Having proper correlations for hydrodynamic forces is essential for successful CFD-DEM simulations of a fluidized bed. For spherical particles in a fluidized bed, efficient correlations for predicting the drag force, including the crowding effect caused by surrounding particles, are already available and well tested. However, for elongated particles, next to the drag force, the lift force, and hydrodynamic torque also gain importance. In this work, we apply recently developed multi-particle correlations for drag, lift and torque in CFD-DEM simulations of a fluidized bed with spherocylindrical particles of aspect ratio 4 and compare them to simulations with widely used single-particle correlations for elongated particles. Simulation results are compared with previous magnetic particle tracking experimental results. We show that multi-particle correlations improve the prediction of particle orientation and vertical velocity. We also show the importance of including hydrodynamic torque.

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In biomass processing fluidized beds are used to process granular materials where particles typically possess elongated shapes. However, for simplicity, in computer simulations particles are often considered spherical, even though elongated particles experience more complex particle–particle interactions as well as different hydrodynamic forces. The exact effect of these more complex interactions in dense fluidized suspensions is still not well understood. In this study we use the magnetic particle tracking technique to compare the fluidization behavior of spherical particles to that of elongated particles. We found a considerable difference between fluidization behavior of spherical versus elongated particles in the time-averaged particle velocity field as well as in the time-averaged particle rotational velocity profile. Moreover, we studied the effect of fluid velocity and the particle's aspect ratio on the particle's preferred orientation in different parts of the bed, which provides new insight in the fluidization behavior of elongated particles.

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The possibility to offer good intermixing between particles is one of the main properties that make fluidized beds such an important industrial appliance. In this work, we use CFD-DEM simulations to compare mixing characteristics of spherical (AR-1) to elongated spherocylindrical particles (AR-4) of aspect ratio In simulation of AR-4 particles, single-particle and multi-particle correlations for hydrodynamic forces are tested. The results show that elongated particles have more vigorous intermixing and lower mixing times compared to spherical particles. Multi-particle correlations have a slight effect on particle mixing, and they increase the difference between AR-1 and AR-4 particles at higher gas velocities. Including hydrodynamic lift force and torque in the case of AR-4 particles leads to more vigorous mixing and lower mixing times.

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In many industrial applications, particles used in fluidized bed clearly deviate from ideal spheres. This leads to an increasing need for better understanding and developing better simulation models for fluidization of non-spherical particles. So far, the literature is quite scarce when it comes to experimental results which can be used for validation of numerical models. Also, the exact difference in fluidization behavior between spherical and elongated particles in dense fluidizing conditions is not well understood. In this work, we apply X-ray tomography to compare the fluidization behavior of a bed of a Geldart D-type spherical particles of aspect ratio 4 to that of volume equivalent spherocylindrical particles for different gas velocities. Even though the beds of both spherical and elongated particles are operating in the slugging regime, due their size and high bed height to width ratio, we see clear differences in their fluidization behavior. Our results indicate that the bed of elongated particles is slugging less than the one with spherical particles. This is indicated by a lower average bubble size in the case of elongated particles, together with a higher bubble rise velocity. The bed of elongated particles has a considerably higher distribution of small and medium bubbles. The slug waiting time distribution and slug frequency distribution indicate that a bed of elongated particles periodically switches between slugging and turbulent fluidization, unlike the bed of spherical particles which remains in the constant slugging regime.

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For the past century, fluidized beds have been standard equipment in many branches of industry. In most applications they are used to manipulate granular and powder-like materials, whose particles can roughly be approximated as spheres. Therefore, numerical models and investigations have focused mainly on fluidized beds with spherical particles. Recent decades witnessed an increase in the use of fluidized beds in biomass processing. Unlike other materials typically used in fluidized beds, biomass is characterized by relatively large and elongated particles. For the sake of simplicity, numerical models for simulating fluidization of elongated particles have so far neglected a lot of specifics that can occur during this process and even applied the same models and conclusions that were developed for fluidization of spherical particles. The goal of this thesis is to define what is necessary for performing physically correct Computational Fluid Dynamics - Discrete Element Model (CFD-DEM) simulations of elongated particles fluidization. This thesis emphasizes the difference in fluidization between spherical and elongated particles and looks into ways to include specific particle and fluid interactions related to elongated particles into numerical (CFD-DEM) model. Results fromCFD-DEMsimulationswere validated using two experimental techniques, magnetic particle tracking (MPT) and X-ray tomography (XRT). This thesis is part of larger project of multi-scale modeling of fluidized beds with elongated particles and is focusing on the middle scale, bridging fully resolved, direct numerical simulations (DNS) with large scale, two fluid model (TFM) or multi-phase - particle in cell (MP-PIC) models, capable of simulating industrial sized fluidized beds. This thesis first looks in to the effect of including shape induced lift force and hydrodynamic torque, which were so far neglected in CFD-DEM simulations of elongated particles. It is shown that including lift force and hydrodynamic torque leads to considerable changes in the particle vertical velocity and particle preferred orientation in the fluidized bed. Looking into the mixing characteristics, as one of the most important parameters of fluidized beds, also considerable differenceswere found. Further differences in fluidization behaviour of spherical and elongated particles, as well as the effect of increasing particle aspect ratio, were shown experimentally, using MPT. Clear differences between spherical and elongated particles were found concerning the particle velocity and rotational velocity distributions. The effect of increasing particle aspect ratio and gas inlet velocity on fluidization of elongated particles was shown. Using XRT, the difference in bubbling and slugging fluidization between spherical and elongated particles was shown. In the end, the effect of newly developed multi-particle correlations for hydrodynamic forces and torque was tested, and it is concluded that they can improve the accuracy of simulations of dense fluidized beds containing elongated particles. The findings of this thesis clearly show that the models and assumptions developed for fluidization of spherical particles cannot simply be transferred to the fluidization of elongated particles. The results presented here give a new insight in the fluidization of elongated particles. They are also valuable for validation and development of larger scale models capable of simulating industrial size fluidized bed with elongated particles. @en

In this paper, we present a number of key numerical methods that can be used to study elongated particles in fluid flows, with a specific emphasis on fluidised beds. Fluidised beds are frequently used for the production of biofuels, bioenergy, and other products from biomass particles, which often have an approximate elongated shape. This raises numerous issues in a numerical approach such as particle-particle contact detection and the accurate description of the various hydrodynamic forces, such as drag, lift, and torque, that elongated particles experience when moving in a fluid flow. The modelling is further complicated by a separation of length scales where industrial flow structures that can extend for many metres evolve subject to solid-solid and solid-fluid interactions at the millimetre scale. As a result, it is impossible to simulate both length scales using the same numerical approach, and a multiscale approach is necessary. First, we outline the direct numerical simulation (DNS) approach that may be employed to estimate hydrodynamic force closures for elongated particles in a fluid flow. We then describe the key aspects of a CFD-DEM approach, which can be used to simulate laboratory scale fluidisation processes, that must be addressed to study elongated particles. Finally, we briefly consider how current industrial-scale models, which concretely assume particle sphericity, could be adapted for the simulation of large collections of elongated particles subject to fluidisation.

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The aim of many industrial processes is to manipulate solid particle aggregates within gas suspensions. Prime examples of such processes include fluidised bed reactors, cyclone separators, and dust collectors. In recent years, fluidised bed reactors have been used in the gasification of biomass particles. When fluidised, these particles are subject to various hydrodynamic forces such as drag, lift and torque due to interactions with the fluid. Computational approaches, which can be used to replicate laboratory and industrial scale processes, offer a crucial method for the study of reactor design and for the formulation of optimal operating procedures. Until now, many computer models have assumed particles to be spherical whereas, in reality, biomass feedstocks typically consist of non-spherical particles. While lift and torque are of minimal importance for spherical particles, non-spherical particles experience varying lift force and torque conditions, depending on particle orientation relative to the direction of the fluid velocity. In this study, we present a numerical investigation on the effect of different lift force and torque correlations on fluidised spherocylindrical particles. We find that lift force has a significant influence on particle velocities parallel to the direction of gravity. On the other hand, particle orientation is dependent on hydrodynamic torque. Results from this numerical study provide new insight with regards to the dynamics of non-spherical particles that can be of paramount importance for industrial processes involving non-spherical particles.

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Particle aggregates are frequently processed in industrial apparatus such as pneumatic conveyors, bed gasifiers and fluidised bed reactors. In numerical models of these processes, particles have traditionally been represented as spheres, thus limiting complexities associated with drag or lift forces. However, spherical particles are not representative of the entities encountered in real systems. For example, non-spherical biomass particles of varying aspect ratios are used in the production of biomass fuels. Thus far, there is limited literature in relation to hydrodynamic forces experienced by non-spherical particles under fluidised conditions. In fluidised beds, particles will experience varying lift force conditions dependent on the orientation of the particle relative to the direction of the fluid velocity. In this study, we investigate numerically the effect of differing lift force coefficient correlations on spherocylindrical particles of varying aspect ratios. We employ correlations derived from previous simulations on non-spherical particles [1] and aerofoil dynamics [2] in simulations. Particle interactions are modelled using the Open Source engine CFDEM, which uses the OpenFOAM computational fluid dynamics (CFD) solver to describe the fluid component and LIGGGHTS to implement discrete element method (DEM) calculations [3]. We investigate the importance of lift forces on non-spherical particles under dense fluidised conditions and compare results to the case of spherical particles where lift forces are often neglected.@en

In industry we encounter many processes that rely on equipment in which particles are suspended by a gas flow, such as pneumatic conveyors, CFB gasifiers, combustors and fluidized bed reactors. In numerical models of these processes, particles have traditionally been represented as spheres, thus limiting complexities associated with drag or lift forces. However, spherical particles are not representative of the entities encountered in real systems. For example, non-spherical biomass particles of varying aspect ratios are used in the production of biomass fuels. Thus far literature is quite limited when it comes to hydrodynamic forces experienced by non-spherical particles under fluidized conditions. In fluidized beds, particles will experience varying lift force conditions dependent on the orientation of the particle relative to the direction of the flow. In this study, we investigate numerically the effect of different lift force coefficient correlations on fluidization of spherocylindrical particles. We employ correlations derived from previous simulations on non-spherical particles and aerofoil dynamics in simulations. We also look into the effect of the Di Felice approximation, in this case applied to take into account the effect of surrounding particles on the lift force. Particle interactions are modelled using the Open Source engine CFDEM, which uses the OpenFOAM computational fluid dynamics (CFD) solver to describe the fluid component and LIGGGHTS to implement discrete element method (DEM) calculations. We investigate the importance of lift forces on non-spherical particles under dense fluidised conditions and compare results to the case of spherical particles where lift forces are often neglected.@en