Liquid Metal Heat Transfer Research Articles

Numerical study of liquid metal magnetoconvection and heat transfer in an electrically conductive square duct

Mixed convection of an electrically conductive fluid in a square duct with imposed transverse magnetic field is studied using Large Eddy Simulation (LES) paradigms. The duct walls are electrically conductive, with the wall conductivity parameter cw ranging from 0 to 0.5. The Reynolds number is Re=5602 and the Prandtl number is Pr=0.0238. The focus of the study is on flows at Hartmann numbers Ha⩽125, Richardson numbers Ri⩽10 and two different thermal boundary conditions are considered: four wall uniform heat fluxes and one-sided heating (fixed wall temperature). The results show that the transition from laminar to turbulent flow depends not only on the ratio Ri/Ha, but also on cw and on the local thermal boundary conditions. In the turbulent regime with one-sided heating, the turbulent heat fluxes play an important role in the total heat transfer, in contrast with the typical behaviours of liquid metals. Moreover, the turbulent and thermal structures are highly dependent on the thermal boundary conditions, which completely alter the flow structure. It is also found that at cw≥0.01 the turbulent heat fluxes decrease.

Structure effect of wall cooling channel on liquid metal heat transfer in aero-engines

In this study, a model of the cooling channel and an experimental system for heat transfer involving liquid metal are studied, to investigate the influence of the wall cooling channel structure of combustor on the heat transfer process. The error between the experimental and simulated outcomes fall is within 3 %, suggesting that the accuracy of the simulation is satisfactory. The simulation results found that the temperature non-uniformity coefficient can be reduced by 13 % and 7 % through adjusting the aspect ratio under the heat flux of 3 MW/m2 and 1.5 MW/m2 respectively, and the wall temperature of gas side can be reduced by 89 K and 47 K respectively. A smaller aspect ratio is favorable for homogenizing the liquid metal temperature distribution. Longer widths, however, can lead to horizontal thermal stratification, which can be unfavorable for heat transfer. With an aspect ratio and rib thickness of about 0.5 and 0.4 mm, respectively, the cooling channel outlet has the lowest non-uniformity coefficient of temperature and enthalpy, which is conducive to the improvement of the wall energy recovery rate. The optimal cooling channel effectively improves the thermal stratification in channel, and provides theoretical guidance for the suitable application of liquid metal to the combustor wall cooling channel.

Study on the turbulent Prandtl number model for liquid metal flow and heat transfer in a narrow rectangular channel

In order to improve the core power density of liquid metal reactors, plate-type fuel assemblies are typically used, which have obvious advantages in heat transfer capacity. For the low Prandtl number fluid, the RANS turbulence model-based CFD method needs to be modified by inserting the turbulent Prandtl number model. This paper investigated the turbulent Prandtl number model for the turbulent heat transfer of liquid metal flow in a narrow rectangular channel. First, the existing turbulent Prandtl number models were summarized and classified as the local turbulence Prandtl number model and the global turbulence Prandtl number model. They were evaluated based on the experimental data. The results showed that the prediction results of each model are not ideal. Then, a typical numerical example was used to analyze the temperature field and flow field in the narrow gap of a narrow rectangular channel. It was shown that the entire flow region only includes the near-wall region and the transition region, and the transition region accounts for the main proportion. A semi-empirical local turbulence Prandtl number model was obtained by solving the turbulent transport equation, and the unknown coefficients were determined iteratively with the experimental data. The error was controlled within ±20%. Finally, the applicability of the newly proposed turbulent Prandtl number model was explored, and the results showed that it has good applicability for the different runner sizes and different liquid metal types. This study can provide a reference for the numerical simulation of turbulent heat transfer of metal liquid in the narrow rectangular channel.

Investigation on the effect of magnetic field on forced convection heat transfer of liquid metals in a microchannel heat sink

Investigation on the effect of magnetic field on forced convection heat transfer of liquid metals in a microchannel heat sink

Numerical study of turbulent heat transfer of annular fuel assembly in a sodium-cooled fast reactor by a SST k-ω-kθ-εθ model

ABSTRACT The annular fuel has two cooling surfaces inside and outside, which can fully take away the heat of the fuel assemblies. It is urgent to conduct research on sodium-cooled fast reactor annular fuel assemblies. The Prandtl number Pr of liquid sodium is much less than 1 (Pr ˂˂ 1). The problem is that using the traditional constant turbulent Prandtl number Pr t of 0.85 ~ 0.9, which will greatly affect the accuracy of the numerical prediction of the thermal-hydraulic properties of liquid sodium. In order to address this issue, the turbulent heat transfer characteristics of the bare and wire spacers 7-pin annular fuel assembly for a sodium-cooled fast reactor are investigated by a SST k-ω-k θ -ε θ model. The open-source CFD software OpenFOAM is used for this calculation. In order to verify the validity of the present calculation method, some classical liquid metal heat transfer correlations are used to compare and analyze the results of the present calculation method (k θ -ε θ model), the results of the Reynolds analogy hypothesis (Pr t = 0.85 model), and the results of the turbulent Prandtl number correlation (Pr t = kays model). It is shown that the SST k-ω-Pr t = 0.85 model overestimates the Nusselt number Nu of the annular fuel assembly. The SST k-ω-k θ -ε θ model agrees well with the SST k-ω-Pr t = Kays model of the bare and wire spacers 7-pin annular fuel assembly . It suggests that the numerical calculation results of the SST k-ω-k θ -ε θ model are reliable in both the simple and complex fuel assembly models. In brief, the SST k-ω-k θ -ε θ model is more capable of predicting the turbulent heat transfer characteristics of the bare and wire spacers 7-pin annular fuel assembly in sodium-cooled fast reactors. In addition, the thermal-hydraulic characteristics of annular fuel assemblies with wire spacers for sodium-cooled fast reactors have been investigated. This research can provide a reference for the study of turbulent heat transfer characteristics of annular fuel assemblies in sodium-cooled fast reactor.

Numerical Investigation of the Effect of the Wall Properties on Downward Mercury Flow and Temperature Fluctuations in a Vertical Heated Pipe under a Transverse Magnetic Field

The work is motivated by the need to study the effect of abnormal quasi-periodic high-amplitude temperature fluctuations in a liquid metal flow in pipes and channels, which arise under conditions close to those specific for liquid metal (PbLi alloy, etc.) hydrodynamics and heat transfer in blankets of tokamak fusion reactors. Such temperature fluctuations in the cooling channels of the fusion reactor blanket pose a threat to the strength of the channel walls. The influence of the physical properties of the channel walls and potential fouling of their inner surface on the generation of these fluctuations has not been studied as yet. Therefore, this problem has been numerically investigated for a MHD mixed convection of a liquid metal in a round tube for a downward flow of mercury with the one-side heating condition under a strong transverse magnetic field using conjugate problem statement by the LES (Large Eddy Simulation) method. Numerical simulation was performed for four cases: (i) the effect of thermophysical and electrophysical properties of the steel pipe wall is neglected, (ii) the effect of wall material properties is included, (iii) the effect of the moderate contact electrical resistance caused by a layer of fouling deposits or oxides on the inner surface of the pipe is accounted for, and (iv) the resistance of the thin fouling layer is assumed to be very high. The predictions for the cases (i) and (iv) are in good agreement with the experimental data and the results of the direct numerical simulation. They demonstrate the existence of quasi-periodic abnormal high-amplitude temperature fluctuations in the fluid with a frequency of approximately 0.14 Hz. With a relatively low electrical resistance of the fouling film (case iii), the frequency of high-amplitude temperature fluctuations was considerably lower (0.07 Hz). In the absence of electrical contact resistance on the inner surface of the steel pipe (case ii), high-amplitude velocity and temperature fluctuations were not revealed in the fluid. Thus, it was shown for the first time that the physical properties of a wall and the electrical resistance between the fluid and the electrically conducting wall were responsible for the development of abnormal temperature fluctuations in the liquid and the wall and control of their amplitude and frequency. The causes of the revealed effects are discussed.

Self-driven thermoelectric cooling contraption for liquid metals under the static magnetic field

The presence of large temperature gradients in liquid metals during heat transfer can also induce thermoelectric effects, which can lead to pumping or stirring of liquid metals under the action of magnetic fields. The thermoelectric effect of liquid metals has potential application background in both nuclear fusion and metal metallurgy. In this paper, an experimental study of flow driven by the Seebeck effect, in which the temperature-dependent voltage difference at an interface between dissimilar metals, in the presence of a magnetic field, can be used to create a Lorentz force. It is proposed that this method could be used for cooling electronics, fusion reactors, and solar technologies. The working fluid is eutectic gallium–indium–tin, and flow measurements are made with ultrasound. The flow velocity tends to increase and then decrease as the magnetic field increases. Two scaling relations are developed to predict the velocity, one for weak magnetic fields and one for strong magnetic fields. Those predictions are combined to estimate the maximum velocity. Temperature gradients and wall conductance ratio have a significant effect on the Seebeck effect self-driven flow. It is found that the self-driven flow velocity caused by the Seebeck effect is positively correlated with the number of channels in the multi-channel experiments. This design idea of self-generated flow and heat transfer of liquid metal in the magnetic field will provide the possibility of pumpless self-driven liquid lithium flow in nuclear fusion reactors and provide new ideas for cooling of electronic products and related energy-saving and emission reduction applications.

Magneto-convective flows around two differentially heated cylinders

Numerical simulations have been carried out in support of an experimental campaign conducted in the MEKKA laboratory at KIT. The aim is investigating liquid metal heat transfer with an imposed magnetic field in a model geometry relevant for the study of water cooled lead lithium blankets for fusion reactors. In the breeding zone of this blanket concept, cooling pipes are immersed in the liquid metal in which convective motion occurs due to significant temperature gradients. The test-section features a rectangular box containing two horizontal cylinders kept at constant differential temperatures in order to establish a temperature gradient that drives the buoyant flow. A magnetic field {textbf{B}} is applied parallel to gravity. The magneto-convective flow, which results from the presence of electromagnetic forces and temperature gradients in the fluid, is relatively complex, since the liquid metal has to move around the cylinders. For weak magnetic fields, a convective recirculation is fed by a jet-like flow formed by the boundary layers that detach from the pipe walls and recombine behind the obstacles. For sufficiently strong textbf{B}, the thermal field resembles that of a conductive regime with vertical isotherms and the fluid is nearly stagnant in most of the cavity except in layers parallel to magnetic field lines and tangent to the cylinders. The rate of convective heat transfer decreases with an increase of the magnetic field. Numerical simulations complement experimental results and give insight into phenomena that cannot be directly analyzed by means of measured quantities.

Numerical investigations of arc plasma characteristic parameters evolution and metal properties in GMAW-based WAAM of Al alloy with an integrated model

Understanding the arc plasma characteristic parameters evolution and the metal thermodynamics is of great significance for guiding the improvement of the forming accuracy and performance of GMAW-based WAAM in engineering. In this study, we have developed a three-dimensional arc plasma-droplet-molten pool integrated model of GMAW-based WAAM of Al alloy, which simultaneously includes the heating of arc plasma, the vaporization of Al metal, the transfer of droplet, the melting and solidification of molten pool, and the relative motion between wire and substrate. Based on this sophisticated model, the effect of Al vapor on the arc plasma characteristics, the dynamic evolution of arc plasma characteristics during the metal transfer process, and the heat transfer and flow behaviors of liquid metal are comprehensively investigated. The simulation results show that the Al vapor is principally generated from the surface of droplet, which leads to the decrease of arc plasma temperature and arc pressure, and the expansion of current path. In a droplet transfer cycle, the average temperature of plasma decreases first and then increases, affected by the distribution of current density. Most of heat for the formation of molten pool comes from the energy transfer of droplet, and the droplet impact effect on the molten pool flow is significantly greater than Marangoni effect. Finally, the simulation results, including droplet transfer process and deposited layer profile, are validated by experimentally measured results, which are in good agreement with each other.

Numerical Study of Heat Transport in Static Liquid Metal Exposed to Plasma with Magnetic Field

Suitable management of excessive heat in liquid metal is one of key roles to accomplish the proper liquid metal usage as future alternative fusion plasma facing components. In this work, theoretical heat transfer in liquid metal in contact with magnetized plasma was carried out, and a numerical solver for investigating temperature and induced velocity by j×B force has been developed. The study suggests the trends: 1. lower bulk temperature in liquid metal at which magnetic field is presented, compared to no magnetic field, by additional convection from j × B, stronger than natural convection; and 2. asymmetric temperature distribution in liquid metal along j × B.

Fundamental and applied investigations of the liquid-metal cooled fast reactor thermal hydraulics (achieved results and further investigation issues)

The results of experimental and computational-theoretical investigations in the area of hydrodynamics and heat transfer of fast reactors and accelerator-driven system with liquid metal coolants are presented, and the problems and tasks for further investigations are formulated. Physical phenomena, effects, laws and process characteristics occurring in reactor power plants, including a flow path and a fast reactor core: the velocity and temperature fields, the structure and characteristics of turbulent transfer of momentum and energy, the hydraulic resistance of channels and fuel rod assemblies, the collector system hydrodynamics, the vibroacoustics, the heat transfer in channels and fuel rod assemblies with liquid metal cooling and inter-channel exchange are considered and analyzed. The data of experimental investigations are presented on a single-tube model of a large-module steam generator and on a fragmentary thermohydraulic model of a steam generator of a reactor power plant with twisted steam-generating tubes operating at subcritical and supercritical water pressure. The results of investigations of temperature and velocity fields on a small-scale water model of a fast reactor vessel with an integral layout in nominal, transient and emergency operating regimes are demonstrated. The investigation results on model of a fast reactor vessel with an integral layout are demonstrated that the effect of thermogravitational forces leads to temperature stratification with stagnant and recirculating formations, internal waves appear at the stratified boundaries, temperature pulsations, thermal fatigue of structural materials and a decrease in the service life of the equipment. It is shown that the liquid metal boiling process in fuel rod assemblies is formed under the influence of various factors, has a complex structure, is characterized by both stable (nucleate, annular-dispersed) and pulsation (slug) regimes with significant fluctuations of technological parameters, which can cause a crisis of heat transfer. Heat transfer was studied, a cartogram two-phase flow was obtained for liquid metal boiling in fuel rod assemblies, the effect of the surface roughness of fuel rods on liquid metal heat transfer and boiling regimes in fuel rod assemblies was found. The possibility of long-term stable cooling of the core during sodium boiling was shown by using “sodium cavity” over the reactor core. The kinetic and mechanical characteristics of the degradation process of the simulated fuel assembly in fast reactor core during the thermal interaction of the uranium-containing simulators of fuel with static sodium and their dependence on the parameters and design system were determined.Information about the key problems of thermophysical investigations of a high-temperature fast reactor with a sodium coolant for the production of hydrogen and reactors cooled by water at supercritical pressure is given.

Numerical Analysis of Liquid Metal MHD Flow and Heat Transfer for Open-Surface Li Divertor in FNSF

Within the ongoing U.S.-based program on the development of liquid metal plasma-facing components, numerical simulations and analyses are performed to address the feasibility of the open-surface Li divertor. In the previous scoping studies (Smolentsev, 2021), heat-removal capabilities of the divertor were assessed using a simplified flow model for a slug-type velocity profile and constant flow thickness. Here, new analyses take into account forces acting on the flowing Li layer. Three reduced-order mathematical models are applied under the conditions of the U.S. Fusion Nuclear Science Facility (FNSF) to access magnetohydrodynamic (MHD) flow development effects, velocity distribution, and surface waves: 1) fully developed MHD flow; 2) quasi-2-D developing MHD flow; and 3) multiphase MHD flow. The obtained results for MHD flows and the surface heat flux computed with the plasma code scrape-off layer plasma simulation for ITER (SOLPS-ITER) are then used as input data to compute the temperature distribution in the divertor by solving the convection–diffusion energy equation.

Experimental study on heat transfer characteristics of LBE flow in a vertical tube under static and rolling conditions

The heat transfer characteristics of liquid metals get much attention because of their low Prandtl number. In this paper, the heat transfer characteristics of a heavy liquid metal (LBE) in a vertical circular test section under static and rolling motions are experimentally investigated. The results show that the additional forces induced by rolling motion cause a periodic change in the flow rate of the forced circulation. The rolling motion reduces the mean circulation flow rate, especially in the low flow rate region. The cycle-averaged heat transfer characteristics are significantly enhanced in the low Peclet number region (<2500). In the high Peclet number region (>2500), the cycle-averaged heat transfer characteristics are not significantly different compared to those under static condition. At low flow rates, flow rate oscillations may lead to counterflow. The amplitude of the Reynolds number oscillation, the cycle-averaged Reynolds number, the roll period and the roll amplitude jointly determine the cycle-averaged Nusselt number. The correlations under static conditions cannot predict the Nussle number under rolling conditions well. Therefore, a correlation is developed to analyze the heat transfer characteristics of LBE under rolling conditions. This study may contribute to the design and safety analysis of liquid metal heat transfer when applied to moving conditions.

Study on flow and heat transfer of liquid metal in the microchannel heat sink

In order to overcome the shortcoming of high resistance of liquid metal flow, three new types of microchannel heat sinks, namely top-slotted microchannel, bottom-extension microchannel and composite microchannel are proposed, and the flow and heat transfer characteristics of liquid metal galinstan are numerically studied. It is found that all three new microchannel heat sinks can reduce the flow resistance effectively. The optimized geometric structure is obtained by analyzing the effect of geometry parameters on total thermal resistance and pump power, including the slot height, the length of extension area, the distance from the entrance to the extension point and the extension height. The lowest flow resistance is obtained in the composite microchannel and the pump power is 54.9% lower than that of the traditional channel with only 0.7% increase of total thermal resistance. Based on the geometry parameter of the slot height, the distance from the entrance to the extension point and the extension height, two factors are proposed to revise the empirical correlation of friction factor f for the top-slotted and bottom-extension channel. The corrected results are in good agreement with the numerical results.

Experimental investigations on the design of a dual-media thermal energy storage with liquid metal

Liquid metals are a promising heat transfer fluid with stability over a wide temperature range and high thermal conductivity. In concentrating solar energy power plants, as well as in the steel processing facilities heat is generated at high temperature level. Many fluids begin to decompose at such high temperatures and gases have poor thermal conductivity, making it difficult to effectively store and extract heat from the source. Liquid metal heat transfer fluids are able to effectively transport energy at a wide range of temperatures (150 ∘C–1000 ∘C). Existing heat transfer fluids, such as molten salts, have a working range between 290 ∘C and 565 ∘C for example. Depending on the metal selected, thermal conductivity can be 30 to 100 times higher than molten salts, but liquid metals have lower energy density. The liquid metal can be paired with a filler material having a higher energy density to develop a system with high energy transport and storage capability. The first lab-scale experiment of thermocline energy storage with liquid metal as a heat transfer fluid and a zirconium silicate filler, called VESPA [Vorversuch EnergieSPeicher Aufbau (ger.), engl. Preliminary test for energy storage setup], was carried out to prove the concept. So far, there are promising, yet only numerical investigations of thermocline energy storage with liquid metal as heat transfer fluid. The storage system under investigation was a dual-media thermocline energy storage system with liquid lead–bismuth eutectic as heat transfer fluid and zirconium silicate as filler material. The experiments were executed at temperatures from 180 ∘C to 380 ∘C, and focused on design aspects of the energy storage system. Different modes of operation of the storage system were investigated, including charging, discharging, and stand-by. Results are presented showing changes in the vertical temperature profile of the energy storage while varying mass flow, inlet temperature, and filler material.

Proposal of a turbulent Prandtl number model for Reynolds-averaged Navier–Stokes approach on the modeling of turbulent heat transfer of low-Prandtl number liquid metal

Because of their high molecular heat conductivity, low-Prandtl number liquid metal is a promising candidate coolant for various designs of advanced nuclear systems such as liquid metal–cooled fast reactors and accelerator-driven sub-critical system (ADS). With the fast-growing computational capacity, more and more attention has been paid to applying computational fluid dynamics (CFD) methods in thermal design and safety assessment of such systems for a detailed analysis of three-dimensional thermal–hydraulic behaviors. However, numerical modeling of turbulent heat transfer for low-Prandtl number liquid metal remains a challenging task. Numerical approaches such as wall-resolved large eddy simulation (LES) or direct numerical simulation (DNS), which can provide detailed insight into the physics of the liquid metal flow and the associated heat transfer, were widely applied to investigate the turbulent heat transfer phenomenon. However, these approaches suffer from the enormous computational consumption and are hence limited only to simple geometrical configurations with low to moderate Reynolds numbers. The Reynolds-averaged Navier–Stokes (RANS) approach associated with a turbulent Prandtl number Prt accounting for the turbulent heat flux based on Reynolds analogy is still, at least in the current state in most of the circumstances, the only feasible approach for practical engineering applications. However, the conventional choice of Prt in the order of 0.9∼unity in many commercial computational fluid dynamics codes is not valid for the low-Prandtl number liquid metal. In this study, LES/DNS simulation results of a simple forced turbulent channel flow up to a friction Reynolds number Reτ of 2000 at Pr of 0.01 and 0.025 were used as references, to which the Reynolds-averaged Navier–Stokes approach with varying Prt was compared. It was found that the appropriate Prt for the RANS approach decreases with bulk Peclet number Peb and approaches a constant value of 1.5 when Peb becomes larger than 2000. Based on this calibrated relation with Peb, a new model for Prt used in the RANS approach was proposed. Validation of the proposed model was carried out with available LES/DNS results on the local temperature profile in the concentric annulus and bare rod bundle, as well as with experimental correlations on the Nusselt number in a circular tube and bare rod bundle.

Direct numerical simulation of turbulent heat transfer in liquid metals in buoyancy-affected vertical channel

This article investigates the impact of buoyancy force in a vertical channel with combined natural and forced convection liquid metal flow using direct numerical simulation (DNS). In particular, a liquid metal with Pr = 0.025 and Re = 4667 is considered in the combined natural and forced convection regime (Ri = 0, 0.25, 0.50 and 1.00). The liquid metal is driven upward while buoyancy aids the flow close to the hot wall and opposes the flow close to the cold wall. The influence of buoyancy on turbulent heat transfer and mean flow are analyzed. Turbulent transport is enhanced in aiding flow and weakened in opposing flow with increasing Richardson number for low-Prandtl-number fluids. The enhancement of normalized Reynolds stresses is more significant in the opposing flow compared to the aiding flow. Turbulent heat flux also increases and, consequently, enhances heat transfer. Besides, the budgets of streamwise and wall-normal turbulent heat flux, temperature variance, Reynolds shear stress and turbulent kinetic energy are investigated and related to the variations of the quantities in mean fields. The budgets of temperature variance and turbulent kinetic energy are nearly balanced by shear production and dissipation. As for the budget of Reynolds shear stress, streamwise and wall-normal turbulent heat flux, temperature pressure-gradient correlation contributes to the budget significantly in addition to production and dissipation. The DNS results can not only shed light on the heat transfer mechanism of turbulence in liquid metal flows, but also be used for validating and improving turbulent models for low-Prandtl-number fluids, which could be useful for developing Gen IV liquid-metal-cooled fast reactors.

Microfluidics Temperature Compensating and Monitoring Based on Liquid Metal Heat Transfer.

Microfluidic devices offer excellent heat transfer, enabling the biochemical reactions to be more efficient. However, the precision of temperature sensing and control of microfluids is limited by the size effect. Here in this work, the relationship between the microfluids and the glass substrate of a typical microfluidic device is investigated. With an intelligent structure design and liquid metal, we demonstrated that a millimeter-scale industrial temperature sensor could be utilized for temperature sensing of micro-scale fluids. We proposed a heat transfer model based on this design, where the local correlations between the macro-scale temperature sensor and the micro-scale fluids were investigated. As a demonstration, a set of temperature-sensitive nucleic acid amplification tests were taken to show the precision of temperature control for micro-scale reagents. Comparations of theoretical and experimental data further verify the effectiveness of our heat transfer model. With the presented compensation approach, the slight fluorescent intensity changes caused by isothermal amplification polymerase chain reaction (PCR) temperature could be distinguished. For instance, the probability distribution plots of fluorescent intensity are significant from each other, even if the amplification temperature has a difference of 1 °C. Thus, this method may serve as a universal approach for micro–macro interface sensing and is helpful beyond microfluidic applications.

Chemical compatibility of F82H and 316L in liquid metal heat transfer mediums Li, Na and NaK

Liquid metals Li, Na and NaK are the candidate heat transfer mediums which are installed in the irradiation capsule of advanced fusion neutron source. The compatibility issue of F82H and 316L in the candidate heat transfer mediums was studied by means of the corrosion tests at the temperature up to 823 K. The depletion of Cr, C and Fe from the steel was caused in liquid Li. The Li diffusion into the steel matrix was analyzed by TOF-SIMS. The corrosion in liquid Na was caused by the Na diffusion into the steel matrix and the surface oxidation. The corrosion in NaK was similar with that in liquid Na.

Direct Numerical Simulation of natural, mixed and forced convection in liquid metals: selected results

Selected results of three Direct Numerical Simulations are presented, on relevant test cases for the thermal hydraulics of liquid–metal-cooled nuclear reactors, encompassing a wide spectrum of turbulent convection regimes. The first test case is a Rayleigh-Bénard cell at a Grashof number Gr=5×107, representative of the conditions in the unstably stratified layer of coolant in a reactor pool in both standard operating conditions and emergency situations, e.g. shutdown of the cooling system. The second case is the mixed convection in a cold-hot–cold triple jet configuration, representative of liquid streams exiting from the core into the pool, and relevant for the modeling of thermal striping and thermal fatigue phenomena on the vessel containment walls. The third case is the fully-developed flow in a vertical bare rod bundle with triangular arrangement and a pitch-to-diameter ratio P/D=1.4, in both forced and mixed convection conditions. These regimes respectively represent normal operation or decay heat removal conditions in reactor cores. The availability of these numerical databases will allows for an in-depth analysis of the turbulent flow and heat transfer in liquid metals under different convection regimes, and is also relevant for the development, calibration and validation of turbulent heat transfer models.