1,721,013 research outputs found
A pilot-scale study of dynamic response scenarios for the flexible operation of post-combustion CO2 capture
Full text linkAbstractThe ability to operate flexibly is critical for the future implementation of carbon capture and storage (CCS) in thermal power plants. A dynamic test campaign examines the response of a CO2 absorption/desorption pilot-scale plant to realistic changes in flue gas flow rates and steam supply, representative of the operation of a Natural gas combined cycle (NGCC) plant fitted with post-combustion capture. Five scenarios, demonstrating the operational flexibility that is likely to be encountered in an energy market with significant penetration from intermittent renewables, are presented, with 30% monoethanolamine (MEA) as the absorbing solvent. It complements a wider effort on dynamic modelling of these systems where a lack of dynamic plant data has been reported.The campaign focuses on analysing critical plant parameters of the response of the pilot plant to a gas turbine shutdown, a gas turbine startup and three enhanced operational flexibility scenarios, including two for power output maximisation and one for frequency response with a rapid increase of steam supply to the reboiler. The campaign also demonstrates the use of continuous in situ solvent lean loading measurement with the use of a novel online continuous liquid sensor.It confirms that no significant barriers to flexible operation of amine post-combustion capture are found, although there remains scope for the improvement of plant response. Solvent inventory and circulation times are found to have a significant effect on capture rate during certain dynamic operations. A large solvent inventory increases total circulation times, which can result in additional time being required for the plant to return to steady state following a perturbation. The plant is forced to operate with a non-optimal capture rate while the solvent loading at the absorber inlet stabilises is identified as a potential impact.Use of interim solvent storage and continuous online measurement of solvent CO2 loading, combined with comprehensive knowledge of liquid circulation times and potential mixing effects, are suggested as methods for improving plant response to dynamic operation, thereby increasing CCS plant flexibility
A Novel Immersed Boundary Method for Direct Numerical Simulations of Solid-Fluid Flows
Full text linkThis thesis has been submitted in fulfilment of the requirements for a postgraduate degree (e.g. PhD, MPhil, DClinPsychol) at the University of Edinburgh. Please note the following terms and conditions of use: • This work is protected by copyright and other intellectual property rights, which are retained by the thesis author, unless otherwise stated. • A copy can be downloaded for personal non-commercial research or study, without prior permission or charge. • This thesis cannot be reproduced or quoted extensively from without first obtaining permission in writing from the author. • The content must not be changed in any way or sold commercially in any format or medium without the formal permission of the author. • When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given
Design and development of catalytic hollow fibre-based reactors for methane emission abatement under extreme conditions
Full text linkNatural gas is playing a key role in the decarbonisation of our energy system. This transition fuel is the cleanest fossil fuel and its combustion produces 50% less carbon dioxide than coal and 30% less than oil per unit energy generated. However, natural gas is itself a greenhouse gas responsible for a third of all global warming since pre-industrial levels. Natural gas-powered processes such as electricity production, industrial operations, residential heating and natural gas-fuelled vehicles inevitably emit small concentrations of unburned methane. Due to their low concentration (i.e. 300-1500 ppmV), these residual methane emissions have long been overlooked. As a result, there is no direct legislation addressing them and, today, an effective technology to abate these emissions is unavailable.
The objective of this work is to develop a residual methane after-treatment technology that is cost-effective, long-lived and compact. The technology must be effective under real conditions and be resistant to sulfur poisoning at low temperature (i.e. 450°C), the main limitation faced by traditional precious metal-based catalysts. In order to evaluate the performance of the technology presented herein, reaction studies under real and extreme conditions as well as thorough characterisation of the materials have been fulfilled.
This work has successfully underpinned the first hollow fibre after-treatment for residual methane emission abatement. This technology combines a non-precious iron and chromium oxide catalyst with a ceramic hollow fibre-based support. The developed catalyst, which is a mixture of hematite, eskolaite and traces of iron (II) chromite, has shown high resistance to sulfur poisoning during the 1000 h reaction study under sulfur dioxide concentrations 20 to 100 times larger than normal levels. Even though the catalyst was initially deactivated, it retained a third of its original activity. The long-term activity of the catalyst has been attributed to the highly sulfur-resistant eskolaite phase. In comparison, state-of-the-art precious metal-based catalysts are deactivated within hours under similar conditions. In addition, when compared to packed bed reactors, hollow fibre reactors have proven to be 3 to 5 times more efficient in terms of methane catalytic oxidation per unit mass of catalyst. Moreover, the technology has proven to be effective under real operating cycles (i.e. 10 thermal shocks) making it the ideal candidate for the development of commercial residual methane emission after-treatments.
Finally, this technology has the potential to reduce an estimated 5.6 billion cubic metres annual residual methane emissions arising from electricity production, industrial and residential heating as well as transportation [1,2]. The global warming effect of these annual emissions over 20 years is equivalent to 470 billion cubic metres of carbon dioxide, a volume similar to the water volume of Lake Erie, one of the five Great Lakes of North America. Furthermore, industrialising this technology will pressure legislators to include residual methane emissions in future greenhouse gas regulations
Numerical study of microfluidic effects and red blood cell dynamics in 'deterministic lateral displacement' geometries
Full text linkThe last two decades have seen microfluidics gaining increasing interest from
the fields of medical diagnostics and bio-chemical processes, due to its immense
potential for point-of-care diagnostic applications. Since blood plays a
crucial role in many physiological and diagnostic processes, red blood cells
(RBCs) have been the focus of a large volume of microfluidics research. The
isolation of red blood cells and other blood components, based on the manifest
morphological characteristics, is required in many applications, e. g. flow
cytometry. The deterministic lateral displacement (DLD) is one such popular
microfluidic technique that has shown great promise toward cellular separations.
The DLD technique separates particles based on their hydrodynamic size.
It has been demonstrated for size-based separations down to unprecedented
size resolutions of ~ 10 nm. The DLD consists of a large number of obstacle pillars
placed in a microfluidic channel. The layout of these obstacles is such that
the obstacle array presents a fixed angle to the average fluid flow through the
microfluidic channel. Size-based separation comes about due to steric interaction
of particles with the pillars. Particles larger than a ‘critical’ size are forced
to move along the obstacle array incline. The larger particles, following the array
incline, are displaced perpendicular to the average flow direction, and are
said to be on the displacement mode. Particles smaller than this critical size
flow along the average fluid flow direction, zigzagging around the obstacles.
The trajectories followed by these smaller particles are classified as zigzag
mode. Micro-particles therefore follow different trajectory modes based on
their size, eventually leading to their spatial separation. The particles are separated
passively, i. e. other than the pressure drop needed to drive the fluid
flow through the DLD micro-channel, there is no need for any external forces
for particle sorting.
Numerous studies since the advent of the DLD have focussed on widening
the scope of applications covered by the technique. In this thesis, I take a more
physical approach, focussing on understanding the microhydrodynamics and
RBC dynamics within the DLD geometries. For these investigations, I have
used an in-house numerical solver that incorporates ingredients for fluid flow
solution, RBC membrane deformation, and an explicit coupling algorithm
between the two. The lattice Boltzmann method is used for obtaining a fluid
flow solution at low Reynolds numbers, and the finite element method is used
for computing the membrane energetics. The immersed boundary method
explicitly couples these two solutions with non-matching boundaries, at each
time step.
Firstly, I investigate subtle flow hydrodynamic effects through DLD obstacle
arrays. Here, fluid-only simulations uncover and map anisotropic flow permeability
of the obstacle arrays. The research reveals that if the unit cell of
the obstacle array geometrically forms a parallelogram, the array induces an
anisotropic pressure gradient normal to the average flow direction. Contrarily,
if the obstacle arrangement reflects a rotated square in its unit cell, anisotropy
is entirely absent. Such anisotropic pressure conditions in the DLD cause local
flow deviations and can lead to unintended particle motion arising from locally
varying critical separation size. I find that elevated levels of such anisotropy
are also brought about by pillar shape design and asymmetric array
gaps. Furthermore, strategies to minimise anisotropic flow effects are proposed.
The research on deformable RBC flow through the DLD tackles both single
and collective cell dynamics in these arrays. Single cell dynamics is studied
for special, non-cylindrical obstacle pillar shapes. In addition to the particle-obstacle
steric contact, dynamic RBC motion leads to effects that influence
cell trajectories in the DLD. Such effects are strongly tied to the interplay
between RBC deformability, dynamic motion (such as tumbling and tank-treading)
and the flow-field generated by the pillar shape. In certain cases,
wall-induced hydrodynamic cell migration becomes significant enough such
that the deformed tank-treading RBC undergoes displacement mode without
steric contact with the pillars. Here, migration velocity experienced by the
cells interacting with special pillar shapes causes a reversal of the phase-bifurcation
trend. The uncovering of this mechanism, opens the door for research
on novel DLD pillar designs that exploit wall-induced soft particle
migration.
Lastly, the research turns to collective RBC dynamics at high volume fractions,
in standard DLD arrays with cylindrical pillars. Here, I research the effect
of increasing cell volume fraction on the displacement and zigzag modes,
with the help of appropriate statistical measures. I find that the displacement
mode suffers a breakdown at higher volume fractions, while the zigzag mode
remains robust. This has important implications for cell separation applications
in the DLD, where smaller particles (e. g. platelets) need to be separated
from a dense background of RBCs and vice versa.
The investigations undertaken in this thesis identify subtle hydrodynamic
and particle effects in DLD arrays that explain previously unresolved particle
behaviour. This research should help improve the design and fabrication of
DLD devices, especially those targeted at improved separation and manipulation
of deformable RBCs
Investigations into CO₂ immiscible displacements from pore scale to core scale
Full text linkThe flow of immiscible displacement in porous media is of great importance in many applications of subsurface transports, such as geological sequestration of CO₂ in saline aquifers and depleted oil/gas fields, enhanced oil recovery (EOR) and removal of nonaqueous phase liquids (NAPLs) from aquifer sediments. In this work, the investigation of immiscible displacement focuses on drainage process, in which wetting fluid residing in porous media is displaced by a non-wetting fluid. Fingering led by unstable displacement is one of the major reasons limiting the efficiency in drainage process. For example, in CO₂ sequestration, fingering of low-viscosity gaseous or supercritical CO₂ limits their access to reservoir rocks and thus only a fraction of reservoir is occupied by injected CO₂. In enhanced oil recovery by CO₂ or water, less than half of original oil in place is recovered due to viscous fingers. The immiscible displacement process in CO₂ injection into the geological formations and reservoirs involves complex cross-scale flow from pore scale to core scale and eventually reservoir scale. Pore-scale flow can provide the most fundamental information of microscopic factors that have significant effects on macroscopic process, which cannot be captured by current continuum-scale models. Simulating a reservoir-scale flow with pore-scale modelling is not possible as it requires enormous computation. Instead, core-scale model is a brilliant option to characterize the flow dynamics of the reservoir to obtain upscaled flow functions (e.g., relative permeability and capillary pressure) for reservoir-scale models. A solid understanding of immiscible displacement from both pore-scale and core-scale perspectives is critically important in evaluating the impacts and efficiencies of displacement processes and providing accurate flow information for the reservoir model.
In this PhD project, immiscible displacements were investigated in the core sample and pore-scale network to evaluate the effect of key factors (namely fluid pressure, temperature, CO₂ phase and injection flowrate) on crucial flow parameters, such as differential pressure, flow patterns, phase saturation, relative permeability and displacement efficiency. The study sheds more light on the impact of capillary and viscous forces on multiphase flow characteristics and
explores the conditions where capillary or viscous forces dominate the flow. In addition, immiscible displacements in homogenous and physical rock networks are compared by numerical simulation.
CO₂-water core flooding experiments were carried out in a sandstone under different subsurface temperature and pressure conditions by considering the effects of CO₂ phase, pressure and CO₂ injection rate. The pressure fluctuation behaviours, capillary displacement pressure, relative permeability and water recovery are investigated for gas CO₂-water, supercritical CO₂-water and liquid CO₂-water displacements. Pressure fluctuations during the displacements are analysed by wavelet decomposition method. The pressure fluctuations are affected by the CO₂ phase but is almost independent of injection rate. The capillary displacement pressure is quantified by the pressure jump occurring at the beginning of CO₂ invasion into the core sample, which agrees well with the value estimated based on Kozeny model. The relative permeability is calculated by the JBN method, and it is found that liquid CO₂-water displacement has a higher relative permeability. Uncertainty of the relative permeability caused by the assumption of incompressible fluids is quantified.
Flow instability in immiscible displacements is predominately affected by the interactions between capillary and viscous forces, which can determine the displacement efficiency and CO₂ storage capacity. CO₂-water and CO₂-oil core flooding experiments with various viscosity ratios and capillary numbers were implemented to understand the interplay of capillary and viscous effects during the displacements by analysis of the pressure behaviours, and the experimental results are further demonstrated by macroscopic capillary number and the Log Nca-Log M phase diagram. In viscous-dominated displacement, differential pressure evidently depends on the injection rate and the pressure decline curve is fitted by a power function. The exponent of the function is found to be significantly larger at the crossover between capillary-dominated and viscous-dominated regions. In capillary-dominated displacement, the pressure profile is characterized by a pressure jump at the beginning and intermittent fluctuations during displacement. Further analysis by wavelet decomposition indicates a transition point existing
in standard deviation of pressure fluctuations when the displacement is transformed from capillary-dominated to viscous-dominated. The experiment results agree well with the macroscopic capillary number, which characterizes the interaction between capillary and viscous forces at a critical value of ᵐᵃᶜʳᵒ꜀ₐ~1.
For further understanding of the effects of capillary and viscous forces on the immiscible displacements occurring on the scale of individual pores and of how these processes determine the invasion patterns, CO₂-water and water-oil immiscible displacements were conducted in the pore-scale network with physical rock structures. Viscosity ratio and capillary number for the displacements were varied by employing different fluid pairs and injection flowrates. The pressure behaviour, flow pattern, invading phase saturation, trapped oil cluster size and macroscopic capillary number were investigated for displacements transforming from viscous-dominated to capillary-controlled. The behaviour of differential pressure is similar to that in core flooding experiments. A transition point exists on the relationship between standard deviation of pressure fluctuations and capillary number when displacement varies from viscous-dominated to capillary-controlled, which is also in accord with the results from core flooding experiments except for the value of the transition point. The non-wetting phase saturation shows great dependence on the capillary number. When capillary force dominates the flow, the non-wetting phase saturation hardly increases with increasing the amount of injection after breakthrough while it increases greatly when viscous force is the dominating force. With increasing capillary number, the trapped oil cluster size become smaller while the interfacial length between two fluids gets larger. The calculated value of macroscopic capillary number accurately predicts the viscous-dominated flow with ᵐᵃᶜʳᵒ꜀ₐ~1, but the value maybe underestimated for capillary-controlled flow due to the underestimation of trapped oil cluster size.
Another key issue considered in the project is the effect of pore structure on the immiscible displacement. A direct numerical simulation method is employed by COMSOL Multiphysics 5.6 software to simulate the immiscible displacement in two microfluidic networks, one with a
homogeneous structure and the other with a physical rock structure. The numerical model is first validated by immiscible displacement in a single capillary. The simulated results are in good agreement with the theoretical solution calculated based on Darcy-Weisbach equation. Then, flow dynamics, such as flow patterns, differential pressure and phase saturation were investigated in these two networks when the displacement is transformed from capillary-controlled to viscous-dominated. Early breakthrough resulted by viscous fingering is observed in both networks and more obviously in physical rock network. Capillary fingering is not clearly detected, properly due to the limitation of the network scale. In both networks, the flow patterns become compact at equilibrium compared with that at breakthrough in displacements with high injection flowrate (or capillary number) while they remain almost unchanged for displacements with low injection flowrate. The behaviours of differential pressure are similar in these two networks other than pressure achieving equilibrium more quickly in homogeneous network under the same injection flowrate condition. The pressure behaviours are in accord with the results from pore-scale experiments except obvious inertia effect at the beginning of displacements. Transition points are discovered on the relationships between pressure fluctuations and capillary number in both networks and are nearly the same, which are around Log Nca = -3.2. The dynamic, breakthrough-time, equilibrium-time saturation of non-wetting phase is evaluated. It is discovered that the saturation is larger in homogeneous network at high injection flowrates while it is about the same at low injection flowrates. The trend of saturation in physical rock network is in good agreement with pore-scale experimental result
From wetting to withering: the life and times of droplets on microstructured terrains
No full textThis thesis presents a detailed exploration into the dynamics of droplet wetting and evaporation on microstructured surfaces, bridging the interdisciplinary fields of physics, chemistry, and surface engineering. Each chapter methodically delves into a distinct facet of this complex interplay, ranging from non-spherical droplet morphologies and nanoparticle deposition patterns to the novel phenomenon of droplet spawning within binary fluid mixtures. The investigation begins with pure fluids and progressively extends to nanofluids and binary mixtures, offering a comprehensive view of fluid behaviour on structured surfaces.
Initially, the research is anchored in understanding the morphological control of droplets on textured surfaces. It places significant emphasis on the influence of surface roughness and microscale topography in shaping droplet wetting and evaporation dynamics. A pivotal finding is how micropillar spacing and geometry critically affect droplet shapes, inducing distinct morphologies and modulating evaporation rates, thereby revealing the nuanced interaction of fluid physics with surface characteristics.
As the thesis progresses, the focus shifts to complex fluids, particularly the behaviour of aluminium oxide nanofluid droplets. Here, the role of interpillar spacing, geometry, and nanofluid concentration is systematically examined. The research uncovers the formation of sophisticated nanoparticle networks, highlighting concentration-dependent self-assembly processes significantly influenced by the geometric constraints of microstructured surfaces.
The study concludes with an innovative investigation into the behaviour of binary ethanol-water mixtures in a hemiwicking state on microtextured surfaces. This research unveils the spontaneous emergence of mini-droplets, a phenomenon driven by the differential volatility of the fluid components and the resultant changes in local surface tension.
Collectively, this thesis offers novel insights into droplet behaviour on microstructured surfaces and lays the groundwork for potential technological applications. By bridging theoretical principles with experimental observations, the research opens new avenues for technological advancements and sets a foundation for future explorations in the evolving landscape of fluid dynamics
A study of passive and active driven motion of droplets on engineered substrates
Full text linkDroplet motion is an everyday phenomenon with potential benefits to multiple
industrial and biological applications. It can be achieved via various methods,
and the understanding and altering of the underlying mechanism are
important to the accurate control of the droplet behaviour and motion. This
thesis focuses on three different mechanisms that induce the droplet motion:
roughness gradient by micro-structure fabrication, thermocapillary motion
with self-rewetting fluid and vapor-mediated droplet motion.
Firstly, the motion of microscale water droplets on the hydrophobic microstructured
surfaces with structural wettability contrast has been studied. The
velocity and displacement of the droplets moving across the wettability
contrasts have been monitored and their relations to the morphological
parameters of the micro-structure have been systematically investigated.
Besides, the dynamic behaviour of the droplets has been investigated and
explained by the mathematical mode proposed.
Secondly, the thermocapillary motion of self-rewetting droplets has been
reported. The behaviour of self-rewetting droplets departed greatly from the
droplets of ordinary mixture and pure fluids. A unique oscillatory behaviour
was observed for self-rewetting droplets, which was related to the nonmonotonic
dependence of surface tension on temperature. Influencing
parameters were studied and IR thermography assisted to reveal the internal
convection.
Last, the motion of sessile mixture or pure droplets induced by vapour was
investigated. The spatial concentration change via the mass transfer through
the liquid-vapour interface near contact line leads to unbalanced surface
tension, which leads to droplet motion. Depending on the concentration of
both droplets and the vapour, repulsive or attractive motion can be observed.
A phase map as well as a critical concentration boundary was proposed for
the mixture of PG and water droplets, which can help to predict the direction
of droplet motion
Using discrete vasculature to model cerebral blood flow and the effectiveness of scalp cooling in ischaemic stroke
Full text linkGlobally, stroke is one of the largest causes of death and disability. For the best
outcomes, treatment should be started as soon as possible. Hypothermia has been
proposed as an early intervention that could extend the time-to-treatment window
or act as a beneficial treatment in its own right. Assessing the nature of stroke in
humans poses an ethical challenge since it is impossible to predict when one will occur.
Animal models can be used to attempt to understand how the changes progress, and
predictions can be made from the observable end-state once a stroke has occurred.
Determining the effectiveness of hypothermia poses an even more significant
challenge. It is difficult to measure temperature changes inside the brain noninvasively,
and surrogate measurements of brain temperature can be far away from
the actual temperature. Work in animal models has shown mixed results, with some
indicating that regional hypothermia can be induced non-invasively. Meanwhile,
others have concluded that the scalp has a shielding effect that prevents any
meaningful change in the brain’s temperature in response to external cooling devices.
Computational modelling of ischaemic stroke can help to solve many of these
problems. For example, computational modelling can produce detailed predictions
of how temperature will vary over the entire brain in response to changing scalp
conditions. Most previous studies have relied on the Pennes Bioheat equation;
however, this has previously been demonstrated to fail to model the changes in
stroke adequately. Vasculature has been shown to play a part. Models which include
vasculature have been developed, such as the Vascular-Porous (VaPor) model. It can
be shown that the input vasculature has a high degree of influence over the results that
are obtained when modelling stroke. Having vasculature with suitable properties is
therefore crucial.
This thesis takes the existing VaPor model and modifies the techniques used to
generate vasculature to model the changes in blood flow and cerebral temperature
following stroke. Previously in the VaPor model, a single 1-dimensional arterial
and venous tree was obtained and extended using a random space-filling algorithm,
weighted only by perfusion. This is shown to be inadequate for directing blood in
the same ways as it is in-vivo. The techniques are extended by providing additional
limitations and weightings to the existing algorithm, resulting in a modified rapidlyexploring
random tree (RRT) approach being taken for the generation of arteries, with veins still being generated using the existing RRT approach. Parameters used in the
modified approach were tested by simulating perfusion using the generated network
and testing this against clinical expectations. This included testing the number of
additional vessel nodes that needed to be added to achieve convergence of perfusion
data.
Once the network achieved good perfusion results, simulations were performed
to understand how the temperature varied over the domain in both normothermic
and hypothermic conditions. Whilst it is more difficult to verify these results, they
lay within physioloically expected bounds. Results of simulations performed using
the VaPor model for ischaemic stroke show that scalp cooling is able to successfully
reduce the temperature of an ischaemic region of the brain. However, they also show
that the results can be highly dependent on the scale and size of the infarction region.
Results from in-silico experiments can provide results that are not possible to
obtain from in-vivo experiments. Crucially including the level of detail in the results
that can be achieved. In-silico experiments allow visulations of the distributions of
temperature in a way that is not possible using current in-vivo techniquies
Electromagnetic characteristics of high temperature superconductor coated conductors applied to electric machines
Full text linkSuperconductor technology has attracted increasing attention during the last few years
because of their advancements made in the material manufacturing technology and the
reduction of cost. As a result, superconducting materials have been widely applied to power
industries, of which one of the most promising and popular applications is the electric machine,
which is the core component of power generation and consumption on the earth. The second-generation (2G) high temperature superconducting (HTS) coated conductor (CC) has become
increasingly appealing among all the superconductors on account of its commercial
availability and advantageous current carrying capacity. Therefore, HTS electric machines are
believed to usher in a period of development opportunities. However, there still exist many
challenges related to the efficiency, cost-effectiveness, reliability, and safety of HTS
machines, and the alternating current (AC) loss of HTS CCs remains one of the most
significant issues. Over the years, the effort of studying the AC loss of HTS CCs has yielded
many outstanding research outcomes; however, most of them have been focused on the loss
estimation at power frequencies under purely sinusoidal currents and magnetic fields. In fact,
the electromagnetic environment in electric machines is abundant in high-frequency ripple
fields and harmonics, especially for high-speed rotating machines. Therefore, the AC loss
characteristics of HTS CCs at high frequencies remain unclear, to some extent. Aiming to
analyse systematically the AC loss properties of HTS CCs applied to electrical machines
within a wide frequency band, from the power frequency to kHz level, this thesis adopts
analytical equations, numerical modelling methods, as well as experimental measurements. In
doing so, this project hopes to contribute to the loss quantification and controlling of HTS CCs
in electrical machines, providing a useful reference for the design of large-scale
superconducting devices.
This thesis starts by providing a comprehensive literature review of the state of the art of AC
loss related studies. The analytical formulae, modelling methods, measurement approaches, as
well as reduction techniques for the AC loss of HTS materials in both low- and high-frequency
fields are systematically summed up. The review work clarifies the research status of the AC
loss of superconducting materials applied to electric machines, elucidating that the
electromagnetic loss characteristics of HTS CCs deserve further investigation, especially at
high frequencies in high-speed rotating machines. Numerical models are an indispensable tool for studying the anisotropic electromagnetic
properties of high temperature superconductors (HTSCs), thus numerical modelling is chosen
as a primary method in this thesis to study the AC loss of HTS CCs employed in electric
machines. The methodologies adopted to build the simulation models are introduced, which
are based on Maxwell’s equations and the finite element method (FEM). The numerical
models here are developed mainly through two formulations, namely T-formulation (T
represents current vector potential) and H-formulation (H denotes magnetic field), which can
be achieved by FORTRAN 90 or incorporated into COMSOL Multiphysics.
Dynamic loss is a crucial component of the AC loss of HTS field windings in
superconducting machines, which occurs when the HTS CC carrying a direct current (DC) is
exposed to an AC magnetic field. Therefore, the dynamic loss of HTS CCs is explored in
detail. The dependence of dynamic loss on the material properties (critical current density and
n-value) is investigated. Then, a novel formulation is derived to describe the full-range
variation of dynamic loss. At last, three new parameters are defined to characterise the non-linearity of dynamic resistance. The proposed analytical formulae and parameters are validated
by the T-formulation based numerical model and experimental measurements.
In superconducting machines, the HTS CCs are usually utilized in the form of stacks and
coils. Therefore, besides a single HTS CC, the transport current loss, magnetization loss,
dynamic loss, and the total AC loss of HTS stacks, coils (circular and racetrack coils), and
trapped field stacks (TFSs) over a wide frequency band, from the power frequency to kHz
level, are studied respectively. The H-formulation based 2D and 3D numerical models are
mainly adopted here, which are validated by published experimental data. It is found that the
widely used thin film approximation in modelling which only considers the superconducting
layer of HTS CCs is inapplicable at high frequencies (higher than 100 Hz for magnetization
loss) due to the skin effect, and the non-superconducting parts (the copper stabilizer, silver
overlayer, and substrate) have to be taken into account. AC loss varies non-linearly with the
frequency of the AC transport current or magnetic field because of the electromagnetic
interactions between different layers. The shielding effect between different turns of an HTS
coil is also explored, which can enhance the dynamic loss in the middle turns of the coil while
the magnetization loss occupies the majority in the outer turns at high frequencies. The
electromagnetic properties of a curved HTS TFS under high-frequency cross fields are
investigated, too, which possesses geometrical applicability for cylindrical rotating shafts. It
is demonstrated that the widely adopted 2D-axisymmetric models are inapplicable to study the
anisotropic electromagnetic distributions of TFSs because of the emergence of the electromagnetic criss-cross. High-frequency ripple fields can drive induced current towards
the periphery of the HTS TFS due to the skin effect, leading to a fast rise of AC loss and even
an irreversible demagnetization of the TFS.
In order to combine AC loss analysis and machine applications, a special magnet made of
HTS coils in the form of a Halbach array is exploited in the designs of an air-cored wind
turbine generator and an electrodynamic wheel (EDW) used for maglev, through numerical
modelling in COMSOL Multiphysics. The HTS Halbach Array magnet (HAM) can focus the
magnetic flux inside the coil loop, greatly increasing the magnetic flux density in the airgap
and the power density of the machine. The HTS HAM represents a generic topology/approach
for the design of fully air-cored superconducting machines. The proposed HTS HAM EDW
can generate higher thrust and lift forces, and greatly reduce the weight of the magnets
compared with the conventional design with permanent magnets (PMs), opening the way to
future on-road maglev vehicles. It is also illustrated that, for modelling the electro-mechanical
performance of large-scale HTS devices, e.g., synchronous electric machines, the HTS field
coils can be reasonably equivalized as conventional ones carrying the same DC so that the
computation complexity can be largely decreased.
This thesis starts with the application of superconductors to electric machines, analyses
thoroughly the loss characteristics of HTS CCs, stacks, coils, and TFSs over a wide frequency
band from the power frequency to kHz level. It is believed that this research work can help
researchers in the communities of applied superconductivity and electrical machines better
understand the electromagnetic properties of different HTS topologies, provide a useful
reference for the quantification and controlling of AC loss, and thus give a significant
guideline for the design of high power density superconducting machines
On passive and active drag reduction of free-falling bodies in quiescent viscous fluid
Full text linkModes of drag reduction on surfaces interfacing with liquids have received a considerable amount of attention from researchers and industries due to the significantly associated advantages in terms of energy-savings and power consumption associated with various applications such as ships, underwater vehicles, piping infrastructures and microfluidic devices. One revolutionary technique to accomplish drag reduction on moving objects in liquids is to introduce a lubricating gas or vapour layer between the object surface and the ambient liquid via different strategies such as surface modification and by inducing the Leidenfrost effect. However, there are many open questions regarding the understanding, effectiveness and implementation of these drag reduction techniques. The main aim of the present study is to investigate the effect of various surface treatment techniques on the drag coefficient of a solid sphere and drag reduction by Leidenfrost effect on deformable liquid droplets in a free-falling experiment. This was accomplished by a newly designed and constructed experimental setup to facilitate the capture of the free-falling motion of both a solid sphere and a liquid droplet through a quiescent continuous viscous fluid phase in a vertical tank. The solid spheres used in the experiments were stainless-steel spheres with a diameter ranging from 4 mm to 7 mm. The spheres were surface-modified by a perflourodecyltrichlorosilane (FDTS) coating, roughened via an etching process and dry ice coating. No significant differences were found for the etched spheres compared with the unmodified spheres. Surprisingly, the drag coefficient of the FDTS sphere was increased by 13%. The dry ice coating successfully produced a substantial gas layer surrounding the free-falling spheres. However, due to issues with the uniformity of the coating, this method was abandoned. Following these, liquid gallium was used as the dispersed phase in free-falling deformable droplet experiments. Firstly, the effect of shape and deformation on the velocity and the drag coefficient of free-falling liquid gallium droplets in water were investigated for droplet diameters (spherical volume-equivalent) ranging from 2.67 mm to 5.56 mm under isothermal conditions with temperatures in the range of 30◦C to 70◦C. The initial shape of the droplets after detachment was found to be spherical. Spherical-oblate oscillations began immediately after the detachment of the droplet prior to the dampening of the oscillations into a final shape of an oblate-spheroid except for the smallest droplet size which remained spherical without any notable change in shape. It was found that the rhythmic change in shape induced the falling velocity to oscillate at a frequency double that of the aspect ratio. Moreover, increasing the viscosity ratio enhanced the amplitude of the oscillations. However, the oscillation frequencies were sensitive to the droplets’ size rather than their associated viscosity ratio. The experimental results reveal that for a deformed liquid gallium droplet with a terminal Reynolds number that varied in the range of 103 to 104, the drag coefficients were found to be larger than those associated with a solid sphere in the same Reynolds number range. Furthermore, the deformation is highly dependent on interfacial surface tension and inertial force, while the viscosity ratio and pressure distribution have negligible effect. Subsequently, the continuous phase was changed to a low boiling point perfluorinated liquid (FC-72) in order to investigate the drag reduction by Leidenfrost effect. The liquid gallium temperature was varied in the range of 40◦C to 170◦C to induce an inverted Leidenfrost effect. The fully-developed Leidenfrost regime was stable at a droplet temperature of 130◦C, and was illustrated by the vapour layer stream moving upward on the droplet surface. Unlike in water, the liquid gallium droplets in FC-72 formed a tear-drop shape. The drag coefficient calculated based on the maximum velocity achieved by the droplets revealed a drastic drag reduction of about 57% for the highest temperature droplet compared with the lowest temperature droplet. Numerical simulation based on the two-dimensional lattice Boltzmann model (LBM) was also carried out to study the velocity field and pressure distribution around a deformable droplet falling through an immiscible quiescent viscous liquid
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