Particle-scale numerical simulation of the thermal behaviour of granular materials
Creators: name: Morimoto, Tokio
Publisher: name: Imperial College London
Year: 2022

description: Granular materials experience temperature changes in a variety of engineering applications and industrial processes including energy geostructures and packed-bed energy storage. Characterising heat transfer and the thermal expansion properties of granular materials is crucial for ensuring efficient and safe operation of many facilities and processes. As a consequence of their multi-phase nature, granular materials exhibit complex heat transfer and thermal expansion behaviours, which impose technical challenges when modelling such materials. This study conducts particle-scale numerical analyses to contribute towards a better understanding and ability to model the complex thermal behaviour of granular materials. In this study, the Discrete Element Method (DEM) is employed to consider the movement of particles and force transmission between particles. The project included development of an existing DEM software to simulate the thermal behaviour of granular materials by considering heat transfer through particles and their thermal expansion. Heat transfer between particles is modelled using a heat pipe network model, which idealizes the system as a network of pipes to simulate heat transfer from each particle to its neighbouring particles. The effect of the fluid between the particles on the effective thermal conductance of the heat pipe is considered and validated by solving the heat equation using the Finite Element Method. Thermal expansion of particles is considered by modifying the particle radius based on the coefficient of thermal expansion and temperature of the particles. The developed thermal DEM is employed to link contact-scale properties of granular materials to sample-scale thermal properties of granular materials. This study considers fluid flow through granular materials using a Pore Network Model (PNM), which considers a fluid pipe connecting adjacent pores. PNMs are known to be a fast method to simulate fluid flow through granular materials; however, hitherto the accuracy of PNMs has been limited due to difficulties in modelling the hydraulic conductance of fluid pipes. This study proposes a novel conductance model based on an analytical study which is validated using Computational Fluid Dynamics (CFD) simulations and which is measurably more accurate than previously proposed models. Building upon the new por-scale fluid flow model, a pore-scale particle-fluid conduction model, which works in a thermal PNM framework, is developed based on previous studies investigating wall-fluid conduction in fluid flow through pipes. CFD simulations for regular and random packings are conducted considering the energy transport equation to validate the proposed particle-fluid conduction model. The developed models will enable accurate and efficient particle-scale numerical simulations of the thermal behaviour of granular materials, which are useful for both industry and research in a variety of fields. Throughout this thesis the developed models are applied to advance understanding of heat transfer and thermal expansion phenomena in granular materials.