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Abstract As an important class of particle flows in industrial applications, hopper flows, particularly those using pebbles (diameters dp ~O(10-2) m), play a significant role in nuclear reactor engineering, e.g. HTGR and ADS reactors. However, the features and influencing factors of the binary mixture discharge have not yet been widely investigated. In this study, the discrete element method (DEM) simulation was adopted to analyze the discharge flow of binary mixtures consisting of ellipsoids and spheres in a hopper. After a model validation, the effects of particle aspect ratio (Ra, the ratio of the major axis to the minor axis) of ellipsoids and component ratio (Rn, the ratio of the ellipsoid number to the sphere number) of ellipsoids to spheres were analyzed. Flow patterns were visualized by colored pebble stripes according to pebbles' initial heights. Particle discharge flow rates were computed to examine their relations to particle aspect ratios and component ratios. The force structure and distributions of the binary mixtures were also explored. Results showed that pebble stripes followed quadratic function profiles. Adding ellipsoids was advantageous for particles discharging at lower particle aspect ratios (Ra≤2), while impedimental at large particle aspect ratios (Ra≥3). The discharge flow rate was inversely proportional to the particle aspect ratio at fixed component ratios, and linearly proportional to the 1/4th power of the component ratio at fixed particle aspect ratios. In addition, the discharge flow rate showed low sensitivity to the initial packing states of particles when the particle aspect ratio and component ratio were fixed.

Abstract The core of pebble bed type reactor (HTGR) is a packed bed composed of spherical pebbles (fuel element and graphite moderator). The restitution coefficient is an important parameter which is directly related to the flow of the core pebbles and affects the motion trajectory and stacking state of the fuel pebbles. Herein, Discrete Element Method (DEM) is used to simulate pebble flows within a thin pebble bed. The packing peaks, apex angles, trajectory and velocity deviations, residence time and residence ratios are analyzed in details. The influence of restitution coefficient on the motion characteristics of pebble flow are studied based on the trajectory of fuel pebble. A new evaluation criterion for the uniformity of pebble flows is put forward, and its influencing mechanisms are explored. The relationship between the restitution coefficient and pebble motion is proposed, which can help understand the flow uniformity of fuel pebbles in nuclear reactor core.

In conjunction with a TiO2 protective layer and FeNiCoOx electrocatalyst, a graphene/Si heterojunction photoanode is demonstrated as a new type of Si-based buried junction with high photovoltage for solar water oxidation.

Vanadium oxide (VOx) nanomaterials are promising candidates for energy storage devices, such as lithium‐ and sodium‐ion batteries and supercapacitors, in which many complicated structural designs and composite strategies are applied to harness the high theoretical capacity of these materials. Herein, a simple yet effective method to achieve improved performance of electrodes via tungsten doping in a green hydrothermal reaction is demonstrated. The evolution of three VOx phases (V2O5, VO2, and V6O13) during the synthesis of the VOx nanostructures is revealed by the systematic investigation of the reaction products. The dopants are critical for the formation of nanocrystalline structures. The as‐fabricated VOx is tested for lithium‐ion batteries, which shows that tungsten doping significantly improves the battery performance, including initial discharge capacity of the VOx (doped VOx = 615.2 ± 41.6 mAh g–1, undoped VOx = 377.9 ± 72.8 mAh g–1, and precursor V2O5 = 393.4 ± 74.0 mAh g–1), cycle stability, and rate performance. This research provides important insights into the understanding of the dopant‐induced phase tuning of VOx nanostructures for energy storage–related applications.

National and global mitigation scenarios consistent with 1.5°C require an early phase-out of coal in major coal-dependent countries, compared to standard technical and economic lifetimes. This appe...

Anaerobic digestion is recognized as a good and promising method for energy recovery from sewage sludge, but it is difficult to select a suitable process from various conventional and emerging technical options. In this study, five processes including mesophilic and thermophilic anaerobic digestion (CAD and TAD), mesophilic and thermophilic high-solids anaerobic digestion (HSAD and THSAD) and anaerobic digestion with thermal hydrolysis pretreatment (THPAD) are compared using life cycle environmental and economic assessment. Particularly, the uncertainty derived from variable sludge organic content and biogas production is analyzed. The results showed that energy output should be the most sensitive factor determining the assessment results. For common high-organic-content sludge, thermophilic processes like THSAD and TAD lead to the least environmental impact while THSAD and THPAD exhibit the best economic performance. Compare with CAD, THSAD have 44% less environmental impact and 118% higher net present value (NPV) for a project with treatment capability of 100 t dry solids per day. However, for low-organic-content sludge, high-solids processes like THSAD and HSAD are much better than the others mainly owing to their less consumption of thermal energy. Using this kind of feed sludge, THSAD can bring 40% less environmental burden and 31% more NPV than CAD.

Abstract Recent increases in vegetation cover, observed over much of the world, reflect increasing CO2 globally and warming in cold areas. However, the strength of the response to both CO2 and warming appears to be declining. Here we examine changes in vegetation cover on the Tibetan Plateau over the past 35 years. Although the climate trends are similar across the Plateau, drier regions have become greener by 0.31±0.14% yr−1 while wetter regions have become browner by 0.12±0.08% yr–1. This divergent response is predicted by a universal model of primary production accounting for optimal carbon allocation to leaves, subject to constraint by water availability. Rising CO2 stimulates production in both greening and browning areas; increased precipitation enhances growth in dry regions, but growth is reduced in wetter regions because warming increases below-ground allocation costs. The declining sensitivity of vegetation to climate change reflects a shift from water to energy limitation.

Abstract Moderate or Intense Low oxygen Dilution (MILD) combustion is a promising technology to meet the ever-stringent emission regulation while maintain high thermal efficiency. In this study, large eddy simulation (LES) in conjunction with transported probability density function (TPDF) method has been carried out for the first time to investigate the impact of reaction and diffusion timescales on the stabilization process of the jet-in-hot-coflow (JHC) CH4/H2 flame emulating MILD conditions. First it is demonstrated that the LES/TPDF simulations yield improved predictions of the species and temperature fields due to its capability in capturing finite-rate chemistry and resolving molecular transport at the filter scale. Then the impact of reaction and diffusion timescales on the stabilization process are investigated. It is found that the attenuation of chemical kinetics results in larger stabilization heights and unstable flame bases. More importantly the variation of stabilization height is found to be linearly proportional to that of auto-ignition delay time, illustrating the crucial importance of chemical kinetics during flame stabilization. The results show that the flame is initiated from the lean mixture away from the shear layer, which implies the importance of molecular transport during flame stabilization. Particle-level budget analysis further shows that the resolved molecular diffusion is important for flame base dynamics by contributing more than half of the overall conditional diffusion rate. Finally, a scaling rule for the characteristic flame stabilization time is proposed based on the auto-ignition delay time and characteristic time of diffusion, and it works reasonably well for all the cases considered. These findings shed light on the key physico-chemical mechanisms of the stabilization process for JHC flames under the MILD combustion mode. Moreover, the assessment on subgrid mixing and resolved molecular diffusion reveals that the simulation exhibits low sensitivity to the mixing model and mixing timescale while being highly sensitive to the resolved molecular diffusion, highlighting the key modelling aspects related to LES/TPDF simulation of this flame.