Ph.D. Theses
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Browsing Ph.D. Theses by Author "Bedir, Hasan."
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Item Core-shell PVA / gelatin nanofibrous scaffolds using multinozzle aqueous electrospinning(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2019., 2019.) Şengör, Mustafa.; Bedir, Hasan.; Altıntaş, Sabri.Biological scaffolds have been used in the reconstruction of the damaged tissues. They have similar morphology and structure to the host tissues. However, they can be produced using materials that can be harmful to humans and the environment. In this context, core-shell nano ber based sca olds, whose mechanical strengths are provided by PVA(poly vinyl alcohol) and recognition sites are provided by gelatin, were fabricated in a non-woven manner using multiple nozzles of electrospinning technique. Instead of widely used toxic, acidic or salt-based ionic solvents, deionized \water" was used as the only solvent for both polymers. Firstly, nano bers were produced from 8 % (w / w) gelatin and 8%(w / w) PVA solutions individually. Limits were determined for parameters such as voltage, feed rate, temperature and polymer concentrations. Although pure gelatin nano bers have diameters of less than 50 nm, they have beaded structure and have lower mechanical strengths. Smooth bers were obtained from 8% PVA. Fibers with PVA: gelatin core shell morphology were then produced at di erent feed rate ratios (FRR). Based on the ber diameter, the optimal FRR with a 15 kV voltage magnitude and 15 cm electrode distance was found to be 1: 1 with an average diameter of 280 nm. The ratio of 1: 3 and 1: 4 was seen as the formation of \beaded" bers and the pealing limit of gelatin over PVA, respectively. Mechanical and water resistance of the produced sca olds was further improved by cross-linking. Core - shell morphology was demonstrated by TEM, SEM, EDS analysis. The secondary structure of the gelatin from collagen and the e ects of the electrospinning were revealed by FTIR and DSC. Approximately 60% of all cross-linked sca olds were degraded in solution using lysozyme enzyme up to day 14.Item Turbulent combustion modelling of fuels using LES(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Güryuva, Serdar.; Bedir, Hasan.Combustion simulations with high fidelity turbulence models and detailed chemistry may suffer from high computational power requirements due to the combined cost of time-scale dissipation and small integration steps. Such a limitation can be avoided by employing a hybrid reaction mechanism reduction method called local self-similarity tabulation (LS2T). LS2T directly solves several dominant species reactions and incorporates the effects of other species on dominant ones by data retrieval from pre-calculated tables. This study is based on the application of the LS2T method to high fidelity 3D combustion simulations with different fuels and combustion physics. The test cases that are selected for demonstration purposes are Sandia Flame-D, the premixed methane combustion, and Sandia Spray-A, the non-premixed n-dodecane combustion. The combustion simulations use large eddy simulation (LES) as turbulence solver and transported probability density function (TPDF) for species transport, to increase the accuracy of the simulation and avoid the use of any additional reaction model. This study is the first demonstrator of LS2T approach application to 3D combustion problem with Sandia Flame-D simulation and it is also the very first Spray-A simulation that is executed using LES and TPDF in a 3D resolved domain. The report consists of wide literature research regarding the LES combustion analyses of both test cases and chemistry reduction techniques applied, a detailed theory behind all the physics solution methods used, the computational methods implemented for the study, results, and discussions of the 0D and 3D simulations. The results show that by the use of the LS2T method, it is possible to have high accuracy and generate results similar to the detailed chemistry while maintaining an acceptable computational effort.