M.S. Theses
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Item Development of a bipropellant rocket engine with a focus on the injector(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) İskender, Enes Oğuz.; Çelik, Murat.Access to space relies on powerful propulsion systems to take a spacecraft out of the Earth’s atmosphere. Currently, chemical propulsion systems are the only propulsion method that are able to provide the thrust levels required to carry out these missions. Among the chemical propulsion systems, bipropellant liquid rocket engines provide the highest specific impulse. In this study, a bipropellant liquid rocket engine, and specifically its injector, is designed, manufactured and tested. The used injector is a fixed area triple impinging injector. The tests include water tests, open injector cold flow tests, open injector hot flow tests and an engine hot fire test. BUSTLab (Boğaziçi University Space Technologies Laboratory) Liquid Rocket Engine is a pressure - fed liquid oxygen - ethanol (75% ethanol - 25% water) engine. The design point for the engine is 5.65kN thrust, 254s of specific impulse, 1.25 oxidizer to fuel ratio, total mass flow rate of 2.27kg/s at 30bar chamber pressure. The engine is hot fire tested at full thrust for 12 seconds. During this test, 0.83kg/ s mass flow rate of oxidizer and 0.89kg/s mass flow rate of fuel are measured which gives 1.72kg/s total mass flow rate and 0.93 oxidizer to fuel ratio. Chamber pressure and thrust measurements during this test are inconclusive due to sealing and load cell issues. With the measured mass flow rates, the chamber pressure is calculated to be 21.2bar and the thrust is calculated as 3.62kN.Item A numerical approach for predicting hemodynamic characteristics of 3D aorta geometry under pulsatile turbulent blood flow conditions using fluid-structure interaction(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Saat, Ahmet.; Atalık, Salim Kunt.Cardiovascular diseases are the leading cause of death all around the world and harm the society in terms of economically, socially, and psychologically. Hence diagnosing cardiovascular diseases as early as possible has become vital circumstance. Since clinicians need reliable and fast numerical approaches for their urgent pre-surgery decisions, individualised risk prediction and virtual treatment planning, CFD has become widespread in biomedical especially in cardiovascular medicine. The main aim of current study is to provide insight to hemodynamic characteristics of 3-D aorta geometry with pulsatile turbulent blood flow. In line with this purpose, blood and vessel mechanism has been evaluated through numerical fluid-structure interaction (FSI) analysis that couples computational fluid dynamics (CFD) and finite element analysis (FEA). Besides, effects of turbulence modelling, viscous effects and solid domain parameters such as artery thickness, elastic modulus and Poisson’s ratio on hemodynamic characteristics have been investigated. The investigations are carried out by using twelve turbulence models, two Non- Newtonian models and different solid domain values to compare output parameters such as oscillatory shear index, velocity field characteristics, von-Mises stress and displacement. Results have shown that SST k-omega with low-Re corrections model seem to be better capable of predicting hemodynamic characteristics. Proposed computational model can be considered as an initial work for the digital twin of cardiovascular system which is described as the realistic virtual model.Item Band structure calculation of 3D ultrawide elastic metamaterials with embedded inertial amplification mechanisms(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Tetik, Zafer Gökay.; Yılmaz, Çetin.In this study, phononic band structure of three dimensional (3D) ultrawide elastic metamaterials with embedded inertial amplification mechanisms are obtained. In order to achieve that, inertial amplification mechanisms with different sizes and geometries are considered by applying periodic boundary conditions, also known as Bloch’s boundary conditions, to the unit cells. First, typical wave propagation problems in one dimensional, two dimensional, and three dimensional periodic structures studied in the literature are investigated and benchmark studies are performed by COMSOL Multiphysics and ABAQUS/MATLAB programs. In this way, these models are tested and verified so that the phonon band structure of the 3D elastic metamaterials with embedded inertial amplification mechanisms can be calculated accurately. Inertial amplification mechanisms have complex geometries and their computational costs can be very high. Thus, analyses are done by using both COMSOL Multiphysics and ABAQUS/MATLAB programs. Also, the comparison of the results obtained using these programs with the FRF results of the 3×2 octahedron array enables the determination of the most accurate model. It is very likely to encounter many problems when applying Bloch’s theorem to a complex system such as a 3D elastic metamaterial with embedded inertial amplification mechanisms. Among many problems, four possible problems are explained and their solutions are presented. Thus, it is shown that the band structure of any geometry can be easily obtained, regardless of the complexity of the geometry. To sum up, in the literature, the widest band gap in 3D is achieved by this method, and the band gap is found to be in between 6.37 - 90.26 Hz, with a ratio of the upper limit to the lower limit of 14.17. Hence, it is demonstrated that the 3D elastic metamaterial with embedded inertial amplification mechanisms impedes waves coming from all directions in a very wide frequency range.Item Design, manufacturing and testing of an ethanol(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Uç, İbrahim Safa.; Çelik, Murat.Rocket engines are getting more popular day by day with the involvement of private companies in the space industry. Reusable rockets and lander vehicles are gaining more importance with the upcoming technologies regarding space exploration. Throttleable rocket engine is one of the key components for these rockets and vehicles. In this thesis, a 5 kN pressure fed and water cooled rocket engine was designed, manufactured and tested as a starting point for developing a throttleable and regeneratively cooled rocket engine. %75 concentrated ethanol-water mixture is used as the fuel and liquid oxygen is used as the oxidizer. Engine is designed to work at 30 bar chamber pressure for 1.01 kg/s and 1.26 kg/s fuel and oxidizer mass flow rates, respectively. A preliminary design procedure for the thrust chamber is explained. Cooling channel geometry, and heat transfer between the hot combustion gases and the coolant flow are investigated. Cavitating venturies are used for the mass flow rate control. Their designs and tests are discussed in detail. A test stand is developed to test the engine. Overview of the test stand and pressure control systems are explained. Engine firing test is conducted and achieved mass flow rates are found out to be lower than the expected. 0.89 kg/s fuel and 0.83 kg/s oxidizer mass flow rates are achieved at the final complete thrust chamber test. Possible causes for this situation are investigated.Item Crack propagation analysis in elastomeric isolation bearings(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Tıknaz, Emirhan.; Özüpek, Şebnem.This thesis study focuses on the crack propagation analysis in elastomeric isolation bearings. One benchmark problem related to crack propagation analysis in a single-edge notch specimen and two problems related to interface crack modelling were studied. The results such as reaction force, J integral and strain energy values were compared with the findings from literature. In the benchmark problem, conventional FEM and extended finite element method (XFEM) were used for single edge notch specimen. The strain energy values determined from conventional FEM and XFEM were in good agreement. Advantages and limitations of XFEM were investigated and it was found that J integral is not calculated in crack propagation modeling using XFEM. Therefore, a variable that would allow calculation of energy release rate was investigated. It was determined that for small crack advances, dissipated energy values obtained from the XFEM are very close to those based on J integral values calculated from FEM. 2D axisymmetric and 3D FE models of a circular elastomeric isolation bearing containing interface cracks and subjected to compression and shear loading were analyzed. For both models, the effects of fillet radius at sharp corners and coefficient of friction on the convergence of the FE analysis were investigated and optimization of these parameters to overcome convergence difficulties was accomplished. In the analysis, three different models were analyzed to improve the run time of the computation while maintaining the accuracy. J integral and reaction force for several stationary cracks were found to be in good agreement with the results obtained from the literature. In the 3D model, partial convergence was achieved for compression. For the converged combined loading the change of the J integral with the crack size followed the correct trend.Item A two parameter characterization of edge cracked NiTi shape memory alloy under plane strain conditions(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Merde, Oğuzhan Fikri.; Alkan, Sertan.Shape memory alloys (SMAs) are metallic systems that exhibit reversible, diffusionless, martensitic phase transformation. Employing finite element analyses, the stress fields and crack tip constraints generated are examined for a NiTi SMA which exhibits superelastic behavior. For this purpose, a single edge cracked configuration satisfying plane strain conditions is subjected to uniform loading. Both pure Mode I and mixed mode (Mode I + Mode II) configurations are elaborated by changing the crack inclination angle. As a novel step, a multi-parameter fracture mechanics approach is adapted to characterize the dependence of stress field components on both asymptotic r−1/2 and radial ro terms around the crack tip. This task is accomplished by generating closed-form fitting expressions for stress components via nonlinear leastsquare regression of the full field data from finite element analyses. It has been shown that ro term plays a significant role on the stress field around the crack tip in NiTi SMAs. In characterization of crack tip constraint in NiTi, stress triaxiality parameter, Q, is utilized in the present work. To quantify the behavior of Q, the material characteristics of NiTi such as transformation start and end stresses, hardening modulus and transformation strain are varied under both pure Mode I and mixed mode configurations. The results show that martensitic transformation has an effect of stress constraint relaxation effect reflected by the decrease of Q parameter. Meanwhile promotion of transformation start stress is found to have a strong contribution in constraining crack tip, the transformation end stress is observed to have negligible effect.Item Plasmonic enhancement of absorption efficiency of nanoparticles for photothermal therapy applications(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Tahmaz, Ege Şükrü.; Ertürk, Hakan.Even though humanity is in a golden age when it comes to medical wonders, can cer is still one of the most common and deadly diseases in the world. One new treatment method is called photothermal therapy, which is the thermal ablation of cancer cells by the intravenous injection of plasmonic nanoparticles. Plasmonic nanoparticles absorb the incident light, converting almost all to heat, and increasing the temperature of the environment. But since the nanoparticles are taken intravenously, their placements in the tissue are randomized. In this thesis, the effects of adding dielectric nanoparticles to a plasmonic nanoparticle system on absorption are studied. Nanorods, nanocones, and bipyramid nanoparticles are considered as the dielectric nanoparticles due to their elongated shape. Effects of geometric parameters, such as the radius, on the absorption of the system are studied separately. Optimal parameters of each dielectric nanoparticle are identified. Multiple simulations are completed for each dielectric nanoparticle type where the nanoparticles is placed randomly in a control volume for an approximation of the randomly scattered nature of the nanoparticles in PTT. Nanocones are found to be the best dielectric nanostructure for improving absorption when the orientation of the dielectric nanoparticle can be controlled, with a 228.5% increase in absorption efficiency. However, the non-symmetric nature of the nanocone diminishes the ab sorption improvement greatly, with only a 68.1% improvement in absorption when the particles are placed randomly. Comparisons between the nanorods, nanocones and the bipyramid nanoparticles show that the slanted shaped dielectric nanoparticles are more suitable for PTT applications, while the symmetric geometry of the bipyramid nanoparticles provides more consistent improvements.Item Analysis of crack initiation in edge cracked NiTi shape memory alloy plate(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Baykara, Onat.; Alkan, Sertan.Shape Memory Alloys (SMAs) are getting more known among scientist and the engineers day by day. Therefore, their industrial, military and also daily life applications are increasing. Their unique properties make them very useful under special circumstances, however also because of that, there are some parameters (such as fracture), behaviors that need to be discovered. This thesis is designed to investigate the fracture behavior of the Shape Memory Alloys. Their super elastic behavior and martensitic transformation, especially around crack tip, is modeled on ABAQUS software followed by some MATLAB fitting evaluations in order to implement data to conventional fracture criterions. The different material properties are changed and their effects on crack angles are studied. As a result of simulations, effect of every material property and crack angle change is discussed on the last chapter. The results are interpreted as the martensitic transformation effects the crack angle according to MTS, Det and Ip crack initiation criterions due to strain hardening.Item Optimum design of stiffened composite cylindrical shells with a cutout for maximum buckling strength(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Değer, Sezer.; Sönmez, Fazıl Önder.Thin-walled composite cylindrical shells have low resistance to buckling and out-of-plane deformations. Introducing a cutout to these structures reduces the load-carrying capacity of such structures drastically. One effective way to recover the load- carrying capacity lost due to a hole is to place stiffeners around the hole. The objective of this study is to find an optimum reinforcement for thin-walled composite cylindrical shells with a cutout to maximize the buckling load and minimize the additional mass due to the reinforcement. A finite element model of a thin-walled composite cylinder with an opening is created and validated using the results of an experimental and numerical study. Then hat-type stiffeners are applied around the cutout. A parametric study is carried out to determine the effect of each stiffener parameter on the buckling strength of the structure and to choose suitable upper and lower limits for optimization. A modified simulated annealing algorithm is used to find the global optimum reinforcement design. Both the FEA and the optimizations are carried out using ANSYS Parametric Design Language (APDL). In the first step , the optimum designs are obtained for stiffeners placed in the axial direction at certain distances to the center of the cutout by varying only the cross-sectional parameters and the length of the stiffener. In the second stage, the distance to the hole center is also optimized. Finally, using the optimum stiffener dimensions optimization is performed by placing additional small stiffeners on the top and bottom of the cutout. Significant improvements are achieved by using optimum stiffener designs. The most effective parameters are found to be the stiffener length, stiffener distance to the center of the opening and stiffener height.Item Design, production and testing of a micro scale ORC(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Bayraktar, Berkhan.; Bedir, Hasan.; Anlaş, Günay.Energy efficiency and renewable energy technologies are becoming more and more important every day. Organic Rankine Cycle is a proven way of generation of power from low temperature heat sources. In this study a 1 kW Organic Rankine Cycle is designed, produced and tested at BURET Laboratory on Boğaziçi University Sarıtepe Campus. A preliminary thermodynamic analysis is done at MATLAB to select the components. After the selection of the components, the test set-up connections are designed. In order to measure temperature, pressure and flow rate data from the system, a data acquisition system is also designed and programmed. The existing oil heating and chiller systems in BURET laboratory are used as the heat source and heat sink of the system. As the expander, an automobile turbocharger is used. Due to unavailability of very high-speed generators, the compressor of the turbocharger is used to measure the power output. Finally, all of the components are connected, the cycle is built, and testing is done. Due to the unexpected behavior of the pressure sensors, limited data are taken at the most efficient part of the experiment. A maximum turbine power output of 1.55 kW is measured at the turbine output. However, due to the low rotational speed of the turbine, the mechanical power generated by the turbine is not used effectively by the compressor, and a maximum compressor power of 33.15 W is obtained.Item Modified numerical method for thin film thermal conductivity calculation with micro-raman spectroscopy(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Meydando, Taher.; Dönmezer Akgül, Fatma Nazlı.Thin film materials have recently drawn a great attention for their wide range of applications in nano- and micro-scale devices. To overcome thermal issues associated with these devices, thin film materials must be thermally characterized since they do not have the same thermal properties as their bulk counterparts. Various analytical and experimental techniques are used to find thin film thermal conductivities. Micro-Raman spectroscopy is a preferred technique among other optical thermal conductivity measurement techniques due to its non-destructive and non-contact nature. However, the thermal size effects originating from both localized heat generation from Raman laser and phonon scattering at boundaries cause erroneous estimation of the thermal conductivities with the current methods. In this study, the gray phonon Boltzmann transport equation (BTE) is used to simulate the real conditions during the Raman experiment. Thermal conductivities from the developed virtual Raman experiment are then compared with a simple slab model in which the deduction of thermal conductivity in sub-micron thicknesses is calculated using the reduced BTE heat flux through the slab, resulting from phonon directional energy densities. Due to the frequency independence of single phonon mode in the gray BTE model, our method stays ahead of most theoretical methods in calculation time while giving adequate agreement with the literature data. The results show that the results from the developed model are in a good agreement with the slab model results as well as literature values.Item Experimental and numerical analysis of foam-filled composite structures subjected to axial crushing(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Demir, Serdar.; Ersoy, Nuri.Crash components in transport vehicles are significant structures for the application of the composite materials due to their high energy absorption capacities. Even better energy absorption values can be achieved by foam filled crash components due to the interaction effect between foam core and structure wall. In this thesis study, axial crushing behavior of composite circular tube structures and foam filled composite circular tube structures were examined experimentally and numerically. Experimental tests on crash specimens were performed as quasi-static axial crushing tests. Numerical analyses carried out in ABAQUS/Explicit Finite Element Analysis software. Main goals of the study are to investigate composite tubular structures and foam filled structures axial crushing behavior by energy absorption and specific energy absorption and to simulate the axial crushing behavior of the specimens accurately. Empty composite tube specimens used in the study were manufactured from plain weave fabric carbon fiber composite with three different diameters of Ø30mm, Ø40mm and Ø50mm. Empty tubes as well as polyurethane (PU) foam filled and syntactic foam filled composite tubes were tested experimentally. Effect of the crushing by using external crush plugs on axial crushing characteristics of empty tubes were also investigated in this study. Crushing characteristics, energy absorption values and specific energy absorption values of various specimens were discussed. Experimental tests were simulated in ABAQUS finite element software by modelling composite materials by using built-in VUMAT subroutine “ABQ_PLY_FABRIC” for intra-laminar damage, and cohesive surface method for inter-laminar damage. Built-in *CRUSHABLE FOAM material model was used to model PU foams. Experimental test results and numerical simulation results were evaluated with load displacement curves and specific energy absorption (SEA) displacement curves. At the end of the study, numerical simulations exhibited coherent results to experimental studies and it is found out that specific energy absorption of the foam filled tubes is lower than empty tubes even the absorbed energy is higher.Item Resource-aware cooperative delivery mission planning for UAV and AGV(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Zabun, Ali Naci.; Öncü, Sinan.In this study, a problem related to cooperative delivery mission planning of an unmanned aerial vehicle (UAV) and an autonomous ground vehicle (AGV) is con sidered. The problem is a new sub- variant of the Traveling Salesman Problem with Drone (TSP-D), known as a variant of the Traveling Salesman Problem (TSP) in the literature, and is called the Flying Sidekick Traveling Salesman Problem (FSTSP). Challenging part of the problem is that the UAV is able to carry only one package per flight and performs only one deliver mission as well. In the study, UAV is considered as able to take-off from warehouse or any delivery points on the route of AGV and land on the AGV on any delivery point on the route of AGV or warehouse. UAV takes the package for the next delivery point while staying on the AGV. Main constraints are taken as payload capacity (regarding weight of the package) and battery capacity of the UAV. Differently from original FSTSP model in the literature, a weight-aware energy consumption model is applied for the UAV and UAV has a chargeable battery with limited capacity. Main objective of the study is to develop a computation-time efficient algorithm by dynamic programming (DP) method to find the exact solution of cooperative delivery mission planning for UAV and AGV. The applicability of dynamic programming method instead of enumeration method was analyzed by considering ex ternal factors and constraints and it was proved that DP method has time efficiency compared to Enumeration method for increased number of delivery points. On the other hand, obtained results showed that determination of the value of battery capac ity and charging rate of the UAV take lead role for the cooperative delivery mission planning for UAV and AGV.Item Ultrawide stop band in a 3D elastic metamaterial with inertial amplification mechanisms having cross flexure hinges(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Otlu, Sedef Nisan.; Yılmaz, Çetin.Inertial amplification is a new method to obtain phononic band gaps in peri odic structures. The aim in this thesis is to obtain an ultrawide stop band in three dimensions by using inertial amplification mechanisms. In order to be used in three dimensions, a two stage remote center flexure mechanism design that allows bending in two orthogonal axes is added to the ends of the inertial amplification mechanism. Moreover, cross flexure hinges that prevent undesired torsional, in-plane and out-of plane bending modes of the inertial amplification mechanism are utilized in order to maximize the stop band frequency range. An octahedron structure is formed with this mechanism, which is also used as the building block of a 3D periodic structure. It is shown that a wide stop band can be achieved with the use of cross flexure hinges and a two stage remote center flexure mechanism. By making design and dimensional changes on the mechanisms forming the octahedron, the stop band of the octahedron is widened. Finally, the stop band is maximized by optimizing the thicknesses of the flexures in the inertial amplification mechanisms.Item Design and implementation of a model predictive trajectory tracking controller for a quadcopter(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Salman, Kübra Hilal.; Öncü, Sinan.In this study, a trajectory tracking controller was designed for a quadcopter unmanned aerial vehicle (UAV), and its performance was evaluated by performing the necessary flight tests. As the controller approach, the model predictive controller (MPC), a newly popular method in aviation, has been chosen. Firstly, nonlinear dynamic equations of quadcopter were derived by Newton-Euler approximation. Then, a control algorithm was created, and a quadcopter system was produced, together with the sensors and flight computers necessary for the algorithm to work. While creating the control algorithm, it was decided to control the angle of the quadcopter UAV with the ArduPilot flight control algorithm, which is open-source software. ArduPilot software is an algorithm consisting of cascaded PID controllers and runs on the Pixhawk flight computer. The MPC algorithm was designed for trajectory tracking by considering the derived linear equations of motion. With the simulation created in the MATLAB environment, trials were made before the real flight tests and the design parameters of the MPC were decided. The tested controller software was written in Python and run on a Raspberry Pi computer. The communication between Raspberry Pi and Pixhawk flight computers was ensured, the system was made ready for flight, and trajectory tracking tests were carried out.Item Design and analysis of ultrawide elastic metamaterial for three- dimensional vibration isolation(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Acar, Berkay.; Yılmaz, Çetin.Metamaterials are materials that are modified to bring in particular desired properties that are not inherited from original substance. Using number of techniques, materials can be engineered to have completely different electrical, optical, chemical or mechanical properties without changing it’s chemical compound. The focus of this thesis is to obtain a broadband elastic metamaterial that does not allow transmission of vibration at low frequencies in three dimensions. In order to achieve broadband vibration isolation at low frequencies, inertial amplification method is utilized. Various unit cell geometries are investigated and optimization studies are conducted to widen the vibration isolation frequency range. The optimized design is manufactured and tested. It is shown that the manufactured prototype has the widest band gap in three dimensions when all the three-dimensional elastic metamaterials and phononic crystals in the literature are considered.Item Quantitative non-destructive characterization of defects in electronic packages using fuzzy inference based thermal tomography(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Muratoğlu, Çağdaş.; Ertürk, Hakan.Thermal challenges have been a roadblock for electronic packaging with the in creasing number of transistors and smaller package sizes. Thermal interface material (TIM) layers play a key role in heat dissipation at all levels within an electronic pack age. The function of TIM is to minimize the thermal contact resistance by filling the microscale gaps between the die and the integrated heat spreader (IHS). For this reason, there have been intensive efforts to achieve a high-quality TIM in electronic packaging. Defects in TIM layers must be identified during the assembly process development to obtain dependable thermal management. Non-destructive characterization techniques such as scanning acoustic microscopy or X-ray tomography have been used to identify such defects and help to advance manufacturing procedures. Thermal tomography is proposed as a low-cost alternative to these qualitative imaging techniques, all of which require high-cost devices and a long processing time. The location and size of defects are identified by evaluating the measured thermal response of a heated electronic pack age. Fuzzy inference method (FIM) is used as an image reconstruction algorithm to solve the resulting ill-posed inverse problem. The feasibility of thermal tomography is studied numerically; therefore, simulated measurements are used in this study rather than experimental data. The results indicate that the fuzzy inference based thermal tomography can be a powerful tool for quantitative non-destructive characterization of defects in the electronic packages, with less cost and shorter processing time than other established methods.Item Current crowding effects on the thermal performance of AlGaN(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Göktepe, Emine.; Dönmezer Akgül, Fatma Nazlı.In the areas, such as automotive, outdoor, and building lighting in which high brightness is very significant, high lumen AlGaN/ GaN light emitting diodes (LED) are preferred due to their low energy consumption vs high intensity light output. Although their high energy efficiency, a large proportion of electrical energy is converted into heat and causes device temperatures to rise. It is important to solve the thermal problems of the devices as the increasing temperatures will affect the properties of the devices such as longevity, lumen flux, and wavelength. The largest heat generation mainly occurs between the p-n junction area where the highest temperature values are observed. All LED luminaire and device designs are possible with accurate detection of junction temperatures. In high lumen applications, non-uniform distribution of current density known as current crowding will affect unexpectedly high junction temperatures may exceed the safe limit and result in degradation in/around thermal hotspots. However, it is very difficult to obtain the junction temperatures of variations at high resolution with current experimental techniques. Junction temperatures can be determined more precisely with multidimensional electro-thermal models by considering the nonuniformities such as current crowding. There are very few studies in the literature where electrical and thermal models are combined. Two-dimensional electrothermal simulations based on finite-element method numerical simulation will be developed to study the electrical and thermal properties of chip level high-lumen LEDs. Junction temperature distribution of LED chips in LED array structures will be obtained considering the effects of current crowding and recombination phenomenon. Hotspot formations in the device structure due to the current crowding effect will be investigated. Simulations will be performed by the commercial software COMSOL Multiphysics Semiconductor Module.Item Dynamic modeling of organic rankine cycle and its experimental verification(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Altun, Ertuğrul.; Bedir, Hasan.; Anlaş, Günay.Renewable energy technologies and waste heat recovery systems have become more important all around the world because of global warming, global climate change, and countries' tendency to decrease fossil fuel consumption. Organic Rankine Cycle is one of the most popular research topics on waste heat recovery systems. In this thesis, a dynamic model of a lab scale Organic Rankine Cycle (ORC) is developed. The main objective of this thesis is to create a dynamic model for ORC system at Bogazici University Renewable Energy Technologies (BURET) laboratory including heat loses and pressure losses in pipes and to validate the model by comparing the simulation results with experiments done at BURET laboratory. The cycle parameters such as temperature and pressure are calculated transiently in the dynamic model. The model is obtained using Modelica language and Dymola program which is an object-oriented modeling software. After the dynamic model testers for each cycle component are created, component models' simulation results are verified by using experimental data taken from ORC setup at BURET laboratory. Once the component models are verified, whole dynamic ORC model is created. Experimental studies are conducted using the ORC setup at BURET laboratory and the results are compared to those of the dynamic model built in this study. It is found that temperature and pressure values for each component's inlet and outlet are accurately simulated by the dynamic model created in this study.Item Investigation of extrudability of different aluminum alloys by using the method of hot extrusion(Thesis (M.S.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2005., 2005.) Çıtak, Hasan Cem.; Altıntaş, Sabri.; Yılmaz, Metin.Aluminum extrusion is a very important forming process in metal industries. Byusing different dies and technologies, high variety of products can be produced byusing the method extrusion. Extrudability is an important parameter for extrusion processes. For evaluating extrudability of a material not only force and energyrequirements must be investigated but also surface quality and mechanical properties ofproduct must be considered properly.In this study, by using the method hot extrusion, extrudabilities of AA 6063 andAA 7075 aluminum alloys are investigated. Furthermore, by using die-bearing angled dies with relief and choke systems, effect of die-bearing angle on the extrudability ofthese alloys is tried to be evaluated. Three different choke dies with die-bearing angleof 1.5o choke , 1o choke , 0.5o angle, one type of relief die with die-bearing angle of 0.3o relief adding to all these a standard extrusion die, with no angle system is used forexperiments. During experiments, AA 6063 and AA 7075 alloys are used as billetswith 30 mm diameter and 80 mm length. A special designed hydraulic hot extrusionpress with 350 bars of maximum extrusion pressure is used for extruding the billets. Inorder to discuss results of experiments properly, SEM images of samples are obtained, for detecting surface qualities of samples roughness test is conducted and for evaluatingmechanical properties of samples, tensile test and hardness test are made. Also forinvestigating flow behaviours of samples during extrusion a special microstructuralstudy is made.