Liquid Metal Magnetohydrodynamic Research Articles

Thermodynamic and mass analysis of a novel two-phase liquid metal MHD enhanced energy conversion system for space nuclear power source

The higher-efficiency and more lightweight space nuclear power source is one of the most important prerequisites for future deep space exploration. Magnetohydrodynamic (MHD) power generation is the potential technical route to achieve significant improvement in energy conversion performance, but technology is not yet mature. By combining the characteristics of MHD and closed Brayton cycle (CBC) power generation systems, a novel quasi-Ericsson cycle with performance advantages is proposed in this study. The estimation method for the MHD power generator is refined and the thermodynamic and mass analysis models for the novel system are established. Comparative studies between the LMMHD-CBC cycle and the traditional CBC cycle are conducted under various boundary conditions and internal parameters. The results indicate that the LMMHD-CBC cycle increases the electricity generation per unit mass flow rate of working fluid under the same temperature conditions, thereby achieving a reduction in the specific mass of the power generation system. The LMMHD-CBC system is more suitable in lower reactor temperatures and higher radiator temperature conditions. Within the parameters range considered in this study, the relative reduction in specific mass compared to traditional CBC systems reaches up to 30.84 %.

On scalable liquid-metal MHD solvers for fusion breeder blanket multiphysics applications

While substantial research effort has been made recently in the development of computational liquid-metal magnetohydrodynamics (MHD) solvers, this has typically been confined to closed-source and commercial codes. This work aimed to investigate some open-source alternatives. Two OpenFOAM-based MHD solvers, mhdFoam and epotFoam, were found to show strong scaling profiles typical of fluid dynamics codes, while weak scaling was impeded by an increase in iterations per timestep with increasing resolution. Both were found to accurately solve the Shercliff and Hunt flow problems for Hartmann numbers from 20 to 1000, except for mhdFoam which failed in the Hunt flow case. A basic inductionless MHD solver was implemented in the Proteus MOOSE application as a proof of concept, using two methods referred to as the kernel method and material method. Future work will aim to build on these studies, exploring more advanced OpenFOAM MHD solvers as well as improving the Proteus MHD solver.

A rapid performance prediction method for Two-Phase liquid metal MHD generators based on Quasi-One-Dimensional model

Liquid metal magnetohydrodynamic (LMMHD) power generation is a novel and highly efficient technology with broad application prospects. In our previous study, a novel LMMHD enhanced Closed-Brayton-Cycle power generation system for hypersonic vehicles was proposed, and its excellent power generation performance was demonstrated by thermodynamic analysis. The most important component of LMMHD system is the power generation channel, and the rapid and accurate performance prediction and optimization design methods of which are the prerequisites for further research. In this study, a quasi-one-dimensional modeling method for LMMHD power generation channels based on a two-fluid model is developed. The performance variation laws with the geometric structure, magnetic field and boundary conditions, as well as the potential loss mechanisms are investigated. Results indicate that the most optimal or appropriate design can be achieved by optimizing the channel geometry and magnetic field, such as the optimal aspect ratio of 1 and the optimal Hartmann number of 2000. But it is difficult to balance the power output and efficiency through the boundary condition optimization. The gas–liquid slip can lead to approximately 20–40% of energy losses, making the elimination and suppression of slip effects a significant research focus in the relevant field.

Experimental and numerical study of a liquid metal magnetohydrodynamic generator for thermoacoustic power generation

Combining a thermoacoustic cycle engine with a liquid metal magnetohydrodynamic (LMMHD) generator will result in a thermal power generation system with no mechanical moving parts and high reliability. This disruptive technology has drawn much attention in space nuclear power generation, especially in recent years. It requires an LMMHD generator to work at a higher frequency than conventional LMMHD generators targeted for ocean wave energy conversion. However, the operating characteristics and loss mechanisms of LMMHD generators at high operating frequencies remain poorly understood, and experimental characterization of such a generator is lacking. In this work, a three-dimensional transient numerical analysis of a high-frequency LMMHD generator is performed based on multi-physics field simulation software COMSOL, to understand the operating characteristics of the generator, and the effects of inlet velocity, load resistance, and operating frequency on the generator’s performance. Furthermore, an LMMHD generator prototype was designed, constructed, and tested under different inlet velocities, load resistances, and frequencies by using a linear compressor for the first time. When the operating frequency and inlet velocity are 15 Hz and 4.3 m/s, the output voltage and current of the generator prototype reached 113 mV and 1720 A, with an output power of 68 W at a corresponding acoustic-to-electric efficiency of 24 %. A discrepancy between the numerical predictions and the experimental results was found, which gave insight into where further improvements can be made. This work reveals the operating characteristics and losses mechanism of LMMHD generators operating at higher frequencies and contributes to the development of high-efficiency generators for thermoacoustic power generation.

A method to optimize the external magnetic field to suppress the end current in liquid metal magnetohydrodynamic generators

Liquid metal magnetohydrodynamic (LMMHD) generators can generate electricity from the movement of liquid metal in an external magnetic field, without any mechanical moving parts. They can be applied in scenarios that require a highly reliable generator, including harvesting ocean wave energy and space nuclear power generation. However, in practice, the presence of end current in LMMHD generators limits the generator efficiency to a relatively low level, which restricts their wide application. After analyzing the generating mechanism of the end current, we report herein a method to optimize the external magnetic field that cancels the electrostatic field with the motional electromotive force and suppresses the end current in LMMHD generators. We use two-dimensional numerical simulations with the proposed method to demonstrate its effectiveness, which we validate with three-dimensional numerical simulations. Both the two- and three-dimensional numerical results show that this method should suppress the end current in the LMMHD generator, thus cutting the internal joule heating nearly in half and improving the generator efficiency by 9.5%. This work provides in-depth insights into both the generating mechanism and the suppression of the end current and contributes to the development of high-efficiency LMMHD generators.

Towards the simulation of MHD flow in an entire WCLL TBM mock-up

In the frame of the EUROfusion breeding blanket research activities, the water cooled lead lithium (WCLL) blanket is considered as reference liquid metal blanket design to be tested in ITER and to be used in a DEMO reactor. For the study of pressure and flow distribution in a WCLL blanket module, magnetohydrodynamic (MHD) phenomena, which result from the motion of the electrically conducting breeder in the intense plasma-confining magnetic field, have to be taken into account. They are due to the induction of electric currents that generate strong electromagnetic forces, which modify the velocity distribution in the blanket compared to hydrodynamic conditions and increase pressure losses. In the WCLL test blanket module (TBM) for ITER a basic geometry, consisting of a rectangular box, representing the breeding zone, is repeated along the poloidal direction. The manifold that distributes and collects the liquid metal features two long poloidal ducts, electrically connected across a common wall. The final goal of the present work is to simulate liquid metal MHD flows in strong magnetic fields in a complete column of a WCLL TBM including poloidal manifolds and 8 breeder units. With this aim, the complexity of the model geometry has been progressively increased and various topologies of computational grids have been considered. First simulations have been performed in a single breeder zone connected with a portion of manifolds in order to test the suitability of the generated grid and the performance of the numerical code for this type of problem. In a second step, the MHD flow at moderate magnetic fields in an entire TBM mock-up has been simulated to get pressure distribution along the manifolds and flow partitioning among breeder units.

Analysis of a novel concentrated solar power and magnetohydrodynamic liquid metal units integrated system with hydrogen production

The engineers are very interested in concentrated solar power (CSP) due to its renewable energy source nature. However, for this technology to grow, it is crucial to integrate efficient, cost-effective subsystems. On the other hand, since liquid metal magnetohydrodynamic (LMMHD) power generation systems can operate at high temperatures of 600 °C–3000 °C, they are ideal for use as a subsystem of a CSP-based plant to improve efficiency. The use of waste heat recovery units is another method of increasing efficiency and preventing exergy losses. Taking these points into consideration, the proposed trigeneration system includes an LMMHD, a CSP, and humidification-dehumidification and proton exchange membrane units to produce power, freshwater, as well as hydrogen, respectively. Performance evaluation of the presented system includes thermodynamic and thermoeconomic considerations. The results show that the presented system produces 11.87 kW of power, 6.1 m3/h of hydrogen, and 860.2 L/h of freshwater with an energy utilization factor of 45.81%, a total exergy efficiency of 4.63%, and a unit cost of 19.57 $/kWh. The receiver is the most destructive component of the system, with 256.9 kW of exergy destruction. Further, the parametric study indicates that it is possible to maximize the energy efficiency of the system by changing the concentration ratio of the receiver.

MHD effects of liquid metal flows through multiple electrically coupling ducts with U-turn bends in fusion blankets

The investigation of liquid metal (LM) magnetohydrodynamic (MHD) flows through multiple electrically coupling ducts with U-turn bends is of crucial importance for accurately predicting the thermal-hydraulic performance of LM blankets in fusion reactors. Moreover, this duct configuration is often adopted in liquid blanket designs. Here, the combination effects of the MHD coupling, 3D MHD and inclined external magnetic fields on LM MHD flows through the multiple coupling ducts with different aspect ratios and wall conductance ratios are systematically investigated by numerical simulations. The results indicate that the MHD pressure gradient caused by MHD coupling effect decreases but the 3D MHD pressure gradient increases firstly and then decreases as the inclined angles of external magnetic fields increase. In addition, the results also indicate that the normalized pressure gradient can be reduced by increasing the wall conductance ratio () through the increase of the wall thickness. Furthermore, an important finding is that the pressure gradients caused by the MHD coupling decrease with the increases of aspect ratios (b/a) of ducts, while the 3D MHD pressure gradient when b/a = 16 increases to an excessive large degree about 40 times of the pressure gradient in a single conductive duct. The related investigation results will provide a theoretical support for the liquid blanket designs in fusion reactors and deepen our understanding on the LM MHD effects.

Proposal of novel exergy-based sustainability indices and case study for a biomass gasification combine cycle integrated with liquid metal magnetohydrodynamics

Exergy is considered a way to sustainability. Exergy-based analyses have been recently widely used for performance assessment and comparison purposes of energy systems from production to end-user while different sustainability related indices or indicators including exergetic concepts have been developed in the literature. In this regard, the present study proposed five different indices: (i) Exergetic Fuel Based Environmental Remediation Index (χ), (ii) Exergetic Product Based Environmental Remediation Index (δ), (iii) Exergetic Fuel Based Total Environmental Remediation Index (β), (iv) Exergetic Product Based Total Environmental Remediation Index (α), and (v) Improved Sustainability Index (ISI). These indices were applied to a novel Biomass-integrated Gasification Combine Cycle (BIGCC) integrated with Liquid Metal Magnetohydrodynamics (LMMHD). They allowed to perform a more complete environmental analysis by considering the exergetic cost of environmental remediation of the process. The average exergy efficiency values for the BIGCC, LMMHD and the overall system were determined as 0.491, 0.222 and 0.688 under daily ambient temperatures for a year and different air to fuel ratio (AFR) conditions, respectively. The average values for χ, β, δ, α and ISI were 1.636, 2.389, 1.949, 2.848 and 0.513, respectively.

Validation and verification of a quasi two-dimensional turbulence model and an eddy detection method for liquid metal magnetohydrodynamics flows

The flow of liquid metals in straight rectangular channels with an externally applied transverse magnetic field is considered for breeding blankets in tokamak fusion reactors. Under the tokamak magnetic field, the liquid metal experiences the magnetohydrodynamic (MHD) effect that enhances the laminarization of the flow. In geometric singularities, or under certain flow conditions, the combination of Lorentz forces, momentum, and buoyant forces may trigger the formation of vortical structures. The generated vortices will align with the direction of the magnetic field, and therefore the use of a quasi-two-dimensional (Q2D) model is convenient to study them assuming that the walls are electrically insulating and the Hartmann number is high enough. Using this model is computationally affordable, allowing extensive parametric analyses. To identify the presence of eddies in the two-dimensional domain, the application of the bi-dimensional fast Fourier transform (FFT2) is foreseen as an adequate detection method. This work presents a methodology to systematically calculate liquid metal MHD flows with a Q2D model and evaluate the formation of eddies in the flow domain. The work includes a description of the validation and verification of the Q2D MHD model and of the FFT2 method proposed to automatically identify eddies. The sensitivity of the detection method is analyzed to subsequently apply it in parametric analyses.

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.

A novel thermoacoustically-driven liquid metal magnetohydrodynamic generator for future space power applications

The generation of electricity in space is a major issue for space exploration, and among the viable alternatives, nuclear power systems appear to present a particularly suitable solution, especially for deep space exploration. Recent developments in thermoacoustic engine and liquid metal magnetohydrodynamic (LMMHD) generator technologies have shown that thermoacoustically-driven LMMHD generators are a promising thermal-to-electrical converter option for space nuclear reactors. In order to improve the power density and capacity of current thermoacoustically-driven LMMHD generators, a novel three-stage looped thermoacoustically-driven LMMHD generator is proposed and investigated in this work. A numerical model of the integrated system including a lumped parameter sub-model for the LMMHD generator is developed and validated. Using this model, the effect of key geometric and operating parameters on the operation and performance of the proposed system are investigated numerically, and acoustic field distributions are presented. The results indicate that when the heat source and sink temperatures are 900 K and 300 K, respectively, a thermal-to-electric efficiency of 27.7% with a total electric power of 4750 W can be obtained at a load factor of 0.92. This work provides guidance for the design of similar systems and contributes to the development of a new thermal-to-electrical conversion technology for space applications.

On the use of CFD to obtain head loss coefficients in hydraulic systems and its application to liquid metal MHD flows in nuclear fusion reactor blankets

When an incompressible fluid flows through a contraction in a conduit, the increase in the kinetic energy of the fluid is accompanied by a pressure drop. This pressure drop is not to be assimilated with head loss. If downstream the fluid encounters an expansion in the conduit, the energy conversion will take place in the opposite way. Therefore, when a geometrical singularity is analysed to assess its contribution to the pumping power requirements of the system, the whole mechanical energy transfer of the fluid in the singularity has to be taken into account, and not only the pressure variation. The first part of the present work establishes a method to obtain head loss coefficients in geometric singularities of hydrodynamic circuits using the results of computational fluid dynamics (CFD) calculations. These coefficients are of interest when modelling the whole system with a 1D system code, for instance. In the second part of the article, the method is applied to a more complex case, involving magnetohydrodynamic (MHD) phenomena. Thus, a prototypical channel singularity in a liquid metal circuit subject to a magnetic field is analysed. The layout is representative of a case that could be found in the liquid metal blankets to be used in nuclear fusion reactors. The influence of the MHD phenomena is studied and the differences with a purely hydrodynamic case are pointed out. The MHD analyses have been done in the Marconi High Performance Computing facility, using 48 cores, each case needing between one and two weeks to complete.

Verification and Validation of COMSOL Magnetohydrodynamic Models for Liquid Metal Breeding Blankets Technologies

Liquid metal breeding blankets are extensively studied in nuclear fusion. In the main proposed systems, the Water Cooled Lithium Lead (WCLL) and the Dual Coolant Lithium Lead (DCLL), the liquid metal flows under an intense transverse magnetic field, for which a magnetohydrodynamic (MHD) effect is produced. The result is the alteration of all the flow features and the increase in the pressure drops. Although the latter issue can be evaluated with system models, 3D MHD codes are of extreme importance both in the design phase and for safety analyses. To test the reliability of COMSOL Multiphysics for the development of MHD models, a method for verification and validation of magnetohydrodynamic codes is followed. The benchmark problems solved regard steady state, fully developed flows in rectangular ducts, non-isothermal flows, flow in a spatially varying transverse magnetic field and two different unsteady turbulent problems, quasi-two-dimensional MHD turbulent flow and 3D turbulent MHD flow entering a magnetic obstacle. The computed results show good agreement with the reference solutions for all the addressed problems, suggesting that COMSOL can be used as software to study liquid metal MHD problems under the flow regimes typical of fusion power reactors.

Magnetohydrodynamic velocity and pressure drop in manifolds of a WCLL TBM

In the frame of the EUROfusion breeding blanket research activities, the water cooled lead lithium (WCLL) blanket is considered as reference liquid metal blanket design to be tested in ITER and to be used in a DEMO reactor. The design of breeding blankets represents a major challenge for fusion reactor engineering because of the performance requirements and the severe operating conditions in terms of heat load and neutron flux. Liquid metal alloys such as lead-lithium, PbLi, are considered as breeder material, due to their lithium content required for tritium production, and as heat transfer medium because of their large thermal conductivity and the possibility to be operated at high temperature. On the other hand, the motion of the electrically conducting breeder in the plasma-confining magnetic field induces electric currents and generates strong electromagnetic forces that modify significantly the velocity distribution in the blanket compared to hydrodynamic conditions and increase pressure losses. Magnetohydrodynamic (MHD) pressure drops have to be carefully quantified, since excessive values can jeopardize the feasibility of the considered blanket concept. The present work investigates numerically liquid metal MHD flows in manifolds of a WCLL test blanket module. Velocity and pressure distributions are analyzed.

A novelsolar-driven power/desalination system based on a liquid metal magnetohydrodynamic unit

In comparison with the conventional turbine-based power unit, liquid metal magnetohydrodynamic (LMMHD) power units have simple structure since they have no moving parts, and hence they are highly reliable systems with low maintenance cost. However, a LMMHD power unit needs a high-temperature prime mover which is conventionally provided by fossil fuels. To cover up this shortcoming, the present study aims at proposing a new LMMHD cogeneration plant for power and fresh water supply, using a concentrated solar power (CSP) system. The devised integrated system was designed and evaluated from the first- and second-laws of thermodynamics perspectives. The results indicated that the proposed cogeneration plant can produce total power of 38.3 kW and fresh water of 881.6 L/h, in which in this case the energy utilization factor (EUF) and total exergy efficiency were computed 99.69% and 5.57%, respectively. Considering the LMMHD power unit as the reference system, it is found that when the idea of cogeneration was employed, the energy and exergy efficiencies of the LMMHD power system can be improved by 150.2% and 4.3%, respectively. Among all constituents, the receiver has the highest exergy destruction of 256.9 kW, followed by the heliostat with exergy destruction of 228.2 kW. The results of parametric study indicated that the energy and exergy efficiencies of the proposed cogeneration system can be improved by increasing the humidifier/dehumidifier effectiveness or decreasing the mass flow rate of the second MHD loop.

Influence of the orientation of the magnetic field on the MHD unaligned isothermal flows

This study numerically explores 3D liquid-metal (LM) magnetohydrodynamic (MHD) flows moving through a channel with dis-alignment subjected to a uniform magnetic field. The channel is composed of an upstream duct, a junction duct and a downstream duct. Considered are the cases with different orientations of external magnetic field. The results show that the maximum and minimum pressure drop are observed in cases with the external magnetic field perpendicular and parallel to the stream-wise direction in the junction duct, respectively. Higher velocities can be seen in the fluid region around the sharp edges of the two right-angle turning sections. Moreover, the inclined magnetic field yields complicated flow structures and electromagnetic characteristics of the aforementioned LM flows. In association with the needs for the minimization of the pressure drop for the optimal design of LM flow in a fusion blanket, the present study on the LM MHD flows in a channel with dis-alignment is performed, which may be helpful to the design of the flow through a fusion blanket.

Influences of the bend angle and the locally different electrical conductivity of the structural walls on the flow characteristics of the LM MHD splitting flows

The minimization of the magnetohydrodynamic (MHD) pressure drop is a hot issue in the design of liquid-metal blanket. The current study aims to investigate the flow distribution and MHD pressure drop in a 3-D magnetohydrodynamic (MHD) flow of a liquid-metal (LM) through a conduit with one inlet and two outlet ducts. CFX software is used to investigate the features of the MHD splitting flows. Here, cases with various electrical conductivity of the structural walls in the two outlet ducts are analyzed with the consideration of different bend angles, which yields various flow distributions in different cases, and also causes mass imbalance in the two outlet ducts. The results show that, when the bend angle is fixed, the mass imbalance increases with the growing discrepancy of the electrical conductivity between the two outlet ducts. Meanwhile, the uneven mass flow rate therein also depends on the variation of the bend angle. Furthermore, this study found that locally different electrical conductivity of the structural walls in T-junction ducts can reduce the MHD pressure drop significantly, and 70% of the pressure drop is reduced within the Test Blanket Module (TBM) when the discrepancy in the electrical conductivity of the two outlet ducts is the largest.

Experimental and numerical investigation of gas-liquid metal two-phase flow pumping

Adapting an efficient method for pumping gas-liquid metal two-phase flow can make liquid metal magnetohydrodynamics (LMMHD) power generation systems economically feasible. An experimental facility was developed using a dual injection gas-lift pump to provide the circulation needed for the liquid metal in the LMMHD system. Also, a numerical simulation was performed in order to understand the behaviour of the two-phase flow through the gas-lift pump and be able to optimize the pump design for the best operational condition of the LMMHD system. In addition, the data were compared to a 1-D drift flux model that predicted the performance of the pump within the loop. The numerical results were found to be in an agreement with the experiments within ±25% which is considered adequate for optimization and design purposes of a two-phase flow system. The dual injection design was evaluated, and the axial injection mode was found to be more efficient in providing the required pumping than radial mode. Moreover, the flow visualization of the two-phase flow patterns seen in the pump riser was captured both experimentally and numerically indicating a swirl-like motion that can potentially be used to enhance heat transfer in the actual operational setting. Also, the optimum condition for pumping was found to occur at a slug-like flow structure in the riser.

Liquid metal MHD flow influence on heat transfer phenomena in fusion reactor blankets

DEMO reactor is projected as a technological demonstrator and a component test reactor for several key systems such as the breeding blanket. The dual coolant lithium lead (DCLL) is one promising breeding blanket candidate that performs the functions of shielding, part of the cooling and breeding with the same liquid metal fluid. PbLi is also in charge of transporting the bred tritium to the Tritium Extraction System. Structural cooling of the blanket walls is performed by pressurized helium flow which also transfers heat to the balance of plant. Keeping a proper distribution of thermal loads between the two coolants is a key aspect to reach a plant high efficiency. The thermal power transferred from the liquid metal to the helium channels depends on the flow regime. While liquid metal flows under the magnetic field, the magnetohydrodynamic (MHD) phenomena will occur, influencing the flow regime and the velocity profile. The heat is volumetrically deposited in the liquid metal coolant due to the interaction between the highly energetic neutrons originated in the plasma and the coolant. The heat deposition profile is roughly exponential causing temperature differences across the same channel cross section. Temperature gradients induce buoyant effects altering the expected MHD profile and affecting the heat transfer rate from the liquid metal to the surrounding walls cooled with helium. It is of utmost interest to identify the effects of the main flow parameters on the heat transfer coefficient under such complex conditions. The work studies computationally a domain consisting of a liquid region and a surrounding solid region electrically and thermally coupled using latest EU DCLL design characteristics. A fully developed flow is assumed where buoyancy is modelled under the Boussinesq approximation. Joule effect has been considered negligible compared to the neutron deposition. The influence of the buoyant MHD phenomena to the heat transfer coefficient is analysed parametrically with a CFD solver implemented in OpenFOAM®. The parameters investigated here are the Reynolds, Hartmann and Grashof numbers, the Grashof ratio, and the wall conductance ratio (cw).