Ph.D. Theses
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Browsing Ph.D. Theses by Author "Ertürk, Hakan."
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Item Advanced coating technologies with spectral alteration for solar applications(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2018., 2018.) Yalçın, Refet Ali.; Ertürk, Hakan.Spectrally selective coatings are used to maximize the efficiency of solar thermal systems and they are designed based on the application. This study focuses on two solar applications; solar thermal energy systems and greenhouses. For solar thermal energy systems, the coating should have high absorptance at solar wavelengths and low emittance at the infrared wavelengths, where absorber emits heat to maximize the heat transfer to the working fluid. For greenhouse applications, coating should provide high radiation at the photosynthetic spectrum and distribute light uniformly and diffusely. This study focuses on fluorescent and non-fluorescent pigmented coatings that consist of a binder and well dispersed nanometer or micrometer sized particles that are known as pigments, selected to achieve the desired spectrally selective behavior based on application. Radiative behavior of coatings depends on coating thickness, pigment size, concentration, and the optical properties of the binder and pigment materials that can be identified by modeling the radiative transfer across these coatings. Models are developed for the problems considered to solve the radiative transfer equation based on the governing physics to predict the spectral reflectance, transmittance and light distributions in conjunction with Lorenz-Mie theory and T-matrix methods that are used for predicting radiative transfer properties. These models are used to design coatings to achieve optimal behavior for considered applications. It is found that the model used for designing pigmented coatings of solar thermal systems can be very critical, and coatings must be designed using a unified model considering the effective medium theory and four flux method together with Lorentz-Mie theory. Besides, it is found that while fluorescent coatings can improve spectral distribution of irradiation for photosynthetic production, they also lead to a significant decrease in the transmittance, decreasing the irradiance when used for traditional greenhouses. However, for vertical farms it is found that using fluorescent particles in coatings both improve distribution of light and effective PAR, resulting around 35% increase in yearly crop production for lettuce.Item Design of spectrally selective coatings for high efficiency power generation devices(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2017., 2018.) Khosroshahi, Ferhad Kazemi.; Ertürk, Hakan.; Mengüç, M. Pınar.This thesis is aimed at designing and optimizing spectrally selective emitters/ filters for optical or thermal applications such as thermophotovoltaic (TPV) devices. Using spectrally selective emitters/filters in these devices is a crucial step to approach to an optimum system. Design of a spectrally selective filter based on one-dimensional Si/SiO2 layers is considered first for improved performance of TPV devices. Spec trally selective filters transmit only the convertible radiation from the emitter as non convertible radiation leads to a reduction in cell efficiency due to heating. The presented Si/SiO2 based filter concept reflects the major part of the unconvertible range back to the emitter to minimize energy required for the process and it is adaptable to differ ent types of cells and emitters with different temperatures since its cut-off wavelength can be tuned. While this study mainly focuses on InGaSb based TPV cell, Si, GaSb, and Ga0.78In0.22As0.19Sb0.81 based cells are also examined. The simulations show that significant enhancement in the overall system and device efficiency is possible by using such filters with TPV devices. In addition, graphene based spectrally selective nano structures are theoretically investigated to achieve absorption and transmission within narrow wavelength bands. Two concepts are identified to control the spectral ab sorptance and transmittance and the results showed that using these two-dimensional, multi-layered structures, with gratings and graphene layers narrow-band absorptance and transmittance can be achieved. The effect of the graphene layer is identified for the emitter structure using power dissipation profiles. The suggested filter structure is then optimized for a TPV system and it is shown that the overall TPV system efficiency can be improved by using the optimized filter. The methodology described in this thesis allows for an improved emitters/filters dItem Molecular dynamics study of water-HBN nanofluid(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2018., 2018.) Akıner, Tolga.; Ertürk, Hakan.; Mason, Jeremy K..This study considers the molecular simulations of nanofluids and the goal is to investigate the thermomechanical mechanisms in nanoscale thermal transport. The enhanced thermal conductivity and limited shear viscosity increase is the fundamental phenomena that makes nanofluids as a hot research topic of the recent thermal-fluid and nanoscience literature, and a potential novel complex liquids for variety of appli cations. The nanofluid problem has been studied from the nanomechanical point of view and molecular dynamics simulations are used to investigate the physical aspects. A water-copper system has been modelled as a benchmark study to understand the nanocolloid concept and the capacity of existing methodologies. Green-Kubo formal ism, pure water system, thermal enhancement and viscosity increase of water-copper nanofluids and Brownian motion effect has been studied and compared with the exper imental results. Potential function improvement has been aimed for a water-hexagonal boron nitride system to obtain a robust mathematical foundation for the molecular dynamics simulations. Therefore, interlayer interactions of hexagonal boron nitride and interface interactions at the water-hexagonal boron nitride interface have been formulated using recent quantum simulation results and experimental data. Thermo mechanical properties of hexagonal boron nitride have been accurately estimated using simulations with derived potentials, and water-hexagonal boron nitride interfacial dy namics have been discussed for the interfacial thermal transport. A new temperature calculation algorithm for non-equilibrium simulations has been introduced and tested for rigid and flexible water model. A new approach has been preliminarily developed to study the agglomeration in nanofluids with orthotropic nanoparticles using simulations and experimental images.