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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Jules Verne or Joint Venture? Investigation of a Novel Concept for Deep Geothermal Energy Extraction

Wachtmeister, Henrik January 2012 (has links)
Geothermal energy is an energy source with potential to supply mankind with both heat and electricity in nearly unlimited amounts. Despite this potential geothermal energy is not often considered in the general energy debate, often due to the perception that it is a margin energy source bound to a few locations with favorable geological conditions. Today, new technology and system concepts are under development with the potential to extract geothermal energy almost anywhere at commercial rates. The goal of these new technologies is the same, to harness the heat stored in the crystalline bedrock available all over the world at sufficient depth. To achieve this goal two major problems need to be solved: (1) access to the depths where the heat resource is located and (2) creation of heat transferring surfaces and fluid circulation paths for energy extraction. In this thesis a novel concept and method for both access and extraction of geothermal energy is investigated. The concept investigated is based on the earlier suggested idea of using a main access shaft instead of conventional surface drilling to access the geothermal resource, and the idea of using mechanically constructed 'artificial fractures' instead of the commonly used hydraulic fracturing process for creation of heat extraction systems. In this thesis a specific method for construction of such suggested mechanically constructed heat transfer surfaces is investigated. The method investigated is the use of diamond wire cutting technology, commonly used in stone quarries. To examine the concept two heat transfer models were created to represent the energy extraction system: an analytical model based on previous research and a numerical model developed in a finite element analysis software. The models were used to assess the energy production potential of the extraction system. To assess the construction cost two cost models were developed to represent the mechanical construction method. By comparison of the energy production potential results from the heat transfer models with the cost results from the construction models a basic assessment of the heat extraction system was made. The calculations presented in this thesis indicate that basic conditions for economic feasibility could exist for the investigated heat extraction system.

Transient reduced-order convective heat transfer modeling for a data center

Ghosh, Rajat 12 January 2015 (has links)
A measurement-based reduced-order heat transfer modeling framework is developed to optimize cooling costs of dynamic and virtualized data centers. The reduced-order model is based on a proper orthogonal decomposition-based model order reduction technique. For data center heat transfer modeling, the framework simulates air temperatures and CPU temperatures as a parametric response surface with different cooling infrastructure design variables as the input parameters. The parametric framework enables an efficient design optimization tool and is used to solve several important problems related to energy-efficient thermal design of data centers. The first of these problems is about determining optimal response time during emergencies such as power outages in data centers. To solve this problem, transient air temperatures are modeled with time as a parameter. This parametric prediction framework is useful as a near-real-time thermal prognostic tool. The second problem pertains to reducing temperature monitoring cost in data centers. To solve this problem, transient air temperatures are modeled with spatial location as the parameter. This parametric model improves spatial resolution of measured temperature data and thereby reduces sensor requisition for transient temperature monitoring in data centers. The third problem is related to determining optimal cooling set points in response to dynamically-evolving heat loads in a data center. To solve this problem, transient air temperatures are modeled with heat load and time as the parameters. This modeling framework is particularly suitable for life-cycle design of data center cooling infrastructure. The last problem is related to determining optimal cooling set points in response to dynamically-evolving computing workload in a virtualized data center. To solve this problem, transient CPU temperatures under a given computing load profile are modeled with cooling resource set-points as the parameters.

Numerical Modeling Of The Shock Tube Flow Fields Before Andduring Ignition Delay Time Experiments At Practical Conditions

lamnaouer, mouna 01 January 2010 (has links)
An axi-symmetric shock-tube model has been developed to simulate the shock-wave propagation and reflection in both non-reactive and reactive flows. Simulations were performed for the full shock-tube geometry of the high-pressure shock tube facility at Texas A&M University. Computations were carried out in the CFD solver FLUENT based on the finite volume approach and the AUSM+ flux differencing scheme. Adaptive mesh refinement (AMR) algorithm was applied to the time-dependent flow fields to accurately capture and resolve the shock and contact discontinuities as well as the very fine scales associated with the viscous and reactive effects. A conjugate heat transfer model has been incorporated which enhanced the credibility of the simulations. The multi-dimensional, time-dependent numerical simulations resolved all of the relevant scales, ranging from the size of the system to the reaction zone scale. The robustness of the numerical model and the accuracy of the simulations were assessed through validation with the analytical ideal shock-tube theory and experimental data. The numerical method is first applied to the problem of axi-symmetric inviscid flow then viscous effects are incorporated through viscous modeling. The non-idealities in the shock tube have been investigated and quantified, notably the non-ideal transient behavior in the shock tube nozzle section, heat transfer effects from the hot gas to the shock tube side walls, the reflected shock/boundary layer interactions or what is known as bifurcation, and the contact surface/bifurcation interaction resulting into driver gas contamination. The non-reactive model is shown to be capable of accurately simulating the shock and expansion wave propagations and reflections as well as the flow non-uniformities behind the reflected shock wave. Both the inviscid and the viscous non-reactive models provided a baseline for the combustion model iii which involves elementary chemical reactions and requires the coupling of the chemistry with the flow fields adding to the complexity of the problem and thereby requiring tremendous computational resources. Combustion modeling focuses on the ignition process behind the reflected shock wave in undiluted and diluted Hydrogen test gas mixtures. Accurate representation of the Shock - tube reactive flow fields is more likely to be achieved by the means of the LES model in conjunction with the EDC model. The shock-tube CFD model developed herein provides valuable information to the interpretation of the shock-tube experimental data and to the understanding of the impact the facility-dependent non-idealities can have on the ignition delay time measurements.

Coupled Heat Transfer Processes in Enclosed Horizontal Heat Generating Rod Bundles

Senve, Vinay January 2013 (has links) (PDF)
In a nuclear fuel cask, the heat generating spent fuel rods are packed in a housing and the resulting bundle is placed inside a cask of thick outer shell made of materials like lead or concrete. The cask presents a wide variation in geometrical dimensions ranging from the diameter of the rods to the diameter of the cask. To make the problem tractable, first the heat generating rod bundle alone is considered for analysis and the effective thermal conductance of the bundle is correlated in terms of the relevant parameters. In the second part, the bundle is represented as a solid of equivalent thermal conductance and the attention is focused on the modelling of the cask. The first part, dealing with the effective thermal conductance is solved using Fluent software, considering coupled conduction, natural convection and surface radiation in the heat generating rod bundle encased in a hexagonal sheath. Helium, argon, air and nitrogen are considered as working media inside the bundle. A correlation is obtained for the critical Rayleigh number which signifies the onset of natural convection. A correlation is also developed for the effective thermal conductance of the bundle, considering all the modes of transport, in terms of the maximum temperature in the rod bundle, pitch-to-diameter ratio, bundle dimension (or number of rods), heat generation rate and the sheath temperature. The correlation covers pitch-to-diameter ratios in the range 1.1-2, number of rods ranging from 19 to 217 and the heat generation rates encountered in practical applications. The second part deals with the heat transfer modeling of the cask with the bundle represented as a solid of effective (or equivalent) thermal conductance. The mathematical model describes two-dimensional conjugate natural convection and its interaction with surface radiation in the cask. Both Boussinesq and non-Boussinesq formulations have been considered for convection. Numerical solutions are obtained on a staggered mesh with a pressure correction method using a custom-made Fortran code. The surface radiation is coupled to the conduction and convection at the solid-fluid interfaces. Steady-state results are obtained using time-marching. Results for various quantities of interest, namely, the flow and temperature distributions, Nusselt numbers, and interface temperatures, are presented. The Grashof number based on the volumetric heat generation and gap width is varied from 105 to 5 ×109. The emissivities of the interfaces are varied from 0.2-0.8 for the radiative calculations. The solid-to-fluid thermal conductivity ratio for the inner cylinder is varied in the range 5-20 in the parametric studies. Simulations are also performed with thermal conductivity calculated in an iterative manner from bundle parameters. The dimensionless outer wall conductivity ratio is chosen to correspond to cask walls made of lead or concrete. The dimensionless thickness (with respect to gap width) of the outer shell is in the range of 0.0825-1, while the inner cylinder dimensionless radius is 0.2. Air is the working medium in the cask for which the Prandtl number is 0.71. Correlations are obtained for the average temperatures and Nusselt numbers at the inner interface in terms of the parameters. The radiation heat transfer is found to contribute significantly to the heat dissipation.

Modeling Material Microstructure and Fatigue Life of Metal Components Produced by Laser Melting Additive Process

Chun-Yu Ou (8791262) 12 October 2021 (has links)
<p>There has been a long-standing need in the marketplace for the economic production of small lots of components that have complex geometry. A potential solution is additive manufacturing (AM). AM is a manufacturing process that adds material bottom-up. It has the distinct advantages of low preparation cost and high geometric creation capability. Components fabricated via AM are now being selectively used for less-demanding applications in motor vehicles, consumer products, medical products, aerospace devices, and even some military projects.</p><p><br></p> <p>For engineering applications, high value-added components require consistency in the fatigue properties. However, components fabricated by AM have large variation in the fatigue properties compared to those by conventional manufacturing processes. To alleviate unpredictable catastrophic failures of components, it is essential to study and predict fatigue life. Previous study reported that fatigue crack initiation process accounts for a large portion of fatigue life, especially for low loading amplitude and high cycle fatigue. However, this major portion of fatigue life prediction is mostly ignored by main stream researchers working on fatigue modeling. For industrial applications, engineers often specify a lower stress condition to obtain a higher safety factor. Under these circumstances, fatigue crack initiation becomes even more important, so it is essential to further study of crack initiation.</p><p><br></p> <p>The objective of this research is to develop a fatigue crack initiation model for metal components produced by AM. To improve life prediction accuracy, the model must incorporate the effect of different microstructures, which are typically produced by AM due to a large number of repetitive cycles of re-heating and re-cooling processes. To fulfill this objective, the tasks are separated into three studies: (1) developing a temperature model to simulate temperature history, (2) modeling the component’s microstructure for the potential crack initiation zone, and (3) developing a fatigue crack initiation model for life estimation. A summary of each task is provided in the following.</p> <p>First, the role of temperature model is to understand the mechanism that leads to the variation of microstructures. The existing temperature models are computationally expensive to obtain an accurate prediction of the temperature history due to repetitive heating and cooling. The main reason is that these models considered entire boundary conditions of all the material points. In this section, we proposed and employed the concept of effective computation zone, which can save the computational time significantly for AM process. </p><p><br></p> <p>Second, it is critical to include the effect of microstructure in the fatigue life model since the microstructure variation at different locations within the real AM component is large. The grain size variation is modeled by using representative volume element, which is defined as a volume of heterogeneous material that is sufficiently large to be statistically representative of the real component’s microstructure. Regarding phase transformation, a continuous cooling transformation (CCT) diagram is a useful tool that can be used with a thermal model for microstructure design and manufacturing process control. However, traditional CCT diagrams are developed based on slow and monotonic cooling processes such as furnace cooling and air cooling, which are greatly different from the repetitive heating and cooling processes in AM. In this study, a new general methodology is presented to create CCT diagrams for materials fabricated by AM. We showed that the effect of the segmented duration within the critical temperature range, which induced precipitate formation, could be cumulative. </p><p><br></p> <p>Third, the existing fatigue crack initiation life model has poor accuracy. One of the reasons for the poor accuracy is the coefficients change due to the variation in microstructure is not accounted for. In this section, a semi-empirical fatigue crack initiation model is presented. The important coefficients include maximum persistent slipband width, energy efficiency coefficient, resolved shear stress and plastic slip rate per cycle. These coefficients are modeled and determined as a function of microstructure, which can improve the accuracy of life estimation.</p><p><br></p> <p>The contribution of this study is to provide a new engineering tool for designing the melting AM process based on scientific research. With this tool, the fundamental mechanism contributing to a large variation of the fatigue life of the metal components made by AM process can be understood, attributed, predicted and improved. The seemly ‘stochastic’ nature of fatigue life of the AM components can be changed to be more deterministic and predictable. This approach represents a major advance in fatigue research on AM materials. The model developed is considered as a tool for research, design, and control for laser-based AM process applications. </p>

Numerical Simulation of a Continuous Caster

Matthew T Moore (8115878) 12 December 2019 (has links)
Heat transfer and solidification models were developed for use in a numerical model of a continuous caster to provide a means of predicting how the developing shell would react under variable operating conditions. Measurement data of the operating conditions leading up to a breakout occurrence were provided by an industrial collaborator and were used to define the model boundary conditions. Steady-state and transient simulations were conducted, using boundary conditions defined from time-averaged measurement data. The predicted shell profiles demonstrated good agreement with thickness measurements of a breakout shell segment – recovered from the quarter-width location. Further examination of the results with measurement data suggests pseudo-steady assumption may be inadequate for modeling shell and flow field transition period following sudden changes in casting speed. An adaptive mesh refinement procedure was established to increase refinement in areas of predicted shell growth and to remove excess refinement from regions containing only liquid. A control algorithm was developed and employed to automate the refinement procedure in a proof-of-concept simulation. The use of adaptive mesh refinement was found to decrease the total simulation time by approximately 11% from the control simulation – using a static mesh.


Hathal, Hisham January 2021 (has links)
Tillskottsvatten i avloppsledningsnät medför stora kostnader i form av underhåll,ökad mängd kemikalier och energi som även belastar miljön. Syftet med detta examensarbetevar att få ökad förståelse av temperaturförhållanden mellan spillvattenoch mark för att förbättra modelleringen av tillskottsvatten. Hypotesen är att spillvattentemperaturensjunker vid inläckage då temperaturen på tillskottsvattnet ärlägre än spillvattnet. Ett spillvattenrör med en längd på ungefär 2,5 km i Umeåundersöktes med givare jämt fördelade längs sträckan. Metoden innebar att medmätvärden på temperaturerna för spillvattnet, luften i spillvattenröret och i markentillsammans med flödet och nivån på spillvattnet modellera värmeutbytet i COMSOLMultiphysics. Resultatet gav fyra liknande funktioner för både spillvattenregionenoch luftregionen som beskrev värmeutbytet mellan spillvattenröret och marken.Effektutbytena hade en linjär tendens som funktion av temperaturdifferensen mellanspillvatten och mark under mätperioden fram till maj månad men med en högspridning då effektutbytet är även beroende på flödet och inte bara temperaturdifferensen.Majoriteten av effektutbytet var mellan ytan på spillvattenröret ochspillvattnet. I regionen mellan luft och yta på spillvattenröret så var majoriteten aveffektutbytet ifrån strålningsutbytet mellan spillvattnet och ytan. Resultaten gaväven U-värden för regionen mellan spillvattenregionen och luftregionen i rörledningenoch dessa var omkring 4 W/(m2K) respektive 0,8 W/(m2K). Tillskottsvattnetmodellerades utifrån värmeutbytet och det visade på ökade flödesnivåer när det varvåtperioder med en noggrannhet på ± 1 kg/s. / Supplemental water in the sewer network entails large costs in the form of maintenance,increased amounts of chemicals and energy that also have a burden on theenvironment. The purpose of this thesis was to gain an increased understandingof temperature conditions between wastewater and soil to improve the modelingof supplemental water. The hypothesis is that the waste water temperature dropswhen leakage occurs and when the temperature of the additional water is lower thanthe wastewater. A wastewater pipe with a length of approximately 2.5 km in Umeåwas examined with sensors evenly distributed along the path. The method involvedmodeling the heat flow in COMSOL Multiphysics with measured values of thetemperatures for the wastewater, the air in the wastewater pipe and in the groundtogether with the flow and the level of the wastewater. The result gave four similarfunctions for both the wastewater region and the air region that described the heattransfer from the wastewater pipe to the ground. The heating effects had a lineartendency as a function of the temperature difference between wastewater and soilduring the measurement period up to the month of May, but with a high spread asthe heat transfer is also dependent on the flow and not just the temperature difference.The majority of the heat transfer was between the surface of the wastewaterpipe and the wastewater. In the region between the air surface on the wastewaterpipe, the majority was the heat transfer from the radiation between the wastewaterand the surface. The results also gave U-values for the wastewater region and theair region in the pipeline and these were around 4 W/(m2K) and 0.8 W/(m2K),respectively. The supplemental water was modeled on the basis of the heat transferand it showed increased flow levels when it was wet periods with an accuracy of ±1 kg/s.

Étude théorique et expérimentale d’une unité de micro-cogénération biomasse avec moteur Ericsson / Theoretical and experimental study of a biomass micro-CHP unit with an Ericsson engine

Creyx, Marie 14 November 2014 (has links)
La micro-cogénération, production simultanée d’électricité et de chaleur à échelle domestique, se développe actuellement en Europe du fait notamment de son intérêt en termes d’économie d’énergie primaire. L’utilisation d’un combustible biomasse dans un système de micro-cogénération contribue à augmenter la part d’énergie renouvelable dans le mix énergétique. L’objet de ce travail est le développement d’un banc d’essai d’une unité de micro-cogénération biomasse composée d’une chaudière à pellets, d’un moteur à air chaud de type Ericsson (décomposé en une partie compression et une partie détente) et d’un échangeur gaz brûlés-air pressurisé inséré dans la chaudière. Des modèles de chacun de ces composants ont été établis pour caractériser leur fonctionnement sur la plage de réglage des paramètres influents et pour dimensionner l’unité prototype. Deux modèles du moteur Ericsson, en régime permanent et en régime dynamique, ont été mis en place. Ils ont montré l’influence prépondérante sur les performances du moteur des conditions de température et pression de l’air en entrée de détente et des réglages des instants de fermeture des soupapes. L’effet de la prise en compte des pertes dynamiques (pertes de charge, pertes thermiques à la paroi du cylindre, frottements mécaniques) sur l’estimation des performances du moteur a été étudié. Deux modélisations de l’échangeur ont permis de caractériser les transferts thermiques qui le traversent, incluant le rayonnement et l’encrassement par des particules de suie du côté des gaz brûlés. Le banc d’essai de l’unité de micro-cogénération mis en place / Nowadays, the micro combined heat and electrical power (micro-CHP) systems are developing in Europe, in particular because of their interest in terms of primary energy savings. The use of biomass fuel in micro-CHP systems enhances the share of renewable energy in the energy mix. The objective of this work is to develop a test bench for a biomass-fuelled micro-CHP unit composed of a pellet boiler, an Ericsson type hot air engine (decomposed into a compression and an expansion part) and a burned gas-pressurized air heat exchanger inserted in the boiler. Models of every component have been established to characterize their working conditions depending on influent parameter settings and to size the micro-CHP unit. Two models of Ericsson engine, with established and dynamic regimes, were implemented. The preponderant influence of the temperature and pressure conditions at the inlet of the expansion cylinder and of the timing of valve closing on the engine performances are shown. The dynamic model shows the effect of considering the dynamic losses (pressure loss, heat transfer at the cylinder wall, mechanical friction) on the estimation of engine performances. Two models of the heat exchanger allow the characterization of the heat transfers crossing it, taking into account the radiation and the fouling by soot particles on the side of combustion gases. Experimental measurements obtained from the test bench of the micro-CHP unit set up were used in the developed models.

Improvements in Engine Performance Simulations and Integrated Engine Thermal Modeling

Aishwarya Vinod Ponkshe (16648650) 26 July 2023 (has links)
<p>One of the major challenges in the field of internal combustion engines is keeping up with the advancements in electrification and hybridization. Automakers are striving to design environment – friendly and highly efficient engines to meet stringent emission standards worldwide. Improving engine efficiency and reducing heat losses are critical aspects of this development. Therefore, accurate heat transfer prediction capabilities play a vital role in engine design process. Current methods rely on computationally intensive 3D numerical analyses, there is a growing interest in reliable simplified models. </p> <p>In this study, a 1D diesel engine model featuring predictive combustion was integrated with a detailed finite element thermal primitive based on the 3D meshing feature available in GT Suite. Coolant and oil hydraulic circuits were incorporated in the model. The model proves to be an effective means to assess the impact on heat rejection and engine heat distribution given by an engine calibration and operating conditions. </p> <p>This work also contributes to the advancement of virtual IC engine development methods by focusing on the design and tuning of complex engine system models using GT Power for accurate prediction of engine performance. The current processes in engine simulations are assessed to identify sources of errors and opportunities for improvements. The methods discussed in this work include isolated sub system level calibration and model evolution specifically address the issue of identifying noise factors and issues in smaller parts. Additionally, the study aims on improving the model’s trustworthiness by computing 1st law sanity checks, replicating real-life compressor map calculations and refining GT’s existing global convergence criteria. </p>

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