The pebble-bed type reactor is one of the mainstreams for high-temperature gas-cooled reactor (HTGR), whose core is a packed bed composed of numerous spherical pebbles. In this work, discrete element ...model (DEM) is used to simulate the pebble flows in a thin pebble bed, and the packing peaks, top angles, trajectory and velocity deviations, retention time, and ratios are analyzed in detail. The effects of the pebble-pebble friction coefficient and pebble-wall friction coefficient, including their difference on the pebble flows are studied based on the trajectory analysis. The friction may bring higher packing peak, sharper top angle, larger diffusions of trajectory and velocity, as well as larger retention time, and ratio. However, if the pebble-pebble friction and pebble-wall friction are different, the effect of them on the pebble flow is quite different. The details and mechanisms are appropriately analyzed to explain the influence of pebble-pebble and pebble-wall friction on pebble flows. The results are helpful for better understanding of the pebble flow behavior under varied frictions.
•A thermal DEM code is proposed for the evaluation of the effective thermal conductivity of ceramic breeder pebble beds.•The Smoluchowski effect was implemented to consider the influence of the gas ...pressure.•Numerical results perfectly resemble the experimental data reported in literature.
In this paper, a Discrete Element Method (DEM) for the evaluation of the effective thermal conductivity of pebble beds in fusion blankets is presented. Pebble beds are multiphase materials in which both the solid and the gas phase filling the voids between particles coexist. The effective thermal conductivity of a pebble bed depends on the thermal properties of the two phases as well as on the system properties (e.g. gas pressure, temperature etc.). In particular, the pressure of the system is a key parameter for the heat transfer in a packed granular assembly since the thermal conductivity of a confined gas decreases with decreasing pressure (known as Smoluchowski effect). In this work, the influence of the gas pressure on the effective thermal conductivity in the Knudsen domain was implemented, to our knowledge, for the first time in a DEM code. The heat transfer mechanisms implemented are: when two particles touch each other the conduction through the contact area between them and, in any case, the conduction through the gas phase in the gap between neighbouring solid particles, may they be touching or not. These mechanisms are expected to dominate the heat transfer in a fusion breeder packed bed. Parametric studies were carried out to investigate the influence of the solid and gas materials, temperature, pressure and compression state. Numerical results were compared with existing experimental literature data and recent experiments carried out at Karlsruhe Institute of Technology (KIT).
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•First report on improved electrocatalytic activity of Ni nano-pebbles decorated Fe3C nanosheets.•Ni@FC/SPCE electrode for direct alcohol fuel cell application (alcohol = methanol, ...ethanol, and ethylene glycol).•Exhibited high mass activity with good anti-CO tolerance and cyclic stability.•Excellent capacity retention after 10000 s of chronoamperometry.
In this work, we report the solid-state reaction-assisted synthesis of the iron-based MAX phase compound (Fe3AlC2 MAX; where M- metal, A- aluminum, and X- carbide), which is solvothermal etched and exfoliated to yield 2D iron (Fe)-MXene. The nanosheet-like morphology of the Fe-MXene is confirmed via TEM and SEM analysis. The 2D Fe-MXene nanosheets are further decorated with nickel (Ni) nano pebbles through the simple electrodeposition technique on a screen-printed carbon electrode (Ni@FC/SPCE). The electrode displays a peak mass activity of 525.5, 742.8, and 910.2 mA/mg for methanol oxidation reaction (MOR), ethanol oxidation reaction (EOR), and ethylene–glycol oxidation reaction (EgOR), respectively. The Ni nano pebbles have excellent electrocatalytic activity owing to their multiple oxidation states (Ni2+/Ni3+) and catalytic active sites which enhance the alcohol oxidation reaction while the metallic sites of the Fe3C2 nanosheets contribute to high electrical conductivity and stability of the electrode. The as-fabricated Ni@FC/SPCE electrode exhibits high stability with outstanding capacity retention of ∼ 88%, ∼95%, and ∼ 75% for MOR, EOR, and EgOR after 10000 s of chronoamperometry analysis. This performance of the Ni@FC/SPCE as a non-noble, cost-effective, and stable nano-catalyst with excellent anti-carbon monoxide (anti-CO) poisoning properties proves it an ideal platform for DAFC applications.
The Xe-100 is a high temperature, helium-cooled, graphite moderated, pebble bed reactor (HTGR) featuring a 6-times through, multi-pass (MEDUL = MEhrfachDUrchLauf) fuelling scheme. It is designed for ...delivering heat in the form of superheated steam at 565 °C and 16.5 MPa. This steam can be used to generate electricity, provide process heat for petrochemical application, or for cogeneration applications, such as the treatment of contaminated water resources, whilst simultaneously generating electricity.
The design is characterized as a Generation IV reactor, with a core that cannot melt and featuring safety characteristics that will eliminate the need at any time to evacuate or displace the public, or that will cause unallowable contamination of the land. The technology is based on proven technology that has demonstrated these safety claims both experimentally and in commercial deployment. The foundation of proof ensures high levels of design and technical readiness.
In the overview presented below a parametric comparison is offered of a 165 MWth design against that of X-energy’s 200 MWth reference Xe-100 design. The excess reactivity in both cases is shown to be limited by the continuous fuelling regime, whilst the core geometry and selected core power density enables passive heat removal. In this way the desired intrinsic safety design features of a typical GEN IV design are guaranteed. It is shown that even in the event of a depressurized loss of forced coolant (DLOFC) design basis event, no significant amount of radiological material will be released. A larger margin of tolerance is afforded in the 165 MWth design case, as expected.
The coupled core neutronic and thermo-fluid dynamics design for the Xe-100 is performed with the VSOP-A and MGT systems of codes. For the equilibrium core, VSOP results demonstrate that the spherical fuel powers (maximum 3.0 kW) and operational temperatures (<1000 °C) fall well within the envelope of the design criteria. Adequate reactivity control and long-term, cold shutdown are provided by two separate, independently actuated systems, while the overall negative reactivity temperature coefficient is illustrated over the total operational range.
•Studied transport behaviour of helium in spherical packed rectangular pebble bed reactor.•Adopted DEM-CFD and porous medium approaches considering radiation heat transfer.•Heat transfer ...characteristics obtained by both the approaches is almost same.•Evaluated the effect of location of inlet and outlet on the flow uniformity in canister.•Pebble bed with diagonally opposite inlet and outlet has higher tritium sweeping efficiency.
The thermal-hydraulic characteristics of helium as a purge gas for the removal of the tritium from a spherical packed rectangular pebble bed reactor are studied in this paper using the DEM-CFDand porous medium approaches. Numerical models are developed for the two approaches considering radiation heat transfer, which are validated with the published experimental results. Further, a comparison is made between the porous medium and the DEM-CFD approaches for predicting the thermal and hydraulic characteristics of the helium. The heat transfer behaviour predicted by both the approaches is approximately the same. The DEM-CFD approach can capture the detailed and localized flow behaviour in the presence of pebble, whereas the porous medium approach can predict the average flow characteristics. Further, the effect of the location of the inlet and outlet on the uniformity of the fluid flow in the canister is studied by considering different combinations of inlet and outlet.It is found that the pebble bed with diagonally opposite inlet and outlet configuration has a higher tritium sweeping efficiency as compared to the existing design of the canister with inlet and outlet at the same face.
•The effective thermal conductivity (keff) of lithium meta-titanate (Li2TiO3) pebble beds is an important parameter for the design and analysis of TBM in ITER.•The keff of Li2TiO3 pebble beds under ...stagnant helium gas have been determined numerically using different uniform packing structures and random close packing (RCP) structures.•keff of Li2TiO3 pebble beds with different packing fractions have been reported as function of temperature; keff of the RCP Li2TiO3 pebble bed is compared with reported experimental results.•The numerically-determined keff of the RCP Li2TiO3 pebble bed agrees reasonably well with the experimental data and Zehner-Schlunder correlation.
The effective thermal conductivity (keff) of lithium meta-titanate (Li2TiO3) pebble beds is an important parameter for the design and analysis of IN LLCB TBM (Indian Lead Lithium Ceramic Breeder Test Blanket Module). The keff of Li2TiO3 pebble beds under stagnant helium gas have been determined numerically using different uniform packing structures and random close packing (RCP) structures. The uniform packing structures of Li2TiO3 pebble bed are modelled by using the simple cubic, body centered cubic and face centered cubic arrangement. The packing structure of the RCP bed of Li2TiO3 pebbles is generated with the discrete element method (DEM) code. keff of Li2TiO3 pebble beds with different packing fractions have been reported as function of temperature; keff of the RCP Li2TiO3 pebble bed is compared with reported experimental results from literature. The numerically determined keff of the Li2TiO3 pebble bed agrees reasonably well with the experimental data.
•Neural network architecture search model is applied to view factor regression of the particle-scale thermal radiation.•Deep neural networks with 3346 feasible architectures are evaluated by the ...HyperBand algorithm to find the optimal solution.•Trained neural architecture search model gives a good prediction of the macroscopic radiative properties in the pebble bed.
In the CFD-DEM simulation of the many dense particulate systems, particle-scale thermal radiation is an important heat transfer mode under high temperatures. In this work, neural network architecture search model with different layer connection matrix is applied to the view factor regression of the thermal radiation in dense particulate systems. Deep neural networks with 3346 feasible architectures are evaluated by the HyperBand algorithm to find the local optimal solution. Neural architecture search model trained by the big data of the view factor gives a good prediction of the macroscopic radiative properties and it is in general agreement with the empirical correlations and experimental data. The particle–wall radiation decreases strongly with the distance and the maximum interaction depth is about 2.0 times the sphere diameter. The trained deep neural network model provides an efficient data-driven closure to discuss the thermal radiation of the particle–particle and particle–wall interactions in particle bed.
The discrete element method (DEM) is used to study the thermal effects of pebble failure in an ensemble of lithium ceramic spheres. Some pebbles crushing in a large system is unavoidable and this ...study provides correlations between the extent of pebble failure and the reduction in effective thermal conductivity of the bed. In the model, we homogeneously induced failure and applied nuclear heating until dynamic and thermal steady-state. Conduction between pebbles and from pebbles to the boundary is the only mode of heat transfer presently modeled. The effective thermal conductivity was found to decrease rapidly as a function of the percent of failed pebbles in the bed. It was found that the dominant contributor to the reduction was the drop in inter-particle forces as pebbles fail; implying the extent of failure induced may not occur in real pebble beds. The results are meant to assist designers in the fusion energy community who are planning to use packed beds of ceramic pebbles. The evolution away from experimentally measured thermomechanical properties as pebbles fail is necessary for proper operation of fusion reactors.
Solar air heaters are the most cost-effective method of converting solar energy into heat and are used for room heating, crop drying, and other industrial uses. However, they suffer from poor thermal ...efficiency. The leading cause behind the poor performance of solar air heaters is heat losses from its different parts. Researchers have used various innovative methods to improve solar air heaters thermal performance by reducing heat losses using energy storage material. The present work demonstrates the state-of-the-art review of different solar air heaters loaded with sensible heat storage materials. This investigation has found that integrating sensible heat storage systems such as pebbles, sand, metal chips, oil and gravels with solar air heaters effectively reduces heat losses and increases thermal efficiency. This study revealed that Therminol-55 gave better efficiency than engine oil. For blacked pebble stones, free convection solar air heater provided better efficiency than forced convection. The cement gave better thermal efficiency than concrete. Gravels integrated with iron chips showed more efficiency than used alone. Pure iron chips contributed maximum efficiency compared to other metal chips. The desert sand furnished better efficiency compared to different types of sand. The overall best performing sensible heat storage material is found as a mixture of desert sand and granular carbon having the highest thermal efficiency of 80.05%; however, the lowest performance is demonstrated by cement with 9.5% of thermal efficiency.
What are the different factors affecting the thermal performance of solar air heaters? How can heat loss from solar air heaters be reduced? Is there any need for sensible heat storage material for solar air heaters? What are the different types of energy storage materials? What are the significant differences between them? Replies to these questions are discovered in subsequent sub-section.
•In a forced convection, the maximum thermal efficiency was recorded for the combination of granular carbon and sand.•In the free convection, the best sensible heat storage material was Iron chips.•The overall best performing material is a mixture of desert sand and granular carbon.
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•Cylinder polydisperse spherical ceramic pebble bed was simulated via DEM.•Parametrical effects on static packing characteristics were presented.•Neither the coefficient of friction ...nor restitution affected the oscillation width.•Peak of force and coordination number distribution left-shifted with increasing friction.•Consideration of effect of difference between coefficients of friction and restitution was essential.
The effects of the coefficient of friction and coefficient of restitution on the static packing characteristics of a polydisperse spherical pebble bed are numerically investigated using the discrete element method. Several important static packing characteristics under different coefficients of friction and restitution are presented and discussed. The results show that the coefficients of friction and restitution impose opposite effects on the packing heights and global packing factor. Neither the coefficient of friction nor restitution affected the oscillation width of the wall, whereas their effects are primarily reflected in the oscillation amplitude of the radial local packing factor and the axial local packing factor distribution at the top of the pebble bed. In both the contact force distribution and coordination number distribution, a left-shifted phenomenon appearing as the coefficient of friction occurred, and only the magnitude of the maximum frequency is affected when the coefficient of restitution changed from 0.1 to 0.9. In all simulation cases, the effects of the coefficients of friction and restitution are similar to that of cross-impact.
