Phd Degree / Doktora

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

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Access type: Open accessSubject: search.filters.subject.Heat transfer

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  • Doctoral Thesis
    Heat and Mass Transfer Characteristics of Adsorbents in Heat Pump and Refrigerator
    (01. Izmir Institute of Technology, 2023) Gündoğan, Şefika Çağla; Çakıcıoğlu Özkan, Seher Fehime
    Due to increasing drought, pandemic and climate crisis in recent years, researchers have increased their studies on environmentally friendly energy use. Although there are technological developments in the production of energy from renewable energy sources, the storage of this produced energy is still a problem that awaits a solution. At this point, adsorption heat pumps with high primary energy efficiency come to the fore as a developing technology and attract the attention of researchers. However, low heat transfer properties in the adsorbent bed reduce the performance of adsorption heat pumps and limit their applications. The aim of this thesis was to improve the performance of the adsorption heat pump by increasing the effective thermal conductivity within the adsorbent bed. Two methods have been employed to enhance the effective thermal conductivity. In the first method, which was unconsolidated adsorbent bed design, it was aimed to increase the thermal conductivity of the bed with metal additives with a continuous structure. In the second method, which is known as consolidated bed design, the effective thermal conductivity of zeolite 13X was tried to be enhance with a high conductive material, reduced graphene oxide. In the experimental studies, it was observed that the thermal conductivity increased from 0.12 W/m.K to 0.28 W/m.K in unconsolidated bed design by means of fin-shaped metal additive. In consolidated adsorbent bed design, the effective thermal conductivity of the Graphene/Zeolite 13X (1 wt% Graphene) was determined as 0.1613 W/m.K. In theoretical studies, the effect of thermal conductivity in the temperature, pressure and adsorbate concentration was investigated. The indirect and direct effects of effective thermal conductivity on specific cooling/heating power (SCP/SHP) and coefficient of performance (COP) values were examined. Although the effect of thermal conductivity on COP was ignored in most of the studies in the literature, it was observed that COP increased from 0.01 to 0.10 when thermal conductivity increased from 0.12 W/m.K to 1 W/m.K.
  • Doctoral Thesis
    Experimental Investigation of a Hybrid Thermal Management System for an Electric Vehicle Battery Module
    (Izmir Institute of Technology, 2022) Coşkun, Turgay; Çetkin, Erdal
    Environmental concerns and limited energy sources of the world are driving force in electric vehicle technology improvements. One of the main components of the electric vehicles is battery cell. Using batteries in electric vehicles brings up new concerns such as safety problems, limit of range and so on. The temperature of the battery cell increases during charging/discharging and operation. There is an optimal temperature range (15ºC ─ 35ºC) for battery cells to maximize efficiency and prevent safety issues. The high temperature values in the battery cells can be result with fire and explosion. In addition, the performance of the battery cells is highly affected by operating temperatures. Therefore, thermal management of the battery cells is a necessity to overcome safety issues and maximize the battery performance. The feasibility of microchannel heat sink for battery cooling is investigated numerically and it is decided to continue with conventional length scales because of the higher pressure drop values in micro scales. Thus, a hybrid cooling system, using air and liquid solely or simultaneously, is developed and is introduced to a battery module. The battery module created by connecting three lithium-ion pouch cells in serial. According to the results, air cooling gives the more homogeneous temperature distribution. The lowest temperature values are observed in hybrid cooling system and temperature difference between the cells are reduced by 30% when compared to the water-cooling system. The temperature profile in air cooling shows that any increase in the ambient temperature (23ºC) or discharge rate will undergo a temperature rise in battery cells and optimal temperature ranges will be exceeded in that case. A step function, in a sequence of various discharge rate, is introduced to the battery module to determine cooling capacity of the air system during operation. The result show that the temperature of the cells is kept below 30ºC. The hybrid cooling is enabled to select cooling systems for the battery module with respect to operating condition; hence, the efficiency of the system is increased.
  • 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
    Experimental and Numerical Analysis of Flow and Heat Transfer in Double Skin Facade Cavities
    (İzmir Institute of Technology, 2016) İnan, Tuğba; Başaran, Tahsin
    In this study, airflow and heat transfer in a double skin facade (DSF) cavity were examined numerically and experimentally under natural and forced flow conditions. An experimental setup was constructed i the laboratory environment. Experiments were performed for two different DSF's airflow modes; buffer zone and external air channel. These experiments vere conducted with and without a solarsimulator integratrd t the system. Furthermore, the effect of pressure drop elements in the cavity of DSF were analyzed experimentally. After the numerical results (CFD and nodal network) were verified with experimental measurements, dimensionless heat transfer correlations were developed for the natural and forced convections. As a result, an extensive experimental data set was obtained for different working conditions of DSF. So, the dimensionless pressure loss coefficients were calculated experimentally based on the geometric configuration of the pressure drop elements in the cavity. In natural convection, with Rayleigh numbers ranging from 8.59*109 to 1.41*1010 and the increasing tendency of the average Nusselt numbers from 142.6 to 168.8 were shown. A correlation for a cavity characteristic length of 0.116 was constructed to evaluate the heat flux. In forced convection, another dimensionless correlations weredeveloped to predict the heat transfer by using. Nusselt numbers with in the Reynolds numbers ranging from 28000 to 56000 for a DSF with an external airflow mode. These correlations could be used for different characteristic length ranged betwen 0.1 and 0.16. These correlations were used for the energy performence of DSF applications for different directions and climatic zones in Turkey and compared with the single skin facede.
  • 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.