LEE-Makina Mühendisliği-Doktora
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Sustainable Development Goal "none" ile LEE-Makina Mühendisliği-Doktora'a göz atma
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ÖgeFabrication and characterization of hybrid nanofiller reinforced polyurethane nanocomposites(Lisansüstü Eğitim Enstitüsü, 2021) Navidfar, Amir ; Trabzon, Levent ; 695278 ; Makina mühendisliğiPolyurethane (PU) foams, which are economic due to their low density, low cost and easy processability, are frequently used in a wider range of applications, such as insulation goals, automotive and electronic industries. However, their applications are limited because of some poor properties. Nanomaterials were generally used to enhance desired properties of polymeric matrices. Combining varied nanofillers with dissimilar dimensions can lead to synergy through effective dispersion. Carbon-nanofillers are known for improving the desired properties of polymers. The dispersion quality of nanofillers in the matrix is vital for the fabrication of high-performance nanocomposites. Hybrid nanocomposites possess better properties in comparison with conventional nanocomposites that lead to the formation of an effective network. The thesis is composed of seven chapters and it is organized as follows: The first chapter will give a detailed literature survey and technical background for the research thesis and the last chapter is a concise but complementary conclusion with inspiring recommendations for future researches. The second, third and fourth chapters are a copy of published articles, where the following two chapters are complementary research findings as well as they are also articles in the press. In brief, the content of chapters related to research findings is to be given. In the second chapter, the effects of nanofiller addition into polyurethane on mechanical properties and thermal stability by means of tensile, Charpy impact, hardness tests, and thermogravimetric analysis were studied. Nanofillers added to polyurethane are multi-walled carbon nanotubes (MWCNT), two types of silica nanoparticles, and MWCNT/silica as hybrid fillers. Hybrid polyurethane/silica/MWCNT nanocomposite with the constant overall content of 0.75 wt% showed higher tensile strength, hardness, and thermal stability than either of nanofillers at this content, which approves a synergistic effect between multi-walled carbon nanotubes and silica nanoparticles. In the third chapter of the dissertation, micromechanical modeling and mechanical properties of PU hybrid nanocomposite foams with MWCNTs and graphene nanoplatelets (GNPs) were investigated through tensile strength, hardness, impact strength and modified Halpin–Tsai equation. Three types of GNPs, with varied flake sizes and specific surface areas (SSA), were utilized to study the effect of GNP types on the synergistic effect of MWCNT/GNP hybrid nanofillers. The results indicate a remarkable synergetic effect between MWCNTs and GNP-1.5 (1:1) with a flake size of 1.5 μm and a higher SSA (750 m2/g), which tensile strength of PU was improved by 43% as compared to 19% for PU/MWCNTs and 17% for PU/GNP-1.5 at 0.25 wt% nanofiller loadings. The synergy was successfully predicted using a unit cell modeling, which the calculated values agree with the experimental results. Combining various carbon nanofillers with different dimensions can lead to a synergistic effect through the formation of an efficient conductive network. In the fourth chapter, hybrid PU nanocomposites containing MWCNTs and GNPs were fabricated to study experimental and theoretical aspects of thermal conductivity (TC) enhancement. The optimization of hybrid nanofillers combinations was done to synergically enhance the TC using various types of graphene, nanofillers concentrations and ratios. A synergistic thermal conductivity improvement with MWCNTs and GNPs was confirmed at low nanofillers contents. The TC of hybrid nanocomposite at 0.25 wt% is approximately equivalent to the TC of individual nanofillers at 0.75 wt%. An analytical model for the effective thermal conductivity of single and hybrid nanocomposites was considered with variables of volume fraction, interfacial thermal resistance, straightness of the nanofillers and the percolation effect, in which the predictions of the modified models agreed with the experimental results. In the fifth chapter, an effective approach for improving dispersion states of MWCNTs and GNPs was employed via hybrid inclusion of the nanofillers in polyurethane matrix to further enhancing viscoelastic properties. Nanocomposites based on MWCNTs, two groups of graphene and hybrid MWCNT/graphene with varied weight fractions and ratios were fabricated via a simple, quick and scalable approach. Dynamic mechanical analysis results indicated an improvement of up to 86% in storage modulus at 25ºC for hybrid MWCNT/GNP-S750 at only 0.25 wt% loading, whereas solely MWCNTs and graphene nanocomposites showed 9% and 15% enhancement at the same content, respectively. The glass transition temperature value was enhanced by about 9.5 ˚C with 0.25 wt% inclusion of well-dispersed three-dimensional MWCNT/GNP-S750 structure, which disclosed a noticeable synergistic effect in thermomechanical properties. In the sixth chapter of the study, the acoustic and dielectric properties of PU hybrid nanocomposites were investigated. PU containing MWCNT and GNPs were used to evaluate the effects of single and hybrid nanofillers on the final properties of nanocomposites. The results showed a synergistic effect between nanofillers, in which the hybrid nanocomposites exhibit better performance, relative to the single inclusion of the nanofillers. These hybrid nanofillers improved dispersion quality in the polymer matrix due to the formation of a GNPs/CNTs 3D architecture, in which acoustic transmission loss and dielectric constant of PU were enhanced by about 51% and 13% at 0.25 wt% loadings, respectively. The overall properties of the hybrid nanocomposite revealed the superiority over the single nanofiller system in multifunctionality, evaluated by a performance index. Finally, to compare diverse nanocomposite systems, the performance index (PI) formula was introduced that is based on mechanical, thermal, acoustic and dielectric results of nanocomposites, in which PU with hybrid GNPs/CNTs showed higher PI compared to single GNPs and CNTs, approving its higher multifunctionality. The high multifunctionality of the hybrid nanocomposites in comparison with single nanofiller included nanocomposites reveals the superiority of the hybrid approach that is appropriate for a wide range of applications.
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ÖgeNumerical and experimental investigation of the effect of refrigerant mixtures on the refrigeration system(Fen Bilimleri Enstitüsü, 2020) Özsipahi, Mustafa ; Güneş, Hasan ; 620640 ; Makine MühendisliğiIncreasing energy costs urge not only developing but also developed countries to have new regulations on energy prevention and recovery. Several protocols have already applied to reduce global warming and ozone depletion potential. These protocols force producers and suppliers to work with less harmful refrigerants in the refrigeration and air conditioning sector. Besides using environment-friendly refrigerants, it is also demanded to increase the energy efficiency of such devices to compete with the rivals of the global market. Reciprocating compressors in the refrigeration systems such as refrigerators and freezers are the main components in the household appliances. Moreover, the largest part of the electric consumption related to refrigerators is caused by compressors. Due to the increasing demands on new refrigerants considering their efficiency and the regulations for the protection of the environment, the natural refrigerants or refrigerant blends are widely used. Within the study, the effect of refrigerant mixtures on the household refrigeration system is studied. In addition to the refrigerant mixtures, the optimum lubricant is investigated for the reciprocating compressors. In the first chapter, a brief introduction to the vapor compression refrigeration systems is given. Lubricants and compressors used in vapor compression refrigeration systems are mentioned and a comprehensive literature survey is presented in this chapter. In the second chapter, experimental studies are discussed. This chapter divided into two parts. In the first part, the effect of refrigerant mixtures on the household refrigeration is studied. In this context, in house refrigerant mixtures test stand is constructed. The performance evaluation of the three different refrigerant mixtures of R600a/R290 is compared with R600a. The performance results of the test stand are compared with calorimeter tests carried out by Arcelik A.S. and the results of both test stands are showed that increase in energy efficiency is possible by using R600a/R290 refrigerant mixture. In the second part, the lubrication system of the hermetic reciprocating compressor is investigated in detail. Oil management and lubrication mechanism in journal bearings may change drastically between compressor maximum and minimum speeds. Nevertheless, sufficient lubrication should be provided to the bearings and all moving parts to avoid any mechanical damage on the compressor. In this context, in house lubrication test bench is built using various instruments. Experiments are conducted to investigate the effect of viscosity and compressor speed on the oil mass flow rate of the compressor. In addition to the oil mass flow rate measurements, flow visualization is performed and flow patterns inside the compressor are given for start-up and steady-state operating conditions. In the third chapter, numerical investigations are presented in detail. This chapter is divided into two sections. Firstly, the numerical modeling of the lubrication system of a compact inverter compressor (CIC) is presented. In the numerical modeling, a finite volume-based ANSYS-FLUENT package is used to model two-phase (air-oil) flow inside the compressor using the Volume of Fluid Method (VoF) method. Transient behavior of the oil flow under laminar flow conditions is both simulated by imposing Sliding Mesh (SM) and the Moving Reference Frame (MRF) methods at various crankshaft speeds varying between 1200 and 4500 rpm. The measurements are used to compare/validate CFD results obtained from SM and MRF methods. Moreover, the start-up behavior of the compressor is studied and the instantaneous flow field is given for both methods. Advantage and disadvantage of the methods are mentioned in the study.