Phd Degree / Doktora

Permanent URI for this collectionhttps://hdl.handle.net/11147/2869

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Author: search.filters.author.Özkol, Ünver

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Now showing 1 - 4 of 4
  • 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, Moghtada
    The 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
    Aerodynamic Optimization of a Transonic Aero-Engine Fan Module
    (Izmir Institute of Technology, 2016) Kor, Orçun; Özkol, Ünver
    Aerodynamic 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.
  • Doctoral Thesis
    Development of a Streamline Curvature Throughflow Design Method for Fan Module of Turbofan Engines
    (Izmir Institute of Technology, 2015) Acarer, Sercan; Özkol, Ünver
    Through-flow modeling of turbomachinery flows is the principle tool for inverse design, off-design analysis and post-processing of test data, due to its capability to simulate the principal aspects of turbomachinery flows, swirling flow with rotors and stators, in the axisymmetric meridian plane with minimum two orders of magnitude smaller computational time compared to three-dimensional analysis methods. Turbomachine energy equation and empirical models for incidence, deviation, pressure loss and blockage are used to define source terms for an axisymmetric compressible flow solution. Even though the subject has been studied in numerous aspects for compressors and turbines, open literature on fully coupled fan and splitter design of turbofan engines is still limited. The present study addresses this void by developing a new split-flow method for inverse streamline curvature flow solution methodology in the course of this thesis. Hybridized empirical models that are compiled from the literature are implemented as a baseline to be calibrated. The method is validated both experimentally and numerically on a total of six different test cases within a three-step validation strategy. Firstly, split-flow solutions of the developed method for three representative duct geometries, but without a turbomachinery, are validated. Secondly, two different single-stream transonic fans, NASA 2-stage fan and a custom-designed fan stage are used to experimentally and numerically validate the empirical models, respectively. Thirdly, experimental data of GE-NASA by-pass fan is used to validate the complete models. It is shown that the accuracy of solutions in the tested cases are within less than 1.6% in pressure ratio, 2.3% in efficiency, 8% in velocity and 1.8 degree in flow angle. With this accuracy level, the proposed method is shown to be valid and can be implemented into existing compressor streamline curvature methodologies with minimal numerical effort.
  • Doctoral Thesis
    An Experimental and Numerical Study on Effects of Pore To Throat Size Ratio on Macroscopic Transport Parameters in Porous Media
    (Izmir Institute of Technology, 2015) Özgümüş, Türküler; Mobedi, Moghtada; Özkol, Ünver
    Heat and fluid flow in porous media are frequently encountered in natural and industrial applications, such as oil recovery, water supply management in hydrogeology, ground heat storage, nuclear waste disposals, and ground water flow modeling. Fluid flow and heat transfer analyses in porous media have gained recent attention. The theoretical analysis of heat and fluid flow in porous media is troublesome. That’s why some methods were developed to overcome the difficulties. One of these methods is the macroscopic method in which the solid and fluid phases are combined and the porous media is represented as an imaginary continuum domain. For the application of the macroscopic method onto a porous medium, the macroscopic transport properties such as permeability and thermal dispersion of the corresponding medium should be known. Many parameters such as pore to throat size ratio, porosity, Reynolds number, solid-to-fluid thermal conductivity ratio influence the macroscopic transport parameters. In this study, the fluid flow and heat transfer in porous media are examined numerically to determine the effects of pore to throat size ratio on permeability, interfacial convective heat transfer and thermal dispersion coefficients. The heat and fluid flow in periodic porous media consisting of rectangular rods are investigated. A representative elementary volume is considered and the continuity, Navier-Stokes and energy equations are solved to determine the velocity, pressure and temperature fields in the voids between the rods. It is shown that the pore to throat size ratio is a significant parameter which should be taken into account to suggest a wide applicable correlation. Based on obtained computational results, correlations for determination of Kozeny constant and interfacial heat transfer coefficient in terms of pore to throat size ratio and other related parameters are proposed. An experimental study was conducted to validate the numerical results of the present study. In the experimental part, a porous channel of square rods is used and the permeability and thermal dispersion coefficient are validated with the aid of experimental measurements. A good agreement between the experimental and numerical results is observed.