Dissertations / Theses on the topic 'Thermal field modeling'

Understanding the magnitude of the temperature in AlGaN/GaN heterostructure fi eld e ffect transistors(HFETs) is a critical aspect of understanding their reliability and providing proper thermal management. At present, most models used to determine the temperature rise in these devices are based on continuum based heat conduction. However, in such devices, the heat generation region can be on the order of or smaller than the phonon mean free path of the heat carriers, and thus, such models may under predict the temperature. The aim of this work is towards building a multiscale thermal model that will allow for the prediction of heat transport from ballistic-diffusive phonon transport near the heat generation region and diffusive transport outside of this zone. First, a study was performed to determine the appropriate numerical solution to the phonon Boltzmann transport equation followed by its integration into a multiscale thermal scheme. The model, which utilizes a Discrete Ordinates Solver, was developed for both gray and non-gray phonon transport. The scheme was applied to the solution of speci fic test problems and then finally to the electrothermal modeling of AlGaN/GaN HFETs under various electrical bias conditions.

Thesis (Ph.D)--Mechanical Engineering, Georgia Institute of Technology, 2010.
Committee Chair: Samuel Graham; Committee Member: Donald Dorsey; Committee Member: Douglas Yoder; Committee Member: Michael Leamy; Committee Member: Sankar Nair; Committee Member: Zhuomin Zhang. Part of the SMARTech Electronic Thesis and Dissertation Collection.

High power Gallium Nitride (GaN) based field effect transistors are used in many high power applications from RADARs to communications. These devices dissipate a large amount of power and sustain high electric fields during operation. High power dissipation occurs in the form of heat generation through Joule heating which also results in localized hot spot formation that induces thermal stresses. In addition, because GaN is strongly piezoelectric, high electric fields result in large inverse piezoelectric stresses. Combined with residual stresses due to growth conditions, these effects are believed to lead to device degradation and reliability issues. This work focuses on studying these effects in detail through modeling of Heterostructure Field Effect Transistors (HFETs) and metal oxide semiconductor hetero-structure field effect transistor (MOSHFETs) under various operational conditions. The goal is to develop a thorough understanding of device operation in order to better predict device failure and eventually aid in device design through modeling. The first portion of this work covers the development of a continuum scale model which couples temperature and thermal stress to find peak temperatures and stresses in the device. The second portion of this work focuses on development of a micro-scale model which captures phonon-interactions at the device scale and can resolve local perturbations in phonon population due to electron-phonon interactions combined with ballistic transport. This portion also includes development of phonon relaxation times for GaN. The model provides a framework to understand the ballistic diffusive phonon transport near the hotspot in GaN transistors which leads to thermally related degradation in these devices.

Welding is used extensively in aerospace, automotive, chemical, manufacturing, electronic and power-generation industries. Thermally-induced residual stresses due to welding can significantly impair the performance and reliability of welded structures. Numerical simulation of weld pool dynamics is important as experimental measurements of velocities and temperature profiles are difficult due to the small size of the weld pool and the presence of the arc. From a structural integrity perspective of welded structures, it is necessary to have an accurate spatial and temporal thermal distribution in the welded structure before stress analysis is performed. Existing research on weld pool dynamics simulation has ignored the effect of fluid flow in the weld pool on the temperature field of the welded joint. Previous research has established that the weld pool depth/width (D/W) ratio and Heat Affected Zone (HAZ) are significantly altered by the weld pool dynamics. Hence, for a more accurate estimation of the thermally-induced stresses it is desired to incorporate the weld pool dynamics into the analysis. Moreover, the effects of microstructure evolution in the HAZ on the mechanical behavior of the structure need to be included in the analysis for better mechanical response prediction. In this study, a three-dimensional model for the thermo-mechanical analysis of Gas Tungsten Arc (GTA) welding of thin stainless steel butt-joint plates has been developed. The model incorporates the effects of thermal energy redistribution through weld pool dynamics into the structural behavior calculations. Through material modeling the effects of microstructure change/phase transformation are indirectly included in the model. The developed weld pool dynamics model includes the effects of current, arc length, and electrode angle on the heat flux and current density distributions. All the major weld pool driving forces are included, namely surface tension gradient, plasma drag force, electromagnetic force, and buoyancy. The weld D/W predictions are validated with experimental results. They agree well. The effects of welding parameters (like welding speed, current, arc length, etc.) on the weld D/W ratio are documented. The workpiece deformation and stress distributions are also highlighted. The transverse and longitudinal residual stress distribution plots across the weld bead and their variations with welding speed and current are also provided. The mathematical framework developed here serves as a robust tool for better prediction of weld D/W ratio and thermally-induced stress evolution and distribution in a welded structure by coupling the different fields in a welding process.
Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Whitaker College of Health Sciences and Technology, 1997.
Vita.
Includes bibliographical references (p. 257-264).
by Daniel Alan Sidney.
Ph.D.

A quantitative understanding of thermal field evolution is vital for quality control in additive manufacturing (AM). Because of the unknown material parameters, high computational costs, and imperfect understanding of the underlying science, physically-based approaches alone are insufficient for component-scale thermal field prediction. Here, I present a new framework that integrates physically-based and data-driven approaches with quasi in situ thermal imaging to address this problem. The framework consists of (i) thermal modeling using 3D finite element analysis (FEA), (ii) surrogate modeling using functional Gaussian process, and (iii) Bayesian calibration using the thermal imaging data. Based on heat transfer laws, I first investigate the transient thermal behavior during AM using 3D FEA. A functional Gaussian process-based surrogate model is then constructed to reduce the computational costs from the high-fidelity, physically-based model. I finally employ a Bayesian calibration method, which incorporates the surrogate model and thermal measurements, to enable layer-to-layer thermal field prediction across the whole component. A case study on fused deposition modeling is conducted for components with 7 to 16 layers. The cross-validation results show that the proposed framework allows for accurate and fast thermal field prediction for components with different process settings and geometric designs.
Master of Science
This paper aims to achieve the layer to layer temperature monitoring and consequently predict the temperature distribution for any new freeform geometry. An engineering statistical synergistic model is proposed to integrate the pure statistical methods and finite element modeling (FEM), which is physically meaningful as well as accurate for temperature prediction. Besides, this proposed synergistic model contains geometry information, which can be applied to any freeform geometry. This paper serves to enable a holistic cyber physical systems-based approach for the additive manufacturing (AM) not only restricted in fused deposition modeling (FDM) process but also can be extended to powder-based process like laser engineered net shaping (LENS) and selective laser sintering (SLS). This paper as well as the scheduled future works will make it affordable for customized AM including customized geometries and materials, which will greatly accelerate the transition from rapid prototyping to rapid manufacturing. This article demonstrates a first evaluation of engineering statistical synergistic model in AM technology, which gives a perspective on future researches about online quality monitoring and control of AM based data fusion principles.

Thesis (Ph. D.)--Ohio State University, 2004.
Title from first page of PDF file. Document formatted into pages; contains xix, 156 p.; also includes graphics (some col.). Includes bibliographical references (p. 152-156).

We are coming to realize that there is an urgent need to reduce energy usage in buildings and it has to be done in a sustainable way. This thesis focuses on the performance of the building envelope; more precisely thermal performance of walls and super insulation material in the form of vacuum insulation. However, the building envelope is just one part of the whole building system, and super insulators have one major flaw: they are easily adversely affected by other problems in the built environment.  Vacuum Insulation Panels are one fresh addition to the arsenal of insulation materials available to the building industry. They are composite material with a core and an enclosure which, as a composite, can reach thermal conductivities as low as 0.004 W/(mK). However, the exceptional performance relies on the barrier material preventing gas permeation, maintaining a near vacuum into the core and a minimized thermal bridge effect from the wrapping of barrier material round the edge of a panel. A serpentine edge is proposed to decrease the heat loss at the edge. Modeling and testing shows a reduction of 60% if a reasonable serpentine edge is used. A diffusion model of permeation through multilayered barrier films with metallization coatings was developed to predict ultimate service life. The model combines numerical calculations with analytical field theory allowing for more precise determination than current models. The results using the proposed model indicate that it is possible to manufacture panels with lifetimes exceeding 50 years with existing manufacturing. Switching from the component scale to the building scale; an approach of integrated testing and modeling is proposed. Four wall types have been tested in a large range of environments with the aim to assess the hygrothermal nature and significance of thermal bridges and air leakages. The test procedure was also examined as a means for a more representative performance indicator than R-value (in USA). The procedure incorporates specific steps exposing the wall to different climate conditions, ranging from cold and dry to hot and humid, with and without a pressure gradient. This study showed that air infiltration alone might decrease the thermal resistance of a residential wall by 15%, more for industrial walls. Results from the research underpin a discussion concerning the importance of a holistic approach to building design if we are to meet the challenge of energy savings and sustainability. Thermal insulation efficiency is a main concept used throughout, and since it measures utilization it is a partial measure of sustainability. It is therefore proposed as a necessary design parameter in addition to a performance indicator when designing building envelopes. The thermal insulation efficiency ranges from below 50% for a wood stud wall poorly designed with incorporated VIP, while an optimized design with VIP placed in an uninterrupted external layer shows an efficiency of 99%, almost perfect. Thermal insulation efficiency reflects the measured wall performance full scale test, thus indicating efficiency under varied environmental loads: heat, moisture and pressure. The building design must be as a system, integrating all the subsystems together to function in concert. New design methodologies must be created along with new, more reliable and comprehensive measuring, testing and integrating procedures. New super insulators are capable of reducing energy usage below zero energy in buildings. It would be a shame to waste them by not taking care of the rest of the system. This thesis details the steps that went into this study and shows how this can be done.
QC 20120228

This thesis presents how to establish a theoretical model to predict risk of thermal cracking in young concrete when cast on ground or an arbitrary construction. The crack risk in young concrete is determined in two steps: 1) calculation of temperature distribution within newly cast concrete and adjacent structure; 2) calculation of stresses caused by thermal and moisture (due to self-desiccation, if drying shrinkage not included) changes in the analyzed structure. If the stress reaches the tensile strength of the young concrete, one or several cracks will occur. The main focus of this work is how to establish a theoretical model denoted Equivalent Restraint Method model, ERM, and the correlation between ERM models and empirical experiences. A key factor in these kind of calculations is how to model the restraint from any adjacent construction part or adjoining restraining block of any type. The building of a road tunnel and a railway tunnel has been studied to collect temperature measurements and crack patterns from the first object, and temperature and thermal dilation measurements from the second object, respectively. These measurements and observed cracks were compared to the theoretical calculations to determine the level of agreement between empirical and theoretical results. Furthermore, this work describes how to obtain a set of fully tested material parameters at CompLAB (test laboratory at Luleå University of Technology, LTU) suitable to be incorporated into the calculation software used. It is of great importance that the obtained material parameters describe the thermal and mechanical properties of the young concrete accurately, in order to perform reliable crack risk calculations.  Therefore, analysis was performed that show how a variation in the evaluated laboratory tests will affect the obtained parameters and what effects it has on calculated thermal stresses.

Práce se zabývá studiem procesů probíhajících při zhášení silnoproudého oblouku ve zhášecí komoře jističe. Je zaměřena na výpočet dynamiky tekutin a teplotního pole v okolí elektrického oblouku. V práci je dále popsán vliv vzdálenosti plechů v komoře a vliv tvarů plechů z hlediska aerodynamických podmínek uvnitř komory. Dalším cílem dosaženým touto prací je poskytnutí informací o vlivu polohy elektrického oblouku na termodynamické vlastnosti uvnitř komory. Toto je důležité, zejména pokud je oblouk do komory vtahován jinými silami, např. elektromagnetickými a během tohoto vtahovacího procesu mění svůj tvar i polohu. Za účelem co nejjednoduššího, ale zároveň co nejefektivnějšího řešení úkolu, byl vyvinut software určen speciálně pro výpočet dynamiky tekutin numerickou metodou konečných objemů (FVM). Tato metoda je, v porovnání s rozšířenější metodou konečných prvků (FEM), vhodnější pro výpočet dynamiky tekutin (CFD) zejména proto, že režie na výpočet jedné iterace jsou menší v porovnání s ostatními numerickými metodami. Další výhodou tohoto softwarového řešení je jeho modularita a rozšiřitelnost. Cely koncept softwaru je postaven na tzv. zásuvných modulech. Díky tomuto řešení můžeme využít výpočtové jádro pro další numerické analýzy, např. strukturální, elektromagnetickou apod. Jediná potřeba pro úspěšné používání těchto analýz je napsáni solveru pro konečné prvky (FEM). Jelikož je software koncipován jako multi–thread aplikace, využívá výkon současných vícejádrových procesorů naplno. Tato vlastnost se ještě více projeví, pokud se výpočet přesune z CPU na GPU. Jelikož současné grafické karty vyšších tříd mají několik desítek až stovek výpočetních jader a pracují s mnohem rychlejšími pamětmi, než CPU, je výpočetní výkon několikanásobně vyšší.

Combining computer fluid dynamic, CFD, models with finite element, FE, models to calculate temperature in fire exposed structures can reduce design temperatures in structures while still obtaining the level of structural fire safety stipulated by society. A better understanding of heat transfer and the concept of adiabatic surface temperatures, AST, the transition of data between models can be simplified and more accurate temperature predictions can be made.The thesis focuses on heat transfer calculations by employing AST in particular, and how this can be used as a means of coupling any CFD and FE-analysis code. The thesis presents a method for performing FE-analysis of the thermal response with input data calculated with the computer code FDS, Fire Dynamics Simulator. Parallel to this, the heat balance equation in FDS is tested and an alternate numerical algorithm is developed and tested.Firstly, a verification model is developed to test the radiative and convective part of the existing heat balance equation in FDS. An alternate numerical algorithm for calculation of the heat transfer at surfaces is developed as a more homogenous alternative for CFD codes.Secondly is a study on how to extract AST from an arbitrary point with direction in a CFD calculation using an infinitesimal surface. Instead of modeling numerous small surfaces for extracting AST, a post processor is developed to calculate AST independent of any modeled surface. For CFD codes, such as FDS that depend on a rectilinear grid, this enables calculation of AST in any direction, not only directions normal to the Cartesian planes.Finally, a comparison is made between different methods for calculating temperatures in steel with AST from numerical fire dynamics/modeling calculations. In this thesis there is a comparison between simplified Eurocode techniques, simple finite element analysis and advanced finite element analysis. This study shows the benefit of understanding heat transfer in numerical codes and to implement the concept of AST in a proper way.This way, the concept of combining numerical fire dynamics calculation with numerical (or simplified) thermal calculations can be better understood and implemented.
Godkänd; 2013; 20131010 (joasan); Tillkännagivande licentiatseminarium 2013-11-15 Nedanstående person kommer att hålla licentiatseminarium för avläggande av teknologie licentiatexamen. Namn: Joakim Sandström Ämne: Stålbyggnad/Steel Structures Uppsats: Thermal Boundary Conditions Based on Field Modelling of Fires Heat Transfer Calculations in CFD and FE Models With Special Regards to Fire Exposure Represented With Adiabatic Surface Temperatures Examinator: Professor Ulf Wickström, Institutionen för samhällsbyggnad och naturresurser, Luleå tekniska universitet Diskutant: Teknologie doktor, Lektor Stephen Welch, the University of Edinburgh, UK Tid: Torsdag den 5 december 2013 kl 13.00 Plats: F1031, Luleå tekniska universitet

Дисертація на здобуття наукового ступеня кандидата технічних наук (доктора філософії) за спеціальністю 05.13.03 «Системи та процеси керування» (151 – Автоматизація та комп’ютерно інтегровані технології) – Національний технічний університет «Харківський політехнічний інститут» Міністерства освіти і науки України, Харків, 2018 р. Дисертацію присвячено аналізу систем керування розподіленими тепловими об’єктами, визначенню їх недоліків для розробки нових енергоефективних методів керування теплопостачанням адміністративних та офісних будівель з використанням систем з прогнозуванням. Розглянуто проблеми розробки енергозберігаючого керування тепловими об’єктами. Проаналізовано розподілені теплові об’єкти. Проведено класифікацію теплових об’єктів керування залежно від кількості використовуваних видів енергоносіїв, структури, режиму роботи опалювального обладнання. Досліджено існуючі методи синтезу систем керування розподіленими тепловими об’єктами та систем керування тепловою енергією. Розглянуто основні чинники, які впливають на температуру в приміщенні. Проаналізовано регулятори з прогнозуванням. Досліджено існуючі тенденції розвитку методів енергозберігаючого керування, які застосовуються у системах теплопостачання. Встановлено, що існуючі регулятори температури з прогнозуванням підтримують температуру в приміщенні в діапазоні від +19 °C до +22 °C з точністю ± 1 °C. Також встановлено, що регулятори, основані на погодозалежному, ПІД- та двопозиційному законах керування, скорочують витрати на енергоспоживання на 15%, а використання регуляторів з прогнозуванням зменшує енергоспоживання на 25–35%. Обґрунтовано перспективність розробки енергозберігаючих методів керування розподіленими тепловими об’єктами з прогнозуванням. Проведено аналіз поведінки розподілених теплових об’єктів керування, їх властивостей. Сформульовано вимоги до синтезу енергозберігаючих законів керування тепловими об’єктами. Визначено область досяжності, початкові і граничні умови при початковій стадії дослідження і моделювання об’єкта для знаходження ефективного рішення. Теоретично обґрунтовані передумови і обмеження для розробки методів енергозберігаючого керування. Проведено класифікацію регуляторів для керування розподіленими тепловими об’єктами в залежності від величини відношення часу транспортного запізнення до часу перехідних процесів. Розроблено алгоритм розв’язання задачі розрахунку розподілення тепла всередині офісного приміщення за допомогою моделі в середовищі ANSYS. Розроблено методику поділу структури приміщення на n об’єктів з зосередженими параметрами з однаковими властивостями простору. Розглянуті основні проблеми керування розподіленими тепловими об’єктами. Набув подальшого розвитку метод керування тепловим об’єктом з розподіленими параметрами за допомогою ступінчастої функції. Розроблено формули, які дозволяють перейти від керування об’єктом з зосередженими параметрами до керування об’єктом з розподіленими параметрами. Розроблено спосіб програмного керування тепловим об’єктом з розподіленими параметрами за допомогою широтно-імпульсної модуляції та прогнозуючого фільтра, де вектор цільової функції формується з помилки керування в поточний момент часу і прогнозованої помилки неузгодженості, що визначається як різниця між заданою температурою і температурою моделі регулятора. Розроблено мікроконтролерну систему керування теплопостачанням, яка складається з підсистеми збору інформації, підсистеми видачі керуючого впливу, центрального обчислювача та пульту керування. Встановлено, що для діагностування несправностей підсистеми збору інформації та видачі керуючого впливу повинні бути замкнуті в кільце. Розроблено алгоритми діагностики справності ліній даних, керування температурою об’єкта та реєстрації перехідних характеристик об'єкта керування. Встановлено, що реалізацію керуючого впливу у вигляді ступінчастої функції можна здійснювати за допомогою паралельного з’єднання нагрівачів різних потужностей. Запропоновано перейти до керування тепловим об’єктом за допомогою широтно-імпульсної модуляції, при якому регулюючий елемент працює в ключовому режимі через наявність недоліків у реалізації керуючого впливу у вигляді ступінчастої функції. На основі вимірів на реальному об’єкті побудована повна тривимірна модель приміщення з урахуванням зовнішніх обводів і точної внутрішньої геометрії. Побудована об’ємна розрахункова сітка з гексаедрів. Задані граничні умови з урахуванням системи опалення, системи вентиляції повітря, теплопровідності стін, температури зовнішнього повітря. Отримано перехідні криві впливу зовнішньої температури, вентиляції з температурою повітря, що дорівнює 20 °C, нагрівача потужністю 1.75 кВт на температуру в приміщенні. Створено програму-макрос (udf файл) на мові С у середовищі ANSYS для проведення експерименту. Задані умови для проведення моделювання: зовнішня температура змінюється від мінус 7 °С до + 5 °С, залежно від часу доби, початкова температура повітря в приміщенні + 18 °С, швидкість приточної та витяжної вентиляції 0.018 кг / с, температура приточної вентиляції + 20 °С. Проведено експеримент з підтримання заданого добового температурного режиму в приміщенні: з 00:00 до 08:00 + 10 °С, з 09:00 до 17:00 + 18 °С, з 18:00 до 00:00 + 15 °С. Порівняно розроблений метод ШІМ-регулювання з прогнозуванням з найбільш поширеними: двопозиційним регулюванням з гістерезисом ± 2 ° C, двопозиційним регулюванням без гістерезису і ПІД-регулюванням. За результатами моделювання встановлено, що найвищу точність показав метод ШІМ-регулювання з прогнозуванням. Найменш ефективним виявився метод двопозиційного регулювання з гістерезисом, рівним ± 2 ° C, який завдяки наявності інерції теплового об’єкта перевищив задану температуру в приміщенні. Отримано сумарний час роботи нагрівача протягом доби. Методи регулювання температурою за допомогою ПІД регулятора і регулятора з прогнозуванням показали приблизно однаковий час роботи нагрівача (39.89% проти 39.24%). Відповідно до місячної вартості за 1 кВт електроенергії, загального часу роботи нагрівача, потужності обчислено витрати на опалення для приміщень з однозмінним режимом роботи без вихідних для добового та місячного режимів. Встановлено, що відмова від безперервної роботи нагрівача і застосування регулятора з прогнозуванням дозволить зменшити час роботи опалювального обладнання на 56%: з 24-х до 10.5 год. За результатами моделювання побудовано температурні зони приміщення, які дають вихідну інформацію, в яких точках приміщення необхідно розташовувати датчики температури. Для експериментальних досліджень об’єктами керування обрано порожнистий сталевий стержень з розмірами: довжина 35.5 см, зовнішній діаметр 3.2 см, внутрішній діаметр 2.8 см, з намотаним на одному кінці нагрівачем довжиною 8.2 см від початку труби опором 19 Ом, і стерилізатор ГП-80. Для здійснення натурного експерименту зібрано експериментальну установку на базі мікроконтролера ATMega16, використано датчики DS18B20, мікросхему пам’яті AT24C256B-PU. Написано програму керування температурою об’єкта для мікроконтролера ATMega16. Для кожного з обраних об’єктів керування отримані перехідні криві та проведено експерименти з підтримання заданих температурних умов. Результати досліджень підтвердили правильність теоретичних передумов, покладених в основу розробки апаратно-програмного комплексу, і перспективність цього напрямку. Вирішено задачу пошуку оптимального співвідношення параметрів потужності нагрівача й інтервалу прогнозування за допомогою методу найменших квадратів. Встановлено, що для зменшення помилки регулювання імпульс керуючого впливу слід виробляти на початку інтервалу програмного регулювання з урахуванням величини перерегулювання. Результати наукових досліджень впроваджені в практику проектування систем автоматичного керування тепловими об’єктами, технологічними процесами підприємства ТОВ «ВО ОВЕН» (м. Харків). Матеріали дисертації використовуються в лекційних курсах «Теорія автоматичного керування» і «Програмні засоби систем керування» на кафедрі «АіУТС» НТУ «ХПІ».
Thesis for granting the Degree of Candidate of Technical sciences (PhD degree) in specialty 05.13.03 «Management systems and processes» (151 – Automation and computer-integrated technologies) National Technical University «Kharkiv Polytechnic Institute» of Ministry of Education and Science of Ukraine, Kharkiv, 2018. The thesis is devoted to the analysis of distributed thermal objects control using prediction systems to determine their disadvantages and development of new energy-efficient heat control methods for administrative and office buildings. The problems of thermal objects energy-saving control development are considered. Distributed thermal objects are analyzed. The author suggests a classification of thermal objects according to the number of used energy sources, structure, working mode of the heating equipment. Existing synthesis methods of control systems for distributed thermal objects and thermal energy control systems are investigated. The main factors affecting temperature in premises are considered. The regulators with prediction are analyzed. The existing tendencies in the development of energy saving control methods applied in heat supply systems are explored. It has been found that existing temperature controllers with prediction maintain temperature in the premises in the range from +19 °C to +22 °C with accuracy ± 1 °C. It has also been demonstrated that regulators based on weather-dependent, PID-, on–off control laws reduce energy consumption by 15%, using regulators with prediction reduces power consumption by 25-35%. Perspective of energy-saving methods control of distributed thermal objects with prediction is substantiated. The author has carried out an analysis of the behavior and properties of distributed thermal control objects. The requirements for the synthesis of energy-saving thermal objects control laws are formulated. Feasible region, initial and boundary conditions at the initial stage of research, modeling of the object for finding an effective solution have been determined. Prerequisites and constraints for the development of methods of energy saving management are theoretically justified. A classification of distributed thermal objects regulators, depending on the ratio of the transport delay time to the time of transient processes, has been carried out. The author developed an algorithm for solving the problem of calculation the heat distribution inside an office premises. A model of the room in the ANSYS environment is created. The method of dividing the structure of the premises into n objects with lumped parameters with the same properties of space is developed. The basic problems of distributed thermal objects control are considered. The control method of thermal object with distributed parameters using a step function have been further developed. Formulas that allow to refuse from an object with lumped parameters control to an object with distributed parameters control has been developed. The author developed a method of thermal object with distributed parameters control using pulse-width modulation with prediction filter, where the target function vector is formed from the control error at the current time and the predicted mismatch error, that is defined as the difference between the given temperature and the temperature of the regulator model. A microcontroller system of heat supply control, that consists of a subsystem of getting information, a subsystem of issuing control influence, central controller and control panel have been developed. It has been established that in order to identify the refusal of the subsystem of information gathering and the issuance of controlling influence, they must be closed in the circle. The algorithms of diagnosing the data linesї efficiency, controlling the temperature of the object and recording the transient characteristics of the control object are developed. It is established that the implementation of control influence in the form of a stepped function can be carried out with the help of a parallel connection of heaters of various power. It is suggested to control the temperature of a thermal object with pulse-width modulation, in which the control element operates in key mode due to the disadvantages of implementing a control effect in the form of a stepped function. On the basis of measurements on a real object, a complete three-dimensional model of the room taking into account external contours and exact internal geometry was constructed. Volumetric netting of hexahedrons was built. Boundary conditions taking into account the heating system, ventilation system, heat conductivity of walls, the temperature of the outside air were specified. Transient curves of influence on the room of external temperature, ventilation with air temperature equal to 20 °C, a 1.75 kW power heater temperature were obtained. A program macro (udf file) on C language in the ANSYS environment for the experiment was created. The conditions for the simulation: the external temperature varies from minus 7 °C to plus 5 °C, depending on the time of day, the initial air temperature in the room +18 °C, the speed of intake and exhaust ventilation 0.018 kg/s, the temperature of the intake ventilation + 20 °C were specified. An experiment to maintain a setting daily temperature mode in the room: from 00:00 to 08:00 + 10 °С, from 9:00 to 17:00 + 18 °С, from 18:00 to 00:00 + 15 ° С was conducted. Method of PWM regulation with prediction to the most common: twoposition regulation with hysteresis ± 2 ° C, two-position regulation without hysteresis and PID-regulation were compared. According to the simulation results, the PWM control with prediction the highest accuracy was exhibited. The least effective method was two-position regulation with hysteresis equal to ± 2 °C, which due to the inertia of the thermal object exceeded the given temperature in the room. The total time of the heater operation during the day is obtained. The methods of temperature control with the PID regulation and PWM with prediction control roughly the same time of operation of the heater (39.89% vs. 39.24%) were showed. According to the monthly cost of 1 kW of electricity, the total operating time of t he heater, heater power for the heating costs for rooms with one-shift operating mode without a weekend for day and month modes were calculated. It has been established that the refusal of continuous operation of the heater and using regulator with prediction will reduce the operating time of the heating equipment by 56%: from 24 to 10.5 h. According to the simulation results, the temperature modes of the premises that give the source information in which points of the room it is necessary to place the temperature sensors are obtained. For experimental studies, a hollow steel rod with dimensions: length 35.5 cm, outer diameter 3.2 cm, internal diameter 2.8 cm, wound on one end with a heater length of 8.2 cm from the beginning of the tube with a resistance of 19 Ohms, and sterilizer GP-80 were selected. For a real experiment, an experimental installation based on the ATMega16 microcontroller, sensors DS18B20, microchip memory AT24C256B-PU was used. A program of the object temperature control on the ATMega16 microcontroller is written. For each of the selected control objects transition curves were obtained and experiments of maintaining the specified temperature conditions were carried out. The correctness of the theoretical prerequisites for the development of the hardware and software complex was confirmed by the results of the research. The problem of finding the optimal ratio of heater power parameters and forecasting interval using the least squares method is solved. It was found that to reduce the control error, the control impulse pulse should be made at the beginning of the program control interval, taking into account the amount of overshoot. The results of scientific research were introduced into the practice of designing systems for automatic control of thermal objects, technological processes of the enterprise VO OWEN (Kharkiv). The materials of the dissertation are used in lecture courses "Theory of automatic control" and "Software tools of control systems" at the department automation and control systems NTU "KhPI".

Дисертація на здобуття наукового ступеня кандидата технічних наук за спеціальністю 05.13.03 – системи та процеси керування.  Національний технічний університет «Харківський політехнічний інститут», Харків, 2018. Дисертація присвячена розробці й удосконаленню методів енергозберігаючого керування розподіленими тепловими об’єктами для застосування в існуючих і нових системах теплопостачання. Запропоновано спосіб керування тепловими об’єктами з розподіленими параметрами з прогнозуванням, де як керуючий вплив використовується ШІМ-сигнал. Набув подальшого розвитку спосіб керування тепловими об’єктами з розподіленими параметрами з прогнозуванням, з керуючим впливом типу ступінчастої функції. Запропоновано формули, які дозволяють перейти від керування об’єктом із зосередженими параметрами до керування об’єктом з розподіленими параметрами в n точках простору, з використанням p нагрівачів. У середовищі ANSYS на основі вимірів на реальному об’єкті з використанням методу скінченних елементів побудована імітаційна модель приміщення. На цій моделі застосовано алгоритми керування температурним полем з використанням ШІМ-керування з прогнозуванням, ПІД-регулятора, двопозиційного керування та промодельована безперервна робота нагрівача. За допомогою розробленого програмно-апаратного комплексу на базі мікроконтролера AtMega 16 для обраних об’єктів керування – сталевого порожнистого стержня і камери стерилізатора – здійснено експериментальні дослідження для підтримання заданої температури за допомогою ШІМ-керування з прогнозуванням. Результати досліджень засвідчили високу ефективність роботи методу з урахуванням зовнішніх збурень і підтриманням температури з точністю не більше ± 1 % від заданого значення температури.
The thesis on Candidate Degree in Technical Sciences: Specialty 05.13.03 – management systems and processes. – National Technical University «Kharkov Polytechnic Institute», Kharkov, 2018. The thesis is devoted to the development and improvement of energy-efficient control methods of distributed thermal objects for using in heat supply systems. The thesis proposes a control method of thermal objects with distributed parameters with prediction filter, where the PWM signal is used as the control action. The method of controlling thermal objects with distributed parameters with prediction using the step function as control effect has been further developed. The paper presents a formalized description of predictive control methods that allow to control temperature of an object with distributed parameters instead of object with lumped parameters temperature control at n points with p heaters. A simulation room model based on measurements of the room and the finite element method in the ANSYS environment was constructed. Using developed predictive controller, experiments of maintaining the assigned daily temperature mode were carried out. The developed method of PWM predictive control with continuous control, two-position, PID-control was compared. Experiments of maintaining the set temperature of the steel hollow rod and the sterilizer chamber using the developed software and hardware complex, based on the microcontroller AtMega 16 and PWM with prediction control method, were carried out.

Field measurement and numerical modeling are the most popular and fundamental approaches for studying mixing pattern in rivers and coastal waters. Due to the limitations associated with both of these methods they should be used together to verify each other. Extensive field measurement was conducted on the effluent plume from the outfall of the Capital Region waste water treatment plant in the North Saskatchewan River. Tracer was injected at the outfall location and the mixing pattern was investigated by tracking the tracer concentration over a 83 km reach of the river. Flow velocity and depth were also measured simultaneously using an acoustic Doppler current profiler. An integrated in situ fluorometer-GPS measurement technique was introduced and used for field tracer studies in meandering rivers. The full transverse mixing length for the river was estimated to be 130 km. A stream-tube orthogonal curvilinear mesh generation algorithm was also developed for numerical modeling of meandering rivers. The method eliminates the effect of transverse velocity field using the stream-tube concept. The field measured velocity data were used for calculating the stream tube width in each cross-sectional strip. The stream-tube grid was used to develop a practical and efficient coupled field-numerical model for estimating the transverse mixing coefficient in meandering rivers. In this model the computational costs associated with solving the hydrodynamic sub-model is reduced by generating the velocity field from measured data. Using the calibrated model, the average transverse mixing coefficient was calculated for the surveyed reach. Extensive field study was also conducted on the near-field and far-field of a thermal plume discharged by the Ras Laffan Industrial City in Qatar. Three-dimensional perspective of the plume behavior was obtained using field measured temperature and velocity data. Different characteristics of the observed plume including the extent of different zones of the plume, plume thickness, detachment depth and variation of the minimum dilution were investigated and compared with available theories. The contribution of each effective mixing mechanism was also calculated using the field measured data. Vertical confinement was found to be the main effective parameter on the near-field mixing rate which reduced the minimum dilution rate up to 80%. An innovative remote sensing technique was introduced to investigate the near-field mixing of thermal surface plumes. The method generates a calibrated thermal image of the plume using LandSat thermal infrared (TIR) satellite images. Using a combination of remote sensing and numerical modeling, the near-field dynamics of the plume was found to be influenced by the wind action. It was also observed that the previous classification for determining the effect of wind on the plume dynamics did not successfully predict the plume behavior in shallow water. Two non-dimensional parameters, WI1=Uwl/U0 (ratio of the long-shore wind speed (Uwl) to the discharge velocity (Uo) and WI2= Uwc/U0 (ratio of the cross-shore wind (Uwc) to the discharge velocity), were introduced to quantify the effect of wind on the plume dilution and deflection. The plume trajectory was found to be sensitive to a longshore wind greater than 2 m/s, which is half of the reported value for deep water conditions. The surveyed coastal outfall was also modeled using a nested coupled hydrodynamic-wave approach. Validation of the model with field measured and remote sensing data showed that the employed approach can be used for engineering applications such as designing outfall systems and environmental impact assessment purposes. The calibrated model was used to investigate the effect of various effective factors on the mixing process such as lateral confinement, wave-flow interaction, wave dissipation factors and turbulence models. Lateral confinement was found to reduce the mixing potential of the outfall by 50% at the end of the near-field.

Energy piles are deep foundation elements designed to utilize near-surface geothermal energy, while at the same time serve as foundations for buildings. The use of energy piles for geothermal heat exchange has been steadily increasing during the last decade, yet there are still pending questions on their thermo-mechanical behavior. The change in temperature along energy piles, resulting from their employment in heat exchange operations, causes axial displacements, thermally induced axial stresses and changes in mobilized shaft resistance which may have possible effects on their behavior. In order to investigate these effects, an extensive field test program, including conventional pile load tests and application of heating-cooling cycles was conducted on three energy piles during a period of six weeks. Temperature changes were applied to the test piles with and without maintained mechanical loads to investigate the effects of structural loads on energy piles. Moreover, the lengths of the test piles were determined to represent different end-restraining conditions at the toe. Various sensors were installed to monitor the strain and temperature changes along the test piles. Axial stress and shaft resistance profiles inferred from the field test data along with the driven conclusions are presented herein for all three test piles. It is inferred from the field test results that changes in temperature results in thermally induced compressive or tensile axial stresses along energy piles, the magnitude of which increases with higher restrictions such as structural load on top or higher toe resistance. Moreover, lower change in shaft resistance is observed with increasing restrictions along the energy piles. In addition to the design, deployment, and execution of the field test, a thermo-mechanical cyclic numerical model was developed as a part of this research. In this numerical model, load-transfer approach was coupled with the Masing's Rule in order to simulate the two-way cyclic axial displacement of energy piles during temperature changes. The numerical model was validated using the field test results for cyclic thermal load and thermo-mechanical load applications. It is concluded that the use of load-transfer approach coupled with the Masing's Rule is capable of simulating the cyclic thermo-mechanical behavior of energy piles.
Ph. D.

Thesis (PhD)--Stellenbosch University, 2015.
ENGLISH ABSTRACT: This study investigates the crustal structure, and assesses the qualitative and quantitative impacts of crust-mantle dynamics on subsidence pattern, past and present-day thermal field and petroleum system evolution at the southern South African continental margin through the application of a multi-disciplinary and multi-scale geo-modelling procedure involving both conceptual and numerical approaches. The modelling procedure becomes particularly important as this margin documents a complex interaction of extension and strike-slip tectonics during its Mesozoic continental rifting processes. Located on the southern shelf of South Africa, the Western Bredasdorp Basin (WBB) constitutes the focus of this study and represents the western section of the larger Bredasdorp sub-basin, which is the westernmost of the southern offshore sub-basins. To understand the margin with respect to its present-day structure, isostatic state and thermal field, a combined approach of isostatic, 3D gravity and 3D thermal modelling was performed by integrating potential field, seismic and well data. Complimenting the resulting configuration and thermal field of the latter by measured present-day temperature, vitrinite reflectance and source potential data, basin-scale burial and thermal history and timing of source rock maturation, petroleum generation, expulsion, migration and accumulation were forwardly simulated using a 3D basin modelling technique. This hierarchical modelling workflow enables geologic assumptions and their associated uncertainties to be well constrained and better quantified, particularly in three dimensions. At present-day, the deep crust of the WBB is characterised by a tripartite density structure (i.e. prerift metasediments underlain by upper and lower crustal domains) depicting a strong thinning that is restricted to a narrow E-W striking zone. The configuration of the radiogenic crystalline crust as well as the conductivity contrasts between the deep crust and the shallow sedimentary cover significantly control the present-day thermal field of the study area. In all respects, this present-day configuration reflects typical characteristics of basin evolution in a strike-slip setting. For instance, the orientations of the deep crust and fault-controlled basin-fill are spatially inconsistent, thereby indicating different extension kinematics typical of transtensional pull-apart mechanisms. As such, syn-rift subsidence is quite rapid and short-lived, and isostatic equilibrium is not achieved, particularly at the Moho level. Accompanied syn-rift rapid subsidence and a heat flow peak led to petroleum preservation in the basin since the Early Cretaceous. Two additional post-rift thermal anomalies related to the Late Cretaceous hotspot mechanism and Miocene margin uplift in Southern Africa succeeded the syn-rift control on maturation. This thermal maturity of the five mature source rocks culminated in four main generation and three main accumulation phases which characterise the total petroleum systems of the WBB. The Campanian, Eocene and Miocene uplift scenarios episodically halted source maturation and caused tertiary migration of previously trapped petroleum. Petroleum loss related to the spill point of each trap configuration additionally occurs during the Late Cretaceous-Paleocene and Oligocene-Early Miocene. The timing and extent of migration dynamics are most sensitive to the geological scenario that combined faulting, intrusive seal bypass system and facies heterogeneity. In fact, for models that do not incorporate facies heterogeneity, predicted past and present-day seafloor leakage of petroleum is largely underestimated. This complex interplay of generation and migration mechanisms has significant implications for charging of petroleum accumulations by multiple source rocks. Due to early maturation and late stage tertiary migration, the syn-rift source rocks particularly Mid Hauterivian and Late Hauterivian source intervals significantly control the extent of petroleum accumulation and loss in the basin. Lastly, the modelled 3D crustal configuration and Mezosoic to Cenozoic thermal regime of the WBB dispute classic uniform lithospheric stretching for the southern South African continental margin. Rather, this PhD thesis confirms that differential thinning of the lithosphere related to a transtensional pull-apart mechanism is the most appropriate for accurately predicting the evolution of basin and petroleum systems of the margin. Also, the presented 3D models currently represent the most advanced insights, and thus have clear implications for assessing associated risks in basin and prospect evaluation of the margin as well as other similar continental margins around the world.
AFRIKAANSE OPSOMMING: Hierdie studie ondersoek die korsstruktuur en evalueer die kwalitatiewe en kwantitatiewe impakte van kors-mantel-dinamika op insinkingspatroon, die termiese veld en petroleumstels evolusie aan die suidelike Suid-Afrikaanse kontinentale grens, in die hede en die verlede, deur die toepassing van ’n multidissiplinêre en multiskaal-geomodelleringsprosedure wat beide konseptuele en numeriese benaderings behels. Die modelleringsprosedure veral is belangrik aangesien hierdie kontinentale grens ’n komplekse interaksie van uitbreidings- en strekkingsparallelle tektoniek gedurende die Mesosoïese vastelandskeurprosesse daarvan dokumenteer. Omdat dit op die suidelike platvorm van Suid-Afrika geleë is, maak die Westelike Bredasdorp Kom (WBK) die fokus van hierdie studie uit, en verteenwoordig dit die westelike deel van die groter Bredasdrop-subkom, wat die verste wes is van die suidelike aflandige subkomme. Om die grens met betrekking tot sy huidige struktuur, isostatiese staat en termiese veld te verstaan, is ’n kombinasie benadering bestaande uit isostatiese, 3D-gravitasie- en 3D- termiese modellering gebruik deur potensiëleveld-, seismiese en boorgatdata te integreer Ondersteunend totot die gevolglike konfigurasie en termiese veld van die laasgenoemde deur middel van hedendaagse temperatuur, soos gemeet, vitriniet-refleksiekoëffisiënt en bronpotensiaal data, komskaal-begrawing en termiese geskiedenis en tydsberekening van brongesteentematurasie, is petroleumgenerasie, -uitwerping, -migrasie en -akkumulasie in die toekoms gesimuleer deur gebruik te maak van ’n 3D-kommodelleringstegniek. Hierdie hierargiese modelleringswerkvloei maak dit moontlik om geologiese aannames en hulle geassosieerde onsekerhede goed aan bande te lê en beter te kwantifiseer, veral in drie dimensies. In die hede word die diep kors van die WBK gekarakteriseer deur ’n drieledige digtheidstruktuur (met ander woorde voorrift-metasedimente onderlê deur bo- en benedekors domeine) wat dui op ’n baie wesenlike verdunning, beperk tot ’n dun O-W-strekkingsone. Die konfigurasie van die radiogeniese kristallyne kors, sowel as die konduktiwiteitskontraste tussen die diep kors en die vlak sedimentêre dekking, beheer grotendeels die hedendaagse termiese veld van die studiearea. Hierdie hedendaagse konfigurasie weerspieël in alle opsigte tipiese eienskappe van kom-evolusie in ’n skuifskeur omgewing. Byvoorbeeld, Die oriëntasies van die diep kors en verskuiwingbeheerde komsedimentasie byvoorbeeld is ruimtelik inkonsekwent en dui daardeur op verskillende ekstensiekinematika, tipies van transtensionale tensiemeganisme. As sulks, is sin-rift-versakking taamlik vinnig en kortstondig, en word isostatiese ekwilibrium nie by die Moho-vlak, in die besonder, bereik nie. Samehangende sin-rift vinnige versakking en hittevloeihoogtepunt het gelei tot petroleum behoud in die kom sedert die vroeë Kryt. Twee bykomende post-rift termiese anomalieë wat verband hou met die laat Kryt-“hotspot” meganisme en die Mioseense kontinentale grensopheffing in Suidelike Afrika het die sin-rift-beheer met maturasie opgevolg. Hierdie termiese maturiteit van die vyf gematureerde brongesteentes het in vier hoofgenerasie- en drie hoofakkumulasie fases, wat die totaliteit van die petroleumstelsels van die WBK karakteriseer, gekulmineer. Die Campaniese, Eoseense en Mioseense opheffings senarios het episodies bronmaturasie gestop en tersiêre migrasie van petroleum wat vroeër opgevang was veroorsaak. Addisioneel vind petroleumverlies gekoppel aan die spilpunt van elke opvanggebiedkonfigurasie tydens die laat Kryt-Paleoseen en Oligoseenvroeë Mioseen plaas. Die tydstelling en omvang van migrasiedinamika is die sensitiefste vir die geologiese scenario wat verskuiwing, seëlomseilingstelsel en fasiesheterogeniteit kombineer. Trouens, vir modelle wat nie fasiesheterogeniteit inkorporeer nie, is voorspellings van vroeëre en huidige seebodemlekkasie van petroleum grotendeels onderskattings. Hierdie komplekse wisselwerking van generasie- en migrasiemeganismes het beduidende implikasies vir die laai van petroleumakkumulasies deur veelvoudige brongesteentes. Vanweë vroeë maturasie en laatstadiumtersiêre migrasie, oefen die sin-rift-brongesteentes, veral middel Hauterivium- en laat Hauteriviumbronintervalle, beduidende beheer oor die omvang van petroleumakkumulasie en -verlies in die kom uit. Laastens weerspreek die gemodelleerde 3D-korskonfigurasie en Mesosoïese-tot-Senosoïesetermiese regime van die WBK ’n klassieke uniforme litosferiese rekking vir die suidelike Suid- Afrikaanse kontinentale grens. Inteendeel, hierdie PhD-proefskrif bevestig dat ’n differensiële verdunning van die litosfeer, gekoppel aan ’n transtensiemeganisme, die beste geskik is om ’n akkurate voorspelling oor die evolusie van kom- en petroleumstelsels van die kontinentale grens mee te maak. Verder, verteenwoordig die 3D-modelle, wat hier aangebied word, tans die mees gevorderde insigte, en het hierdie modelle dus duidelike implikasies vir die assessering van verwante risiko’s in kom- en petroleum teikene valuering van die kontinentale grens, so wel as van ander soortgelyke kontinentale grense regoor die wêreld.

An experimental characterisation of a pre-commercial spiral wound permeate gap membrane distillation module was carried out to test its performance at different operating conditions for the purpose of seawater desalination. The experimental setup consisted of a flat plate solar collector field indirectly coupled to the permeate gap membrane distillation module via an inertia tank. The operating parameters varied were the condenser inlet temperature (from 20 °C to 30 °C), evaporator inlet temperature (from 60 °C to 80 °C) and seawater feed flow rate (from 200 l/h to 400 l/h). Within this operational boundary, it was found that the maximum permeate/distillate flux was 4.135 l/(h∙m2) which equates to a distillate production/flow rate of close to 21.3 l/h. The maximum potential distillate production rate is expected to be significantly higher than this value though as the maximum manufacturer specified feed flow rate is 700 l/h and the maximum evaporator inlet temperature is rated at 90 °C. Both these parameters are positively related to the distillate production rate. The minimum specific thermal energy consumption was found to be 180 kWh/m3. A mathematical model of the overall system was developed, and experimentally validated, to mathematically describe the coupling of the membrane distillation module with a solar collector field. The effectiveness of internal heat recovery of the membrane distillation module was found to be an accurate and simple tool to evaluate the thermal energy demand of the distillation process at a given set of operation parameters. The mathematical model was used to further investigate the experimental findings and provide insights into the operational dynamics of the membrane distillation module. It was also used to determine some external conditions required for steady state operation, at a given distillation operating point, such as the minimum solar irradiation required for operation and the auxiliary cooling required in the solar collector loop for maintaining steady state conditions. Finally, general guidelines are provided toward better operational practices to improve the coupling of a solar thermal collector unit/field with a membrane distillation system using a storage tank or inertia tank.

The main questions today concerning thermal systems are their economical and environmental impacts. These entities are generally, at present, assessed on the basis of operation performances of newly installed/designed systems, during an assumed lifetime period. While this is the common way of perceiving thermal systems, performance-over-time will change as an effect of degradation, and not solely of different operation scenarios. How and to what extent is the question that needs assessing in order to evaluate if these changes will jeopardise the intended system performance requirement, hence service life (SL). The lack of knowledge/approaches and tools for assessing durability and performance-over-time of thermal systems complicates the task of incorporating these aspects in engineering. In turn, this pro-active assessment and analysis is in line with today’s performance based directives, laws and regulations; of which the working life is an essential part. The durability of materials, components and systems is not a topic that is an end in itself, but becomes a vital part in a comprehensive perspective as sustainability. The lifetime performance assessment of thermal systems, as presented in this thesis, shows that it is a vital part of the R&D in the quest of sustainable energy/thermal systems and energy use. This thesis gives knowledge to the thermal (energy) system/technology R&D and engineering sector, regarding durability and lifetime performance assessment methodologies; but also to the durability of construction works sector, regarding the needs for assessing lifetime performance of materials and components in relation to system performance. It also presents descriptions of requirements on construction works. Specifically, the studies presented in the thesis show how durability and lifetime performance assessment of thermal systems may be sought, with knowledge on: methodologies, exposure test set-ups, modelling and the attainment and use of adequate tools. The main focus is on performance-over-time modelling, tying material/component degradation to altered thermal performance, thereby attaining performance-over-time assessment tools to be used in order to incorporate these aspects when engineering thermal systems; hence enabling the forecasting of SL. The presented work was predominantly done in association to the EU project ENDOHOUSING. The project developed a solar-assisted heat pump system solution, with heat storage, to provide the thermal energy to meet space heating, cooling and hot water requirements for domestic houses in different regions of the EU. The project constituted the platform for the work presented in this thesis, thereby outlining the main context with studies on durability and lifetime performance of: flat plate solar collectors ground heat sources/storages and interaction with a heat pump system evaluation of the ENDOHOUSING solar-assisted heat pump system The thesis also presents a study of SL prediction and estimation of district heating distribution networks (an additional thermal system application). In this particular context, the Factor Method is proposed as a methodology. The main issue of lifetime performance of thermal systems is how and to what extent performance reduction in individual materials or components influence the overall system performance, as the essence of energy/thermal system sustainability is system performance.
QC 20100810

Depuis leur découverte, les alliages à mémoire de forme (AMF) one connu un développement croissant de leur applications industrielles (e.g implants médicaux, aérospatial) en relation avec leur propriétés mécaniques remarquables(effet mémoire et pseudo-élasticité). La transformation martensitique est au coeur de ces propriétés, ce type de transformation couplés (les alliages à mémoire de forme magnétiques font figure d'exception puisque pour ces alliages, le chargement magnétique est une troisième source de transformation ; ils ne seront pas traités dans ce mémoire). Un des obstacles majeurs à une utilisation beaucoup plus générale de ces matériaux est un manque de modèle de comportement robuste. Il s'agit en effect de restituer un comportement non-linéaire généralement dissipative dans un cadre thermomécanique multiaxial. Les approaches multi-échelles, utilisant un jeu de variables internes et de potentiels écrits à une échelle appropriée, peuvent permettre de relever ce défi, avec bien entendu une mise en place d'outil de localisation et d'homogénisation appropriés. Le modèle développé pendant ce travail de thèse, par une approche stochastique du calcul des fractions de variantes du milieu (l'approche peut s'appliquer à n'importe quel AMF), permet une généralisation du modèle multi-échelle réversible développé dans la thèse de Anne Maynadier (2012) dans un cadre irréversible. Une campagne d'essais expérimentaux a été réalisé en parallèle avec du development du modèle, permettant d'en identifier les paramètres intrinsèques et de le valider via un certain nombre de simulations numériques. Le matériaux retenu est un NiTi équiatomique traité pour être pseudo-élastique. Dans ce cadre , un banc d'éssai in-situ et une procédure d'identification totalement novatrice ont été développés : le bands d'éssai permet de combiner simultanément les mesures du champ cinématique par corrélation d'images numériques (CIN) et de champ de phase par diffraction des rayons X(DRX). Le tout au cours d'un essai de traction uniaxial, puis lors d'essais multiaxiaux proportionnels et non-proportionnels. Le champ cinématique permet d'avoir une description continue du phénomène de localisation en bandes de transformation; la mise en place d'une procédure de décomposition en modes propres spatio-angulaires (POD) a permis une identification robuste des champs de phases cohérente avec le champ cinématique, et mettent en évidence des phénomènes robuste d'interface et de sélection de variantes jusqu'icic jamais reportés dans le literature. Cette comparison multi-champs permet d'avoir une meilleure compréhension du phénomène de localisation en bandes et suggère une piste potentielle de développement d'un modèle de structure pertinent
Since their discovery, shape memory alloys (SMA) have experienced an increasing development of their industrial application (e.g medical implants, aerospace) in relation to their remarkable mechanical properties (memory effect and pseudo-elasticity). The martensitic transformation is at the heart of these properties, this type of transformation being able to be induced by a thermal loading, mechanical loading, pr a coupled thermomechanical loading (magnetic shape memory alloys are exceptions since for these alloys the magnetic loading is a third source of transformation: they will not be treated in this work). One of the major obstacles to a much more general use of these materials is a lack of robust constitutive model. Indeed, the behavior that is expected is a non-linear and generally dissipative behavior in a thermomechanical multiaxial framework. Multiscale approaches, using a set o f internal variables and potentials written at an appropriate scale can meet this challenge, with of course, the implementation of appropriate localisation and homogenization procedures. The model developed in this thesis, with a stochastic approach for the computation of the variants volume fractions (this approach can be applied to any SMA), allows a generalization of the reversible multi-scale model developed in the PhD of Anne Maynadier (2012) to an irreversible framework. An experimental test campaign was carried out in parallel with the development of the model, making it possible to identify ins intrinsic parameters and to validate it via a large number of numerical simulations. The material retained is a pseudo-elastic equiatomic NiTi. In this context, an in-situ test bench and a completely innovative identification procedure have been developed. the test bench makes it possible to combine the kinematic field measurements by digital images correlation (DIC) and phase field measurement by X-ray diffraction (XRD) simultaneously, during a uniaxial tensile test first, then during proportional and non proportional multiaxial loading tests. The kinematic field allows a continuous description of the localization phenomenon in transformation bands to be obtained; the implementation of a spatial-angular proper order decomposition (POD) method allows a robust identification of the coherent phase field with the kinematic field, and highlights interfaces and variant selection phenomena until now never reported in the literature. This multi-field comparison provides a better understanding of the band localisation phenomenon and suggest a potential track for developing a relevant structure model

Le champ électrique d'une puissance suffisante peut provoquer une augmentation de conductivité et perméabilité de la membrane cellulaire. L'effet est connu comme l'électroporation, attribuée à la création de voies aqueuses dans la membrane. Quantifier le transport de la matière dans le cadre d'électroporation est un objectif important. Comprendre ces processus a des ramifications dans l’extraction du jus ou l’extraction sélective des composés de cellules végétales, l'amélioration de l'administration de médicaments, et des solutions aux défis environnementaux. Il y a un manque de modèles qui pourraient être utilisés pour modéliser le transport de la matière dans les structures complexes (tissus biologiques) par rapport à l'électroporation. Cette thèse présente une description mathématique théorique (un modèle) pour étudier le transport de la matière et le transfert de la chaleur dans tissu traité par l’électroporation. Le modèle a été développé en utilisant les lois de conservation et de transport et permet le couplage des effets de l'électroporation sur la membrane des cellules individuelles au transport de la matière ou la chaleur dans le tissu. Une solution analytique a été trouvée par une simplification, mais le modèle peut être étendu avec des dépendances fonctionnelles supplémentaires et résolu numériquement. La thèse comprend cinq articles sur l'électroporation dans l'industrie alimentaire, la création de modèle pour le problème de diffusion, la traduction du modèle au problème lié à l’expression de jus, validation du modèle, ainsi que des suggestions pour une élaboration future du modèle. Un chapitre supplémentaire est dédié au transfert de la chaleur dans tissu
An electric field of sufficient strength can cause an increase of conductivity and permeability of cell membrane. Effect is known as electroporation and is attributed to creation of aqueous pathways in the membrane. Quantifying mass transport in connection with electroporation of biological tissues is an important goal. The ability to fully comprehend transport processes has ramifications in improved juice extraction and improved selective extraction of compounds from plant cells, improved drug delivery, and solutions to environmental challenges. While electroporation is intensively investigated, there is a lack of models that can be used to model mass transport in complex structures such as biological tissues with relation to electroporation. This thesis presents an attempt at constructing a theoretical mathematical description – a model, for studying mass (and heat) transfer in electroporated tissue. The model was developed employing conservation and transport laws and enables coupling effects of electroporation to the membrane of individual cells with the resulting mass transport or heat transfer in tissue. An analytical solution has been found though the model can be extended with additional dependencies to account for the phenomenon of electroporation, and solved numerically. Thesis comprises five peer-reviewed papers describing electroporation in the food industry, model creation for the problem of diffusion, translation of the model to the mathematically-related case of juice expression, model validation, as well as suggestions for possible future development, extension, and generalization. An additional chapter is dedicated to transfer of heat in tissue

Le regioni idrotermali sono interessate da una grande varietà di fenomeni naturali, tra cui episodi di rigonfiamento e subsidenza del suolo. Tra di esse, le caldere consentono di studiare le interazioni tra i processi magmatici e le dinamiche dei fluidi di cui sono imbevute le rocce porose nella crosta superficiale. Un'area idrotermale soggetta a instabilità periodiche è la caldera dei Campi Flegrei, nell'Italia meridionale. Una delle crisi più recenti, oggetto di numerosi studi e ricerche, ha avuto luogo nel biennio 1982-84. La tesi si propone di sviluppare un modello di sorgente deformativa che prevede una regione termo-poro-elastica inclusa in un solido poro-elastico semi-illimitato, nel caso in cui questa regione abbia forma cilindrica e subisca cambiamenti di temperatura e pressione di poro al suo interno. Viene fornita una soluzione semi-analitica per spostamenti e sforzi così generati all'interno e all'esterno della sorgente deformativa, e i risultati sono paragonati a quelli ottenuti tramite un approccio completamente numerico, insieme a quelli di altri tre modelli. Lo spostamento verticale risultante alla superficie libera è in buon accordo con quello prodotto dal modello di Mogi, e riproduce il pattern di deformazione verticale osservato presso i Campi Flegrei durante la crisi del 1982-84. Tuttavia, vi sono differenze nelle ampiezze delle componenti dello spostamento tra il nostro modello e gli altri due considerati. I risultati per il campo di sforzi sul piano mediano della sorgente indicano un regime compressivo al suo interno, mentre si evidenzia un regime distensivo nella regione sovrastante, e un regime trascorrente nella regione esterna del semispazio. Questo è in accordo con la distribuzione eterogenea dei meccanismi focali ottenuti dai dati sismologici relativi allo stesso episodio presso i Campi Flegrei. Alla fine dell’opera, sono discussi i limiti di applicabilità del modello e ne sono indicati alcuni possibili sviluppi ulteriori.

Cette thèse est dédiée à la caractérisation des vents descendants de vallée en terrain complexe d'orographie modérée à moyenne latitude, dans le contexte de la réglementation des rejets atmosphériques de Cadarache. Cadarache est un des centres de recherche du "Commissariat à l'énergie atomique et aux énergies alternatives" (CEA), installé dans une petite vallée (CV) confluente à la vallée de la Durance (DV). Ces deux vallées se distinguent par leur taille, et sont le siège d'écoulements aux caractéristiques différentes en stratification stable. Un forçage synoptique faible associé à un ciel dégagé sont dans la région des conditions fréquentes qui favorisent la stabilité atmosphérique et consécutivement la mauvaise dispersion des polluants, faisant de cette situation un sujet d'intérêt majeur. La campagne de mesure KASCADE (KAtabatic winds and Stability over CAdarache for Dispersion of Effluents) constitue le volet expérimental de l'étude. Réalisée pendant l'hiver 2013 elle a couvert 3 mois d'observation continue et complétée de 23 périodes d'observation intensive (POI). L'analyse montre que les écoulements descendant les vallées de Cadarache (CDV) et de la Durance (DDV) dominent pendant toute la période d'étude. La stabilité s'installant dès le coucher du soleil, le courant CDV s'épaissit progressivement. Le profil de vent en forme de jet présente son maximum à environ 30 m où il atteint 2 à 3 m s-1. Il se maintient toute la nuit et disparaît avec l'inversion de stabilité. Comme la station météorologique du centre manque de capteur de vent dans la CV même, une méthode a été développée pour diagnostiquer le CDV en exploitant l'instrumentation actuelle. Ainsi, si la prévision de ce vent n'est pas à la portée du modèle méso-échelle WRF avec une résolution kilométrique, cette méthode le permet en combinant une descente d'échelle dynamique et statistique. Le vent DDV est identifié comme un vent qui suit l'axe de la vallée, fortement corrélé à la stabilité à l'échelle régionale car il n'apparaît que la nuit lorsque le forçage synoptique est faible. Ce vent n'arrive à Cadarache que 6 à 9 heures après le coucher du soleil avec une grande variabilité. D'un autre côté, il est à son maximum au lever du soleil avant que les processus convectifs ne démarrent, et présente un jet autour de 200 m avec des vitesses de 4 à 8 m s-1 et dont la hauteur est corrélée à la profondeur de la vallée. Dans les simulations avec WRF, malgré des défauts, la DV étant bien résolue avec une maille de 1 km, l'occurrence de ce vent est assez bien simulée. Par ailleurs l'examen de ses caractéristiques spatiales montre qu'il s'agit soit d'un écoulement de drainage, soit d'un écoulement canalisé forcé. Bien qu'on ne dispose pas de données suffisantes pour élucider le mécanisme dominant de déclenchement du vent DDV, les deux précédemment identifiés sont de bons candidats
Stable stratification can be one of the most penalizing condition concerning pollutants in the atmospheric boundary layer. Over complex terrain under these conditions, the relief may modify the flow. Therefore the knowledge of down-valley wind characteristics influencing the wind field at Cadarache and its close surroundings is crucial for safety regulation in the context of sanitary impact of the site. Cadarache is a CEA research centre and located in the Prealps of southeast France. It is embedded in a small valley, the Cadarache Valley (CV), which is one of the tributaries of the larger Durance Valley (DV). The two valleys are distinct in size and therefore react differently to stable conditions, and are investigated by means of observations (field experiment KASCADE : KAtabatic winds and Stability over CADarache for Dispersion of Effluents) and simulations (the Weather Research and Forecasting (WRF) model). To investigate the valley wind behaviour, the KASCADE campaign has been designed and conducted in the winter of 2013, covering a 3-month period and 23 intensive observation periods (IOP). It resulted in a well-documented campaign, from which the analysis shows that the Cadarache and Durance down-valley (CDV and DDV respectively) winds are both dominant flows during the period of investigation. The CDV wind is a thermally driven flow, with regular wind speeds up to 2 - 3 m s-1 up to 50 m agl. It persists throughout the night and disappears in the early morning with the stability. The current observational network of Cadarache lacks means of measurement for inside CDV wind. This work shows that it can be nowcasted from available meteorological tower observations. Due to the CV small scale, currently a wind forecast on kilometer resolution is out of reach, but the methodology developed here can be used to forecast the wind through a combination of dynamical and statistical downscaling. The DDV wind has been recognized as down-valley oriented, and strongly related to stability at a regional scale, as it exists only after sunset when synoptic forcing is very weak. DDV wind arrival at Cadarache is mostly observed 6 to 9 hours after sunset, but however dominantly present around sunrise, when convectively driven processes are not yet established. Jets are observed mostly at around 200 m agl with wind speeds between 4 and 8 m s-1. Despite some (general) deficiencies of the WRF model, the DDV wind is simulated close to reality thanks to the 1-km resolution allowing a correct representation of the Durance valley orography. The ensemble of 23 simulated IOPs allowed further to characterize the flow in a spatial sense and to recognize drainage and flow channelling as most important candidates for the flow mechanism

The accessibility of the Martian atmosphere to spacecraft provides an opportunity to study an ionosphere that differs from our own. Yet, despite the half century of measurements made at Mars, the current state of the neutral atmosphere and its embedded plasma (ionosphere) remains largely uncharacterized. In situ measurements of the neutral and ionized constituents versus height exist only from the two Viking Landers from the 1970s. Subsequent satellite and remote sensing data offer sparse global coverage of the ionosphere. Thermal characteristics of the plasma environment are not well understood. Patchy crustal magnetic fields interact with the Martian plasma in a way that has not been fully studied. Hence, investigating the coupled compositional, thermal and crustal-field-affected properties of the ionosphere can provide insight into comparative systems at Earth and other planets, as well as to atypical processes such as the solar wind interaction with topside ionospheric plasma and associated pathways to escape. Ionospheric models are fundamental tools that advance our understanding of complex plasma systems. A pre-existing one-dimensional model of the Martian ionosphere has been upgraded to include more comprehensive chemistry and transport physics. This new BU Mars Ionosphere Model has been used to study the composition, thermal structure and dynamics of the Martian ionosphere. Specifically: the sensitivity of the abundance of ions to neutral atmospheric composition has been quantified, diurnal patterns of ion and electron temperatures have been derived self-consistently using supra-thermal electron heating rates, and the behavior of ionospheric plasma in crustal field regions was simulated by constructing a two-dimensional ionospheric model. Results from these studies were compared with measurements and show that (1) ion composition at Mars is highly sensitive to the abundance of neutral molecular and atomic hydrogen, (2) lighter ions heat up more efficiently than heavier ones and provide additional heating sources for cooler plasma, and (3) crustal field morphology affects plasma dynamics and structure at Mars in a way that is consistent with observations. Finally, model predictions of ion composition and plasma temperatures are provided for observations to be made by several instruments on board the upcoming 2013 MAVEN orbiter.

Large scale and cost-efficient synthesis of carbon nanostructured materials has garnered tremendous interest over the last decade owing to their plethora of engineering and bio-science applications. One promising method is roll-to-roll radio frequency chemical vapor deposition and this work presents a computational investigation of the capacitively coupled radio frequency plasma in such a system. The system operates at moderate pressures (less than 30 mbar) with an 80 kHz square wave voltage input. The computational model aids the understanding of plasma properties and α-γ transition parameters which strongly influence the nanostructure deposition characteristics in the system. One dimensional argon and hydrogen plasma models are developed to characterize the effects of input voltage, gas pressure, frequency, and waveform on the plasma properties. A hybrid mode which displays the characteristics of both α and γ discharges is found to exist for the low cycle frequency 80 kHz square wave voltage input due to the high frequency harmonics associated with a square waveform. The threshold voltage at which the transition between the different regimes occurs is higher for hydrogen than for argon owing to its diatomic nature. Collision radiative modeling is performed to predict the argon emission intensity in the discharge gap. The results are found to lie within 16% of the optical emission spectroscopy measurements with better agreement at the center of the discharge, where the measurement uncertainty is low and the emission by ions is not significant. A quasi-zero dimensional steady state chemistry model which uses the hydrogen plasma properties as inputs predicts high concentrations of C₂H, C₂H₂, C₂H₃⁺, C₂H₄⁺ and C₂H₆⁺ during carbon nanostructure deposition.

Carbon nanostructures are popularly used as field emitters. Field emission based microplasma actuators generate highly non-neutral surface discharges that can be used to heat, pump, and mix the flow through microchannels and offer an innovative solution to the problems associated with microcombustion. They provide a constant source of heat to counter the large heat loss through the combustor surface, they aid in flow transport at low Reynolds numbers without the use of moving parts, and they provide a constant supply of radicals to promote chain branching reactions. This work presents two actuator concepts for the generation of field emission microplasma, one with offset electrodes and the other with planar electrodes. They operate at input voltages in the 275 to 325 V range at a frequency of 1 GHz which is found to be the most suitable value for flow enhancement. The momentum and energy imparted by the charged particles to the neutrals as modeled by 2D Particle-In-Cell with Monte Carlo Collisions (PIC/MCC) are applied to actuate flow in microchannels using 2D Computational Fluid Dynamics modeling. The planar electrode configuration is found to be more suitable for the purpose of heating, igniting and mixing the flow, as well as improving its residence time through a 10 mm long microcombustor. The combustion of hydrogen and air with the help of 4 such actuators, each with a power consumption of 47.5 mW/cm, generates power with an efficiency of 28.8%. Coating the electrode surface with carbon nanostructures improves the combustion efficiency by a factor of 2.5 and reduces the input voltage by a factor of 6.5. Such microcombustors can be applied to all battery based systems requiring micropower generation with the ultimate goal of “generating power on a chip'”.

碩士
國立成功大學
材料科學及工程學系
104
Laser surface modification is a technique that treats the materials locally to change the material properties. Complex thermal histories are induced at different sites during the laser surface modification process because of the various laser parameters and scanning paths. Diverse thermal histories would lead to different phase transformations that finally decide the microstructure and material properties. In this study, a finite element model and a mathematical model were built and validated to provide reliable temperatures as a function of time and location and simulate the phase transformations respectively. The hardness was evaluated by the rules of mixture on the basis of the simulated phase compositions and validated by experimental results. Eventually, the simulations were conducted at different laser scanning speeds. The faster laser scanning speed leads to a smaller heat-affected zone but a larger hardness value of the heat-affected zone.

The control of the thermal cooling conditions at the start-up phase of the Direct Chill (DC) casting process for aluminum sheet ingots is difficult, and is critical from the standpoint of defect formation. Firstly, boiling water heat transfer governs the secondary cooling experienced by the ingot surfaces as they emerge from the mould. This results in varying rates of heat transfer from the ingot faces as the surface temperature of the ingot changes with time during the start-up phase. Moreover, if the ingot surface temperature at locations below the point of water impingement is high enough to promote film boiling, the water is ejected away from the surface. This can result in a sudden decrease in heat transfer and the formation of local hot spots. Also, the chill water may enter into the gap formed between the ingot base and the bottom block with the evolution of the butt curl. This process of water incursion alters the heat transfer from the base of the ingot, and in turn affects the surface temperature of the ingot faces. A comprehensive mathematical model has been developed to describe heat transfer during the start-up phase of the D.C. casting process. The model, based on the commercial finite element package ABAQUS, includes primary cooling to the mould, secondary cooling to water, and ingot base cooling. The algorithm used to account for secondary cooling to the water includes boiling curves that are a function of surface temperature, water flow rate, impingement point temperature, and position relative to the point of water impingement. In addition, the secondary cooling algorithm accounts for water ejection, which can occur at low water flow rates (low heat extraction rates). The algorithm used to describe ingot base cooling includes the drop in contact heat transfer due to base deformation (butt curl), and also the increase in heat transfer due to the process of water incursion between the ingot base and bottom block. The model has been extensively validated against temperature measurements obtained from two 711 x 1680 mm AA5182 ingots, cast under different start-up conditions (non typical "cold" practice and non-typical "hot" practice). Temperature measurements were taken at various locations on the ingot rolling and narrow faces, ingot base, and top surface of the bottom block. Ingot base deflection data were also obtained for the two test conditions. Comparison of the model predictions with the data collected from the cast/embedded thermocouples indicates that the model that accounts for the processes of water ejection and water incursion, is capable of describing the flow of heat in the early stages of the casting process, satisfactorily. The research programme represents a significant improvement over existing thermal models that do not quantitatively describe the important phenomena related to the effects of water ejection and water incursion, which are specific to the transient start-up phase of the process. The thermal model, which has been extensively validated by the industrial data, not only provides an insight into the link between ingot base cooling and secondary water cooling heat transfer during the start-up phase, but also emerges as a basis for the development of thermomechanical models, based on fundamental principles, which can be used as a powerful tool for process optimization and quality control.

In recent years, the use of first-principles based atomistic modeling technique has become extremely popular to gain better insights on the various locally modulated electronic properties of nano materials and structures. Atomistic modeling offers the benefit of predicting crystal structures, visualizing orbital distribution and electron density, as well as understanding material properties which are hard to access experimentally. The single layer MoS2 has emerged as a suitable choice for the next generation nano devices, owing to its distinctive electrical, optical and mechanical properties like, better electrostatics, increased photo luminescence, higher mechanical flexibility, etc. The realization of decananometer scale digital switches with the single layer MoS2 as the channel may provide many significant advantages such as, high On/Off current ratio, excellent electrostatic control of the gate, low leakage, etc. However, there are quite a few critical issues such as, forming low resistance source/drain contacts, achieving higher effective mobility, ensuring large scale controlled growth, etc. which need to be addressed for successful implementation of the atomically thin transistors in integrated circuits. Recent experimental demonstration showing the coexistence of metallic and semiconducting phases in the same monolayer MoS2, has attracted much attention for its use in ultra-low contact resistance-MoS2 transistors. Howbeit, the electronic structures of the metallic-to-semiconducting phase boundaries, which appear to dictate the carrier injection in such transistors, are not yet well understood. In this work, we first develop the geometrically optimized atomistic models of the 2H-1T′ hetero-phase structures with two distinct phase boundaries, β and γ. We then apply density functional theory to calculate the electronic structures for those optimized geometries. Furthermore, we employ non equilibrium Green’s function formalism to evaluate the transmission spectra and the local density of states in order to assess the Schottky barrier nature of the phase boundaries. Nonetheless, the symmetry of the source-channel and drain-channel junction, is a unique property of a metal-oxide semiconductor field effect transistor (MOSFET), which needs to be preserved while realizing sub-10 nm channel length devices using advanced technology. Employing experimental-findings-driven atomistic modeling technique, we demonstrate that such symmetry might not be preserved in an atomically thin phase-engineered MoS2- based MOSFET. It originates from the two distinct atomic patterns at phase boundaries (β and β*) when the semiconducting phase (channel) is sandwiched between the two metallic phases (source and drain). Next, using first principles based quantum transport calculations we demonstrate that due to the clusterization of “Mo” atoms in 1T′ MoS2, the transmission along the zigzag direction is significantly higher than that in the armchair direction. Moreover, to achieve excellent impedance matching with various metal contacts (such as, “Au”, “Pd”, etc.), we further develop the atomistic models of metal-1T′ MoS2 edge contact geometries and compute their resistance values. Other than the charge carrier transport, analysing the heat transport across the channel is also crucial in designing the ultra-thin next generation transistors. Hence, in this thesis work, we have investigated the electro-thermal transport properties of single layer MoS2, in quasi ballistic regime. Besides the perfect monolayer in its pristine form, we have also considered various line defects which have been experimentally observed in mechanically exfoliated MoS2 samples. Furthermore, a comprehensive study on the phonon thermal conductivity of a suspended monolayer MoS2, has been incorporated in this thesis. The studies presented in this thesis could be useful for understanding the carrier transport in atomically thin devices and designing the ultra-thin next generation transistors.

The relatively recent discovery of the giant electrocaloric effect in ferroelectric ceramics may lead to new solid state cooling technologies that are energy efficient, reliable, portable, and environmentally friendly. This phenomenon, along with many other novel field-coupled properties of ferroelectrics, such as piezoelectricity, pyroelectricity, the electro-optic effect, phase changes, and polarization switching, make these materials useful for a wide range of technological applications including sensors, ultrasound, infrared cameras, sonar, diesel engine fuel injectors, ferroelectric random access memory, electro-optic modulators, vibration control, and electrocaloric cooling devices. Most of world’s current cooling and refrigeration technology is based upon the vapor-compression cycle of a refrigerant. Refrigeration systems that are based on this technology are bulky, require moving parts in the compressor and some of them have a less than optimal environmental impact. Thin film devices that utilize the electrocaloric effect could have a significant impact on refrigeration, heat pumps, air conditioning, energy scavenging, and computer cooling systems. Especially for the latter ones, the fan-based solutions are not likely to be able to keep up with the increases in computing power and the resulting current densities in integrated circuits. The ability to make quantitative predictions of the behavior of ferroelectric structures is of significant importance given the experimental efforts on the synthesis of barium titanate nanodots, nanorods, nanowires, and nanotubes, and lead zirconate titanate (PZT) thin films, and nanoparticles, and the potential for technological applications of these structures. The research contained herein implements a full thermo-electro-mechanical continuum framework and numerical methods based on phase-field modeling to study the domain and phase structure evolution associated with the electrocaloric effect in barium titanate ferroelectric nanodots.
text

Modeling microbial inactivation has a great influence on the optimization, control and design of food processes. In the area of food safety, modeling is a valuable tool for characterizing survival curves and for supporting food safety decisions. The modeling of microbial behavior is based on the premise that the response of the microbial population to the environment factors is reproducible. And that from the past, it is possible to predict how these microorganisms would respond in other similar environments. Thus, the use of mathematical models has become an attractive and relevant tool in the food industry.

This research provides tools to relate the inactivation of microorganisms of public health importance with processing conditions used in nonisothermal and non-thermal food processing technologies. Current models employ simple approaches that do not capture the realistic behavior of microbial inactivation. This oversight brings a number of fundamental and practical issues, such as excessive or insufficient processing, which can result in quality problems (when foods are over-processed) or safety problems (when foods are under-processed). Given these issues, there is an urgent need to develop reliable models that accurately describe the inactivation of dangerous microbial cells under more realistic processing conditions and that take into account the variability on microbial population, for instance their resistance to lethal agents. To address this urgency, this dissertation focused on mathematical models, combined mathematical tools with microbiological science to develop models that, by resembling realistic and practical processing conditions, can provide a better estimation of the efficacy of food processes. The objective of the approach is to relate the processing conditions to microbial inactivation. The development of the modeling approach went through all the phases of a modeling cycle from planning, data collection, formulation of the model approach according to the data analysis, and validation of the model under different conditions than those that the approach was developed.

A non-linear ordinary differential equation was used to describe the inactivation curves with the hypothesis that the momentary inactivation rate is not constant and depends on the instantaneous processing conditions. The inactivation rate was related to key process parameters to describe the inactivation kinetics under more realistic processing conditions. From the solution of the non-linear ordinary differential equation and the optimization algorithm, safety inferences in the microbial response can be retrieved, such as the critical lethal variable that increases microbial inactivation. For example, for nonisothermal processes such as microwave heating, time-temperature profiles were modeled and incorporated into the inactivation rate equation. The critical temperature required to increase the microbial inactivation was obtained from the optimization analysis. For non-thermal processes, such as cold plasma, the time-varying concentration of reactive gas species was incorporated into the inactivation rate equation. The approach allowed the estimation of the critical gas concentration above which microbial inactivation becomes effective. For Pulsed Electric Fields (PEF), the energy density is the integral parameter that groups the wide range of parameters of the PEF process, such as the electric field strength, the treatment time and the electrical conductivity of the sample. The literature has shown that all of these parameters impact microbial inactivation. It has been hyphothesized that the inactivation rate is a function of the energy density and that above a threshold value significant microbial inactivation begins.

The differential equation was solved numerically using the Runge-Kutta method (ode45 in MATLAB ®). The lsqcurvefit function in MATLAB ® estimated the kinetic parameters. The approach to model microbial inactivation, whether when samples were subjected to nonisothermal or to non-thermal food processes, was validated using data published in the literature and/or in other samples and treatment conditions. The modeling approaches developed by this dissertation are expected to assist the food industry in the development and validation process to achieve the level of microbial reduction required by regulatory agencies. In addition, it is expected to assist the food industry in managing food safety systems through support food safety decision-making, such as the designation of the minimal critical parameter that may increase microbial inactivation. Finally, this dissertation will contribute in depth to the field of food safety and engineering, with the ultimate outcome of having a broad and highly positive impact on human health by ensuring the consumption of safe food products.

Tese de doutoramento em "Civil Engineering" (ramo do conhecimento em "Geotechnics")
The use of fully probabilistic approaches to account for uncertainties within dam engineering is a recently emerging field on which studies have been mostly done concerning the safety evaluation of dams under service. This thesis arises within this framework as a contribution on moving the process of risk analysis of dams beyond empirical knowledge, applying probabilistic tools on the numerical modelling of a roller compacted concrete (RCC) dam during its construction phase. The work developed here aims to propose a methodology so as to account for risks related to cracking during construction which may compromise the dam’s functional and structural behaviour. In order to do so, emphasis is given to uncertainties related to the material itself (i.e. strength, water-to-cement ratio, among others) as well as to ambient conditions during the construction phase of RCC dams. A thermo-chemo-mechanical model is used to describe the RCC behaviour. Concerning the probabilistic model, two aspects are studied: how the uncertainties related to the input variables are propagated through the model, and what is the influence of their dispersion on the dispersion of the output, assessed by performing a global sensitivity analysis by means of the RBD-FAST method. Also, spatial variability of some input parameters is accounted for through bi-dimensional random fields. Furthermore, a coupling between reliability methods and finite element methods is performed in order to evaluate the cracking potential of each casted RCC layer during construction by means of a cracking density concept. As an important outcome of this applied research, probability curves for cracking density within each casted layer as functions of both age and boundary conditions are predicted, which is believed to be an original contribution of this thesis. The proposed methodology may therefore be seen as a contribution to help engineers understand how uncertainties will affect the dam behaviour during construction and rely on it in the future to improve and support the design phase of the dam project.
A aplicação de métodos probabilísticos para o estudo de incertezas no ramo da engenharia de barragens é um campo em crescente ascensão no qual a grande maioria dos estudos realizados se concentra na avaliação da segurança de barragens durante o período de serviço. Este trabalho de tese situa-se neste contexto, pretendendo contribuir para a abordagem de análise de risco em barragens em betão compactado com cilindros (BCC) durante a fase de construção. Assim, é proposta uma metodologia na qual são tidos em conta riscos relacionados com a fissuração do BCC durante a sua construção, o que poderá comprometer o comportamento funcional e estrutural da barragem. As incertezas consideradas integram algumas propriedades do material (i.e. resistência, rácio água-cimento, entre outras) bem como as condições climatéricas que se observam durante a fase de de construção de barragens em BCC. Para descrever o comportamento do BCC é utilizado um modelo termo-químico-mecânico. O modelo probabilístico considera, por um lado, a propagação das incertezas relacionadas com as variáveis de entrada e, por outro, permite avaliar qual a influência que têm na dispersão da resposta do modelo. Essa influência é avaliada através de uma análise de sensibilidade global, recorrendo ao método RBD-FAST. A variabilidade espacial de alguns parâmetros de entrada é também tida em conta através de campos aleatórios bi-dimensionais. O acoplamento entre métodos de fiabilidade e elementos finitos permite avaliar o potencial de fissuração de cada camada de BCC durante a construção da barragem. Para tal é introduzido o conceito de densidade de fissuração. Esta abordagem constitui uma contribuição original, com a obtenção de curvas de probabilidade para a densidade de fissuração, avaliadas ao nível de cada camada e em função da idade e condições de fronteira. A metodologia desenvolvida constitui uma contribuição para a compreensão da influência de determinadas incertezas no comportamento da barragem durante a sua construção, podendo servir no futuro como um importante suporte à fase de projecto de barragens.
Contribution pour le controle des incertitudes dans la modelisation numerique de la performance de barrages. Application a un barrage en BCR. L’application des approches probabilistes pour tenir compte des incertitudes dans le domaine des barrages est un sujet en developpement. Cependant, la plupart des etudes ont ete realisees sur l’evaluation de la securite des barrages pendant leur service. Ce travail de these vise a appliquer ce type d’approches et a faire une contribution a l’analyse de risque des barrages en beton compacte au rouleau (BCR) des sa construction, a l’aide d’une simulation numerique. Les travaux presentes dans ce manuscrit proposent l’application d’une methodologie qui vise a quantifier la vulnerabilite vis-a-vis de l’apparition de la fissuration pendant la construction du barrage, ce qui peut aflecter a long-terme la permeabilite et par consequent, compromettre son comportement structurel. Pour ce faire, l’accent est mis sur les incertitudes liees a quelques caracteristiques des materi- aux (e.g., resistance, rapport eau-ciment, entre autres) et aux conditions environnementales pendant la phase de construction. Un modele thermo-chemo-mecanique est utilise pour decrire le comportement du BCR. En ce qui concerne le modele probabiliste, deux aspects sont etudies: i) comment les incertitudes liees aux variables d’entree sont propagees dans le modele, et ii) quelle est l’influence de leur dispersion par rapport a la dispersion totale de la sortie. Ce dernier est evalue par l’intermediaire d’une analyse de sensibilite globale eflectuee avec la meth- ode RBD-FAST. En outre, la variabilite spatiale des parametres d’entree est aussi prise en compte a travers des champs aleatoires bidimensionnels. Par ailleurs, un couplage entre des methodes de fiabilite et la methode d’elements finis est eflectue de facon a evaluer le potentiel de fissuration dans chaque couche de BCR lors de sa construction en utilisant un concept de densite’ dc fissumtion. Comme resultat important issu de ce travail de recherche, des courbes de probabilite pour la densite de fissuration sont obtenues au niveau de chaque couche en fonction de leur age et des conditions aux limites, ce qui est considérée comme étant une contribution originale de cette these. La méthodologie proposée peut etre utilise pour aider a comprendre comment les incerti- tudes vont affecter le comportement du barrage pendant sa construction et servir d’appui dans le futur pour améliorer et soutenir la phase de conception du projet de barrage. Mots-clés: Barrages BCR, Comportement thern1o-chemo-nqécanique, Incertitudes, Meth- odes de fiabilité, Analyse de sensibilité, RBD-FAST, Champs aléatoires.
The financial support by the Portuguese Foundation for Science and Technology (FCT) PhD grant (SFRH/BD/63939/2009, QREN POPH - Tipologia 4.1).