Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
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Master Thesis Numerical Study on Gasoline Direct Injection Sprays(Izmir Institute of Technology, 2015) Güleç, Yağmur; Diez, Alvaro; Mobedi, MoghtadaGasoline direct injection technology is a combination of the advantages of diesel engine and spark ignition engines. Hence, the gasoline fuel is injected directly to combustion chamber. It provides finer atomization to allow evaporation more rapidly than traditional gasoline engines and less emissions in terms of NOx and particulate than diesel engines. The understanding of spray behaviour is a significant point to control optimal air/fuel mixture and misfiring. This thesis presents a numerical investigation of a GDI injector under flash and non-flash boiling conditions. The study is performed with sprays injected into a constant volume chamber. The purpose of the thesis is to develop a CFD model to compare it with experimental spray penetration and spray angle results. Firstly, the model is validated in terms of spray penetration. Secondly, the model is applied to nine different conditions including flash and non-flash boiling conditions. The spray penetration and angle comparisons between numerical and experimental results were done.Master Thesis Stochasting reactor model for surrogate diesel fuels(Izmir Institute of Technology, 2015) Sandal, Gökçen; Mobedi, Moghtada; Diez Rodriguez, Alvaro; Mobedi, MoghtadaThe objective of this thesis is to develop of various surrogate fuels with computational engine simulation tools based on stochastic reactor models (SRMs) in direct injection engines (DI). The SRM is based on probability density function (PDF) approach which has important ability to calculate complex reactions, energy equations and mass transfer. It is also successful to control the effects of inhomogeneities and turbulence with low computational cost. Since the diesel fuel is very complex to solve in numerical tools, surrogate fuels were tested in this study. N-heptane, n-heptane / toluene and n-heptane / isooctane mixtures were used as surrogate fuels due to their physical and chemical properties were very close to diesel fuel. These fuels were tested under different injection timings and the calculated results were compared with an experimental study’s data. Comparison of ignition delay results demonstrate a good agreement between theoretical and experimental data, but soot formations were different when compared to literature and reference study. Fuel properties, structures or discrepancy between engine operating conditions and numerical model could be the cause of these differences.Master Thesis A Study on Cop Improvement of a Household Refrigerator by Using an Adsorption Heat Pump(Izmir Institute of Technology, 2015) Arslan, Gizem; Barışık, Murat; Mobedi, MoghtadaThe thesis aims to increase the coefficient of performance of a domestic type household refrigerator. A household refrigerator is a kind of mechanical heat pump which works based on the vapor compression cycle. On the other hand, adsorption heat pump is a thermal heat pump and it can directly operate with any kind of third energy source such as solar, waste heat. The heat released from the condenser of a mechanical heat pump provides the desorption of the adsorbate in the adsorbent bed and helps the transfer of the adsorbate to the condenser of the adsorption heat pump. Then, the adsorbent bed which has low level adsorbate concentration provides the evaporation of the adsorbate in the evaporator and generates a cooling effect. By this way, an additional cooling effect can be generated without the increase of energy consumption of refrigerator. In this thesis, a detailed literature survey on the combination of hybrid cooling systems is done. The suggested hybrid cooling systems are classified, and their advantages and disadvantages are discussed. In order to analyze heat and mass transfer of an adsorption heat pump, different conditions for adsorbent bed design are studied, theoretically. To design an adsorption heat pump the effects on the heat and mass transfer should be well known. Theoretical studies on heat and mass transfer in a rectangular adsorbent bed is performed to understand what can be effected on the heat and mass transfer in an adsorbent bed. Heat and mass transfer equations for a rectangular adsorbent bed are derived for non-uniform pressure approaches and numerically solved to determine temperature and concentration profiles in the bed with using Comsol Software. Furthermore, our CFD program coded in FORTRAN language. The code is used to validate the obtained results from Comsol. An experimental setup was designed and constructed in the light of these numerical studies. The enhancement of performance of a household refrigerator is calculated by using an adsorption heat pump.Master Thesis Numerical Determination of Permeability and Interfacial Convective Heat Transfer Coefficient for Non-Isotropic and Periodic Dual Scale Porous Medium(Izmir Institute of Technology, 2015) Sabet, Safa; Barışık, Murat; Mobedi, MoghtadaIn this study, the fluid flow and heat transfer in a periodic, non-isotropic dual scale porous media consisting of permeable square rods in inline arrangement is analyzed to determine permeability and interfacial convective heat transfer coefficient, numerically. A periodical representative elementary volume (REV) with the dimensions of H×H is chosen as the computational domain. The flow in the REV is assumed fully developed and periodical. The permeable square particles are placed with in-line arrangement. There are two symmetrical intraparticle pores considered here which are in longitudinal flow direction. The continuity, Navier-Stokes and energy equations are solved to obtain the velocity, pressure and temperature distributions in the unit structures of the dual scale porous media. The obtained fields are upscaled by using volume average method to obtain the intrinsic inter and intraparticle permeabilities, bulk permeability tensor, interfacial convective heat transfer coefficients and the corresponding Nusselt numbers of the dual scale porous media for different values of inter and intraparticle porosities. The study is performed for interparticle porosities between 0.4 and 0.75 and for intraparticle porosities range of 0.2 to 0.8. A correlation based on Kozeny-Carman theory in terms of interparticle and intraparticle porosities and permeabilities is proposed to determine the bulk permeability tensor of the dual scale porous media. The intraparticle porosity value increase the flow rate passes through the porous media and the particle becomes more permeable. However; for high interparticle porosity values, the intraparticle porosity does not have importance effect on bulk permeability. Additionally, the results predicts that the interfacial convective heat transfer coefficient increases with increase of Reynolds number and the ratio of intra to interparticle porosity, while the increase rate shows variation with the porosity ratio and Reynolds number values.Master Thesis A Research on Design of Heating, Ventilation and Air Conditioning of Hygienic Spaces in Hospitals(Izmir Institute of Technology, 2008) Anıl, Orkun Baki; Mobedi, MoghtadaThere are various sterile spaces in hospitals which are highly at risk in terms of infection. HVAC systems play an important role on the infection risk in sterile spaces.HVAC systems for sterile spaces focus on number of particles, number and types of microorganisms, pressure difference between the sterile and its neighboring spaces, supply air velocity and air distribution in addition to the comfort parameters which are temperature, relative humidity and fresh air rate. Therefore, the design of HVAC systems for sterile spaces is more difficult and complicated compared to the comfort applications.This study firstly reviews the design parameters of HVAC systems of sterile spaces in hospitals. A literature survey is conducted about the effects of the design parameters on infection transmission in sterile spaces. Also a literature survey on various standards and design guidelines for HVAC design parameters of sterile spaces used by different countries is performed. Secondly, an experimental study was conducted in an operating room to reveal the system performance during operation.The reviewed standards and guidelines are compared via tables. The weak and strong points of the reviewed references are noted. Despite well known effects of HVAC systems on infection transmission, significant differences among standards and guidelines are observed. Based on the obtained experimental results, considerable differences of design parameters between .in operation. and .at rest. states were observed.Master Thesis Conjugate Natural Convection Heat Transfer in a Cavity With Finite Wall Thickness(Izmir Institute of Technology, 2009) Hakyemez, Erinç; Mobedi, MoghtadaThe effects of a heat barrier, located in the thick ceiling wall of a square enclosure, on conjugate conduction and natural convection heat transfer are investigated numerically. The analysis is performed by numerical solution of the continuity, unsteady momentum conservation and energy equations with finite difference solution method based on the streamfunction-vorticity formulation. The vertical walls of the enclosure are differentially heated and horizontal walls are adiabatic. A thin heat barrier, having infinite thermal resistance, is located in the ceiling wall at different locations. The calculations are made for different Rayleigh numbers (103 Ra 106), thermal conductivity ratios (1 K 100), dimensionless locations of heat barrier (0<Xh<1) and two dimensionless ceiling wall thicknesses (D . 0.05 and D . 0.20). By using the results of the computer program, streamlines and isotherms are plotted. Heatline visualization technique is used to simulate heat transport and the effect of heat barrier is presented by comparing and plotting heatlines for the cavity and for the solid region with and without heat barrier. The study is performed for air with Prandtl number 0.71. It is found that the effect of heat barrier is more significant in the cavity with high thermal conductivity ratio but low Rayleigh number. There are certain reductions in the average Nusselt number at the vertical walls of the cavity and dimensionless heat transfer rate of the solid region walls for high conductivity ratios, but the reduction in dimensionless heat transfer rate is greater.Master Thesis A Study on Mixed Convection Heat Transfer Through a Channel Partially Filled With Porous Medium(Izmir Institute of Technology, 2012) Çelik, Hasan; Mobedi, MoghtadaA study on mixed convection heat transfer in a laminar, fully developed, vertical channel is performed for three different cases: i) clear fluid channel ii) saturated porous medium filled channel iii) partially porous medium filled channel. For the all analyzed cases, motion and heat transfer equations are solved both analytically and numerically. The governing equations are presented both in dimensional and dimensionless forms. The dimensional forms of the governing equations are solved by numerical method while dimensionless equations are solved analytically. The dimensional results, obtained by numerical method, are converted into dimensionless values and compared with dimensionless results of analytical solutions. Good agreement between analytical and numerical results is observed. Based on the obtained results, velocity and temperature profiles are plotted for different values of Gr/Re, Da and porous layer thickness. A detailed discussion is performed on the obtained results. Moreover, heatline functions are obtained and plotted for different values of Gr/Re, Da and Peclet number. It is found that flow reversals in the channel highly depends on Gr/Re value and flow reversals occurs in the channel if Gr/Re exceeds threshold value. It is also found that for low Peclet numbers (i.e., Pe = 0.01), the path of heat flow is independent of Gr/Re and Darcy number. However, for high Peclet numbers (i.e., Pe = 5), the ratio of Gr/Re, Darcy number and thermal conductivity ratio influence heatline patterns, considerably.Master Thesis Investigation of Flow Through a Semi Axial Centrifugal Pump(Izmir Institute of Technology, 2006) Karamanoğlu, Yılmaz; Mobedi, Moghtada; Mobedi, MoghtadaThe aim of present study is to perform a numerical work to investigate flow inside a semi axial centrifugal pump. The results of the study can be used to improve the design of the pump.The Navier Stokes equations with appropriate boundary conditions are solved for a 3 dimensional rotating geometry. To solve the governing equations, Fluent program is used. Fluent is a commercial CFD program, which has been developed based on the finite volume method. The mesh for the flow volume is created by Gambit.The k -e turbulence model is used to handle the turbulence inside the flow. Obtained results are compared with experimental test results. An acceptable agreement between the numerical and experimental results is observed.Based on the obtained results, the velocity vectors, pressure distributions on impeller and diffuser blades and flow patterns are plotted for three different flow rates.The necessary discussions are performed for these results. It is observed that the design of the impeller provides a regular flow inside the channel; however some vortexes are observed in the channel between the diffuser blades.The effect of the surface roughness is also investigated and the problem is solved for four different roughness values (0, 50, 100 and 250 m). It is found that the roughness of the surface affects the characteristic curves of the pump. By increasing the roughness, the head and efficiency of the pump decrease however the consumed power is almost constant.Master Thesis A Study on Isotherm Characteristics of Adsorbent-Adsorbate Pairs Used in Adsorption Heat Pumps(Izmir Institute of Technology, 2011) Yıldırım, Zeynep Elvan; Mobedi, Moghtada; Ülkü, SemraAdsorption heat pumps are promising systems due to their ability to recover heat at low temperature levels and to provide cooling and/or heating effects. It has advantages such as using renewable energy sources, being environmental friendly, having no vibration and lower operation costs. In this study, the most common pairs used in adsorption heat pumps were reviewed and their thermophysical and adsorption behaviors were discussed. The adsorption equilibrium phenomena were explained in details. The different experimental methods for determination of adsorption equilibrium for pairs were explained and compared. The change of heat of adsorption according to the varying adsorbed amount was studied numerically for two different equilibrium equations. It is found that, the heat of adsorption for pairs fitting Dubinin-Astakhov equation has a decrease with increasing adsorbed amount, while the heat of adsorption for pairs fitting Freundlich equation does not depend on adsorbed amount. A numerical study was performed to investigate the performances and cooling capacities of three different adsorption chillers for six different pairs. The results showed that when S40 silica gels - water pair is used, the chiller cooled with low temperature source gives the highest performances.In the experimental section of the present study, a volumetric setup was designed and constructed. Experiments were performed for a conventional pair of silica gel - water. It was observed that the maximum adsorption capacity of the experimented silica gel -water pair was 21% (kg water vapor /kg silica gel) at 35 °C, 19% (kg water vapor/kg silica gel) at 45 °C and 11% (kg water vapor /kg silica gel) at 60 °C.Master Thesis A Theoretical Study on Enhancement of Heat Transfer in a Solar Air Heater Collector by Using Porous Media(Izmir Institute of Technology, 2011) Çekmer, Özgür; Mobedi, MoghtadaIn this study; a theoretical work is performed on heat and fluid flow in a solar-air heater collector partially filled with porous medium. The most of the efforts are paid to understand the characteristics of heat and fluid flow in a duct filled with the porous media. Various porous-clear media configurations are investigated. The buoyancy effects are neglected. The air flow in the channel is assumed to be incompressible, laminar, and hydrodynamically fully developed. For the porous regions, Darcy and Darcy-Brinkman models are used to obtain the velocity profile. In all chapters, dimensionless forms of the governing equations for the heat and the fluid flow are used. In the chapters with asymmetric cases, three different Nusselt numbers are defined; two individual Nusselt numbers for the upper and the lower wall, and an overall Nusselt number. The study is mainly focused on heat transfer enhancement by using porous media and only for a few cases pressure-drop analyses are performed. It is concluded that the use of a porous medium affects the heat transfer significantly, and the rate of effect is influenced by the thickness and location of porous media. Finally, a twodimensional study including thermally entrance region for an air-solar collector with partially filled porous medium is performed. It is observed that the outlet and mean temperature along the channel are significantly affected by upper wall energy loss, and thickness and conductivity of the porous medium.