport of momentum and heat. Thus specific models and validation data with low-Pr fluids are required for representing safety-related thermal-hydraulic scenarios, such as heat transfer in fuel assemblies. Aiming to achieve a better understanding of these flow scenarios, in the European FP7 cooperation project THINS (2010–2014) this subject is investigated at three complementary levels. An experimental campaign consisting of an electrically heated 19-pin hexagonal rod bundle cooled by lead-bismuth eutectic (LBE) is carried out at typical reactor conditions in terms of operating temperature, power density and velocity. Both pre- and post-test analyses using existing numerical tools are performed for evaluating the differential pressure and heat transfer characteristics of the test section. Moreover, advanced turbulence models and numerical techniques are developed and applied to this geometry. Overall, the goals of this project are achieved. The experiments show good degree of repeatability and provide reliable validation data. For intermediate flow rates a good agreement is observed with the results of the heat transfer simulations, based on a constant turbulent Prandtl number. Two advanced approaches for representing the turbulent heat transport considering look-up tables and a four-equation model are successfully tested and overcome the limitations of using a constant turbulent Prandtl number. Using a coarse-grid CFD approach the turbulent momentum transport along two bundles is studied, yielding a good accuracy with a 1000-fold mesh reduction.
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triangular rod bundle simulations of a cfd κ ϵ κθ ϵθ heat transfer turbulence model for heavy Liquid Metals
Nuclear Engineering and Design, 2014Co-Authors: S Manservisi, F MenghiniAbstract:Abstract Heavy Liquid Metals are often considered as coolant fluids for fast nuclear reactors because of their physical and neutronic properties. Nevertheless, these fluids are not well known as ordinary fluids, such as air or water, and the use of reliable numerical tools for the simulation of turbulent heat transfer inside reactor cores refrigerated with Liquid Metals is still an open issue. In the present work we propose the use of a κ - ϵ - κ θ - ϵ θ turbulence model to improve the prediction of turbulent heat transfer for Liquid metal flows in triangular rod bundle geometry with different pitch-to-diameter ratios and different Peclet numbers. Nusselt number comparisons between experimental correlations and different heat turbulence models are reported for many test cases.
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a cfd four parameter heat transfer turbulence model for engineering applications in heavy Liquid Metals
International Journal of Heat and Mass Transfer, 2014Co-Authors: S Manservisi, F MenghiniAbstract:Abstract In ordinary fluids, such as water or air, similarity between thermal and dynamical fields holds and it is commonly accepted that implementing a Computational Fluid Dynamics (CFD) code for a two-equation turbulence model with the hypothesis of a constant turbulent Prandtl number in the range 0.85–0.9 is sufficient to obtain reliable results both for velocity and temperature fields. In heavy Liquid Metals such as sodium, lead and Lead–Bismuth Eutectic (LBE) with low Prandtl number (Pr ≈ 0.025) the time scales of temperature and velocity fields are rather different, because heat transfer is due mainly to molecular diffusion. In these fluids a standard constant turbulent Prandtl number model fails to reproduce correlations build from experimental data and predicts a too high heat transfer. Heavy Liquid Metals are promising coolant fluids for achieving the necessary requirements of fast nuclear reactors and many European projects have been started with the purpose of developing CFD codes able to correctly predict turbulent heat transfer for these fluids. The present work addresses an effort to improve the prediction of turbulent heat transfer for Liquid metal flows in plane and cylindrical geometries assessing a κ–∊–κθ–∊θ four parameter turbulence model. In particular the simulations aim to reproduce fully developed thermal and velocity profiles by using a standard finite element implementation of the Navier–Stokes equations coupled with the energy and momentum turbulence models. A modified κ–∊ system with low-Reynolds model functions is used for the turbulent velocity field while a κθ–∊θ system is employed to compute the turbulent temperature field. The results of the simulations are compared with Direct Numerical Simulations (DNS) data and with heat transfer experimental correlations in order to validate the four parameter turbulence model. Different uniform heat flux boundary conditions with zero and constant temperature fluctuations at the wall are presented.
Ralf Uhlig - One of the best experts on this subject based on the ideXlab platform.
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Liquid Metals as efficient coolants for high intensity point focus receivers implications to the design and performance of next generation csp systems
Energy Procedia, 2014Co-Authors: J Pacio, Andreas Fritsch, Csaba Singer, Ralf UhligAbstract:Abstract To a large extent, the economic viability of CSP projects is influenced by the main parameters characterizing the receiver, such as concentration ratio, heat-flux-intensity and distribution, fluid outlet temperature and, naturally, the receiver efficiency. The design ranges for these parameters are in turn limited by the selection of the heat transfer fluid (HTF). In this work, two candidate Liquid Metals (LMs), namely sodium (Na) and lead-bismuth eutectic (LBE), are proposed as efficient HTFs that shall allow extending these design ranges beyond the current state-of-the-art, and thus contribution to the development of next-generation point-focus central receiver systems (CRSs). LMs offer two significant advantages compared to other HTFs used in CRSs. First, very high heat transfer coefficients can be achieved; roughly one order of magnitudes higher than with other Liquids like molten nitrate salts, and many times the typical values of pressurized air; allowing to operate at higher heat flux intensities. Second, high fluid outlet temperatures can be achieved within stable Liquids up to their boiling point (at 1 bar, Na: 883 °C, LBE: 1533 °C). Design considerations for implementing advanced concepts based on LM-cooling are analyzed in this work, evaluating their advantages and limitations. Previous experiences and theoretical evaluations indicate that a superior performance can be achieved with LMs, and the current state of technology reached a satisfactory maturity level for operating large-scale facilities
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thermodynamic evaluation of Liquid Metals as heat transfer fluids in concentrated solar power plants
Applied Thermal Engineering, 2013Co-Authors: J Pacio, Cs Singer, Th Wetzel, Ralf UhligAbstract:Abstract Concentrated solar power, and in particular central receiver systems, can play a major role as a renewable energy source with the inherent possibility of including a thermal energy storage subsystem for improving the plant dispatchability. While current commercial projects are dominated by direct steam generation and molten nitrate salt concepts, next-generation systems will require higher operating temperature and larger heat-flux densities in order to increase the efficiency and reduce costs. In that context, Liquid Metals are proposed in this work as advanced heat transfer fluids that can face those challenges. The main advantages, regarding higher temperature and improved heat transfer performance, are discussed and quantified using simplified models. Indirect thermal storage solutions are proposed for compensating their relatively small heat capacity. Overall, provided that some practical challenges can be overcome, Liquid Metals present large potential as efficient heat transfer fluids.
S Manservisi - One of the best experts on this subject based on the ideXlab platform.
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triangular rod bundle simulations of a cfd κ ϵ κθ ϵθ heat transfer turbulence model for heavy Liquid Metals
Nuclear Engineering and Design, 2014Co-Authors: S Manservisi, F MenghiniAbstract:Abstract Heavy Liquid Metals are often considered as coolant fluids for fast nuclear reactors because of their physical and neutronic properties. Nevertheless, these fluids are not well known as ordinary fluids, such as air or water, and the use of reliable numerical tools for the simulation of turbulent heat transfer inside reactor cores refrigerated with Liquid Metals is still an open issue. In the present work we propose the use of a κ - ϵ - κ θ - ϵ θ turbulence model to improve the prediction of turbulent heat transfer for Liquid metal flows in triangular rod bundle geometry with different pitch-to-diameter ratios and different Peclet numbers. Nusselt number comparisons between experimental correlations and different heat turbulence models are reported for many test cases.
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a cfd four parameter heat transfer turbulence model for engineering applications in heavy Liquid Metals
International Journal of Heat and Mass Transfer, 2014Co-Authors: S Manservisi, F MenghiniAbstract:Abstract In ordinary fluids, such as water or air, similarity between thermal and dynamical fields holds and it is commonly accepted that implementing a Computational Fluid Dynamics (CFD) code for a two-equation turbulence model with the hypothesis of a constant turbulent Prandtl number in the range 0.85–0.9 is sufficient to obtain reliable results both for velocity and temperature fields. In heavy Liquid Metals such as sodium, lead and Lead–Bismuth Eutectic (LBE) with low Prandtl number (Pr ≈ 0.025) the time scales of temperature and velocity fields are rather different, because heat transfer is due mainly to molecular diffusion. In these fluids a standard constant turbulent Prandtl number model fails to reproduce correlations build from experimental data and predicts a too high heat transfer. Heavy Liquid Metals are promising coolant fluids for achieving the necessary requirements of fast nuclear reactors and many European projects have been started with the purpose of developing CFD codes able to correctly predict turbulent heat transfer for these fluids. The present work addresses an effort to improve the prediction of turbulent heat transfer for Liquid metal flows in plane and cylindrical geometries assessing a κ–∊–κθ–∊θ four parameter turbulence model. In particular the simulations aim to reproduce fully developed thermal and velocity profiles by using a standard finite element implementation of the Navier–Stokes equations coupled with the energy and momentum turbulence models. A modified κ–∊ system with low-Reynolds model functions is used for the turbulent velocity field while a κθ–∊θ system is employed to compute the turbulent temperature field. The results of the simulations are compared with Direct Numerical Simulations (DNS) data and with heat transfer experimental correlations in order to validate the four parameter turbulence model. Different uniform heat flux boundary conditions with zero and constant temperature fluctuations at the wall are presented.
