Effect Of Packing Structure Research Articles

Effect of packing structure on anisotropic effective thermal conductivity of thin ceramic pebble bed

Helium cooled solid breeder blanket as an important blanket candidate of the Tokamak fusion reactor uses ceramic pebble bed for tritium breeding. Considering the poor effective thermal conductivity of the ceramic breeder pebble bed, thin structure of tritium breeder pebble bed is usually adopted in the blanket design. The container wall has a great influence on the thin pebble bed packing structure, especially for the assembly of mono-sized particles, and thin pebble bed will appear anisotropic effective thermal conductivity phenomenon. In this paper, thin ceramic pebble beds composed of 1 mm diameter Li4SiO4 particles are generated by the EDEM 2.7. The effective thermal conductivity of different thickness pebble beds in the three-dimensional directions are analyzed by three-dimensional thermal network method. It is observed that thin Li4SiO4 pebble bed showing anisotropic effective thermal conductivity under the practical design size. Normally, the effective thermal conductivity along the bed vertical direction is higher than the horizontal direction due to the gravity effect. As the thickness increases from 10 mm to 40 mm, the effective thermal conductivity of the pebble bed gradually increases.

Effect of packing structure on mechanical properties of the pebble beds and the probability of particle crushing

Abstract The tritium ceramic breeder pebble bed, used for tritium breeding and in nuclear fusion reactor, is a key constituent of helium cooled solid breeder blankets. The mechanical properties of the pebble beds, along with the integrity of the particles are very important for the extraction of tritium and safe operation of the solid blanket. In this paper, analysis is carried out on the effect of packing structure, with varying bed thicknesses and particle smoothness, on mechanical properties and the probability of particle crushing. Pebble beds are randomly packed by a single type of Li 4 SiO 4 particle, 1 mm in diameter. It is found that when the thickness of the Li 4 SiO 4 pebble bed (ranging from 15 mm to 25 mm) becomes thinner, or the coefficient of friction (ranging from 0.1 to 0.6) increases, the strain of the pebble bed and the probability of particle crushing increases under the same mechanical load. Pebble beds with different packing structures have the same probability distribution function of normalized contact forces. The results could serve to characterize the tritium pebble beds and to optimize the design of helium cooled solid breeder blankets.

Evaluation of effective thermal conductivity in random packed bed: Heat transfer through fluid voids and effect of packing structure

Effective thermal conductivity (ETC) is an important parameter in packed-bed systems and is affected significantly by packing structure. Heat flow through a packed bed can be divided into three parallel paths: the solid-fluid-solid path, the solid-solid path and the fluid path. The models to evaluate ETC in packed beds for the first two paths were previously developed. In the present work, a comprehensive model to consider the heat transfer through the voids of a randomly-packed bed is further developed using the Delaunay tessellation. The results show that heat conduction through the fluid-filled voids becomes significant when the solid-to-fluid conductivity ratio decreases to a certain level (<5). The effect of packing structure on the ETC of packed beds is then investigated based on the packing generated by the discrete element method (DEM). Four cases with different heat transfer mechanisms or paths are considered. The results indicate that the probability density distribution of heat flow is generally dependent on the packing structure, represented by porosity. With an increase in porosity, the heat conduction and radiation via the solid path decrease, while the heat conduction via the fluid-filled voids increases.

The effects of packing structure on the effective thermal conductivity of granular media: A grain scale investigation

Structural characteristics are considered to be the dominant factors in determining the effective properties of granular media, particularly in the scope of transport phenomena. Enhancing heat management requires an improved fundamental understanding of thermal transport in granular media. In this study, the effects of packing structure on heat transfer in granular media are evaluated at macro- and grain-scales. At the grain-scale, a gas-solid coupling heat transfer model is adapted into a discrete-element-method to simulate this transport phenomenon. The numerical framework is validated by experimental data obtained using a plane source technique, and the Smoluchowski effect of the gas phase is found to be captured by this extension. By considering packings of spherical SiO2 grains with an interstitial helium phase, vibration induced ordering in granular media is studied, using the simulation method developed here, to investigate how disorder-to-order transitions of packing structure enhance effective thermal conductivity. Grain-scale thermal transport is shown to be influenced by the local neighbourhood configuration of individual grains. The formation of an ordered packing structure enhances both global and local thermal transport. This study provides a structure-based approach to explain transport phenomena, which can be applied in properties modification for granular media.

Effect of Packing Structure on CO2 Capturing Process

The increasing concentration of carbon dioxide (CO2) in the atmosphere is a primary global environmental concern due to its detrimental impacts on climate change. A significant reduction in CO2 generation together with its capture and storage is an imperative need of the time. CO2 can be captured from power plants and other industries through various methods such as absorption, adsorption, membranes, physical and biological separation techniques. The most widely used systems are solvent based CO2 absorption method. The aim of this study was to analyze the effect of various random and structured packing materials in absorption column on CO2 removing efficiency. Aspen plus was used to develop the CO2 capture model for different packing materials with Monoethanolamine (MEA) solvent in order to optimize the system. It was found that the lowest re-boiler duty of 3,444 kJ/KgCO2 yield the highest rich CO2 loading of 0.475 (mole CO2/mole MEA) by using the BX type of structured packing having the highest surface area. The surface area of the different packing materials were inversely proportional to the temperature profiles along the column. Furthermore, the packing materials with higher surface areas yielded higher CO2 loading profiles and vice versa. The findings of this study and recommendation would help further research on optimization of solvent-based CO2 capturing technologies.

The effect of packing structure of powder particles on warping during sintering

In this study, aqueous alumina suspension consisting of constant solid loading was chosen as a model system in order to observe the effect of particle packing structure in green tape on sintering deformation. The specific particle packing structure in consolidated green tape was intentionally formed during tape casting, for instance, a non-uniform particle packing structure (Type-I) and a relatively uniform particle packing structure (Type-II). The development of deformation in sintered specimens was explainable from anisotropic packing structure. Alumina tapes with Type-I structure particularly showed warping which was transverse to the casting direction. Tapes with Type-II structure appeared with irregular distortion which was expected as a result of the localized non-uniform particle packing structure.

充填層の分離性能に及ぼす粒子充填構造及び表面処理の影響

充填層の分離性能に及ぼす粒子充填構造及び表面処理の影響

Effect of Packing Structure on Separation Performance of Packed Bed.

分割真空充填法による微粒子の充填カラムを作成し, そのカラム内を流体が透過する際の分離性能に及ぼす局所空隙率の影響を液体クロマトグラフィー装置を用いて測定した.また, カラム内を透過する流体の挙動をSIMPLE アルゴリズムを用いた数値計算によって求め, 両者を比較検討した.その結果, 実験で得られた理論段数からカラム内の充填構造をある程度推定できることが明らかになった.また, 理論段数はカラム半径方向の空隙率分布に最も大きく影響を受けるのに対し, 多段充填の際に生じる粒子境界多段充填面は理論段数の低下にほとんど影響を与えない.さらに, 分割多段充填操作及びタッピング操作を併用することによりカラム全体にわたって微粒子をより均一に充填でき, カラムの分離性能は向上するという知見を得た.