Phd Degree / Doktora
Permanent URI for this collectionhttps://hdl.handle.net/11147/2869
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Doctoral Thesis Improvement of It-Sofc by Tailoring the Microstructure of Lscf Cathode and Gdc Electrolyte(Izmir Institute of Technology, 2019) Sındıraç, Can; Akkurt, Sedat; Büyükaksoy, AligülThe high operating temperature of solid oxide fuel cells (SOFC) brings about some restrictions like the high cost of fuel cell, long start-up time, thermal stress, and decreasing lifetime. Thus, lowering the operating temperature is vital for improvement of SOFC. However, reducing the operating temperature leads to some negative effects on solid oxide fuel cell performance by increasing the electrolyte and electrode resistances. This dissertation focuses on tailoring the cathode and/or electrolyte layers to obtain improved electrochemical performances. For this purpose, some strategies are proposed. These are (i) thin film cathode nanocomposite cathode layer formation, (ii) infiltration of porous GDC by LSCF/LSCF+GDC, (iii) infiltration of porous GDC by GDC solution to improve densification at lower temperature and finally (iv) electrochemical characterization of GDC densified by infiltration.Doctoral Thesis Macro-micro robotic manipulation: A laser cutting case study(Izmir Institute of Technology, 2019) Uzunoğlu, Emre; Dede, Mehmet İsmet Can; Kiper, GökhanThis dissertation focuses on investigation and devising of proper methodology to utilize a special type of kinematic redundancy, namely macro–micro manipulation, in the scope of robotic science. Briefly, macro-micro manipulation is comprised of two kinematically different mechanisms that have distinct characteristics, which work on macro and micro-scale. Aim of this dissertation is to present the most convenient motion planning and control algorithm to resolve kinematic redundancy for macro-micro manipulation concept. Controller designs, including the motion planning algorithms are devised taking into account the selected industrial case study. Additionally, a general framework controller is developed for macro-micro manipulation. Experiments with industrial setup and simulation verifications are done for the proposed methodologies. It is proven with simulation test results and experiments that the task completion duration for a laser cutting machine is reduced by enhancing the acceleration capability with the macro-micro manipulation. Although the proposed methodologies are implemented for a specific case, it can also be used for other systems considering the versatility of the proposed methodology. The core novelty of this research is the introduction of methodologies in order to achieve the maximum efficiency for the combined use of macro and micro-scaled manipulators. As an outcome of this study, a redundant laser cutting machine that can move with higher accelerations by making use of macro-micro manipulation is developed as a first of its kind in the Turkish machine industry and third of its kind in the world.Doctoral Thesis Modeling, Simulation and Analysis of Type-Iii Composite Overwrapped Pressure Vessels for High-Pressure Gas Storage(Izmir Institute of Technology, 2019) Kangal, Serkan; Tanoğlu, MetinIn this thesis, multi-layered composite overwrapped pressure vessels (COPVs) for high-pressure gaseous storage were modeled by finite element (FE) method and manufactured by filament winding technique. Two liners with distinct geometries were utilized for containing gas and forming a basis for composite filament winding. 34CrMo4 steel as a load-sharing metallic liner was selected for investigation of hybridization effects. Glass and carbon filaments were overwrapped to the liner with a winding angle of [±11°/90°2]3 to obtain a fully overwrapped composite reinforced vessel with non-identical front and back dome endings. The other type of liner was made of Al 6061-T6 and chosen for containing high-pressure gas such as hydrogen and its better strength-to-weight ratio suitable for onboard applications. Doily layers were implemented to the structure for inducing safe burst modes and increasing the burst pressure of the aluminum-based COPVs. All vessels were hydrostatically loaded with increasing internal pressure up to the burst pressure. The mechanical performances of pressure vessels were investigated by both experimental and numerical approaches. In numerical approaches, FE analysis was performed featuring a simple progressive damage model available in ANSYS for composite section. The metal liners were modeled as elastic-plastic material with two different hardening approaches; bilinear and multilinear hardening. The results from steel based COPV indicate that the FE model provided a good correlation between experimental and numerical strain results for the vessels with indications that the composite interlayer hybridization has positive effects on radial deformation of the COPVs. The constructed model for aluminum-based COPVs was also able to predict experimental burst pressures within a range of 8%.Doctoral Thesis Development of Energy-Efficient Personalized Thermal Comfort Driven Control in Hvac Systems(Izmir Institute of Technology, 2018) Turhan, Cihan; Gökçen Akkurt, Gülden; Simani, SilvioIncreasing thermal comfort and reducing energy consumption are two main objectives of advanced HVAC control systems. Studies conducted in the last decade show that intelligent HVAC systems can geatly affect thermal comfort, health, satisfaction, and productivity of building occupants while decreasing the energey consumption. Also, personelized thermal comfort driven control of the HVAC systems is the most effective way of saving energy and maintaining thermal comfort. In this thesis, an energy-efficient personalized thermal comfort control algorithm is developed to improve HVAC control systems. The thesis presents a complete system to control algorithm which includes the deployment of wireless sensor network. First a novel control algorithm is developed to perceived comfort conditions of occupants and to save energy. Then, a prototype of the personalized thermal comfort driven controller (PTC-DC) is manufactured an tested in a case building at İzmir Institute of Technology Campus, İzmir/Turkey. The proposed control strategy is tested betwen July 3rd, 2017 and November 1st, 2018, and compared with conventional controller in terms of energey saving and boath energetic and exergetic approaches of thermal comfort. The results showed that PTC-DC satisfies neutral thermal comfort for 92% of total measurements days while AM=0 for only 6% of total measurement days for conventional controller. From energy consumption point of wiev, PTC-DC decreased energy consumption by 13.2% compared to conventional controller.Doctoral Thesis The Penetration Behavior of Repeated Hemisphere Core Sandwich Structures: an Experimental and Numerical Study(Izmir Institute of Technology, 2018) Turan, Ali Kıvanç; Taşdemirci, Alper; Güden, MustafaIn this study, penetration behavior of novel core structure consisting hemispherical and cylindrical parts was investigated. Core units were manufactured with deep drawing method in two thicknesses to have monolithic form without any sort of assembly method or element. Produced specimens were then subjected to penetration tests at low and intermediate velocities against blunt, conical and hemispherical tipped indenters using special fixtures and apparatuses on conventional testing equipment. Effect of heat treatment on penetration behavior was investigated to observe whether residual stresses arise from manufacturing process changes the penetration behavior. Confinement effects were studied experimentally with a special fixture, allowing tested specimen to be radially confined with other core units as in an armor structure. Finally, experimental work was finished by conducting a case study where core units were subjected to spherical projectile impact up to impact velocities of 180 m.s-1 in a composite sandwich structure. Results show that each indenter geometry showed unique deformation characteristics in testing of both core units and both of the core geometries were able to hold a steel spherical projectile with mass of 110 g without full perforation at impact velocity of 180 m.s-1. Details of experimental results were presented in each chapter. Study also included modeling parts where core units were numerically produced with residual stresses and strains and good correlation was noted where thickness was compared with actual measurements on core units. Test conducted on single core structure in as-received and heat-treated condition were also repeated in numerical environment, where numerical study exhibited good correlation on both forcedisplacement curves and deformation of core units with tests. Correlation achieved with experimental study has led into further investigations of strain rate and micro-inertia where behavior of core units was studied at numerical impact velocities of 300 m.s-1. Results show that both strain rate and micro-inertia increase the local maximums and average of force levels. Effect of strain rate and micro-inertia is clearly distinguished for a threshold displacement level where micro-inertia is further dominant on behavior.Doctoral Thesis Optimum Design of Carbon/Epoxy Composite Laminates for Maximum Fatigue Life Using Multiaxial Prediction Models(Izmir Institute of Technology, 2017) Deveci, Hamza Arda; Artem, Hatice Seçil; Artem, Hatice SeçilIn this thesis study, the aim is to propose a methodology on the optimum stacking sequence design of carbon/epoxy composite laminates under various cyclic loading conditions for maximum fatigue life. In this respect, first, fatigue life prediction models, Failure Tensor Polynomial in Fatigue (FTPF), Fawaz-Ellyin (FWE), Sims-Brogdon (SB) and Shokrieh-Taheri (ST) are selected to investigate their prediction capabilities in multidirectional laminates and optimization capabilities in laminate design for maximum fatigue life. An experimental correlation study is performed for different multidirectional composite materials to evaluate the prediction capability of the models by comparing to each other. The predictions of the models give accurate and close results for all the composites in many lay-up configurations. Then, the optimum designs for maximum fatigue life are obtained for glass/epoxy composite laminate from the literature using different powerful hybrid algorithms to determine the optimization capability of the models. The results of the optimization imply that FTPF and SB models produce more consistent fatigue-resistant designs than FWE and ST models. After obtaining reasonable theoretical derivations, the methodology for fatigue-resistant design is applied to carbon/epoxy composite laminates under proper cyclic loading conditions. For this, first, quasi-static and fatigue strength properties of the carbon/epoxy laminates are determined by experimental procedure. Then, many problems including different design cases are solved using the FTPF model and hybrid PSA-GPSA algorithm, and multidirectional laminate designs with maximum fatigue life are determined. The results show that fatigue strength of the composite laminates can be seriously increased by appropriate stacking sequence designs.Doctoral Thesis Toughening of Carbon Fiber Based Composites With Electrospun Fabric Layers(Izmir Institute of Technology, 2017) Beylergil, Bertan; Tanoğlu, Metin; Aktaş, EnginThe objective of this PhD thesis is to investigate interlaminar Mode-I fracture toughness of carbon fiber/epoxy composites interleaved by micro and/or nano scaled PA66 nonwoven veils. Also, the effects of electrospun PVA nanofibers on the mechanical performance of these composites were investigated. Additionally, this thesis also deals with the effects of aramid nonwoven veils on the mechanical properties of CF/EP composites. The produced nanofibers produced by electrospinning were directly deposited on carbon fiber fabrics. Then, reference and nano-modified laminates were manufactured by vacuum infusion method. A series of mechanical tests such as tensile, compression, three point bending, Charpy-impact, interlaminar shear strength and open hole tensile tests (OHT) were carried out on the prepared specimens. Double cantilever beam (DCB) tests were conducted on reference and interleaf-modified laminates. The effect of PA 66 nanofiber areal weight density was also evaluated with varying electrospinning time. Scanning electron microscopy (SEM) was used to investigate the fiber morphology and to understand the toughening mechanisms. Dynamic mechanic analysis (DMA) was used to investigate the thermo-mechanical behavior of reference and interleaf-modified composite specimens. Differential scanning calorimetry (DSC) was used to determine the thermal properties of micro and electrospun PA66 nonwoven veils. Comparing the mechanical test results, the most effective nonwoven interleaving system was determined in terms of higher delamination resistance and in-plane mechanical properties. Finite element method (FEM) was used to evaluate the effects of electrospun PA66 nonwoven veils on the CF/EP composites. Numerical simulations of Mode-I fracture toughness tests were carried out using ANSYS Workbench. The results showed that the most effective material was electrospun PA66 nonwovens considering the higher delamination resistance. Additionally, the electrospun PA 66 nonwovens also improved Charpy-impact and interlaminar shear strength of the reference CF/EP composites. Numerical results showed good agreement with the experimental ones.Doctoral Thesis An Experimental and Numerical Study on Interfacial Convective Heat Transfer Coefficient and Thermal Dispersion Conductivity of a Periodic Porous Medium Under Mixed Convection Heat Transfer(Izmir Institute of Technology, 2017) Çelik, Hasan; Özkol, Ünver; Mobedi, MoghtadaThe need on effective heat transfer enhancement has been increasing day by day. Because of that, researchers/engineers who work on heat transfer are required to obtain new techniques to address raising accumulation of heat transfer. Heat transfer can be enhanced by active and passive methods and passive methods are mostly chosen, as no external power input is required. Porous media is one of the most popular passive heat transfer techniques. Porous media can be divided into periodic and stochastic structures. In this thesis, the analysis of heat and fluid flow in 2D periodic structure and 3D aluminum and ceramic foam structures under mixed and forced convection heat transfer are studied. The governing equations are solved at pore scale and volume-averaged transport parameters as permeability, inertia coefficient, interfacial heat transfer coefficient and thermal dispersion are obtained by using volume averaging of the obtained pore scale velocity, pressure and temperature. For the change of periodic structure, the interfacial heat transfer coefficient and thermal dispersion with respect to Reynolds, Richardson and porosity under mixed convection are studied probably for the first time in literature. For foam structure, the changes of permeability, inertia coefficient, interfacial heat transfer coefficient and thermal dispersion with respect to Re are discussed. The determination of thermal dispersion by using tomography method is probably reported for the first time. For 2D periodic structures, the interfacial convective heat transfer coefficient successfully found while for the thermal dispersion conductivity the Volume Averaging Technique fails for high Richardson numbers under mixed convection. Based on good agreement between the computational values of this study and reported correlation in literature, it is observed that the use of micro-tomography technique for determination of volume-averaged transport parameters yield satisfactory results if properly used. The comprehensive methods for inspection, verification and validation of the obtained computational results for 3D digitally generated foam are suggested.Doctoral Thesis Finite Element Simulations of Impact Test for Light Alloy Wheels(Izmir Institute of Technology, 2016) Pehlivanoğlu, Uğur; Yardımoğlu, BülentStatic and dynamic finite element models for the simulation of the wheel impact test defined in ISO7141 were developed for the AlSi7Mg and AlSi11Mg alloy wheels. The dynamic model consists of the striker, the wheel with radial pneumatic tire, and the hub adapter structure. Two types of tire models, composite and simplified, are formed in this study. The finite element model in the dynamic model, referred to as composite tire, involves bead, bead core, casing and crown plies, tread, and side walls. A simplified tire model that does not include bead cores, casing and crown plies is also generated. Although these items are not used in the second model directly, they are considered using their equivalent effects. It is shown that a simplified tire model can be used instead of the composite tire model. The dynamic model is validated by experimental studies. Such studies are related to the plastic deformations at the impact point of the wheel. It is shown that simulation of the failure of the wheel during impact tests can be determined using von Mises and effective plastic strain occurs in the wheel. In total, forty-one experiments are done to see the wheel behaviors and whether it performes according to the standard. The experimental results and the corresponding simulations focusing on von Mises stresses along with effective strains are shown in box plots. Thus, critical values for design are found. The static model consists of the wheel with simplified tire and the lumped model of the hub adapter structure. The stiffness characteristic of the impact point of the wheel is determined by using the static model. It is shown that the maximum von Mises stress that occurs in the wheel due to impact load is found using energy conversions. Significant time can be saved by this manner.Doctoral Thesis Aerodynamic Optimization of a Transonic Aero-Engine Fan Module(Izmir Institute of Technology, 2016) Kor, Orçun; Özkol, ÜnverAerodynamic design of an aero-engine fan blade is a multi-step process with multi-variables. The general purpose in aerodynamic design is to obtain proper blade angles and flowpath geometry providing the necessary pressure ratio with maximum efficiency, while respecting the structural and aerodynamic constraints. The throughflow design in aerodynamic design procedure is a key step where one can obtain a basic aero-design which generally fixes 80% to 90% of the final fan geometry, by adjusting parameters like blade exit angle distribution, solidity, hub and shroud contour, meridional chord length, etc. Throughout this procedure, the aim of the designer is to obtain an optimum (i.e. light, reliable and robust) system with highest efficiency. Among optimization methods, zero order methods are reported to fit best for turbomachinery problems, due to their good performance in discrete and non-differentiable problems and their ability to find the global optimum. Genetic algorithm is the most widely used optimization method in turbomachinery optimization. Methods inspired by swarm intelligence are reported as promising global optimizers, whereas, to the author’s knowledge, there are no reported studies that employs such algorithms in turbomachinery throughflow optimization. These methods can find the neighborhood that provides the globally optimum design, rather than exactly finding the global design. This drawback is overcome by hybridizing genetic/swarm inspired algorithms by first order (gradient based) methods. Within this aspect, the present study focuses on developing genetic and swarm inspired algorithms hybridized with gradient based algorithms to find the optimum throughflow design of a transonic aero-engine fan module.