497 research outputs found

    A study of the charge structure and energy utilisation in a Stirred Media Detritor using DEM-SPH

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    The Stirred Media Detritor, (SMD), is a grinding device used for fine and ultra-fine grinding applications in mineral processing. The SMD has a vertically orientated shell that supports a shaft, with protruding impeller arms for agitating the charge. There is currently limited understanding of charge structure and motion in the SMD, particularly the interaction of the media and the slurry. Additionally, the number of arms and their arrangement on the shaft, are important aspects of the impeller that determine flow, energy consumption and grinding efficiency. Impeller geometry choices affect these characteristics of the process. This work focuses on studying the flow of grinding media and slurry for the industrial scale SMD 1100- E. This information is used to explore charge dynamics and energy utilisation in the SMD. To investigate the effect of impeller arm configuration on the operational behaviour of the SMD, the commercially available impeller configuration of the industrial scale SMD 335-E is used as the base case. Mill charge dynamics, transport and mixing, patterns of energy absorption on the mill surfaces are examined for the base case and compared to three different impeller arm arrangements. A two-way transient coupled Discrete Element Method (DEM) and Smoothed Particle Hydrodynamics model is used to achieve this. The ceramic grinding media is represented by the DEM component of the model, which is fully resolved, while the slurry (water and fine particles) is represented by the Smoothed Particle Hydrodynamics (SPH) model. The focus is on steady state operation therefore discharge from and feed into the mill are omitted. A nominal media size of 8 mm is used. The rotational action of the impeller forces the charge to the mill wall creating vortex centred on the mill shaft. The vortex is conical with a large diameter at the bottom, which decrease towards the bottom of the mill. Abrasion is found to be the dominant breakage mechanism in the SMD. Mixing behaviour is complex with media transfer past layers of impeller arms being influenced by the fall distance of media between impeller arm encounters

    Non-Equilibrium Dynamics of Driven and Confined Colloidal Systems

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    [eng] In this thesis, I study the behavior of confined colloidal particles in aqueous suspension driven through an optical potential. For this purpose, I use micro-meter polystyrene particles, which I confine in the optical potential created with a system of optical tweezers. With the help of an Acousto Optical Deflector (AOD), which varies the laser position at a high frequency, I can create multiple quasi-simultaneous optical traps. This way, I can easily manipulate the particles and define the desired experimental conditions for the potential. I record videos of the particles' dynamics using optical microscopy. Thus, I obtain position information over time, which allows me to extract the necessary data to analyze the mechanisms that develop during forced transport. The results presented in this thesis expose the importance of Hydrodynamic Interactions (HI) when the transport of particles occurs due to a fluid drag. In addition, different situations are compared, including the change in the relative particle size concerning the separation between potential wells. In addition, I present a study on the emergence of solitons propagating in the opposite direction to the drag force. This situation, which appears when the experimental system is overcrowded, presents a mechanism where the transport dynamics accelerate, increasing the systems' efficiency.[spa] En esta tesis estudio el comportamiento de partículas coloidales en suspensión acuosa cuando son forzadas a moverse a través de un potencial óptico. Para ello, utilizo partículas micro- métricas de poliestireno, las cuales confino en el potencial óptico creado con un sistema de pinzas ópticas. Con la ayuda de un Deflector Acusto Óptico (AOD), el cual varía la posición del láser a una alta frecuencia, puedo crear múltiples trampas ópticas de manera casi simultánea. Esto me permite manipular las partículas con facilidad y definir las condiciones experimentales deseadas para el potencial. A través de microscopía óptica, obtengo imágenes en vídeo de la dinámica de las partículas. Así, obtengo la información de la posición a lo largo del tiempo, lo que me permite extraer los datos necesarios para analizar los mecanismos que se desarrollan durante el transporte forzado. Los resultados expuestos en esta tesis ponen de manifiesto la importancia de las Interacciones Hidrodinámicas (HI) en el transporte de partículas cuando son arrastradas por el fluido. Además, se comparan diferentes situaciones en las que se incluye el cambio en el tamaño relativo de las partículas respecto a la separación entre pozos de potencial. Además, presento un estudio sobre la aparición de solitones que se propagan en la dirección contraria en la que se ejerce la fuerza de arrastre. Esta situación, que aparece al sobrepoblar el sistema experimental, presenta un mecanismo en el que el transporte de materia se acelera, lo que incrementa la eficiencia

    CFD modeling of biomass combustion and gasification in fluidized bed reactors

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    Biomass is an environmentally friendly renewable energy source and carbon-neutral fuel alternative. Direct combustion/gasification of biomass in the dense particle-fluid system is an important pathway to biomass energy utilization. To efficiently utilize biomass for energy conversion, a full understanding of biomass thermal conversion in lab/industrial-scale equipment is essential. This thesis aims to gain a deeper understanding of the physical and chemical mechanisms of biomass combustion/gasification in fluidized bed (FB) furnaces using computational fluid dynamics (CFD) simulations. A three-dimensional reactive CFD model based on the Eulerian-Lagrangian method is developed to investigate the hydrodynamics, heat transfer, and gasification/combustion characteristics of biomass in multiple-scale FB furnaces. The CFD model considered here is based on the multi-phase particle-in-cell (MP-PIC) collision model and the coarse grain method (CGM). CGM is computationally efficient; however, it can cause numerical instability if the clustered parcels pass through small computational cells, resulting in the over-loading of solid particles in the cells. To address this issue, a distribution kernel method (DKM) is proposed. This method is to spread the solid volume and source terms of the parcel to the surrounding domain. The numerical stiffness problem caused by the CGM clustering can be remedied using DKM. Validation of the model is performed using experimental data from various lab-scale reactors. The validated model is employed to investigate further the heat transfer and biomass combustion/gasification process. Biomass pyrolysis produces a large variety of species in the products, which poses great challenges to the modeling of biomass gasification. A conventional single-step pyrolysis model is widely employed in FB simulations due to its low computational cost. However, the prediction of pyrolysis products of this model under varying operating temperatures needs to be improved. To address this issue, an empirical pyrolysis model based on element conservation law is developed. The empirical parameters are based on a number of experiments from the literature. The simulation results agree well with the experimental data under differentoperating conditions. The pyrolysis model improves the sensitivity of gasification product yields to operating temperature. Furthermore, the mixture characteristics of the biomass and sand particles and the effect of the operating conditions on the yields of gasification products are analyzed. The validated CFD model is employed to investigate the fluidization, combustion, and emission processes in industrial-scale FB furnaces. A major challenge in the CFD simulation of industrial-scale FB furnaces is the enormous computational time and memory required to track quadrillions of particles in the systems. The CFD model coupling MP-PIC and CGM greatly reduces the computational cost, and the DKM overcomes the unavoidable particle overloading issue due to the refined mesh in complex geometry. The CFD predictions agree well with onsite temperature experiments in the furnace. The CFD results are used to understand the granular flow and the impact of operating conditions on the physical and chemical processes in biomass FB-fired furnaces

    Colloids with perception-dependent motility: Dynamics and structure of rotating aggregates and directed swarms

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    In this thesis we focus on two-dimensional systems of colloids governed by Brownian dynamics that are able to sense their neighbors via a visual-type of perception, then they can switch their motility between passive and active depending on a given perception parameter. Our setup corresponds to experiments performed in Bechinger's lab in Konstanz University, where they have considered cases of quorum-sensing (isotropic perception) and visual-type of perception (anisotropic perception). Here we study the case when the perception is both anisotropic and also misaligned with respect to the self-propulsion orientation vector. The purpose of this thesis is to characterize the emergence of collective behaviors in this model, as well as the dynamics and structural changes of the system. We provide novel strategies where the interplay between perception and motility of the agents allows them to self-organize into rotating aggregates and directed swarms. Our study sheds light in the understanding of active automatons with adaptable collective states, and can be implemented for example in macroscopic swarms of robots, or microscopic colloids activated by light. In chapter 2 we introduce the ingredients necessary to perform particle-based numerical simulations, like the integration method, interaction forces, boundary conditions, and optimization techniques. We also briefly comment on the organization and design of the Brownian dynamics code we developed to obtain results shown in this thesis. In chapter 3, we consider systems of colloids with discontinuous motility and misaligned visual perception. We explain how this type of interaction generically leads to aggregation and rotation of cohesive structures. Then, we characterize the resulting dynamics for different system parameters. In chapter 4 we characterize different types of circular structures that emerge in this model, as a function of the perception threshold and misalignment angle. We also derive analytical expressions from conservation equations corresponding to a solid-body rotation of a continuum aggregate driven by activity at the interface. We find an agreement between theory and numerical results for the density, size, and angular velocity of the aggregates as a function of the system parameters. In chapter 5 we consider a binary mixture of particles with different misalignment angle. Under given conditions, we find the striking case where the system aggregates, self-sorts into species subdomains which counter-rotate leading to a self-propulsion of the overall system. We characterize this process by means of dynamic parameters and their averages in steady state. We find cases where the directed swarms can either dilute or remain robust, or where the aggregate is species homogeneous and its center of mass describes random motion. We also study the swarms shape and how it can change for varying misalignment angle. In chapter 6 we study cases when the mixture is non-equimolar. In this case the system self-organizes into swarms describing helical trajectories. We also show an example of an externally guided system, where we dynamically change the misalignment angle of the particles, leading to a swarm performing run-and-turn motion

    Coupled point neutron kinetics and thermal-hydraulics models of transient nuclear criticality excursions in wetted fissile uranium dioxide (UO2) powders

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    This thesis describes a phenomenologically based mathematical and computational methodology for the simulation of a postulated transient nuclear criticality excursion initiated by the incursion of water, from a fire-sprinkler system, into a bed of dry UO2 powder. These potentially hazardous multi-phase dispersed particulate systems may form as a result of a fire or explosion in a nuclear fuel fabrication facility. The models proposed in this thesis aim to support nuclear criticality safety analysis and assessment. In addition, the development of these models aims to support emergency planning and preparedness. The point neutron kinetics equations are coupled to phenomenological models of water infiltration, sedimentation, fluidisation, nuclear thermal hydraulics, radiolysis and boiling, through the use of multivariate reactivity feedback components. The spatial and temporal solution of this set of equations enables the modelling of postulated transient nuclear criticality excursions in highly dispersed three-phase particulate systems of UO2 powder. The results indicate that there is the potential for large positive reactivities to be added to a UO2 powder system as pores become filled with water. Generally, thermal expansion and Doppler broadening are insufficient to control the transient, leading to significant radiolysis and boiling on the surface of the UO2 powder particles. Radiolytic gas and steam bubble induced fluidisation and sedimentation significantly alters the characteristics of a transient nuclear criticality excursion and should be carefully considered. Research has also been undertaken examining transient nuclear criticality excursions in weak intrinsic neutron source UO2 powder systems by solving the forward probability balance equation and using a Gamma probability distribution function to estimate mean wait-time probability distributions. Significant variations in the potential initial peak power are predicted for highly enriched, wetted, UO2 powders as a function of the stochastic behaviour associated with criticality excursions in low neutron population systems.Open Acces

    Simulating the impact of wind and radiation on snow dynamics across linear disturbances in boreal forests

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    Boreal forests are Earth‘s second largest forest biome, covering an area of 12.0–14.7 million km2. Winters are typically long, cold and dry, creating ideal conditions for sustaining snowpacks throughout this period. The spatial and temporal distribution of snow cover in boreal forest environments plays a crucial role in hydrological and ecological processes at local and regional scales. The dynamics of snow accumulation and melt reflect the interplay between such processes as the wind-driven redistribution of snow and the net energy balance at the snowpack surface. The presence of a forest canopy exerts a modifying effect on these processes; snow on the forest floor is typically sheltered from wind and direct solar radiation, whilst receiving enhanced longwave radiation from the surrounding canopy. However, the balance between these effects can be complex, particularly in the case of discontinuous forest canopies where clearings allow wind and light to penetrate down to the underlying snowpack. Understanding how the interplay between environmental factors drives spatially and temporally varying patterns of snow cover across forest edges is of particular importance and relevance in boreal regions where rates of climate change are high and forest fragmentation is increasing. In this thesis I explore how linear clearings, such as roads and tracks, may alter patterns of wind flow and incoming radiation, and consequently modify the dynamics of snow accumulation and melt across discontinuous forest canopies. This investigation uses field data collected during this research project and observations from long-running monitoring at the Arctic Research Centre of the Finnish Meteorological Institute (FMI-ARC), in northern Finland. Using a Met Office wind flow model (BLASIUS) I simulate patterns of wind flow across forest discontinuities and show that the clearing width is a key influence on these dynamics. There is less drag on the wind flow within the clearing relative to the forest canopy. Sufficient distance (approx. 100 m) is required for the wind flowing across the gap to adjust to this change in the boundary conditions. A region of reversed flow as the wind enters the gap was found for all gap widths. Within the 100 m gap, the wind speed then increases with distance across the gap until it is slowed by the presence of the downwind canopy edge. Narrow gaps (<30 m wide) have less impact on sub-canopy wind speeds as there is insufficient distance for the flow to fully adjust to the change in conditions. The influence of a forest gap on sub-canopy wind dynamics becomes negligible for very narrow gaps (approx. 3 m wide). Canopy height and density have a second-order effect on the wind flow dynamics across the gap. Increasing the canopy height accentuates the region of reversed flow, and faster flowing air above the canopy is not drawn down as deeply down into the gap. A denser forest canopy results in greater vertical velocities at the canopy edges. Reducing the canopy density results in greater overall wind speeds across the model domain. The wind flow model was coupled to a forest snow model (a simplified version of FSM2) using a linear scaling relationship observed between above- and below- canopy wind speeds, and similarly for surface friction velocity. This coupled model was used to explore the interactions between forest canopy, wind, and the surface energy balance on snow accumulation and melt across a range of forest gap scenarios. The simulated snow mass accumulates sooner and at a greater rate within the gap compared to under the forest canopy. In wider gaps (>50 m) there is an asymmetric pattern of snowmelt, with melt occurring sooner towards the exposed downwind edge of the gap and persisting for up to a month longer towards the sheltered upwind edge. Snow melts more evenly across narrower gaps. In the simulated scenarios I show that turbulent heat fluxes drive the spatial pattern of snowmelt across a gap. Simulated snowmelt patterns in the wider gaps correlate with sub-canopy wind speeds across the gap; higher wind speeds lead to greater fluxes of sensible heat and therefore earlier onset of melting and higher melt rates. Radiative fluxes provide a secondary influence on snow melt and have most impact on the melt dynamics towards the upwind edge where wind speeds are lowest. The canopy density influences the amount of sub-canopy snow accumulation and modulates the snowmelt patterns set by the energy fluxes across the gap. In the widest gap (100 m), increasing the LAI leads to later snow disappearance. The findings from this thesis demonstrate that introducing clearings into boreal forests produces a significant change in the local wind flow dynamics and snow hydrology. The width of the clearing is important, with canopy characteristics providing a secondary, modulating effect. The modifications to wind and snow induced by the presence of a gap in the canopy are greatest in the widest gaps. However, even narrow canopy gaps may have a significant impact if their orientation aligns closely with the prevailing wind direction. While the effects of an individual gap may be localised, they could become regionally significant in areas of boreal forest undergoing extensive fragmentation

    Coarse-grained modelling of blood cell mechanics

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    This thesis concerns development of mechanically realistic in silico representations of human blood cells using coarse-grained molecular dynamics (CGMD), ultimately building a new model for a lymphocyte-class white blood cell (WBC). This development is approached successively, evaluated through simulation of experimental testing methods common to past in vitro studies on blood cell mechanics. Considering both their biophysical simplicity and the extensive associated literature, the red blood cell (RBC) is first considered. As a foundation, I thus used the CGMD RBC model of Fu et al. [Lennard-Jones type pair-potential method for coarse-grained lipid bilayer membrane simulations in LAMMPS, Fu et al., Comput. Phys. Commun., 210, 193-203 (2017)]. Chapter 3 establishes implementation of this model, and in silico implementations of the three chosen testing methods. In doing so, the first quantitative assessment of the "miniature cell" approach is conducted - being the down-scaling of the physical cell size to make feasible simulation times, as was done in the original article presenting the model. The RBC model is then used as a foundation from which to develop a new whole-cell WBC lymphocyte model. This is approached sequentially. Firstly (Chapter 4), the morphology and mechanics relevant to the existing RBC model are adapted to those of a lymphocyte. As such, a quasi-spherical morphology is induced, and elastic membrane-associated parameters brought in line with the literature on isolated lymphocytes in vitro. A semi-rigid nucleus is then added to the cell interior, again parameterised to produce elastic properties consistent with the literature. These developments produce a cell having macroscopic mechanical properties much more consistent with a WBC, with an "optimal" parameterisation established. After the membrane and nucleus, the entity most influential to the mechanics of nucleated cells (such as WBC) is their complex intracellular actin-based cytoskeleton (CSK). Therefore, Chapter 5 attempts to represent such a system within our new lymphocyte model. This is approached in three successive stages, assessed against recognised CSK mechanical properties, in particular those also common to soft glassy materials. As such, a novel CSK representation is developed, inspired as a discretisation of soft glassy rheology (SGR). It is proposed that the resulting system has characteristics comparable to having undergone a glass-like transition, as relatable to a real CSK. Therefore, the resulting lymphocyte model may lay a foundation for future development towards mechanically accurate representations of other cells - in particular, a circulating tumour cell
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