1,721,158 research outputs found
Development of a novel hybrid field and zone fire model
Full text linkThis thesis describes the design and implementation of a novel hybrid field/zone fire model, linking a fire field model to a zone model. This novel concept was implemented using SMARTFIRE (a fire field model produced at the University of Greenwich) and two different zone models (CFAST which is produced by NIST and FSEG-ZONE which has been produced by the author during the course of this work). The intention of the hybrid model is to reduce the amount of computation incurred in using field models to simulate multi-compartment geometries, and it will be implemented to allow users to employ the zone component without having to make further technical considerations, in line with the existing paradigm of the SMARTFIRE suite.
In using the hybrid model only the most important or complex parts of the geometry are fully modelled using the field model. Other suitable and less important parts of the geometry are modelled using the zone model. From the field model‘s perspective the zone model is represented as an accurate pressure boundary condition. From the zone model‘s perspective the energy and mass fluxes crossing the interface between the models are seen as point sources.
The models are fully coupled and iterate towards a solution ensuring both global conservation along with conservation between the regions of different computational method. By using this approach a significant proportion of the computational cells can be replaced by a relatively simple zone model, saving computational time. The hybrid model can be used in a wide range of situations but will be especially applicable to large geometries, such as hotels, prisons, factories or ships, where the domain size typically proves to be extremely computationally expensive for treatment using a field model. The capability to model such geometries without the associated mesh overheads could eventually permit simulations to be run in ‘faster-real-time’, allowing the spread of fire and effluents to be modelled, along with a close coupling with evacuation software, to provide a tool not just for research objectives, but to allow real time incident management in emergency situations.
Initial ‘proof of concept’ work began with the development of one way coupling regimes to demonstrate that a valid link between models could allow communication and conservation of the respective variables. This was extended to a two-way coupling regime using the CFAST zone model and results of this implementation are presented. Fundamental differences between the SMARTFIRE and CFAST models resulted in the development of the FSEG-ZONE model to address several issues; this implementation and numerous results are discussed at length. Finally, several additions were made to the FSEG-ZONE model that are necessary for an accurate consideration of fire simulations.
The test cases presented in this thesis show that a good agreement with full- field results can be obtained through use of the hybrid model, while the reduction in computational time realised is approximately equivalent to the percentage of domain cells that are replaced by the zone calculations of the hybrid model
Modelling and CFD simulation of a fluidized bed process for the capture of C02 from fossil fuel combustion sources
Full text linkFossil fuels provide the main source of energy for power generation in existing power plants. A mitigation option to reduce carbon dioxide (CO2) emission from existing power plants with fossil fuel combustion is the sequestration of carbon dioxide and storage in geological formations, in the ocean or for use in industrial processes. CO2 capture from combustion exhaust gases by mineral carbonation using a fluidised bed is studied in this project. CFD modelling has been used to study the efficiency of CO2 capture in a fluidized bed reactor containing a solid sorbent Calcium Oxide (CaO). This present work seeks to maximize CO2 conversion by a systematic modification of the flow domain. In particular, it is intended to use a convergent-divergent geometry to control the velocity of particles in the reaction domain thereby keeping the particles in the domain as long as possible. This is expected to improve the performance of the system as more time is allowed for any remaining CO2 to react with CaO and then be removed in the calcination stage. Further the effect of other key parameters such as particle size, CO2 concentration of flue gas and mass loading of solid sorbent have also been investigated.
A Lagrangian/Eulerian scheme has been developed for this purpose, which uses a particle tracking model to describe CaO particle trajectories and mass, momentum and energy exchange with the carrier gas, entering the reactor in a typical flue gas composition. A steady-state condition is assumed, with each trajectory representing a parcel of particles of a given mass and diameter. The number of particles entering the fluidised bed is kept constant, and the fluidization velocity is chosen so that particles remain in the reactor. As the carbonation progresses, heavier well-reacted particles are collected at the bottom of the reactor. In the case of a non-uniform size distribution, fine particles would escape from the top of the reactor; in order to keep such particles within the domain the geometry was modified to increase the residence time of particles and to obtain maximum conversion. CO2 reduction of the order of 90% was achieved in a single pass, with a mass loading of 2.5 times of equivalent solid sorbent to CO2 in gas
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Strategy development for object-oriented multiblock grid generation and adaptation to complex geometries
Full text linkThe first part of the thesis deals with the strategy development for multiblock structured grid generation to complex geometries. Based on the grid generation practices over years, a set of grid construction rules is developed to provide the CFD engineer an object-oriented method for grid design.
The essential core of the object-oriented method is to decompose a complex meshing task into a set of sub-tasks, which are treated individually at a lower level of both geometry and topology. The grid construction rules cover the questions of dealing with selection of meshing direction, generation of surface description and block topology building. To explain this grid design method, an example, dealing with a highly complex geometry, is demonstrated.
The second part of the thesis deals with the strategy development for multiblock structured grid adaptation. Since a grid can be adapted with or without a flow solution, the terminologies passive and active grid adaptation are introduced to describe a solution-dependent or a solution-independent grid adaptation.
Passive grid adaptation is performed by generating adequate block topologies, such that a local enrichment of grid points can be achieved. It consists of three concepts: one-dimensional clustering of grid lines, block encapsulation, and smart block. The method for solution-dependent grid adaptation is developed based on the idea of grid optimization. A grid is adapted by minimization of objective functions, in which relations among weight functions and grid line distribution are formulated. The measures for grid quality, such as smoothness, cell aspect ratio, and orthogonality, are formulated as control terms of the objective functions to improve grid quality. In addition, a concept of smart cell used for solution-dependent grid adaptation is proposed in this thesis
Numerical modelling of electrodeposition process for printed circuit boards manufacturing
Full text linkPrinted circuit boards (PCBs) are used extensively in electronic products to connect assembled components within a system. The so-called vertical interconnect access (via) is a vertical hole or cavity in the PCB filled with metal to facilitate conductivity. The current trend, particularly for high technology products (e.g., 3D packaging), is to manufacture PCBs with high aspect ratio (AR) vias. Typically, the size of such a via is at the micrometer scale (this is why they are termed micro-vias).
The most widely used technique for manufacturing micro-vias is electrodeposition of metal (e.g., copper), where the PCB is immersed into a plating cell filled with an electrolyte solution. Using standard conditions, electrodeposition usually does not produce micro-vias with the required quality. This is due to a lack of copper ion transport into the via. This has lead to studies of various ways of enhancing the ion transport. This thesis documents the results from a modelling study into the electrodeposition processes for fabricating high aspect ratio micro-vias. This includes basic electrodeposition and techniques that enhance ion transport such as forced convection (using a pump) and acoustic streaming (using transducers).
In this work, a novel numerical method for explicitly tracking the interface between the deposited metal and the electrolyte is implemented and validated under the conditions of basic electrodeposition using experimental data. Results from a parametric study have established a set of design rules for micro-vias fabrication.
When ion transport is enhanced by forced convection (e.g., pumping) in the plating cell, we apply a multi-scale modelling methodology that provides interaction between models at the macro level (the plating cell) and the micro level (the interior of a via). Numerical simulations can then be used to verify how ion transport into the micro-via is improved. These results can then be used to identify process conditions for the plating cell which will result in the required flow behaviour at the micro-via.
Megasonic agitation can also be used to enhance electrolyte convection in the plating cell. This is achieved by placing megasonic transducers into the plating cell. This leads to several phenomena, one of which is known as the acoustic streaming. Models have been developed for predicting megasonic agitation both at the macro and micro-scales, and a number of designs have been investigated for both open and blind micro-vias
Computational electrohydrodynamics for fabricating polymer microstructures
Full text linkThe aim of the work presented in this thesis is the development of two computational models of two processes that can be used to shape molten polymers on a micro-scale, namely Electrohydrodynamic Induced Patterning (EHDIP) and Electric Field Assisted Capillarity (EFAC). These related processes both use the dielectric forces at the interface between a polymer and another dielectric such as air. When the molten polymers are placed in a shaped electric field the imbalance in these dielectric forces causes the polymer to flow in a controlled way creating shapes in the polymer melt, this is the basis for the EHDIP process. The shaped electric field is controlled by the morphology of the top mask which acts as an electrode. This process is further extended by introducing a heavily wetted surface on the top mask which results in capillary forces that cause the polymer melt to coat the top mask creating a fully enclosed shape. This process can be used to create enclosed micro-channels or micro-capsules. Thus results and discussion presented herein highlight several possible application routes for industrial manufacturing. The process is discussed here for microstructures of 1 µm to 200 µm in size. The range at which the processes work is not fully understood, however the EHDIP process has been shown to work at a nanoscale producing structures around 100 nm in size.
From a comprehensive literature review, the underlying theory and mechanisms of this process were identified and the governing equations derived. Computational models were developed based on the underlying physics. These models were initially developed in PHYSICA version 3g and later they were implemented into COMSOL Multiphysics as the latter proved to be more stable. The results from the computational models were compared to the limited experimental data available.
The results from the computational models show that the mask shape was found to have the largest effect on the final structure of the shaped poly-mer. Due to capillary forces the shape of the microstructure at the top mask mimics the shape of the mask. In the lower section of the enclosed microstructure there is a force balance between surface tension, dielectric forces and internal pressure, giving a rounded morphology. Furthermore, by wetting the lower mask, flat bottomed structures can be produced. By both shaping and wetting the lower mask the shape of the microstructure can be even further modified. However, sharp cornered masks are unsuitable for this process. The effects of other key parameters such as air gap, contact angle, polymer permittivity and applied voltage were investigated through a sensitivity analysis.
Changing the permittivity is shown to have an effect on the final microstructure. The change is small; however the permittivity does affect the speed of the process. The contact angle between the top mask and the polymer modifies the thickness of the polymer at the top of the structures. Increasing the contact angle causes a decrease in polymer thickness due to a reduction in the capillary force. The depth of the structures can be altered by changing the air gap; hence a larger air gap gives a deeper structure. The initial polymer thickness has no effect on the top of the structure but determines the thickness, shape and curvature of the lower part of the structure.
The applied voltage controls the electrostatic forces and hence the speed of the process. For a low voltage the electrostatic forces are not strong enough to initiate the process and an enclosed microstructure does not form. If the voltage is too high, the structure forms quickly and bubbles can be entrapped at the top mask.
With the correct mask shapes the processes can produce a wide variety of microstructures. These would have a wide range of applications either in the communications sector as fibre-optical wave-uides or in the biomedical sector as microstructures used in BioMEMS. Further development of the process is required to ensure that the process can be controlled. The models presented here are initial investigations of this but further experimental work is required along with the expansion of the model into three-dimensions
A model to predict the lifetime of pneumatic conveyor bends
Full text linkBursts in pneumatic conveyor pipelines in industrial processes are a well-known hindrance to the smooth operation of any plant. Unanticipated stoppages can have serious financial implications for any company using pneumatic conveying technology, and health and safety factors are also paramount. This thesis describes an attempt to enable improved prediction of bend-wear and bend lifetime, so that more cost-effective survey work can be undertaken in anticipation of bursts. This work delivers a tool that allows bend lifetime prediction to be made according to: the bend geometry and material; the material conveyed and its rate of transportation; and bend wall thickness.
Firstly, a computational model based on the coupling of CFD and particle tracking techniques is created in order to encompass the mechanics of the erosion process. This erosion process is assumed to be dominated by impact damage, and predictions of bend lifetime are made using empirical erosion algorithms gleaned from laboratory experiments, commercial CFD code (PHOENICS, and GENTRA its particle tracking sister code), and custom erosion modelling code that employs a three-dimensional toroidal geometry.
Secondly, matrices of predictions are built up using the mathematical modelling technology mentioned above. These predictions are collated behind a friendly interface to produce a far more accessible piece of PC software that an engineer can employ quickly and easily. More general bend-life predictions are interpolated from this fundamental dataset using behaviours established in the course of this work, according to the particular conveying conditions input by the user.
Predictions in the desktop tool are calibrated to actual bend lives as established by experimentation on a full-scale pneumatic conveyor. This experimental work was an integral part of this EPSRC-funded project, and allows some estimation of error magnitude in the predictive tool
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Dynamic grid adaption using the LPE equation
Full text linkThis thesis describes the development and implementation of a dynamic adaptive grid method for general two and three dimensional static and transient fluid flow problems solved over structured grids. The technique automatically manipulates the location of grid points within the domain of interest to concentrate cells in regions of high solution activity, thus aiming to improve the accuracy of the overall simulation for a given number of initial grid cells. To achieve this aim the Laplace Poisson Equidistribution equation is used. Furthermore, the work also covers different types and treatment of weight functions needed to represent areas of high solution activity and a range of techniques necessary to make the use of adaptive grids practical, including geometry modelling and grid quality control. The technique is implemented on simple functions and within the commercial CFD code PHOENICS, on fluid flow problems ranging from convection driven flows to shock capturing. The ability of the technique to be used for general grid manipulation is demonstrated by using it to couple PHOENICS with a stress code in the simulation of a deflecting beam in a uniform flow. In addition, a novel technique to adapt grids to solution phenomena using neural nets is demonstrated
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