THERMAL CONDUCTION IN GRANULAR MEDIA: FROM INTERFACE, TOPOLOGY TO EFFECTIVE PROPERTY

Granular media are particulate substance featured by their unique discrete structure, which are commonly seen in daily life and extensively used in industry. Differently from those continuum materials whose properties are mostly defined by their chemical formulas and status, granular media further require clarification about the effects of their topology on their properties. Therefore, effective properties are used to emphasise this distinction in measuring and describing granular media. In this study, we focus on the effective thermal conductivity of generalised gas-filled granular media, which is highly related to energy technologies and advanced fabrication processes. With particularly concentration on the topological transitions in vibrated granular media, how the topology influences the effective thermal conductivity is explored. Aiming at revealing the mechanisms governing the heat conduction of granular media, a bottom-up consequence scheme is employed in this study by decomposing the macroscale phenomena into grain-scale interactions. Under such scheme, the objectives of this work are further divided into (1) investigating the heat conduction mechanisms at inter-grain contact interfaces and (2) integrating the thermal contact units based on the topology of granular media. To accomplish the former investigation, the finite element analysis is implemented to model the gas-solid thermal interaction contributed by the Smoluschowski effect that gives rise to coupling dependence of gas pressure and grain size. With a systematic study on the heat conduction of individual units, the later objective is tackled by introducing the grain-scale thermal interaction into discrete element methods. With the combination of these cross-scale studies, a numerical framework is established. Furthermore, the thermal measurement system based on transient plane source techniques is applied to experimentally characterise correlations between the effective thermal conductivity and external mechanical loading. These experimental results as well as available literature data are used to quantitatively verify the proposed numerical method. In order to figure out the topological influence on the effective thermal conductivity, the discrete element method is further employed to examine the mesoscale behaviours of agitated granular media. The grain-scale structural characterisation unravels the topological transitions in vibration. Granular crystallisation, a process prompting the disorder-to-order transition, is identified as the major phenomenon and its boundary dependent mechanisms are iii proposed. Moreover, the topological influence on the effective thermal conductivity can be assessed with respect to the crystallisation, i.e., the degree of structure order, of granular media. With the fundamental research in this thesis on the heat conduction mechanisms and the granular crystallisation, the effective thermal conductivity is studied in a full range of scales from individual grains to bulk media. In summary, we demonstrate and experimentally validate a multiscale framework to solve the thermal problems in granular media that can also be applied to other effective conduction properties

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. Towards improved heat management, thermal transport in granular media requires an improved fundamental understanding. 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 Smoluschowski 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 methods 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 approach to explain transport phenomena, which can be applied in properties modification for granular media.Comment: 11 figures, 29 page

Modes of wall induced granular crystallisation in vibrational packing

Granular crystallisation is an important phenomenon whereby ordered packing structures form in granular matter under vibration. However, compared with the well-developed principles of crystallisation at the atomic scale, crystallisation in granular matter remains relatively poorly understood. To investigate this behaviour further and bridge the fields of granular matter and materials science, we simulated mono-disperse spheres confined in cylindrical containers to study their structural dynamics during vibration. By applying adequate vibration, disorder-to-order transitions were induced. Such transitions were characterised at the particle scale through bond orientation order parameters. As a result, emergent crystallisation was indicated by the enhancement of the local order of individual particles and the number of ordered particles. The observed heterogeneous crystallisation was characterised by the evolution of the spatial distributions via coarse-graining the order index. Crystalline regimes epitaxially grew from templates formed near the container walls during vibration, here termed the wall effect. By varying the geometrical dimensions of cylindrical containers, the obtained crystallised structures were found to differ at the cylindrical wall zone and the planar bottom wall zone. The formed packing structures were quantitatively compared to X-ray tomography results using again these order parameters. The findings here provide a microscopic perspective for developing laws governing structural dynamics in granular matter

Pore-scale Modelling of Gravity-driven Drainage in Disordered Porous Media

Multiphase flow through a porous medium involves complex interactions between gravity, wettability and capillarity during drainage process. In contrast to these factors, the effect of pore distribution on liquid retention is less understood. In particular, the quantitative correlation between the fluid displacement and level of disorder has not yet been established. In this work, we employ direct numerical simulation by solving the Navier-Stokes equations and using volume of fluid method to track the liquid-liquid interface during drainage in disordered porous media. The disorder of pore configuration is characterized by an improved index to capture small microstructural perturbation, which is pivotal for fluid displacement in porous media. Then, we focus on the residual volume and morphological characteristics of saturated zones after drainage and compare the effect of disorder under different wettability (i.e., the contact angle) and gravity (characterized by a modified Bond number) conditions. Pore-scale simulations reveal that the highly-disordered porous medium is favourable to improve liquid retention and provide various morphologies of entrapped saturated zones. Furthermore, the disorder index has a positive correlation to the characteristic curve index (n) in van Genuchten equation, controlling the shape of the retention characteristic curves. It is expected that the findings will benefit to a broad range of industrial applications involving drainage processes in porous media, e.g., drying, carbon sequestration, and underground water remediation.Comment: 22 pages, 8 figure

Modes de cristallisation granulaire induite par la paroi dans un compactage vibratoire

International audienceGranular crystallisation is an important phenomenon whereby ordered packing structures form in granular matter under vibration. However, compared with the well-developed principles of crystallisation at the atomic scale, crystallisation in granular matter remains relatively poorly understood. To investigate this behaviour further and bridge the fields of granular matter and materials science, we simulated mono-disperse spheres confined in cylindrical containers to study their structural dynamics during vibration. By applying adequate vibration, disorder-to-order transitions were induced. Such transitions were characterised at the particle scale through bond orientation order parameters. As a result, emergent crystallisation was indicated by the enhancement of the local order of individual particles and the number of ordered particles. The observed heterogeneous crystallisation was characterised by the evolution of the spatial distributions via coarse-graining the order index. Crystalline regimes epitaxially grew from templates formed near the container walls during vibration, here termed the wall effect. By varying the geometrical dimensions of cylindrical containers, the obtained crystallised structures were found to differ at the cylindrical wall zone and the planar bottom wall zone. The formed packing structures were quantitatively compared to X-ray tomography results using again these order parameters. The findings here provide a microscopic perspective for developing laws governing structural dynamics in granular matter.https://doi.org/10.1007/s10035-019-0876-8La cristallisation granulaire est un phénomène important dans lequel des structures de garnissage ordonnées se forment dans une matière granulaire sous vibration. Cependant, comparée aux principes bien développés de la cristallisation à l'échelle atomique, la cristallisation dans la matière granulaire reste relativement mal comprise. Pour étudier plus avant ce comportement et relier les domaines de la matière granulaire et de la science des matériaux, nous avons simulé des sphères mono-dispersées confinées dans des conteneurs cylindriques afin d'étudier leur dynamique structurelle lors de la vibration. En appliquant une vibration adéquate, des transitions désordre à ordre ont été induites. Ces transitions ont été caractérisées à l'échelle des particules par le biais de paramètres d'ordre d'orientation de liaison. En conséquence, la cristallisation émergente était indiquée par l'amélioration de l'ordre local des particules individuelles et du nombre de particules ordonnées. La cristallisation hétérogène observée a été caractérisée par l'évolution des distributions spatiales via la granulation grossière de l'indice d'ordre. Les régimes cristallins se développaient par épitaxie à partir de gabarits formés près des parois du conteneur lors de vibrations, appelé ici effet de paroi. En faisant varier les dimensions géométriques des récipients cylindriques, il s'est avéré que les structures cristallisées obtenues différaient au niveau de la zone de paroi cylindrique et de la zone de paroi de fond plane. Les structures de garnissage formées ont été comparées quantitativement aux résultats de tomographie à rayons X en utilisant à nouveau ces paramètres d'ordre. Les résultats présentés ici fournissent une perspective microscopique pour l'élaboration de lois régissant la dynamique structurelle dans la matière granulaire.https://doi.org/10.1007/s10035-019-0876-

Measurement of effective thermal conductivity of compacted granular media by the transient plane source technique

To successfully realise industrial applications handling granular media, especially those involving heating and cooling processes, the temperature fields must be properly evaluated according to the accurate thermal properties of the media. The knowledge the effective thermal conductivity is regarded as one of the fundamental aspects. However, due to the complicated relations between the effective thermal conductivity and the heterogeneity and complexity in the structures and composition of the granular media, the quantitative prediction of the conductivity is challenging. Therefore, experimental investigation of the effective thermal conductivity becomes desired and this can provide first-hand data for industrial reference and serve as the benchmark for the theoretical analysis. In this study, the transient plane source technique is employed to investigate the effective thermal conductivity of compacted granular beds by the application of the commercially available Hot Disk system. The granular beds of different particle size ranges are characterised under different mechanical loading conditions by different sensors. Experimental results are discussed and suggestion to achieve reliable experimental designs is provided

Les effets de la structure de la garniture sur la conductivité thermique effective des milieux granulaires: étude à l'échelle du grain

International audienceStructural characteristics are considered to be the dominant factors in determining the effective properties of granular media, particularly in the scope of transport phenomena. Towards improved heat management, thermal transport in granular media requires an improved fundamental understanding. 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 Smoluschowski 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 methods 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 approach to explain transport phenomena, which can be applied in properties modification for granular media.https://doi.org/10.1016/j.ijthermalsci.2019.04.028Les caractéristiques structurelles sont considérées comme les facteurs dominants dans la détermination des propriétés effectives des milieux granulaires, en particulier dans le cadre des phénomènes de transport. Pour une meilleure gestion de la chaleur, le transport thermique dans les milieux granulaires nécessite une compréhension fondamentale améliorée. Dans cette étude, les effets de la structure de la garniture sur le transfert de chaleur dans les milieux granulaires sont évalués à l’échelle macro et granulaire. À l'échelle du grain, un modèle de transfert de chaleur à couplage gaz-solide est adapté en une méthode à éléments discrets pour simuler ce phénomène de transport. Le cadre numérique est validé par des données expérimentales obtenues à l'aide d'une technique de source plane, et l'effet Smoluschowski de la phase gazeuse est capturé par cette extension. En considérant les garnissages de grains de SiO2 sphériques avec une phase d’hélium interstitielle, on étudie l’ordonnancement induit par la vibration dans un milieu granulaire, à l’aide des méthodes de simulation développées ici, afin de déterminer comment les transitions désordonnées de la structure d’emballage améliorent la conductivité thermique. Il a été démontré que le transport thermique à l’échelle des grains était influencé par la configuration des grains individuels au voisinage local. La formation d'une structure d'emballage ordonnée améliore le transport thermique global et local. Cette étude fournit une approche structurelle pour expliquer les phénomènes de transport, qui peut être appliquée à la modification des propriétés de milieux granulaires.https://doi.org/10.1016/j.ijthermalsci.2019.04.02